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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: 14 Mar 12 at 10:54am
Originally posted by JimC

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.


That paper seems to contradict itself. It clearly details big numbers of lift from the kite.
But the numbers are based on a model,

 " Any kind of mathematical model is only as good as the data you can put into it, and ideally one would like to feed in some real data from measured performance on the water. Mikko has assumed an "across the deck" wind speed of around 20kts, apparent wind at 70 degrees to the centreline, and a forward velocity of around 15 knots."
20knots across the deck is a lot downwind!
So I would sat that's an interesting document, but not a complete story.

I think the key thing with a big assymetric is that you cannot directly compare the boat with and without it, becuase it changes the speed and apparent wind so much.  Whereas with a yacht, you can put a kite up, the apparent hardly changes as you go 1knot fast, but you notice the bow lift sometimes.
Clearly when the front 2/3 of the boat is out of the water, the weight of the boat is pulling the front down. The forward component of the rig force is pivoting the front down, and the lift component is err, lifting it. The weight of the crew is not much behind the centre of lift from the water, but this may vary from boat to boat.
All the forces balance, on average (the boat isn't static!, neither is the water!).
As the boat hits a wave and slows, the water lifts the middle of the boat, now the crew weight is pulling the stern down. The bow is lifted and that compensates for the increased drive pushing the bow  down. That's (sort of) stable.
Of cource if you start off with the crew forward, trimmng the bow down, the force from the kite will have a bigger forward component and less lift, in the limit, pulling the bow further down. Then as you hit a wave, the bow goes down, the boat slows, the apparent goes further back giving more down force and splat!
Positive feedback as we say.

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Post Options Post Options   Quote r2d2 Quote  Post ReplyReply Direct Link To This Post Posted: 14 Mar 12 at 10:56am
Originally posted by JimC

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.

 
you seemed to agree earlier in this thread that that the kite has an effect if not of lifting the bow, then of stopping it falling - are you really sure you arn't kidding yourself that there isn't an upwards force provide by the kite?
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Post Options Post Options   Quote 2547 Quote  Post ReplyReply Direct Link To This Post Posted: 14 Mar 12 at 11:05am
Originally posted by RS400atC

Originally posted by JimC

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.


That paper seems to contradict itself. It clearly details big numbers of lift from the kite.
But the numbers are based on a model,

 " Any kind of mathematical model is only as good as the data you can put into it, and ideally one would like to feed in some real data from measured performance on the water. Mikko has assumed an "across the deck" wind speed of around 20kts, apparent wind at 70 degrees to the centreline, and a forward velocity of around 15 knots."
20knots across the deck is a lot downwind!
So I would sat that's an interesting document, but not a complete story.

 
Agreed ... the paper is interesting but just because it a paper dosn't mean it's correct or reflective of all case ... some people seem to believe stuff just because it was churned out of a computer model.
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tgruitt View Drop Down
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Post Options Post Options   Quote tgruitt Quote  Post ReplyReply Direct Link To This Post Posted: 14 Mar 12 at 11:32am
Originally posted by JimC

You sort of "hang" on the kite as the bow pitches down - that's for sure. You can see it especially in 12 videos.


This is what I'm going to agree with, this is the feeling I get when sailing downwind too. The reason the bow stays up is because the kite stops it dropping so quickly, especially with all the weight at the back of the bus.
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Post Options Post Options   Quote Menace Quote  Post ReplyReply Direct Link To This Post Posted: 14 Mar 12 at 7:56pm
Originally posted by seamonkey

Originally posted by RS400atC

Originally posted by JimC

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.


That paper seems to contradict itself. It clearly details big numbers of lift from the kite.
But the numbers are based on a model,

 " Any kind of mathematical model is only as good as the data you can put into it, and ideally one would like to feed in some real data from measured performance on the water. Mikko has assumed an "across the deck" wind speed of around 20kts, apparent wind at 70 degrees to the centreline, and a forward velocity of around 15 knots."
20knots across the deck is a lot downwind!
So I would sat that's an interesting document, but not a complete story.

 
Agreed ... the paper is interesting but just because it a paper dosn't mean it's correct or reflective of all case ... some people seem to believe stuff just because it was churned out of a computer model.
 
It's fairly subjective for a paper, a lot of "I think" and not enough of other supporting evidence outside the computer program. Was mid 90s and even now, I think computer packages struggle modelling what kites do. Know some who have got close with CFD. At the end of the day, I believe what my boat tells me, 17knots downwind in a 49er is fairly civilised with the kite up, without, it's a ball ache.
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Post Options Post Options   Quote Skiffybob Quote  Post ReplyReply Direct Link To This Post Posted: 15 Mar 12 at 10:45pm
Originally posted by tgruitt

Originally posted by JimC

You sort of "hang" on the kite as the bow pitches down - that's for sure. You can see it especially in 12 videos.


This is what I'm going to agree with, this is the feeling I get when sailing downwind too. The reason the bow stays up is because the kite stops it dropping so quickly, especially with all the weight at the back of the bus.
Quite Tom & Jim
 
On a 12 for example, it's not actually the kite that lifts the nose into the air, it's usually due to the boat bouncing off a wave (you'd be surprised on the effect that a relatively small wave can have on a short light boat).  When we sail the 12 on flat water (i.e inland), the boat sits pretty flat as most others do.  The kite does act as a bit of a parachute once the nose goes up though, holding it up there for longer, and then letting it down more slowly.  This is what give the illusion that the crew is standing the boat on it's arse (which they aren't).  Without standing at the back, the big main would simply shove the bow straight back home (to Australia).
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Post Options Post Options   Quote JimC Quote  Post ReplyReply Direct Link To This Post Posted: 16 Mar 12 at 4:02am
When you think about the situation - C of G pretty much on the stern and all the rest of it there's very little stopping the bow getting pitched up on a 12 with the crew weight and sail forces in equilibrium. You could push it up with one finger!

My guess is that the kite must have a pretty big dampening effect on pitch in any kind of waves, and I suspect this is at least one of the factors that makes it easier to get downhill with the rag up. I reckon what's going on is b****y complicated though. Potentially much more interesting than the simplistic and wrong "kite lifts the bows" nonsense I used to believe until I had it explained to me all those years ago on rec boats racing (Try here if you want to read a dialogue which includes me advancing many of the dumb arguments I now attempt to quash), but maybe too complicated to work out without some very serious work.

When I review the thread I see we also managed to mention that other old chestnut: "direct downwind faster than the true wind". I dunno, seventeen years on and still talking about the same stuff. Probably why I get a bit terse when the topic comes up yet again. I hope I've learned a bit more over those years though. Nothing like your own contribution to antique threads to make you cringe: those of you who were yet to be born when we were wibbling away on usenet are yet to find this out...

Edited by JimC - 16 Mar 12 at 4:16am
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Post Options Post Options   Quote Skiffman Quote  Post ReplyReply Direct Link To This Post Posted: 19 Mar 12 at 6:46pm
Not sure I remember telling Alain to keep the kite trimmed when the bows on its way down or there is a big set of waves in front... To be fair I never say anything unless its a big set and the words used are slightly more colourful, with the meaning of FLOG!

We pull the main in, heel the boat to windward and flog the kite at the right time to stop the kite pushing the bow straight down into the next wave. 

The difference is that you can sail downwind with the main close to the centre line with the kite up. If you had the kite down you would need the main out, if you pulled it in to where you have it with the kite up I promise that you will not pitchpole as easily as with the kite up. The main pushes the bow down a lot more than the kite ever does, especially on these square top rigs with the vang let off.


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