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Why does a gennaker/kite lift the bow?

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Post Options Post Options   Quote r2d2 Quote  Post ReplyReply Direct Link To This Post Posted: 13 Mar 12 at 8:38am
so in that figure, which doesn't show the crew's weight, the kite is just about the only thing preventing the boat from an immediate nosedive, as all of the other forces act to rotate the boat nose down
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Post Options Post Options   Quote RS400atC Quote  Post ReplyReply Direct Link To This Post Posted: 13 Mar 12 at 8:48am
Originally posted by r2d2

so in that figure, which doesn't show the crew's weight, the kite is just about the only thing preventing the boat from an immediate nosedive, as all of the other forces act to rotate the boat nose down


Without the kite, the bow would go down a bit, the centre of buoyancy moving forward as a result. When the centre of buoyancy is far enough forward of the centre of gravity, the moment (leverage) of the buoyancy cancels the moment of the rig thrust and balance is restored.
As the boat is moving forwards, there will also be lift off the bow.
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Post Options Post Options   Quote x1testpilot Quote  Post ReplyReply Direct Link To This Post Posted: 13 Mar 12 at 9:27am
Lift is very dependant on the rig - clearly a kite-surfer gets lift. Decades ago my grandfather also proved he could get a boat to go upwind with an ordinary kite. Definitely lift there!
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Post Options Post Options   Quote Flick-Flock Quote  Post ReplyReply Direct Link To This Post Posted: 13 Mar 12 at 5:50pm
Even if it doesn't actually lift the bow, could the kite be reducing the pitching motion of the boat? That could explain the fact that a boat will pitchpole less with the kite up.


Edited by Flick-Flock - 13 Mar 12 at 5:51pm
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Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 13 Mar 12 at 6:56pm
You sort of "hang" on the kite as the bow pitches down - that's for sure. You can see it especially in 12 videos.
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Post Options Post Options   Quote Peaky Quote  Post ReplyReply Direct Link To This Post Posted: 13 Mar 12 at 7:32pm
Plus of course, if the kite gets you planing that will lift the bow anyway.
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Post Options Post Options   Quote BarnsieB14768 Quote  Post ReplyReply Direct Link To This Post Posted: 14 Mar 12 at 8:20am
Just a quick one, the kite will generate some lift due to the shape and where the centre of effort is. The lift is not that great but it is there. The hull shape and sea state aer also contributing factors.

In boats where the kite is say 20 sq.m. and above or smaller ones in bigger breeze, you will see the top guys have their mains nearly on centre going downwind and so reducing resistance to lift the bow. Ease the main and the main input changes and the bow drops, so acting as a trim tab. Work through the physics and you'll see how it works. A conventional kit, will also generate lift if deep broad reaching to running but here the main may change the dynamics here.

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Post Options Post Options   Quote 2547 Quote  Post ReplyReply Direct Link To This Post Posted: 14 Mar 12 at 9:02am
Originally posted by Medway Maniac



So, compared to sailing with main and jib alone, sailing with the kite will result in a relatively reduced bow-down tendency. 
 
 ... if with the kite up we have relatively reduced bow-down tendancy we have lift in the combined system as a results. THat is what people experience and that is what matters.
 
I think the subtle differences in designs and sail shape also make a big difference on the point of intersection relative to the point of contact on the water and so different classes will feel this differently.
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Post Options Post Options   Quote RS400atC Quote  Post ReplyReply Direct Link To This Post Posted: 14 Mar 12 at 9:56am
Originally posted by seamonkey

 
 ... if with the kite up we have relatively reduced bow-down tendancy we have lift in the combined system as a results. THat is what people experience and that is what matters.
 
....

well put, sir!
In an RS400, if you round the windward mark in moderate waves and pull the pole out, then dither getting the kite up, you will sometimes be sailing along with the bowsprit in the water.
Once the kit is set,   you speed up, the bow comes up and the world is a better, more stable place!
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Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 14 Mar 12 at 10:23am
Originally posted by seamonkey

if with the kite up we have relatively reduced bow-down tendancy

Which we don't have of course. Its all in the document I linked to at the start.
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