Low planing speed

Discussion in 'Boat Design' started by Steboe, Oct 26, 2008.

  1. Steboe
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    Steboe Junior Member

    Which hull design features produce high L/D ratios and low planing speeds
     
  2. marshmat
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    marshmat Senior Member

    Long, light and well balanced?

    You may have to be a little more specific in your question. The question "Which hull design features produce high L/D ratios?" is its own answer: high L and low D. Can you elaborate on what information you need, and/or why?
     
  3. Tcubed
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    Tcubed Boat Designer

    For the lowest possible planing speed you want the lowest possible weight per square area of useful lifting surface. Efficiency, once planing is actually achieved, is actually favored by low L/B-at the expense of other factors.
     
  4. Steboe
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    Steboe Junior Member

    Light weight low powered dinghy, 10~11ft long, 350~400lbs total mass
     
  5. Landlubber
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    Landlubber Senior Member

    A honeycomb door with an outboard planes at very low speeds
     
  6. PAR
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    PAR Yacht Designer/Builder

    A 10' - 12', 350 - 400 pound dinghy isn't light, unless you're counting crew weight too (still not light though). Dry hull weight for a taped seam skiff of that size could be in the 80 to 150 pound range with lower weights possible at a cost.

    How fast do you want to go and what dinghy are you playing with? If it's not a commonly understood model or a unique design, then images of some sort would be helpful.

    Landlubber, a 5 HP short shaft on a honeycomb door is a real flyer. Remove the knobs and hinges for better performance . . .
     
  7. tom28571
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    tom28571 Senior Member

    I guess everybody knows that a flat bottom has the highest L/D ratios relative to V hulls. Not many like a completely flat bottom for general use in a planing boat though. Given a reasonably sharp entry coupled with a moderate aft deadrise of 10 to 11 degrees, the boat will work well in waves and have low L/D numbers. Additionally, there must be no convexity in the aft bottom surfaces.

    Assuming the above, the ease of getting on plane at low speed rests on low bottom loading in lbs per square of planing contact area. What kind of loading are we looking at? I know that 25 lbs/ft sq will easily do the trick. Lower will be quicker and higher will still work. Well designed small sailboats meet this goal very easily and will move onto plane quickly at low speed ( in the hands of a skilled sailor, of course) . Powerboats have a harder time of getting loading this low, which is why most of them don't come close to planing at low speed. By low speed, I mean speed at or just above theoretical hull speed.

    There are lots of other factors that will always influence the design and no boat is built with your premise as the only goal. It's possible to add other little mods like transom wedges, trim tabs, steps, etc that will affect low speed planing ability but, in the end, weight is the most important single element of design and building. I add building to the equation because it is a lot harder and takes more attention to detail to build light than heavy.

    Edited to add: Calculating the actual bottom loading is a little obtuse though. There is the static loading which is the projected waterplane (area projected on a flat plane) and then there is the projected water contact area when the boat is on full plane. The latter is about 65% to 70% of the static area at low planing speed and far less at very high speed. When the boat is just starting to lift out, the load area varies from 100% to 70% when you can say that the boat is fully planing or almost all the weight is supported by dynamic rather than buoyancy forces. Confusing factors not withstanding, it's still light weight that wins out.

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    Landlubber, a 5 HP short shaft on a honeycomb door is a real flyer. Remove the knobs and hinges for better performance . . . Cute, Paul LOL
     
  8. Landlubber
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    Landlubber Senior Member

    PAR, wondered why i kept going around in circles, Ta mate.
     
  9. Landlubber
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    Landlubber Senior Member

    Hey fellas, just a little hint, when i was younger i raced an outboard boat, we made the transom/hull corner edges razor sharp edges, and it increased the speed....normaally they are quite round about 3/8", especiallly in glass boats of course, but we made it seriously sharp and it worked well.
     

  10. PAR
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    PAR Yacht Designer/Builder

    That's an old trick 'Lubber. On our sailing dinghies as young men, we did the same thing, even taking it a step further and attaching pieces of aluminum angle, screwed to the edges of the transom, with the trailing edge sharpened. Of course, we eventually got caught and had to remove them, but it worked for a while.
     
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