# Lifting Spinnakers:does it lift the bow?

Discussion in 'Sailboats' started by Doug Lord, Nov 24, 2006.

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### KiteshipSenior Member

I give up, fellas. You can't get a spi's resultant to 45 degrees, though your argument requires that it must, and vertical LCB is as much as 12' behind the bow less than 1% of the time, which your argument also requires, but fine, it's all smoke and mirrors. You got me. Probably explains why the crew sits so far forward in high wind--to hold that bow down--and also why high wind failure modes so often include that well-known reverse pitchpole, as lift exceeds the boat's weight and the bow skys.

D.

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### PI DesignSenior Member

Kiteship, I have to say I find your attitude somewhat distasteful. I realise that you have a comercial incentive for making people believe you, but you are wrong.
The 14ft boat I was using to illustrate my point was a 'typical' boat, not an Int 14 specifically. However, if you want a longer pole, it only increases the anticlockwise (in my example) moment. Obviously I realise that a moment doesn't act 'up' or 'down' - that is why I put them in inverted commas to start with, and then in a later post actually gave a frame of refence so that clockwise and anticlockwise had some meaning. I notice that you hadn't done this before.
Splitting a force (which is by definition a vector, so don't be all patronising please), into two components that run parallel to the axis system we wish to use is entirely justified and normal practise. I think this is the root of our disagreement. There is a 'tug of war' and whichever moment (up * distance to LCF v forward * distance to VCF) is greater will determine whether the spinnaker lifts the bow or not.
Actually, a boat trims about its Centre of Flotation, not its Centre of Buoyancy, so I believe that the CF makes a better origin to take moments about. I have been entirely consistent with this. The LCF is generally further aft than the LCB. I fully understand all the points you are making (I think), but you repeatedly ignore the fact the the upward component of the spinnaker's resultant force also produces a moment, which absolutely, 100%, certainly does oppose the moment produced by the forward component of all the sails. As I, wateraddict and others have tried to explain, whether this oppsosing moment is greater than the bow down moment is dependent on the geometry of the boat.
I have not made a mistake in my calculation for the 300sqft kite. You have misunderstood it. The net moment from the kite is -1262 clockwise, which is the same as 1262 anticlockwise. I do not claim that it is zero. The main and jib have a clockwise moment of 1050. 1050+-1262=-212 ie anticlockwise. By the way, you are wrong to claim that zero moment is not possible. It is entirely possible for a kite to have no net moment, but this is not the example I have used. In your parlance, it is when the kite's resultant passes through the CB.
What else can I say? I never mentinded 45 degrees, so don't know why you started using this value. In the example I gave, the displacement mode LCF is not used, so I don't care if you want to move it 12 inches forward. The CofE of the main may be lower than you think, due to twist. At any rate, I am surprised that a deeply cambered spinnaker trimmed properly produces less drive force than a flat, twisted, mainsail that is trimmed to keep the boat flat and reduce leehelm rather than provide maximum drive. The jib's CofE would not be 8ft above the deck. Assuming the sail was triangular, that would mean it extended 24ft up the mast. As I have only assumed a 20ft mast this seems unlikely.
One thing that I think we are agreed on is that there is no downward force produced by the kite, which is the claim that started this thread in the first place. This is what I originally disputed. It was only later when you claimed categorially that a spinnaker ALWAYS has a bow down pitching moment that I disputed that claim as well. It is not always, but it is not never either.

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You proposed the 45 degree vector, merely restated your proposition for illustrative purposes.
A skiff will be in planing mode a whole lot more than 1% of the time. And if it's strong winds off the wind the chute will largely be out in front of the boat.
Crew position - planing on small aft portion of hull reduces wetted surface makes the boat go faster.
Pitchpole is often a dynamic issue not a static one.

Insulting a professionals' judgement when they spent years in training and have made a career doing this stuff is not the way to promote your product. Insisting a bow down moment for spins isn't gonna make everyone "see the light" and buy a kite rig, especially when you piss them off.

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### PI DesignSenior Member

Here, here!
1. Our argument does not require the resultant to be at 45 degrees. This is only the case if the kite's CofE is exactly the same distance above the VCF as it is forward of the LCF
2. Vertical LCB? Who's the one making up terms?
3. The crew need to sit further back when the kite is not being flown. This may suggest that the kite is (relatively) lifting the bow, but not enough to actually lift it. It may still be pitching down, but less than without the kite. This is case 1 in my calculations. There are many factors other than the aerodynamic forces and moments that dictate a crew's position.
4. Culp culpa?

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### PI DesignSenior Member

By the way, the link provided by gggGuest about the analysis on a Cherub kite suggests a resultant of 41.4 degrees (68kg forward, 60kg up).

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### KiteshipSenior Member

A) Apologies if you either think the purpose of the argument is to "sell" kites. I've long ago learned that traditional thinking and kites are like oil and water, and "selling" a kiting concept to luddites is like wrestling with pigs--sooner or late you come to realize the pig is enjoying himself. I was asked to enter this thread as an "expert witness" by it's starter, have promulgated the concepts stated here for decades--since long before I began selling spinnaker replacement kites. If I were "selling" kites, I'd be doing it elsewhere than in a professional design forum. I'm here for the entertainment value, lads.

B) Apologies for the inconsistency regarding 45 degrees versus 30 degrees. I'd made a note that PI used the term "equal forces forward and upwards." The note was in error, he said 30 degrees. I altered one post to conform, forgot that the other used the 45 degrees claim. Apologies; I meant the same thing PI meant, 30 degrees. For the record, I continue to dispute this, I don't think an asym capable of forces anywhere near this high, but without wind tunnel or spi-only CFD data, I cannot further defend the position. For fun, take any of the photos I provided in my last email; for instance the last one. Take a wild guess at the aero CE of the spi, draw a line back at 30 degrees, defining the force vector. Please demonstrate to me how this causes a pitch up moment in the spi? Forget the total of moments from all sails--just show me a pitch up moment form the spi.

C) I don't believe I've promulgated the idea, here or elsewhere, that spis--or any sail--cause a downforce. All cause bow-down pitching moments.

D) Water, I'm sorry, personally, if I've offended you. It is not my intention to insult anyone--people aren't stupid, but even pros (including myself; including yourself; even including PI) make mistakes in logic, then set their watch by them. I try very hard to speak to the issues--the ideas, not the messenger. I can't speak for you, but I often learn new insight by not just thinking outside the box, but by removing the box altogether, and having a fresh look. Your "insulting a professional's judgment" comment is interesting. Right back atcha, my friend.

E) A skiff rarely--rarely--planes with 80-90% of its hull clear of the water. I wasn't saying skiffs rarely planed; I said skiffs rarely plane with only 2' immersed. Watch videos or races, not photographs. Photoggers always pick the most dramatic shot, not a representative average. It never ceases to shock me how many observers--even seasoned pros, seem to fall for this.

F) Last, each of the two of you, PI and Water, seem satisfied with the conclusion that it's just *possible* that an asym spi could be made to pitch the bow up. This is fine with me, it *is* theoretically possible for such a sail to do so. It just doesn't exist on any actual boats.

F) Small nitpick; PI, in a boat at equilibrium, CF cannot be in a different location from CB or there would be a torque, thus no equilibrium. I suspect this is a semantics thing; where your use of "CF" and mine of "CB" are different (perhaps to do with planing forces?). For the sake of this argument it does not matter; one can take the moments around any origin and will arrive at the same conclusion. Starting with the boat's "center of rotation" just simplifies the equation; same as using a single force from each sail.

Cheers,

Dave

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Point D - My comments were strictly that depending on the specific geometry, a moment causing the bow to pitch upward is possible. You seemed to take the stance that this is never the case. So we disagree. I would propose that point A that you make above could be interpreted as insulting. Maybe I am overly sensitive.

Point E - the skiff does not need to have 80-90% of its hull clear of the water for the centroid of vertical hydro force components to be 80-90% aft.

Your comment about CF and CB is wrong. LCF and LCB are nearly always in different places. Check basic nav arch texts.

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### KiteshipSenior Member

Maybe.

And yet you were happy when I stipulated that it is near the leading edge of a planing surface. Hmm... Splitting hairs to win semantics arguments, or are we actually trying to make headway here?

Yeah, sometimes by as much as 1-2% of LWL. So we've answered the above question; we're splitting hairs. Enjoy.

Dave

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### KiteshipSenior Member

And yet, the study still found a net bow-down pitch moment for the spi alone; significantly more than that of the main. Taken together, the moment is more than twice as large as the main alone. Is this what you wanted to point out?

Dave

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LCB and LCF can be much different than 1-2% of LWL. Not trying to split hairs, just pointing that out. They can be off by as much as 20% of LWL or more, depending on the hull.

Not sure to what you are referring regarding the stipulation of leading edge of planing surface.

12. ### Chris OstlindPrevious Member

Been done before

That's a Sanford and Son version of one of Dave Culp's kites.

Yes, folks, they really do lift the bow.

And no, Dave didn't ask me to float these images for him. He's capable of that on his own. I would, however, like to see a small sized version of these bad boys so I can use it on my 17-22' trimarans.

Chris Ostlind

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13. Joined: Jun 2004
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ok,
drew a scaled picture in cad, 15 foot hull, 6 foot pole. CE of kite at (0, 10).
Projected line of action from CE at angle of 41 degrees, put CG circle at estimate 4.4 ft fwd of transom. Sure looks like in the realm of possibility with these somewhat normal looking dimensions that a bow-up moment is possible.

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### raykSenior Member

This is how I see it....

Dont get out your rulers but those vectors combine and lift.

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15. ### Chris OstlindPrevious Member

Pick up the phone, Bubba

Really, guys... hasn't anyone queried some of the bigger sail companies like North, et.al., as well as places like Stanford Univ. who do so much tunnel testing of their design and research studies?