Email This Page to a FriendNotes on Boat Design, Part 1 — The Case for Going Slower
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October 2, 2012
Off Center Harbor blogs seem like they might be a good place to try to discuss some technical issues in boat design. There are lots of books on these subjects, many of them written by people with greater credentials and writing skill than I possess. But still, the blog-and-comment format available here would seem to offer some advantages. I can ramble on about some issues that seem to constantly come up in conversations with customers who are interested in design, and respond to readers' questions about stuff that they could never quite otherwise get—and they might even get an answer that makes sense. But I make no promises.
To the extent possible, I am going to try to avoid relying on numbers and formulas to explain technical points. Sometimes this may be impossible, but generally it will be a good challenge for me, and possibly a relief for readers. There are lots of books out there that are full of formulas, and anyone who wants to delve into a subject in greater technical detail should carry on! David Gerr is a particularly lucid and practical writer, and for those who wish to see the numbers behind my generalities, I can heartily recommend his books.
I should also state from the start that my approach to boat design could be described as low-tech meat and potatoes. Although many of the concepts discussed here can apply to anything that floats, from a toy boat to a supertanker, my comfort zone extends principally from dinghies to mid-sized pleasure boats. Anything I say on cutting-edge racers, submarines, or cargo vessels should be regarded with deep skepticism. I am very interested in the aesthetics of boating, not just of the boats themselves, but of the entire process. Even the most beautiful boats lose some of their elegance if they are used clumsily or aggressively, or if they are bogged down with unnecessary gear or technical gizmos. In their classic Elements of Style, William Strunk and E. B. White advise us to “omit needless words.” I would argue that a similar prescription should apply to boats: omit needless stuff.
There are countless boating magazines and on-line forums that tend to discuss the same issues ad nausium. I am hoping to discuss different issues here, and be forewarned that I plan to spend some time up on my soap box. To start, I'd like to discuss powering and fuel issues. For a second opinion on this subject, pick up a copy of Polluting for Pleasure, by Andre Mele.
PART ONE: THE FORCES FOR “MORE”
When it comes to installing power in boats, the forces for MORE are firmly arrayed. The engine manufactures, the boatbuilders, the maintenance mechanics, the fuel dock operators, the magazine editors – almost everyone stands to profit from installing more. With a chorus like this leading the hymns, it would take a confident individual with a secure ego to sing a different song. Toss in some testosterone, cheap fuel, fully-packed leisure schedules and fish that might get away, and you have a lobby that would make the NRA weep with envy. In the other corner, there is only Mother Earth and you.
One of my first questions for anyone asking about a power boat design is “what speed is required”. A typical conversation might go something like this; “Got to have twenty.” “You could get nineteen out of a smaller engine.” “Nope, got to have twenty.” “Why?” “It's a safety issue. If bad weather comes up, I got to be able to get in quick.”
Now, ignoring the fact that we now have weather forecasts that allow us to avoid most bad weather, and the fact that in said bad weather you would be hard pressed to maintain 20 knots, and the fact that 19 knots is pretty darned close to twenty, what is wrong with adding a little extra horsepower? Well, a bigger engine weighs more – not only the engine, but everything attached to it. Higher speed boats need to have heavier construction in order to withstand the higher forces they create. And you'd better install more tankage, or else your range will head down the dumpster. All this extra weight, together with the immutable fact that every extra knot above hull speed is more difficult to achieve, means that your bigger engine doesn't get you much. And, you pay the price for this extra weight, both in initial construction costs and operating costs, even if you decide to slow down. Then there are the environmental costs.
Others may disagree, but I have the feeling that the human race may survive long enough to regret roaring through a cheap and incredibly versatile energy source that took the earth billions of years to create.
(PARTS TWO AND THREE OF THIS SERIES TO FOLLOW SOON, SO STAY TUNED.)
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Finally at 63, my first thought is, “how small of a motor can I use, and how small of a boat can I get away with”?
It took a while but now I’m “cruising” on my 45 year old, 19′ O’Day Mariner.
For my 20′-0″ outboard-powered launch, now stretched to around 23′-0″ with motor-in-a-well, the designer David Payne recommended 15 to 20 hp. The Yamaha retailer here asked first “What speed”, then said “You don’t need 20hp or even 15hp, For your 6 to 7 knots hull speed you’d do very well with a Yamaha High-Thrust 9.9hp.” When I checked with David Payne he agreed. I look forward to finding out!
I’ll give two more reasons. At 8 knots you can read underway while on autopilot. At 8 knots you get time to “inspect” those other classic boats that you’re now passing more slowly.
Doug, I work in the marine industry and it is always about more, more horsepower, more lights, more cupholders. In fact when co-workers look at my Lyman Islander, and see none of that stuff they disregard the boat all together. Until, they find out that with a 60 hp engine I can do the same 18 MPH they do with a 150 hp engine!
Thanks. Great explanation, take time to get salty and understand the logic that you explain clearly, after 10 years having planing yachts (cruise over 20 knts 44′ to 55′), I moved to semi-displacement (44′ to 65′) and with this experience I choose a 50′ slow semi-displacement I enjoyed cruise at 10 knts, fast cruise rarely used 16 knts.
Looking fwd for more
You have my attention. Two years ago I decided to buy a 34 yr old trawler with just enough power to get around Chesapeake Bay at 7 knots or so. What impressed me the most about trawler designs was they offered lots of cabin space & comfort, were ultra stable in most sea conditions and operated pretty efficiently on a single diesel engine. While two diesel engines offered redundancy, I didn’t want to have to maintain two and decided one was adequate. As a result I have met more and more trawler owners and find they use their boats whereas the folks with the gas powered boats park them in the slips most of the time.
My Town Class sloop (17′) has oarlocks, should Mother Earth be feeling sleepy…
We don’t even have a motor. Ole Roz sails pretty good without.
Thanks Doug. Looking forward to more.
Love my Lawton Tender. Oars move it nicely in Florida inland waterways at 2-4 knots, and if I’m feeling lazy, my 2 hp Honda will get it all the way up to 4-6 knots. ‘Bout as fast as a need to go to enjoy the ride.
Wooden sailboats: Best of both worlds. Built from renewable resource- driven by free, unlimited power.
I sail on Elephant Butte Reservoir, New Mexico. When full it is about 40 miles long. Plenty big for my small sailboat giving me opportunity for some long days on the water if I choose. I watch the 70 mph plus bass boat roaring around easily getting to one end of the lake or the other in a few minutes. To me that would make me feel like I was on a farm pond. Of course they might not enjoy ghosting along at a few knots.
When I bought a new 4 hp outboard for my 22′ Oday I was wondering if it was enough. Although it was on a calm day, and no current, my question was somewhat answered when I towed a 30′ Catalina back to port. Also when motoring alone I reach hull speed at slightly less than full throttle.
I love going slow. We have a cabin motor launch with a 32 hp diesel that gets around comfortably at seven knots. That’s just fast enough to keep me attentive to avoiding the ever present lobster buoys where we live, but slow enough that I can enter into the conversations aboard. From July 4 through Labor Day we were under way for fifty hours and used forty gallons of fuel. Thus we got a ‘good deal’ this summer on a DP/H basis (Dollar and Pleasure per Hour) and can’t wait for next season.
At 89 years old, I’m slowly realizing these truths. Thanks.
Thanks for the logic. When I’m at a show displaying my creek boats a question that is always asked several times is… “Now, how big a motor can I put on her?” I cringe and try to explain.