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Why does a gennaker/kite lift the bow?

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RS400atC View Drop Down
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Post Options Post Options   Quote RS400atC Quote  Post ReplyReply Direct Link To This Post Posted: 12 Mar 12 at 7:21pm
Part of the story is that with the kite up, you go faster, moving the apparent wind forward, where it isn't pushing the rig forward over the hull like a main in 'blow along mode'.

On a yacht, a kite can give lift without increasing the speed that much, on a much broader reach (apparent wind) by raising the clews, the top of the kite is nowhere near vertical, it gives lift, pulling up on the guy and sheet much more than it's pulling down on the halyard, which is pulling more forward than down.
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G.R.F. View Drop Down
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Post Options Post Options   Quote G.R.F. Quote  Post ReplyReply Direct Link To This Post Posted: 12 Mar 12 at 7:21pm
I doubt the kite lifts the boat, it's more likely that the more power and increased speed uses the nose rocker to lift the nose. If it did then boats wouldn't go down the mine would they?
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Post Options Post Options   Quote rb_stretch Quote  Post ReplyReply Direct Link To This Post Posted: 12 Mar 12 at 7:28pm
I most noticed the upward lift on an asymmetric twin trapeze boat when you come of a big bit of swell. Without the spinnaker you nose dive and pitch pole, with it you seem to almost parachute down making the jumps far less dramatic.
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Rupert View Drop Down
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Post Options Post Options   Quote Rupert Quote  Post ReplyReply Direct Link To This Post Posted: 12 Mar 12 at 7:47pm
Anyone know on which bow lifting thread the diagrams are on? I was unconvinced about the lack of lift thing till I saw them. 
Firefly 2324, Lightning 130, Puffin 229, Minisail 3446
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Medway Maniac View Drop Down
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Post Options Post Options   Quote Medway Maniac Quote  Post ReplyReply Direct Link To This Post Posted: 12 Mar 12 at 10:07pm
J im C gave us a nice diagram last time this was discussed which explained things well, and which, in anticipation of a reprise of the topic, I saved!:


What it shows is that while the kite does generate lift, the line of action is such that it still tends to push the bows down. However, it doesn't push the bows down as much as the main or jib.  So, compared to sailing with main and jib alone, sailing with the kite will result in a relatively reduced bow-down tendency. 

This explains nicely why on the 3k we need to move right aft on a windy two-sail reach in order to stop the bow burying, but we don't need to move aft much at all with the kite up (even if it sometimes pays to do so for other reasons).

Edit: looking again, I see Jim was anticipating the 505 class going asymmetric  Smile


Edited by Medway Maniac - 12 Mar 12 at 10:08pm
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Peaky View Drop Down
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Post Options Post Options   Quote Peaky Quote  Post ReplyReply Direct Link To This Post Posted: 12 Mar 12 at 10:38pm
Pretty sure I drew that picture, but heck I ain't precious! the upshot is that the kite can lift the bow, but only if the kites force passes forward of the LCF, which is unlikely for most rigs that you see.
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JimC View Drop Down
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Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 12 Mar 12 at 11:01pm
Originally posted by Peaky

Pretty sure I drew that picture

Oh good, because I was pretty sure I hadn't.
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Medway Maniac View Drop Down
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Post Options Post Options   Quote Medway Maniac Quote  Post ReplyReply Direct Link To This Post Posted: 12 Mar 12 at 11:40pm
Sorry Peaky.  Bask in your reputation, Jim.
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Post Options Post Options   Quote r2d2 Quote  Post ReplyReply Direct Link To This Post Posted: 13 Mar 12 at 7:18am
Its a lovely picture, but shouldnt the wind hitting the genny be coming from much further forward (ie in front of the boat)? Or are those arrows forces rather than wind?

Edited by r2d2 - 13 Mar 12 at 7:27am
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Peaky View Drop Down
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Post Options Post Options   Quote Peaky Quote  Post ReplyReply Direct Link To This Post Posted: 13 Mar 12 at 7:38am
That's right. They are the force produced by the sail. (technically I think it is Jim's boat and my arrows).
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