Input desired on 20ft row/kite boat design

Discussion in 'Boat Design' started by W9GFO, Oct 15, 2015.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    The armour piercing bullet shape just sacrifices stability for no real gain, in other areas.
     
  2. W9GFO
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    W9GFO Senior Member

    ??? The only wedge shape advertising that I can recall is that of the Triumph TR7 - it was "the shape of things to come" or something like that.

    My rationale for the fine entry is that I want it to penetrate chop and I do not want to add volume forward where it is not needed to support the boat - and I want to retain the length. There are many examples of boats with such a fine entry. Mostly multi hulls but there are some mono hulls as well that I have found.

    Agreed, but are we remembering that 50% of the displaced weight of this boat is it's (moveable) crew? It seems unlikely to me that with the CG so far back that it would not have sufficient angle of incidence or bow lift to initiate a plane.

    The power source, a kite, will also be contributing to lifting the bow.
     
  3. W9GFO
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    W9GFO Senior Member

    Can you explain the trouble? Please don't take this the wrong way but I'm not likely to go changing my design unless I can agree with the reasoning for changing it. I need to know the "why" of these comments, otherwise they are of no use to me.

    It does look like a bullet, that was not intended. I understand that this boat looks different than most, and that alone can cause people to poo-poo it.

    What stability specifically do you feel is being sacrificed?
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Anything that depresses the bow, like plunging into the back of a wave under your kite power, and it will plunge deeply there is no buoyancy to stop it, just detracts from your already modest stability, and will also tend to broaching. You could easily make the bow a lot fuller and lose virtually nothing in resistance terms, in fact you would probably gain by less wetted area when the bows do dip. And have a more stable platform as a bonus. Don't assume a super-slim bow has an advantage in lower resistance to forward motion, at the modest speeds here it has no such attribute.
     
  5. W9GFO
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    W9GFO Senior Member

    There is buoyancy though, the bow is almost entirely reserve buoyancy. More than you would find in a similar sized catamaran. Why would this boat tend to broach more readily than a catamaran?

    I agree that a fuller bow would have benefits when running with the waves but I feel that it would be less favorable for going into the waves. I want to optimize it for windward performance.

    Wave piercing was what I was going after. At some point the size of the waves will make the piercing a liability - I'm not sure what that point is.
     
  6. messabout
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    messabout Senior Member

    You could build a model of your design to see how it behaved. Perhaps a 2 inch to the foot or one sixth scale model that would be 40 inches long and have a scale weight of 4 pounds 3 ounces. That weight would simulate a full sized weight of 900 pounds. Ideally it would be an RC model with electric power.

    The RC equipment, servo, speed controller, motor, receiver, transmitter, battery, and charger, can be had cheaply by taking the parts out of a toy grade RC car or truck, under $50. That might be a good winter project to prove or disprove the merits of your design preferences. Beside that it will be fun to play with come spring time.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Unlike a catamaran, your boat is full and buoyant at the stern, but a stiletto at the bow. And you can't avoid pitching by a "wave piercing" bow, the whole boat is subject to the forces that cause pitching. You will actually increase the pitching amplitude at the bow by having the point of pitch so far aft.
     
  8. W9GFO
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    W9GFO Senior Member

    Makes sense, but the "point of pitch" in this boat is right where the people are. That seems desirable to me. With more volume forward the axis of rotation would be at the midpoint of the boat, ahead of the cockpit.

    My comparison to catamarans was that they tend to dig their bows into waves quite well, and come out of them pretty well too, except in extreme cases. Bow steering does concern me since the CG is well aft and the bottom flat in the stern, I think it will have a tendency to swap ends if the skeg is not adequate.

    I do see what you are saying. I think I will soften the keel/forefoot and increase the volume forward a bit - probably not as much as you'd like though. :D
     
  9. W9GFO
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    W9GFO Senior Member

    I had thought of doing just that. RC equipment is something that I have in relative abundance. I do need to settle on a design to try first.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Is the hitching position of your kite going to heel the boat ? I assume it must, and with your dagger board to reduce leeway, so widening the boat forward will decrease the heel, assuming that is desirable. A few things to juggle here.
     
  11. W9GFO
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    W9GFO Senior Member

    Generally no. I want to have an arc shaped track with the center of the arc just forward of the centerboard. The center of the arc will be about a foot higher than the ends. The line of pull when beating will be through the gunnel roughly inline with the CG of the boat. Of course the position of the kite in the sky will change the heeling moment. When the kite is real low it will tend to heel slightly in the normal direction but when the kite is higher the track will allow it to seek a position where it stays inline with the CG and should not really make any significant heeling moment.

    If I were to add a method for fixing it's position on the track it could be made to heel opposite of the normal direction.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Are there any boats of this type (kite) on the market, and what do they look like ?
     
  13. W9GFO
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    W9GFO Senior Member

    There is the "Kite Tender", it is smaller and not intended for rowing.

     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    I sounds like a tough contract to get right, I think I would err on the side of a boat that is stiffer along both axes, in case getting the hitching set-up right turns out to be a headache.
     

  15. W9GFO
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    W9GFO Senior Member

    Here it is with the volume shifted forward a bit.
     

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