Key Differentiating Factors
There are many ways to acquire a trawler capable of passage making. You can buy production off the shelf and customize it or not; buy custom; DIY from scratch or a kit; convert a fishing trawler; buy used and renovate it; and so on.
This blog-book is intended for the person starting with a bare hull, in the 40- to 65-ft range, who wishes to develop a specification for its completion, based on total system performance (TSP). You might be giving this specification to a builder, or using it for a DIY project in your backyard.
The blog-book is based on the specification for Sharina, a 55-ft trawler the author hopes to build. The objective of Sharina’s design is to create a small coastwise and offshore power yacht for living aboard, with the additional requirement of being a capable long-range ocean voyager.
The blog-book describes the intent of the design, to explain why certain design choices were made. Not every choice is discussed – only those that are key differentiating factors in terms of total trawler system performance. Also, in certain areas, emerging technologies are flagged for you to watch in the next five to ten years.
It is evident that many builders, while obviously expert in hull construction and ship’s mechanicals, do not consider total system performance. For example, many trawlers are fitted with a well-known brand-name refrigerator that has marketing cachet but poor performance. This is not surprising. No one can be an expert in everything; and brand names are often a reality in successful sales. Solutions are more difficult to sell than products, and in the end customers get what they think they want.
Fig 1-1 - Bruce Roberts Trawler 62 built by Yachtsmiths International- Photo with permission © Bruce Roberts Yachts
To illustrate TSP, consider that it is a given that navigation lights will operate on direct current (DC), not alternating current (AC). This has been sufficiently understood for so long that there is nothing to discuss here in DC vs. AC. It’s a “no brainer”.
But should the DC be 12 or 24 or even 150 Volts (V)? This is an interesting question for a smaller boat, with numerous pros and cons from a TSP perspective. Choosing between the common 12 or 24 V impacts many systems on the trawler (e.g., motors, lights, toilets, furnace, navigation instruments, engine controls, stereo, etc.) and its infrastructure (e.g., wiring size, length of wiring runs, batteries, chargers, inverters). The DC voltage is an example of a key differentiating factor. It is what makes your boat significantly different from the next one, because your functional and performance needs are different.
Deciding the best choice of voltage requires an analysis of impact on the total trawler system, not just one part of it. The analysis must also consider the availability and cost of equipment in one voltage vs. the other. (There is a much wider range available in 12 V and almost none for 150 V.) At 55 ft, Sharina is well within the capability of a 12 VDC system. This is what I planned until I came to specify the watermaker and the solar power system. That sent me back to the drawing board for a re-think, and an investigation of DC-DC converters. Similarly, my estimates of daily water consumption per person have evolved from 25 to 60 gal. TSP is an iterative process.
Here’s another example: The traditional trawler you’re looking at probably has a single screw and a three-bladed propeller. Yet research shows twin screws give superior thrust efficiency and we know they cancel stern walking. Theory says that fewer blades on a propeller increase efficiency; although we know just one will shake, rattle ‘n roll. Yet many modern propellers have four or five blades. What does this mean?
A single blade might be more efficient theoretically but it is unbalanced and so has serious practical side effects: vibration. Then, practical experience shows four blades are best with a large single screw; three blades with smaller double screws.
If you don’t feel safe with a single engine and want an emergency get-me-home wing engine, with a tiny emergency propeller, think again. Should you instead go for twin engines and screws, even though the fuel efficiency will be less?
If redundancy is important the real choice boils down to a single engine with a four-bladed propeller or twin engines and twin screws with three-bladed propellers. There is no point in having a wimpy get-me-home auxiliary engine and propeller that are not likely to do the job in heavy weather, no matter how good it makes you feel.
Other combinations are possible, such as two engines and one screw mated with an expensive mechanical combining transmission; or one engine and two screws with a hydraulic or electric drive. From the perspective of a single-point of failure neither of these is attractive. However, there might well be other good reasons for such a choice. (We will explore this in more depth in a later chapter.)
These are the kinds of things you must think through as you develop your specification to meet your unique operational and psychological needs.
When you purchase a production boat, of course the designer and boat yard have considered many options and trade-offs to achieve a specific price point. They will have brought years of experience to the task. When you develop your own specification for a bespoke boat, you have to resolve these issues yourself. But, like building a custom home, in the end you will have a unique and better product suited to your needs.
The design considerations are complex. It is not easy to work through the trade-offs and understand the impact on performance and safety in the areas of propulsion, electrical, heating and air conditioning, water and plumbing. For example, did you know the performance of reverse-osmosis water makers deteriorates the colder the water [1]?
Both performance and safety are critically important if you intend to venture offshore. It is my hope that as we discuss the decisions made for Sharina, you will be able to make informed decisions for your own boat, even if they are different from mine.
Summary
Developing a specification for a trawler involves many complex trade-offs. By focusing on key differentiating factors (what makes your boat better than the next one) and their impact on total system performance, you can build a better boat that others will admire. Main areas considered are propulsion, electrical, heating and air conditioning, water and plumbing. This analysis is iterative, with consideration given to safety, performance and traditional value.
References
1. Passagemaker Magazine, Big Ship, Little Ship, p.135, July/Aug 2003, http://www.passagemaker.com/
© 2008 David Shaw
david.shaw.x23@gmail.com
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