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Thread: Good read on boat design part 2

  1. #1
    Sponsor potbuilder's Avatar
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    Default Good read on boat design part 2

    In summary
    The semiplaning boat is lighter, narrower, more comfortable and less expensive to buy and own than the high-powered planing boat. You could likely buy a 40-foot SP boat for the price of a 33- to 35-foot planing boat. When you consider you can get out on the SP 40 on windy days that would keep the 35 tied to the dock, larger for the same price becomes an appealing proposition. Given the price of diesel horsepower, you could save more than $100,000 in engine costs alone between our two 40-footers, to say nothing of the cost of maintaining and overhauling them down the line.
    You can also cover a lot of ground in eight or 10 hours at 18 to 20 knots and, as Huntís Winn Willard has pointed out, you often canít make 30 knots anyway, because real-world ocean conditions tend to slow a boat with a prudent operator. At 18 to 20 knots you can also hear yourself think, chat with the person five feet away without screaming above the diesel-and-wind din, walk from one side of the boat to the other without hanging on and generally enjoy the process of getting there.
    Also, consider how nice it would be to cruise at any speed you want from 1 to 20 knots. Getting over the hump in an SP is almost a non-event, so you can cruise along at 10, 11, 12 or 13 knots with little bow rise and even less complaint from your propulsion system. The Latitude 40 will be a lot less money than its 30-knot counterpart, which means you can have more boat for the money. More boat means more comfort and seaworthiness, more space and range, and so on.
    Just donít succumb to the temptation to make your 20-knot semiplaning boat a 30-knot planing boat because that requires double or more the horsepower, more fuel and a more heavily built bottom, which increases bottom loading so you no longer have a boat that performs well in the SP region. Thatís really the take-home from this whole discussion. A planing hull can run in the SP region, though poorly; it will be sluggish and ponderous at these speeds, with its high bottom loading, light gear ratios, small props and smaller rudders. A 40-foot SP boat at 20 knots is fully planing and, because it has a lower-horsepower engine, carries less fuel, is more simply outfitted and has lighter hull scantlings itís as agile as a cat in this realm.
    Simplicity has other rewards, too ó not as much stuff to break and maintain means more time cruising and less time fixing. You can get a bow thruster if single inboard power worries you ó and joystick control if you add a stern thruster and integrate the main engine. Itís still a lot less weight and cost than twin diesels. And one small diesel instead of two big ones means more room for a second or third ďcabinetteĒ below the saloon.
    With the world trying to pull out of the Great Recession, and peopleís attitudes about money, environmental responsibility and legacy (what kind of example am I setting for my kids), the time is right for semiplaning boats.
    Eric Sorensen is a consultant to boat- and shipbuilders and to the government. He was founding director of the J.D. Power and Associates marine practice and is the author of ďSorensenís Guide to Powerboats: How to Evaluate Design, Construction and Performance.Ē A longtime licensed captain, he can be reached at ericllc.com
    This article originally appeared in the January 2012 issue.

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    Member jrogers's Avatar
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    Thanks for post, Potbuilder, it is an interesting read. When they talk about the wave length and speed of the wave, this is a concept that I have not heard of before. I did know about hull speed, but I did not (and still don't) fully understand the physics behind it. It is the bow of the boat making the wave, but it is the overall length at the waterline that determines the wavelength and therefore the hull speed for a particular vessel. Would this mean that a boat pulling a log tied directly to it's stern would have a higher hull speed than the same boat without that log? How does the most aft few feet of a boat's length effect the length of the wave it makes at it's bow?
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    Steve, you've run your lobstah rig for a year now - what have you observed getting around PWS? Cruising 16 knots, blowing through 3-foot chop like it's nothing? What are you seeing for fuel economy?

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    Quote Originally Posted by Vek View Post
    Steve, you've run your lobstah rig for a year now - what have you observed getting around PWS? Cruising 16 knots, blowing through 3-foot chop like it's nothing? What are you seeing for fuel economy?
    I only fished the boat last summer and so far i love the boat, the more i ran it the more it surprised me, turns in its own length, steers and handles like a sports car, put it in gear and at idle it goes 5-6 knots(not a troller!), with the power steering, its 2 turns block to block, i steer with just my thumb on the wheel, honestly never had it out in any bad weather so can't comment on running through a chop in the Sound but i did seatrial it in 4-6 footers and never had to slow down, running at 2000rpm i cruise at 16 knots and burn 8 gals/hr, the John Deere will turn up to 2600 so i'm pretty will coasting along at 2000. Boat goes the same empty or loaded with 40 pots and 500lb of ice! Wish i would of got one of these boats sooner. I'm looking forward to spending a lot of time on the boat this year.
    There is a builder in Maine that just came out with a 23 footer that i bet would be a killer boat for the Sound i'll try and dig up some pictures to post.

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