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Why does a gennaker/kite lift the bow?

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Category: Dinghy classes
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Topic: Why does a gennaker/kite lift the bow?
Posted By: 17mika
Subject: Why does a gennaker/kite lift the bow?
Date Posted: 12 Mar 12 at 5:49pm

As simple as the subject above; but still I cannot get the answer. Anyone can help?

I mean, I know (or at least I think I know)  that a kite is a highly tapered and swept wing,  with more and more taper as you go upwards to the head of the sail. But then, again, what does it has to do with upward lift?
 
Thanks in advance for the answers; I know that once I get it, I'll be at least a couple of knots faster around the course LOL



Replies:
Posted By: Lukepiewalker
Date Posted: 12 Mar 12 at 5:56pm
Run away! Run away! Before JimC gets here...

Or, to put it another way, it doesn't.


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Posted By: winging it
Date Posted: 12 Mar 12 at 6:01pm
oh god, I'm getting my coat....

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Posted By: 2547
Date Posted: 12 Mar 12 at 6:01pm
Of course it lifts the bow ...
 


Posted By: JohnW
Date Posted: 12 Mar 12 at 6:04pm
All the photos of 12 foot skiffs prove it!!
Big smile


Posted By: winging it
Date Posted: 12 Mar 12 at 6:22pm
nah, it's because they all stand at the end of the boat....LOL

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Posted By: x1testpilot
Date Posted: 12 Mar 12 at 6:34pm
It's the angle of the sail to the boat/wind/water. The force on the sail is perpendicular to the sail. think of a wooden board held in the wind, if you hold it to the top (leading edge) is into the wind and the bottom at 45 degrees to the leading, you will feel lift (also a real kite).

The leverage of a long spin pole will make the effect more pronounced.

Having said that the effect can't be that significant as 14's skiffs etc do tend to stand at the back of the boat + the wind is pushing the main forward too.

I expect a better explanation will be forthcoming!



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Posted By: JimC
Date Posted: 12 Mar 12 at 6:57pm
It doesn't.

There may be a lifting effect on the boat as a whole, but the bow is always pushed down.

http://www.devboats.co.uk/cherubweb/liftbow.htm

If it did you would see trapeze crews going for the front of the boat to get the weight forward when a gust hits.


Posted By: RS400atC
Date Posted: 12 Mar 12 at 7:12pm
I think Jim's argument is a little spurious, because in most of the bow up pictures, the crew are back but the upward force of the water is also concentrated aft.
The sum of the vectors of the 3 strings on a kite pulls in a certain line.
Whether that is pulling the bow up or down depends also on where the reaction force (drag on the hull etc) is ahead or astern of that line.


Posted By: Rupert
Date Posted: 12 Mar 12 at 7:15pm
There are many tales of boats going down the mine less once the kite is up, but this doesn't mean it gives lift. What does happen is beyond me, though - probebly to do with centres of effort or something.


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Posted By: RS400atC
Date 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.


Posted By: G.R.F.
Date 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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Posted By: rb_stretch
Date 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.


Posted By: Rupert
Date 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. 

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Posted By: Medway Maniac
Date 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


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Posted By: Peaky
Date 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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Posted By: JimC
Date 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.


Posted By: Medway Maniac
Date Posted: 12 Mar 12 at 11:40pm
Sorry Peaky.  Bask in your reputation, Jim.

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Posted By: r2d2
Date 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?


Posted By: Peaky
Date 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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Posted By: r2d2
Date 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


Posted By: RS400atC
Date 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.


Posted By: x1testpilot
Date 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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Posted By: Flick-Flock
Date 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.


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Posted By: JimC
Date 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.


Posted By: Peaky
Date 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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Posted By: BarnsieB14768
Date 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.

Bye for now


Posted By: 2547
Date 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.


Posted By: RS400atC
Date 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!


Posted By: JimC
Date 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.


Posted By: RS400atC
Date 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.



Posted By: r2d2
Date 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?


Posted By: 2547
Date 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.


Posted By: tgruitt
Date 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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Posted By: Menace
Date 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.


Posted By: Skiffybob
Date 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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Posted By: JimC
Date 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 ( http://groups.google.com/group/rec.boats.racing/browse_thread/thread/bb19d37380914ff2/549f06e0afb98390?lnk=gst&q=spinnaker+lifting+bows#549f06e0afb98390 - 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...


Posted By: Skiffman
Date 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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