Propulsion Systems

Propulsion systems are based on direct mechanical drives, diesel-electric or diesel-hydraulic drives. The ultimate goal is to achieve efficiency at the propeller.

Design Considerations

At its simplest, a propulsion system consists of an engine, reduction gear, propeller shaft and bearings, and a propeller. When I first started looking at propulsion systems for trawlers, I was perplexed. Most designs feature a long rigid drive shaft to get the correct propeller angle. A lot of attention is paid to getting the shaft and engine aligned. Periodic re-alignment is not uncommon. The engine can suffer wear and tear because it bears the end thrust from the propeller. The engine is hard mounted to its bed, resulting in a lot of vibration and noise transferred through the hull.

I was perplexed because my experience was with racing and performance cars, trucks, and tractors and farm implements. Racing cars have long used flexible Constant Velocity (CV) joints, with engines of over 1000 hp. Tractors use flexible drive shafts running from a power-take-off (PTO) on the back. These shafts are extensible on splines, and use universal joints for alignment. There is little direct loading on the PTO bearings. My last heavy-duty diesel tractor had a fully hydraulic transmission and drive, including a separate ro-ro lever for rapidly rocking back and forth. I couldn’t understand the rigid arrangements in small trawlers. I thought there must be some double triple [1] marine secret nobody was talking about.

It turns out that the secret is this: (1) tradition; (2) hard fixed mounts are cheaper; (3) simpler is better (more reliable); and (4) tradition (i.e., mistrust of unknown technologies). There is something to be said for #1 and #4. All of the alternatives are more complicated, which is not what you want with a breakdown in a crisis.

The other reality is that the marine market for small boats is too small for many manufacturers to develop specialized versions of their drives for continuous-duty marine applications. This is why some areas innovate slowly, and at great expense.

Propellers

How Propellers Work

Like many aspects of ship design, designing propellers is a mixture of science and art in the form of experience. There is a lot going on at the stern of a ship that is poorly understood. This is not surprising. Every hull form, engine-propeller combination, interaction with the rudder wake region, and sea condition is different. Wave action introduces a random vertical moment into this equation.

For these reasons, most ship propellers operate at efficiencies around 56%, with the principle design problem being cavitation as rotational speed is increased. In comparison, aircraft propellers operate at 90% efficiency. On a ship a propeller efficiency of less than 50% indicates a poor propeller/hull design. Efficiency is measured as a ratio of the propulsive energy produced to the energy consumed in rotating the propeller shaft.

A propeller works by accelerating a mass of water. This generates an equal and opposite reaction in the propeller/shaft/engine assembly. This is the thrust that drives the boat forward. In short, the propeller pushes the engine – more about this poor state of affairs later.

When the propeller turns it accelerates a column of water through the screw. The pitch of the screw determines the acceleration and the distance traveled for each turn. In essence, this is like a nut turning on a screw. Increasing the pitch increases the weight of water moved and the horsepower required.

Since for efficiency a displacement boat should not exceed its hull speed (a function of the waterline length), and a propeller cannot exceed the available space under the stern, a propeller must be designed for the hull and the amount of thrust it must impart to overcome friction at the maximum hull speed. Arbitrarily increasing the speed of the propeller or making it larger without changing the design of the screw will not make the boat go faster. Indeed, at some faster rotational speed, the water around the blades cavitates excessively, and the boat will go slower.

Cavitation is the formation of a vacuum along parts of the blade. This lowers the vapour pressure of the water, causing air bubbles to form, just like in boiling water. This causes slip, loss of efficiency and pitting of the metal. Increasing the pitch of a propeller also increases its propensity for cavitation.

Consistency (accuracy) in pitch and camber are important to reduce the potential for cavitation. This also reduces vibration: as much as 80% of hull vibration can be caused by a propeller. Propellers should be balanced both statically and dynamically.

Fixed propellers are the most efficient, strongest and least expensive. But they are at their most efficient only at their designed rpm and hull speed. At other speeds, their efficiency falls off. A fixed propeller is ideal for a boat that cruises long distances at a constant speed. Boats that stop and start a lot or change speed frequently are better with a controllable-pitch propeller.

Number of Blades

The ideal propeller has one large blade, but this would be very unstable. It would set up a lot of vibration in the shaft.

Traditionally, trawler propellers were three-bladed. Modern propellers are four, five, six and seven-bladed designs. Six-bladed propellers, at first, were unsuccessful. Prior to the development of thrust systems, submarines were using designs with seven skewed (curved) blades. Generally, trawler blades are an aerofoil shape called B.Troost.

With more blades, vibration and noise are reduced, and thrust is more evenly distributed. For example, a boat with 200 shaft horsepower and three blades has 66.6 hp per blade. With five blades, the power distribution is 40 hp per blade. With more blades, a propeller can be smaller. However, larger propellers are better at moving water. Small blades move less water. To compensate, they have to turn faster. But increasing rpm reduces efficiency. Other factors also come into play. For example, higher block coefficients in the hull design reduce propeller efficiency.

Again, like so many boat design issues, the optimum is a compromise between several factors. Generally, the choice is between large slow turning and small fast turning propellers with an increased pitch angle.

Experience shows that the optimum for a single-screw boat is a four-bladed propeller with some skew, to compensate for the disturbed inflow. A four-bladed propeller may, however, increase sympathetic vibration, pushing you back to re-considering three or maybe five blades. A double-screw boat is better with two smaller three-bladed propellers turning slightly faster.

Propellers for nozzles (see below) should be square-tipped Kaplan designs. All propellers should be manufactured to ISO 484 Class 1 standards.

Controllable Pitch Propellers

The pitch of a propeller can be fixed-true, fixed-variable, flexible or controlled. Think of pitch as the number of threads per inch on a screw.

Fixed propellers have a fixed (true) or variable pitch from the tip of the blades to the hub. A fixed-pitch propeller has a constant pitch at any radius from the hub to the tip. Think of this as a screw with a constant number of threads per inch.

A variable-pitch propeller has a pitch that increases from the hub to the tip, but is still fixed in the metal. Think of this as a variable number of threads per inch, i.e., the threads are getting further and further apart. Variable pitch propellers reduce cavitation and are used on high-speed boats, or on very large boats to compensate for varying inflow velocities due to the draft of the hull.

Flexible-pitch (pitch-adapting) propellers are made of a metal hub with carbon-fibre composite blades inserted. The blade pitch angle adapts as the load on the blade changes, reducing cavitation and increasing hydrodynamic efficiency by around 15% [44]. Designed initially for high-end yachts, they are being tested starting in 2004 in larger sizes for the US Navy.

A controllable-pitch propeller has blades whose pitch angle can be adjusted mechanically while the propeller is turning. This requires a special hollow propeller shaft for the mechanical bits. This means the engine can run at its optimum rpm all the time. To speed up, or slow down, you change the pitch of the propeller. To go from forward to reverse, you simply reverse the pitch of the blades.

Controllable-pitch propellers yield the best overall fuel economy when vessel speeds vary. They are also an advantage in areas like Europe where fuel costs are higher. When vessel speed is constant most of the time, a fixed-pitch one is better because of the reduced complexity and cost.

Kort/Rice Nozzles

When a blade rotates in water, it creates a high-pressure behind the blades and low pressure in front (towards the hull). This differential provides thrust. However, at the edge of the blades, high pressure leaks to the low-pressure side, resulting in loss of thrust. In the 1930s, this caught the attention of Ludwig Kort, an aeronautical engineer from Hanover.

He developed nozzles (shrouds or ducts) for propellers. They prevented water from escaping from the blade tips. This improved propulsion efficiency at speeds under 10 knots.

Decreasing the exit side of the shroud, thus forcing the water to accelerate as it exited the propeller, made further improvements to the Kort nozzle. Eventually, several types of Kort nozzle were developed, with the Type 19 most suited to a trawler. Today, many companies make Kort Nozzles.

Later, the Mexican company Rice Propulsion [46] improved on the Kort nozzle by using aerodynamic duct shapes derived from wing sections displaying the highest lift and lowest drag.

Twin Screws are Better

Hydrodynamic studies show that twin screws are clearly superior to a single screw [15, 16]. The twin-screw environment under the stern is less chaotic. This means the propeller-wake interaction is more constant and, therefore, more efficient.

Twin-screw ships are generally easier to manoeuvre. A single-screw ship suffers from stern walk, a tendency for the stern to counteract the rotation of the screw by moving sideways. Twin screws are designed to rotate in opposite directions, cancelling any stern walk. Also, the two shafts can be used to "twist" the ship in docking manoeuvres.

To drive twin screws, you need two main engines or one engine with hydraulic or electric drive. As mentioned in Chapter One, twin engines are less fuel efficient.

One Rudder is Best

Contrarily, note that one rudder is better than two. Two rudders make the stern environment more chaotic again, and require a complicated steering system like on the front wheels of a car.

Unlike propellers, rudders are more likely to be a Naca shape (symmetrical). As a rule of thumb, rudder area = 0.5 * (waterline length * displacement). Articulated/deflector/high-lift-flap rudders common on work boats are appearing on yachts. They can reduce turning circles by 50%.

Engines

Number of Engines

From an emergency perspective, the debate over one engine or two will never be settled. Each person’s comfort zone is different when it comes to managing risk. From this perspective, there are these choices:

• No backup: One main engine with one propeller.
• Emergency power: One main engine with one propeller, plus a small wing engine with a small propeller.
• Redundant power: One main engine with dual electric drive, two propellers and battery backup; or two main engines with two propellers (drive systems are unimportant).

If you’re comfortable psychologically with the proven reliability of diesel engines, you will happily go to sea with one engine and one propeller, and save some money in the bargain. You will be in the excellent company of most small fishing trawlers.

If your mind baulks at the risk this represents, then, hang the cost!, you will want two engines and two propellers.

Realistically, a small wing engine with a wimpy propeller is unlikely to have enough power in a severe-weather emergency. Two main engines is the only way to provide true redundancy. Alternatively, you might consider a single engine with an electric drive – it can be configured to use the house bank as an emergency source of power.

From a layout perspective, twin engines require a larger engine room, and may dictate an amidships layout. Twins mean twice as much cost and maintenance, and duplicate charging and fuel systems (not necessarily bad). My own experience is that many farmers have more than one tractor, in case the primary one breaks down at a critical time in harvesting. Usually they end up with two tractors in some degree of disrepair at harvest.

For cost reasons and fuel efficiency, Sharina will have one main engine and shaft; and a life raft.

Diesel

The engine of choice for a passagemaker is a marine diesel from a manufacturer such as: Caterpillar, Cummins, Detroit Diesel, Deutz, Lugger, Man, Perkins, Vetus, Volvo Penta, Westerbeke, Yanmar, ZF-Marine [19-30]. Diesels are more efficient per gallon of fuel than a gasoline engine and, as discussed in Chapter One, much much safer. The only significant alternatives on the horizon are fuel cells. BoatDiesel.Com has a wealth of information on diesel engines, transmissions and propellers.

Invented by Rudolf Diesel circa 1898, diesel engines are pressure ignited. They use the familiar four-stroke Otto Cycle of intake-compression-combustion-exhaust [31]. Unlike a gasoline engine, which uses spark plugs to ignite a compressed mixture of air and fuel, diesel engines use the heat of compression to ignite the fuel-air mixture. Diesels use very high compression to do this. Like all modern engines, diesels use fuel injectors to spray fuel into the combustion chamber at the precise best moment.

A good diesel engine will produce about 20 hp per gallon/hour compared to 12 hp per gallon/hour for a gasoline engine [34]. Put another way, a 300-hp diesel will consume about 15 gallons an hour while a similar gasoline engine will consume about 25 gallons. At prices of $1.74 and $1.94/gal, this works out to $26.10 and $48.50/hour, respectively [35].

Diesels are very reliable and robust, but they are only as good as the fuel system. Most troubles with diesel engines stem from the fuel system, and can be prevented through maintenance and a high-quality filtering system. The engine should also be fitted with a starter button, bypassing the one on the bridge; and a dry sump and drip pan.

Fuel System

Diesel fuel deteriorates when stored. (So does gasoline, for that matter.) Diesel fuel has a shelf life of approximately 100 days. The chief causes of deterioration are oxidation, condensation, micro organisms, corrosion and sedimentation. Oxygen combines with the hydrocarbon molecules, forming particulates, water and gum. Condensation inside the tank deposits more water. Bacteria and fungi collect and grow at this water-fuel interface in the fuel tank. They eat diesel fuel and give off more water and acids. Corrosion in the fuel tank releases metal into the fuel, accelerating the oxidizing process.

Water, microbial growth, and stray voltages can cause corrosion in steel tanks. In addition, fuel can be contaminated with water and sediments on delivery; and air can be drawn into fuel lines where fuel is leaking. Sediments, moisture, and air can block fuel filters or disrupt an injector. Sediments such as clay and sand originate in the crude oil; others from the sludge that precipitates as the fuel deteriorates.

Good practice is to keep tanks full to avoid condensation, and minimize mixing fuel from different sources, as this can reduce the stability of the fuel. Condensation on the outside of a tank is a good sign! It means the dew point inside the tank will not be low enough for condensation inside. Religiously maintain filters, injectors and fuel pumps, and inspect fuel lines for leaks. It also helps to use a petroleum solvent such as Pri-D or Run-Right [32-33], but avoid the regular use of biocides, as they are corrosive. Note also that diesel substitutes like bio-diesel, made from vegetable oils, animal fats, and recycled cooking oils, introduce new sets of problems.

A high-quality filtering system consists of at least three micron filters, water separator and a circulating fuel system to polish fuel. The term polishing means cleaning the fuel. A polishing system should have a high flow rate to turn over a tank quickly and also because most filters are designed internally to work better with a higher flow. Depth-type filters are better than edge-type filters.

Avoid using a day tank. It is better to constantly polish the fuel in the main tanks. A day tank holds enough fuel for a day’s run, but if you don’t use all of the fuel, the day tank becomes just another fuel tank with the attendant problems.

Fuel should be polished at least three times a week; and always returned to the tank of origin. A top-outlet fuel tank eliminates the risk of dumping fuel if a line breaks.

Tanks should not have open cross-connections as this will reduce stability. As the boat heels, fluid will run to the lower side, adding weight. Consider a fuel-shut-off valve outside the engine room.

A polishing system for two tanks and one engine is shown. It is designed to polish fuel with or without the engine running. When running, the filters can be bypassed if they clog and you need to hot-swop them. If the engine fuel pump fails, the polishing pump serves as a backup to keep the engine going. The vacuum gauge after the filters monitors their condition. A new set of filters will show about 3-inches of mercury. An engine will begin to starve around 10 inches as the filters become restricted [36].

Lubrication

Finally, to protect the engine from lack of lubrication during heeling, the engine should have a dry sump. Wet sumps are the most common. In a wet-sump engine, the lubricating oil, usually several quarts, is contained in a sump (oil) pan bolted underneath the engine block. During heeling, the oil runs to the side and, in the worst case, can starve the engine of oil, causing damage to the bearings. Oil can also slosh onto the lower cylinder walls, reducing power.

In a dry-sump engine, the oil is kept in an external tank. The tank can be as large as you want. Typically, there are two oil pumps, one for pressurising the oil galleries, another for scavenging the oil from the sump. While this provides protection during stormy weather, when the boat is being tossed around, it doesn’t provide full engine protection in a knockdown. The engine won’t suffer oil starvation in a knockdown, but it can still be flooded by oil if the scavenging pickup sucks air.

An ignition shut-down switch activated by oil pressure is also advisable. Such a switch (e.g., a universal voltage switch) should also be fitted with an alarm.

Cold Weather Starting

Consider adding a block heater, controlled by a thermostat, to the engine for starting in cold weather.

Fuel Cells

There are no alternatives to diesel fuel for ocean-going boats. On the horizon are two fuel-cell technologies: Proton Exchange Membrane Fuel Cell (PEMFC) and Direct Methanol Fuel Cell (DMFC) [14]. Fuel cells combine hydrogen and oxygen to produce electricity. The electricity is used to turn a shaft motor, just like in a diesel-electric system. With world reserves in rapid decline, and fuel-cell technology improving, fuel cells may well be widespread in marine use by 2010. They are already powering submarines and yachts.

With the USA declaring a 25-year war on terror, the world is not focussing on the depletion of oil supplies as it should. Estimates of world reserves range from 1,000 to 3,000 billion barrels of conventional oil still to be produced, the latter estimated by the US Geological Survey's World Petroleum Assessment 2000. At current and forecast rates of consumption, that is good until oil supply peaks about 2030 [9]. Availability may decline well before then, because of rising prices. The problem with peaks is that they can only be identified after the fact. In other signs of change, many lakes in the USA now only allow electric power for boats.

There are several fuel-cell technologies but only the PEMFC and DMFC are suitable for transportation. They operate at a fairly low temperature (50-100 C and 50-200 C, respectively) compared to other fuel cell technologies, warm up quickly and don't require expensive containment structures. Current devices the size of a small piece of luggage can power a car.
Fuel cells are rapidly occupying niches. On April 22, 2004, Kiel-based HDW launched the first of four fuel-cell submarines being built for Greece. The U212-class submarine is equipped with an air-independent 300-kW fuel cell propulsion system developed with Siemens. This submarine is 65 meters long and displaces 1,700 tons [10]. Several experimental yachts are also powered with fuel cells [11], and HaveBlue makes 20-80 kW fuel-cell gensets [12].

In October 2004, NEC unveiled a prototype laptop computer powered by a fuel cell. Several experimental cars, such as Honda’s 160-kmh (100-mph) 2004 FCX and BMW’s 296-kmh (185-mph) 2004 H2r also use fuel cells. Sharina would require a fuel cell producing about 150 kW.

The oxygen required for a fuel cell comes directly from the air. The hydrogen is not so readily available. It is difficult to store and distribute. The practical solution is to use a reformer to generate the hydrogen from some other fuel, such as methanol (wood alcohol). A reformer turns hydrocarbon or alcohol fuels into hydrogen. A basic fuel cell is about 80% efficient at producing electricity. But methanol reformers and fuel cells in combination are only about 30-40% efficient, so the overall efficiency drops to 24 to 32%, given a conservative electric motor efficiency of about 80%. DMFCs are around 40% efficient [14]. Methanol used to be distilled from wood but today is usually processed from natural gas.

Another approach generates hydrogen from the electrolysis of water, using electricity generated from solar panels or wind turbines. This would make a passagemaker completely independent of shore-based energy.

Exhaust System

The exhaust system can be a dry stack routed up the mast, or a wet system exiting the stern or side under water. In the wet system, seawater circulates in a jacket around the exhaust to cool it. Cast iron jackets tend to rust out after a few years.

Intuitively, I don’t like large underwater openings protected by an inverted plumber’s trap. Also, the pumping losses on the engine will be greater with a wet exhaust. But anecdotally more boats have had stack fires than have been swamped by wet exhausts. Wet exhausts are cooler, and use flexible hosing. Wet exhausts are also quieter, use a smaller muffler, and don’t broadcast soot or PM10 pollutants on the boat deck. They only soot up the hull when the boat rocks or yaws. Contrarily, Sharina will have a dry stack. Just like Norwegian ice-rescue boats.

Marine Drives

A standard marine drive system consists of an engine, reduction gear, propeller shaft and bearings, and a propeller. Alternate drives consist of an engine, hydraulic pump or alternator, shaft motor (hydraulic or electric), propeller shaft and bearings, and a propeller.

Mechanical Drives

Hard Mounts

In a standard mechanical-drive system, the engine is hard mounted to the engine bed. A reduction-gear transmission such as ZF reduces the engine rpm to the rpm required by the propeller. The propeller shaft is fixed, and great care is taken to align the shaft with the engine. Periodic re-alignment may be necessary. The advantages are simplicity, low cost, reliability, ease of repair, and maximum mechanical efficiency of around 95%.

The disadvantages are increased noise and vibration, central engine-room placement, and stress on the engine. Noise and vibration are transmitted directly through the hull. To get the right angle on the propeller shaft, the engine room usually has to be placed amidships. Finally, the thrust of the propeller pushes directly against the engine.

Constant Velocity

In a constant-velocity (CV) anti-vibration system such as the AquaDrive® [17], there is a propeller shaft and a separate drive shaft, coupled with a CV joint. The propeller shaft ends in a thrust bearing mounted to the hull. This bearing takes the end-thrust from the propeller and transmits it to the hull. The drive shaft has a CV joint at both ends, and is splined inside so that its length is adjustable. This allows the engine to be mounted on flexible rubber mounts.

The advantages are reduced noise and vibration, slightly more flexibility in the placement of the engine, and no stress on the engine.

The disadvantages are higher cost, higher maintenance due to the CV joints, and increased complexity of repair. But don’t take this too much to heart. CV joints are in most cars, trucks, buses, tractors and racecars. Other noise-reducing systems include the Centaflex AGM [49] and Rubber Design [50].

Mechanical V-Drive

A mechanical V-drive is a compact reduction-gear transmission that reverses the direction of the drive shaft. It allows the engine to be placed aft, facing aft, with the output shaft facing forward. The V-drive reverses the direction of the output to connect to the propeller shaft, usually at an angle of 10-15 degrees. Typical manufacturers are: Borg Warner, Halibrand, Twin Disc Marine Transmissions, Yanmar, and ZF.

With a V-drive, there is no reason not to have the engine room fully aft, taking full advantage of the forward hull for accommodation.

Hydraulic Z-Drive

A hydraulic Z-drive such as the Thrustmaster [37] or Olympic [38] is the next step up from a V-drive. In this layout, the engine drives a hydraulic pump. This connects to a hydraulic shaft motor via hydraulic lines. The shaft motor turns the propeller shaft. Typically, the shaft and propeller are mounted so that the propeller can swivel through 360 degrees. Thus, a Z-drive removes the need for stern thrusters. The engine does not have to be inline, and can be put anywhere convenient. Z-drives allow the engine room to be aft.

There is a bias against hydraulic drives because of their additional complexity, and perceived lack of efficiency and robustness. Hydraulic systems lose energy mainly through torque losses, and are typically 80-85% efficient. This is not quite as efficient as a straight mechanical or an electrical drive. If you have doubts about robustness, go watch some heavy equipment in operation, or the Yamaha TT600 dual-wheel drive motorcycle [18]. Z-drives of 4,000-hp and up are routinely fitted to tugs. The only thing against hydraulic drives is their cost starting at $50,000 and up.

Azipod®

An Azipod® (azimuth thruster) is a form of rudderless diesel-electric propulsion. They were adapted from icebreakers and developed in 1990 by ABB Group [52], primarily to drive large cruise ships like the Carnival Elation [53]. A pod combines the main electric drive motor and propeller in a casing beneath the ship. Typically the propeller is on the front of the pod. The pod rotates through 360 degrees, providing steering, stern thrusting and reversing.

Like other forms of diesel-electric propulsion, pods reduce fuel consumption because the diesel generator runs at an optimum fixed speed. Turning circles are reduced by as much as 30%.
Pods have also been designed with contra-rotating propellers, one at the front and one at the rear of the pod, to further increase fuel efficiency. Pods have been used on icebreakers like the USCG Mackinaw and on cargo ships like the MT Tempera [54]. Robust pods have been developed for icebreakers with strengthened propellers to chop the ice. And pods are starting to appear in larger yachts. Azipod® is a registered trademark of the ABB Group

Diesel-Electric

Diesel-electric propulsion systems are not new. They have been used in submarines, trains and buses. In a diesel-electric system, the engine drives a high output DC generator. The current from this drives one or more electric motors connected to the propeller shaft. The chief advantage of an electrical drive is increased performance throughout the speed range. This results because the engine can be run at its optimum RPM, while the speed of the boat is regulated by the electrical drive.

Much is said about the higher efficiency of mechanical drives, but electric drives are more or less as efficient as mechanical ones. The efficiency of an electric motor is measured in Watts-out/Watts-in, where Watts out are measured in hp x 746. Generally, modern electric motors (NEMA standard) have an efficiency between 87 and 97%, with larger motors being more efficient. Relevant standards are IEEE 112, JEC 37, and IEC 34-2. The main energy loss in electric drives is through energy transformed to heat [5, 6, 7].

Diesel-electric drives seem to have evolved at different times in different sectors. Rudolf Diesel, a refrigeration engineer, invented and developed the diesel engine between 1893-97 [2]. In World War I, British C-Class submarines in 1906 were gasoline-electric. The D-Class, in 1908, introduced diesel-electric [3]. In the railroad sector, in the United States, General Electric introduced gasoline-electric locomotives in 1913, switching to diesel in 1917 [4]. By the time I was on Onondaga, diesel-electric submarines had electric drives in excess of 6,000 shaft horsepower.

Electric drives are very reliable, with a MTBF of 25,000 hours (about 2.8 years of continuous running). Brushless (permanent magnet) motors are more robust and are becoming cheaper as patents expire. Commercial trawler systems such as FEYS are warranted for 10,000 hours (1.1 continuous years). Some other factors to consider are lower noise and fuel consumption, and costs around $20,000 for a 300-hp system [8].

Continuously Variable Transmission

Continuously Variable Transmissions (CVT) are mentioned here only for completeness. A CVT has a nearly infinite range of gear ratios. Like a controllable-pitch propeller, a CVT allows the engine to operate at its optimum rpm regardless of the speed. Until recently, CVTs were too expensive and unreliable for use in cars. They are unlikely to be adapted for marine use.

Thrusters

Bow Thrusters

The bow of a boat follows the stern. To see this, push a pencil across your desk, steering it from the stern. Imagine trying the same in wave action or currents, or when trying to dock. Enter the
bow thruster, a small electric or hydraulic motor and propeller system mounted in the bow to steer it. A bow thruster is essential on boats over 45 feet. They are appropriately named. Performance is dependent on the thrust of the system, not the horsepower of the motor. The amount of thrust developed is a function of horsepower, propeller design and tunnel diameter. Thrusters are available from such as Dickson, Great Water, Shipwrights, Vetus, and Wesmar [39-43]. Costs are in the range of $2,000 - $20,000.

Bow thrusters are mounted in a transverse tunnel that runs through the hull. For maximum efficiency (turning moment), they should be located as far forward and as low as possible. Make sure you can reach the propeller and motor in the tunnel, and the zincs, for servicing. If the system is mounted in a bulbous bow, consider installing a watertight hatch for access to the interior of the bulb.

Electric thrusters, available in 12, 24 and 48 VDC and from two to thirty horsepower, are suited to boats up to 50 ft. They work intermittently in short bursts of three to five minutes. A few continuous-duty units are available. Electric thrusters draw a very high current, from 100-600 Amps. For example, a 15-hp unit operating at 24 VDC draws 466 Amps. In practice, bursts will be five to ten seconds, so intermittent duty at this current is not a problem. It is impractical to run cables for this kind of current from the main batteries. The cables would be very thick and unwieldy, and difficult to route. There would be a large voltage drop across the cable. For these reasons, install a separate battery bank close to the thruster. It should be at least 24 VDC to reduce cable size and heat production.

For batteries, use either a gel cell or AGM (absorbed glass mat) type, with a suitable charger. These battery types are completely sealed and maintenance-free. Lead-acid batteries produce hydrogen gas while charging. This is harmful to you and the fittings in the accommodation. Hydraulic models operate continuously. They can produce over 100 hp. They are best for boats longer than 60 ft or where conditions warrant continuous use, such as docking on a river where there’s always current. Hydraulic units are much more expensive than electric ones. They make sense in larger boats requiring more horsepower, or boats that already have a hydraulic system.

With either electric or hydraulic, fit your system with a time delay. When changing direction, this allows the propeller to come to a stop before it reverses. This takes a major shock load off the drive train.

Tunnel diameters run 5-12 inches. In general smaller tunnels are better because they accelerate the water more. They can be placed further forward. They have greater structural integrity. A smaller opening also reduces drag from the aft wall of the tunnel, and disrupts laminar flow less. Consult your designer about putting an eyebrow fairing in front of the tube, or a scallop behind the tube, to reduce drag. Don’t put fine grids over the openings, as they clog up. Paint the inside of the tunnel with antifouling paint.

Bow thrusters come with either single or twin propellers. Each has its advocates. The efficiency of one versus the other is unclear. Twin propeller designs contra-rotate and are more complex. Propellers are usually square-bladed, made of Dupont Delrin® or Zytel® plastic, and designed to be efficient in both turning directions.

The thrust required is a function of displacement, waterline length, lateral water resistance, wind on the superstructure, the forward thrust of the stern, and the turning point on the hull (usually the transom). Some manufacturers’ recommendations are given below. Be sure to consult your designer/builder about the appropriate size for your boat.

Thruster Size (Based on Manufacturers’ Guidelines)
LWL (ft)	Thrust (ft-lb)
20-28	55-66
26-36	97-121
31-39	130-165
40-50	177-210
45-55	196
50-60	265-352
55-70	302
70-95	488

Stern Thrusters

Stern thrusters are not essential on boats under 60 ft. They let you make a nice show by parking sideways. They are redundant if you have a Z-drive. They should be placed well aft, for example, under the swim platform but keep in mind safety considerations.

David Myers has developed an innovative stern thruster mounted on the rudder. This Variable Angle Stern Thruster (VAST) can double as a get-home drive. [51]

Steering

Hydraulic rams are the standard steering mechanism; although DC stepping motors are beginning to appear. Steering is a vital single-point of failure, so a duplicate system is advised if you can afford it.

Summary

A modern propeller has four or five blades. Twin screws give superior efficiency and cancel stern walking. To drive twin screws, use two main engines or one engine with hydraulic or electric drive. Engine type should be diesel. A fuel polishing system is mandatory. Fuel cells are on the 10-year horizon. With a mechanical drive, use a controllable pitch propeller, CV joints and flexible engine mounts. Chief disadvantage of a mechanical drive is an amidships engine room. To maximize the accommodation by positioning the engine room aft, use a mechanical V drive, electric or hydraulic drive. Hydraulic drives are fixed or a 360-degree Z-drive that provides stern thrusting. Bow thrusters are a must-have on boats more than 45 ft. If the boat has a hydraulic system, thrusters should be hydraulic; otherwise electric is cheaper for boats under 50 ft. Stern thrusters are a nice-to-have. Hydraulic steering is necessary on larger boats.

References

1. Corrie Czenka introduced me to this phrase.
2. About, http://inventors.about.com/library/inventors/bldiesel.htm
3. Royal Navy Submarines (D-Class 1908 to 1911), http://members.iinet.net.au/~eadej/dclass.html
4. The Evolution of the Diesel Locomotive in the United States, Benn Coifman, 1994, the yardlimit, http://yardlimit.railfan.net/guide/locopaper.html
5. AC Motor Efficiency Guide, Rockwell Automation, http://www.reliance.com/mtr/b7087_5/b7087_5_3.htm
6. Australian Technology Showcase, http://www.ats.business.gov.au/ats-members/t-flux_brushless-ironless_d.htm [page no longer available]
7. Advanced Energy, http://www.advancedenergy.org/progressenergy/motor_efficiency.html
8. FAST Electric Yacht Systems, Inc., http://www.feys.org/
9. BBC, http://news.bbc.co.uk/1/hi/sci/tech/3623549.stm
10. Fuel Cell Today, http://www.fuelcelltoday.com/FuelCellToday/IndustryInformation/IndustryInformationExternal/NewsDisplayArticle/0,1602,4309,00.html
11. Fuel Cell Today, http://www.fuelcelltoday.com/FuelCellToday/IndustryInformation/IndustryInformationExternal/IndustryInformationDisplayArticle/0,1588,752,00.html
12. HaveBlue, http://www.haveblue.com/
13. Auto Stuff, HowStuffWorks, http://auto.howstuffworks.com/fuel-cell2.htm
14. Center for Renewable Energy & Sustainable technology, http://solstice.crest.org/hydrogen/hydrogen_fuelcell_intro.html
15. Naval Science 302: Navigation and Naval Operations II, OlDominion University, http://www.odu.edu/webroot/orgs/ao/mo/nrotc.nsf 0/714fcf01b722201185256a0100447b19?OpenDocument
16. J. D. Van Manen and P. Van Ossanen, Principles of Naval Architecture, Second Revision, Volume II: Resistance, Propulsion, and Vibration, Society of Naval Architects and Marine Engineers, Jersey City, New Jersey USA, 1988, E. V. Lewis, Editor.
17. AquaDrive, http://www.aquadriveusa.com/advantage/advantage.htm
18. Yamaha Design Café, Yamaha, http://ymedc.introweb.nl/en/archive/enduro/lars_interview.shtml
19. Caterpillar, http://www.caterpillar.com/
20. Cummins, http://www.cummins.com/
21. Detroit Diesel, http://www.detroitdiesel.com/
22. Deutz, http://www.deutz.de/
23. Lugger, http://www.northern-lights.com/
24. MAN Engines & Components, http://www.MAN-MEC.com
25. Perkins, http://www.Perkins-Sabre.com
26. Westerbeke, http://www.westerbeke.com/
27. Vetus, http://www.Vetus.nl
28. Volvo Penta, http://www.volvo.com/volvopenta/se/sv-se/
29. Yanmar, http://www.yanmar.com/
30. ZF-Marine, http://www.ZF-Marine.com
31. How Engines Work, http://www.keveney.com/Engines.html
32. Power Research Inc., http://www.priproducts.com/
33. C.A.T. Products, Inc., http://www.run-rite.com/
34. Fuel Horsepower, Tony Athens, http://boatdiesel.com/Articles/
35. Gasoline and Diesel Fuel Update, http://tonto.eia.doe.gov/oog/info/gdu/gasdiesel.asp
36. Fuel Filters, Tony Athens, http://boatdiesel.com/Articles/
37. Thrustmaster of Texas, Inc., http://www.thrustmastertexas.com/
38. Summer Equipment, Olympic Steerable Drive, http://www.summerequipment.com/
39. Western Marine Electronics, http://www.wesmar.com/
40. Vetus den Ouden, http://www.vetus.com/
41. Greater Water, Inc,. http://www.great-water.com/pages/product_pages/ql_prod_list.shtml
42. Shipwrights Inc,. http://www.shipwrightsinc.com/
43. Dickson Thruster, http://dickson-thruster.com/
44. A.I.R. Fertigung-Technologie GmbH, http://www.air-composite.com/englisch/html/unter/frameset_unternehmen.html
45. National Museum of American History, Fast Attacks and Boomers, http://americanhistory.si.edu/subs/operating/propulsion/propulsion/
46. Rice Propulsion, http://www.ricepropellers.com/
47. Blank
48. Trans Atlantic Diesel, http://www.tadiesels.com/borg_warner-vdrive.html
49. CENTA, http://www.centa-uk.co.uk/
50. Rubber Design, http://www.rubberdesign.nl/
51. QUEST V A VAST-ly Different Boat, Robert M. Lane, PassageMaker Magazine, April 2006, http://www.passagemaker.com/
52. ABB Group, http://www.abb.com/
53. Ship Technology, Elation – Fantasy Class Cruise Liner, http://www.ship-technology.com/projects/elation/index.html#elation7
54. Wikipedia, http://en.wikipedia.org/wiki/Azipod

© 2008 David Shaw
david.shaw.x23@gmail.com

The Arrangement

Although you can customize a trawler no end, there are only a few possible variants in the layout of the accommodation. The choice of propulsion system and location of the galley will have a major impact on the arrangement.

Design Considerations

Deciding the arrangement of Sharina was very vexatious. I changed my mind numerous times. Being in a fluid situation with relationships didn’t help. I didn’t know if I were designing for a couple, a family, weekend stag parties or just myself. But as it turns out, in a 50-60-ft boat the arrangement boils down to just a few key questions. The main considerations in deciding the arrangement are:

• Pilothouse
• Widebody or side decks
• Engine room location
• Galley up or down
• Number and location of berths
• Number and location of heads

Pilothouse

The pilothouse can be fore, raised or not, or aft.

In a smaller trawler a three-deck arrangement is the norm: below decks, main deck and then a boat deck with or without an inline flying bridge. The standard configuration is a forward pilothouse with a Portuguese bridge. In this arrangement, the pilothouse is inline with the saloon, sometimes with a step down. The pilothouse and saloon share the main deck, usually with a cockpit aft.

If the pilothouse is raised, the saloon can be arranged with a small step on a line with the forward accommodation. This one-level living is generally more comfortable. The shorter steps are better for aging knees, too. The disadvantages are increased cost and height and the need for a deeper keel if you plan to go blue water. The increased height will limit inland cruising while the deeper keel will limit inland and coastal cruising. Many inland bridges only have a 21-ft clearance and canals often have a maximum depth of 5 ft.

An aft pilothouse is raised by definition. It places the boat deck forward, where the dinghy may be exposed to more seas. In general, an aft pilothouse is more suited to a boat that is 75 ft or larger.

Standard practice is to have a forward-slanting windshield to eliminate reflectance. This requires especial attention to wipers and washing systems, as with poor design the wipers can fall forward off the glass.

Widebody or Side Decks

On the main deck traditional designs have full side decks, allowing you to walk around the boat. An alternative design is the widebody. In the widebody, the width of the saloon is extended to the sides of the boat. Often there are steps up to the top deck from the side deck alongside the pilothouse. A variation is the staggered widebody, which keeps a side deck usually on the starboard side.

The singular advantage of a widebody is that it increases living space. The smaller the boat, the more important this may be. Two extra feet will often make the difference between feeling like you’re in a trailer house and being comfortable. A secondary advantage is that it increases security somewhat, by restricting areas that can be accessed by boarders.

There are several disadvantages. Boarding may only be possible aft at the cockpit. Docking is more difficult. You will need cleats on the top deck for fenders. To get from the bow to the cockpit you will have to run through the saloon or up to the top deck and down. This may be especially troublesome going through a lock that has vertical ropes for tie-ups that need constant re-adjustment. Lifesaving will only be possible from the cockpit or the top deck. Washing the saloon windows will be more difficult, and they will lack shading from the sun. This will increase air conditioning costs.

This decision is largely a lifestyle choice. If your boat will be used mainly as a live aboard, a widebody makes sense. If you plan blue water expeditions, side decks are safer. You will be able to conduct any kind of operation from either side of the boat. Although one side deck appears to be a neat compromise, it won’t help recoveries at sea in a storm if it’s on the wrong side.

If your boat will be less than 45 ft or used primarily inland or as a live aboard, consider a widebody or one side deck for the extra living space it affords. Over 50 ft or for extensive coastal or blue water expeditions, consider two side decks.

Engine Room

The location of the engine room decides whether or not the accommodation will be split in two. In turn, the location is dependent on the type of propulsion system, and its attendant cost. With a standard mechanical propulsion system, the engine room has to be amidships to align the engine with the shaft at a reasonable angle. Typically the master cabin is aft and several berths are forward.

This arrangement of two separate living quarters has advantages for privacy. But the space allocated to mechanical systems always seems disproportionate. And amidships just happens to be the most comfortable location for a cabin.

Also, engine-room bulkheads transfer heat to the accommodation. An amidships engine room adversely impacts air-conditioning more than an aft engine room, since there are twice as many bulkheads to transfer heat (and noise).

It also complicates heating and air conditioning distribution systems; and increases the general noise level aboard. Contrarily, putting the mass of the engine amidships helps the centre of gravity. However, overall the optimum layout has the engine room aft.

There are several ways of reducing the impact of the engine room, but they all add cost and complexity. The space occupied by a central engine room can be minimised by using a sideways engine layout and hydraulic or electric drives.

With hydraulic or electrical drive, the engine can be bedded in any convenient spot. It doesn’t have to be inline with the prop shaft. This enlarges the space for the accommodation, and allows more variety in layouts.

The trade-off is that the drive system is more expensive, from $20,000 to $50,000, and the larger accommodation increases the fitting out cost. So the trawler is substantially more expensive to build. Repair expertise is harder to find for hydraulic and electric drives; although construction and farming use hydraulic extensively.

With an electric drive, build costs can be contained somewhat by using only one engine. If the engine fails, an electric drive can use the house bank for full emergency power.

Even more space can be reclaimed by placing the engine room aft, and making it smaller, using a mechanical V-drive, hydraulic Z-drive or electrical drive. Rotatable Z-drives eliminate any need for a stern thruster. All of the below deck accommodation will then be forward, substantially simplifying the layout of electrical wiring, plumbing, heating, ventilation and air-conditioning. The main trade-off is in the position of the galley, especially if it has cold-plate refrigeration. A cold-plate refrigerator should be as close as possible to the engine room.

Galley

The location of the galley – up or down – is a lifestyle choice. After the location of the engine room, this decision will have the largest impact on the use of your trawler. The current trend is galley up, in the salon, supposedly to keep women happier. It’s also easier to reach from the pilothouse, when you want a quick snack or coffee. This is a bit like entertaining in your kitchen, or having a bar kitchen in your living room. Many small apartments are arranged this way.

Putting the galley and dinette below decks maximises space for lounging in the salon, while reducing the space for cabins. A galley below decks will have small port lights, while a galley above will have lots of light from windows and be adjacent to the entertainment centre. Either way, there are equally good reasons. You decide.

Berths

The number of berths is even harder to decide; although it comes down to two options and three questions. The options are: two cabins or several. The questions are: Do you want a home for two people and an occasional guest? Do you have children? Do you want to cater to large groups?

If you want a home for two and occasional guests (i.e., one or two friends or a couple) then you need either two staterooms or a stateroom and a cabin with two berths. The rest of the space can be used for an office, library, workshop or whatever tickles your fancy. That’s the two-cabin option.

If you have children, then you must plan the cabin arrangement to suit. Ideally, each child should have its own cabin. On the other hand, if you’re single and want to have a blast with the boys every weekend, then you need as many berths as you can manage (and enough safety equipment to go around). That’s the several-cabin option.

It seemed likely that I would either cruise alone, with a companion, or with another couple. As a basic requirement, Sharina had to be capable of accommodating and carrying stores for four people for four weeks, on a summer cruise or ocean passage. This was my main consideration. But in looking at my business acquaintances, I thought that overnight cruises for eight to ten or weekend cruises for six people would be desirable. Obviously, in the overnight case, people would be roughing it a bit. This argued for the larger salon.

But even though I wanted this flexibility I didn’t want space wasted on empty berths. In addition to the owners’ stateroom, the options for additional berths are:

• Port cabin
• Starboard cabin
• Pilot berth or pilot cabin on a larger boat
• Settee berths in salon (2)
• Settee berth in dinette

I decided it made good sense to have a berth in the pilothouse, and have the settees in the salon convertible into berths for occasional use. Any of these could be used for overnight guests, and the pilot berth would be used on long passages. On a larger boat, the pilot berth could be a cabin, which would be very useful for chartering. Making the dinette convertible, common in many sail boats, didn’t appeal to me. Too many sardines in the can.

The two forward cabins were the big issue. One was required for another couple, or perhaps two singles. So it should have a wide lower berth and an upper bunk. The upper bunk could fold-down to convert the lower berth into a settee.

The other cabin could be an office (or what have you). But I planned to use the pilothouse as an office. As a well-practiced road warrior, I didn’t need much room for office work. Also, the pilothouse, surrounded by large windows, would be a cheerier place to work. Alternatively, with a laptop computer and wireless (wi-fi) network, I could work in the salon or dinette, or the aft cockpit on a nice day.

Another choice would be to combine the berth space in a roomier stateroom for a second couple. In the end, I decided to make both forward cabins similar, with Pullman type berths. Coward. This gave me a total of nine berths.

There is another aspect to this convoluted analysis: the more you customize the accommodation to your needs, the longer it will take to find some one with similar needs when you re-sell.

Heads

Heads also have lifestyle considerations. For example, I’m not a shower guy. I like to soak in a bathtub. This can probably be traced to the fact that we didn’t have a shower when I was a child in post-war England. On a smaller trawler, where space is at a premium, you might have one or two heads with showers. On a trawler with an engine room amidships, you would want a head ensuite with the aft master stateroom, and another one forward in the main accommodation. If size and layout permits, a day head above decks is handy, especially if you are entertaining a gang.

Summary

Placing the engine room aft will maximize accommodation space and comfort but increase your build costs substantially if you have to use an indirect drive. The least expensive indirect option is a V-drive, followed by electric and hydraulic. Putting the galley above or below deck is a lifestyle decision. Similarly the number of berths reflects the intended use of your trawler. A master cabin and a guest cabin are the minimum configuration. Heads are like bathrooms in a house, the more the merrier, but watch the water consumption.

References

1. Nelson Trawlers, http://www.nelsontrawlers.com/trawlers/defever44.htm
2. Bruce Roberts International Yacht Design, http://www.bruceroberts.com/

© 2008 David Shaw
david.shaw.x23@gmail.com