Effect of aft warp?

Discussion in 'Boat Design' started by ezombie, May 12, 2007.

  1. tom28571
    Joined: Dec 2001
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    Location: Oriental, NC

    tom28571 Senior Member

    Take a look at this 10' dinghy just planing with a 2hp motor and a lightweight crew sitting amidships. The aft bottom is similar to yours but a bit stretched out since it is primarily a sailboat. It will plane under sail much more readily than with power but is just capable with either. I have planed mine at far greater than hull speed and I weigh 170lbs. Of course it had ot be windy and I had to set up and balance the boat just right.

    At 8', your options are very limited. If you really want to plane, you need to either make the aft bottom a constant deadrise by raising the chines in line with the keel or drop them to the level of the keel. If you want to emphasize displacement speed, put in some rocker as advised.
     
  2. Willallison
    Joined: Oct 2001
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    Willallison Senior Member

    Are you getting the hint yet ez?;)
    TAKE THE ROCKER OUT OF THE BOTTOM IF YOU WANT THE BOAT TO PLANE!
    As far as the looks go, I'd drop the transom just a smidgeon (or raise the bow). To my eye, it looks just a little high at the back....
     
  3. ezombie
    Joined: May 2007
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    Location: Chehalis

    ezombie Junior Member

    I do plan to take back out the rocker, but in order to actually test slow speed efficiency, I either build two protoypes, or one with the 'tucked chines' and two wedges to screw on while testing.

    I can temporarily decrease deadrise after the boat is built just fine, but I cannot increase it. Not being able to plane is not a deal breaker for this project, but being unstable on the beach or at rest, or being dog slow, is.

    As for the wake, I did some more fiddling with the flat bottomed version scale model, and the skegs appear to make a BIG difference with the bow wake, as well as the decreasing the speed to plane significantly. Perhaps they are 'persuading' the water flow to go under the craft instead of around or away to the side?

    Since they are not in the plans yet, I'll describe them: There is an 1" x 4" outer keel from bow to 20 inches short of the transom, and two 1" x 3" straight and parallel skegs on both sides of the outer keel. The outboard ones are spaced 2" in from the transom bottom corner and run from transom to 10" aft of the forward most station. The inboard skegs are spaced equally between the outer keel and outboard skegs, also parallel and running from the transom to 4" foward of the forward most station. The fronts are tapered into the hull with a wedge shape. When seen from the bottom, the front of these battens line up in an big V pointing toward the bow, with bow wave being just cut by them.

    I attached a simple drawing of the setup, with the observed waterline during semi-planing. Need a much longer fish tank to see the flow while planing ;)

    BTW, an old long fish tank is *great* for testing 1/12th scale models. You just set the motor to the speed you want with an ESC, then have a helper hold it in the water at one end and let go while you are watching from the side.

    tom, was there supposed to be a picture or link? I'd love to see more examples of small, efficient planing boats in actual use.
     

    Attached Files:

  4. tom28571
    Joined: Dec 2001
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    Location: Oriental, NC

    tom28571 Senior Member

  5. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Tom;
    Spindrift is a spiffy little dink. For some reason it reminds me of a Penguin, maybe because of the arced bottom. The penguin is somewhere near an example of what Ezombie first contemplated. (rising chines aft) Given the choice between Spindrift and Penguin, no contest, gimme the Spindrift. It is sure to give more long term user satisfaction than any pram or garvey that I can think of. Are you reading this Ezombie? Key words; user satisfaction.
     

  6. ezombie
    Joined: May 2007
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    ezombie Junior Member

    That is a nice little boat... too bad these will *never* be sailed. These are for a small "convenience" fleet, cruising and fishing duty only. And by people that likely know little about boats in general. My thought is to keep the balance issue simple by not giving them the choice... as in: either sit here or here buddy, either will give a decent trim.

    The client wants a framed boat if possible, and I do have to build these at the rate of one per week ( weekends and evenings - about 15-20 hours a boat ). Hence the pram configuration and integrated frame/bulkhead/seat structures. If I thought I could get away with a pointed bow, then I might try that.

    Yeah, the transom could loose a few inches. If I carried the sheer up to a pointed bow, then it *would* be a few inches higher then the back of the boat. Perhaps a bow and transom that have a modest arc on top? Atkins style, eh... this would bring the aft sheer down an inch or two, and raise the top of the bow up a corresponding amount. I'll have to draw up one with that feature and a slightly narrower bow and transom. Something in me wants more curves on the boat ;)

    Yesyesyes! I finally got a hold of one of these:

    [​IMG]
    ( High Speed Small Craft - Third Edition - by Peter Du Cane )

    And for $20 delivered, no less! I already have Glen's big book and have read it front to back a couple times now. Now I will really have some meat to sink my teeth into :D
     
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