Help With Economical Semi-Planing Designs

Discussion in 'Powerboats' started by SAQuestor, Apr 5, 2007.

  1. charmc
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    charmc Senior Member

    Longliner, I'm proud of you, my friend. On another thread I questioned your 1 gph @ 20 knots (that's knots, Ron, not "knots per hour") claim for the JC hull. But the overall performance ... being able haul 6000 lb loads at displacement speeds at only 1 gph, get over 20 knots when needed, and still get a week or more fishing offshore on 300 gal of fuel.... no wonder JC never had to advertise, the word of mouth in the commercial fishing community had them able to sell more boats than they could build. Obviously, that's one efficient hull.
     
  2. charmc
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    charmc Senior Member

    OK, here's another modern application of a box keel in a semi-displacement hull, if the claimed performance is accurate, this is very efficient:

    http://www.alsphere.at/dg/index.shtml
     
  3. longliner45
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    longliner45 Senior Member

    let me be more clear,,1 gallon per hr ,,,at 6 or 8 knots,,,longliner
     
  4. u4ea32
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    u4ea32 Senior Member

    No. Its light weight that makes any boat fuel efficient.

    I've been looking into exactly this same question for years, and the experimental, observable answer is always the same: Same weight planing boats (constant deadrise) are always, at all speeds, more fuel efficient that semi-planing boats (variable deadrise, or round bottom).

    The full planing boats do burn more gal/hr when they are going faster, but they burn LESS gal/hr when they are going the same speed as the semi planing hulls (except, perhaps at 3 knots, but at that speed, who cares what the fuel burn is -- its trivial in any boat). And of course as soon as you start talking about mpg, then the planing boat wins hands down.

    The reason is obvious, and well supported by tank tests for a century: once its making a bow wake, the planing boat has (a) less wetted surface and (b) more weight of the boat carried by dynamic lift. There is a reason there are zero commerical airlines using blimps.

    You can simulate a semi-planing boat with a planing boat very easily, if you have big enough trim tabs. Push those tabs down, and the speed drops and the fuel flow stays the same. Yup, you are pushing the lift way aft like having a warped bottom, and getting the forefoot in the water. Smooth, yes. Lots of wetted surface, lousy dynamic lift, and you've got a semi-planing boat, and lots of fuel burn.

    Its weight, pure and simple.

    That said, there is an advantage to semi-planing hulls, but its not efficiency: its smooth ride in a seaway.

    But even a smooth ride is trivially easy to achieve with a planing hull: make the deep vee constant deadrise (better, stepped like a Fountain) hull looooooong. Like 8:1. You'll get all the benefits of a very low hump, low vertical accelerations, but you will still be able to go really fast if you feel like it.
     
  5. Wayne Grabow
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    Wayne Grabow Senior Member

    "Same weight planing boats (constant deadrise) are always, at all speeds, more fuel efficient that semi-planing boats"

    At all speeds.... even when not planing, with the planing hull dragging its transom underwater creating form turbulence and having greater immersed surface area? At low speeds, the semi-planing hull acts similar to a displacement hull; is the planing hull then superior to the displacement hull also (at speeds from, say, 3 knots up to hull speed)? If so, then should we start thinking about more full-planing daysailer designs? A square-sterned (planing) racing canoe? Lighter is certainly better. Flat bottomed boats plane easiest. So, is the jon boat the ultimate design for fuel efficiency? I appreciate much of what you are saying, but I can't yet accept the idea that a constant deadrise transom-sterned planing hull trumps all other shapes at all significant speeds. Help me out with some more details. How does rocker figure into this situation?
     
  6. charmc
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    charmc Senior Member


    I'm not an NA or professional designer (not a rocket scientist, either :) ), so I'm not quoting charts or curves. I question the "always, at all speeds" claim for constant deadrise planing hulls, however. The speed range Wayne seems to be describing is the one at which the planing hull is at its worst. On the hump, beyond hull speed but not yet on plane ... bow high, stern buried, unable to get out of the hole and climb over the bow wave, dragging a huge stern wave ... it's ugly and I'm convinced it's not at all fuel efficient. True, no operator with any skill at all would remain in this speed range for long, but I believe, as Wayne said, the planing hull in this admittedly narrow speed range is not more efficient.
     
  7. Willallison
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    Willallison Senior Member

    Regardless of type, I'd agree that for a given vessel lighter is always going to be more fuel efficient. As for the rest of it - constant deadrise boats being more efficient at all speeds etc - well, without wanting to be rude....that's just bollocks.;)
     
  8. Willallison
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    Willallison Senior Member

    Another perspective on the same issue

    "In general, a hull should be shaped for the cruising speed the boat is locked into by the power installed. If a hull is powered such that it will never reach planing speeds, it should not have a planing (hard-chine) bottom shape. (Ever see a duck with chines?) Water does not like hard corners unless true planing occurs; that is, when the water cleanly separates from the transom and chines at speed. Otherwise, a semiplaning (or penetrating) hull form will be more efficient. There are far too many boats designed with planing bottoms that never have a prayer of planing—except, maybe on the face of a 50-foot wave."

    Tom Fexas, "The Spectator", PMY, Nov 2001
     
  9. tom28571
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    tom28571 Senior Member

    I am not sure just what David is getting at. Will's comments seem spot on about the "at all speeds other than 3mph" statement.

    In the first place, it would be very difficult to compare planing and semi planing hulls of the same weight. By it's nature the semi-planing hull would have convex bottom surfaces. This means it would have shorter waterline length, narrower beam or both. If both hulls are to have the same displacement, the above must be true. If one of the boats is designed optimally and both are the same weight and general dimensions, the other one will be certainly not be optimally designed for its use.

    This is actually not a good area of argument. Both hulls have their proper place but neither is altogether happy operating in the other's "best" speed range, especially when the measure of "best" is fuel use.
     
  10. Frosty

    Frosty Previous Member

    Bollocks???

    Probably but when you talk semi displacement catamarans then they are the most economical craft on the planet.

    At the speed they were designed to do.
     
  11. u4ea32
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    u4ea32 Senior Member

    Hmmm. Let me use less hyperbole.

    And please, first restrict your consideration to very light displacement boats. Very light. If they are not very light, efficiency is already lost.
    And if they are not long, they are not efficient either. I don't care about piggy boats that don't fit both of these criteria.

    If you are worrying about transom drag being a significant portion of drag at any speed in a long thin light displacement vessel, you are thinking too much and you are not rowing or sailing enough. Try it, and you'll see that everything else is more important than how much transom is immersed.

    Shape is only involved at low speed if the shape leads to less wetted surface. Semi-planing hulls have a far higher wetted surface shape than a deep vee hull form. Therefore, at very low speed (when fuel burn simply does not matter at all) the advantage should go to the fixed deadrise hull.

    As we go from 1 to 2 times the sqrt WL, wave drag rapidly overtakes wetted surface. The non-planining shape starts to sink into the water due to the negative pressure underneath, and therefore displaces MORE water, and its wave drag increases even more steeply than it should.

    A planing boat starts to develop dynamic pressure under the hull, reducing its displacement, and therefore wave drag. A long thin light and especially stepped planing hull exhibits very little to no bow rise. And more importantly, with a stepped hull the wave length is quickly halved or less, thereby GREATLY decreasing wave drag. If you don't understand this, you don't understand planing boats and why their wave trains disappear at speed.

    Therefore, it does make sense that light planing boats always beat light semi-planing boats.

    But before arguing some more, here is some data you can explore yourself and come to the same conclusion. Compare the two 42 foot power offerings by Beneteau. The planing boat always beats the semi planing boat in nmpg.
     
  12. SAQuestor
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    SAQuestor Senior Member

    Indeed.

    Now, you've made some claims but I've seen no references cited. I've seen no links to online sources. When one wants to make a point that is - shall we say - thought provoking, one normally cites references so those that actually care to verify the data can do so with ease.

    Without citing sources or providing online links, then discussions like this become simple he said-she said arguments when one person expresses opinion and another expresses opinion. Nothing gets resolved and little if any actual learning takes place.

    And you know about opinions right? They're like a-holes, everybody's got one and they all stink. Mine, yours, everyone's. So let's get away from expressing opinion by your providing citations and/or links to data that back up your recent posts.

    Best,

    Leo
     
  13. Willallison
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    Willallison Senior Member

    To be fair, Leo, I think David intended to post some examples from Beneteau...probably forgot to add the links..?
     
  14. SAQuestor
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    SAQuestor Senior Member

    OK Will - Benefit of the doubt in that case. Regardless, there have some statements that could stand some supporting documentation. As ol' Ronny Ray-Gun once said, "Trust, but verify."
     

  15. tom28571
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    tom28571 Senior Member

    If a boat is on motion on the water, it must be supported by either buoyancy, hydrodynamic lift, aero forces or a combination of these. Waves do not disappear at high speed. Once a boat is planing and almost all support is from hydrodynamic force, they do get smaller the faster the boat goes but the energy in them is nearly constant relative to time. That is, the work done by the water in supporting the boat must be constant, disregarding aero forces. Work done has a time element so the waves are spread out over a greater area of water due to the speed of the boat, thus appearing to be of lesser importance. Unless we can somehow get rid of Newton, this must be true.

    Dynamic lift and momentum imparted to the water are exactly equal and opposite vectors. Doesn't matter whether it's a regular planing monohull or a stepped hull. Momentary imbalance of these forces do occur but the average must be equal to the weight of the boat or the boat will either sink or fly. There are other minor considerations but the above covers the basics.
     
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