Lifting Spinnakers:does it lift the bow? [Archive]

Doug Lord

11-24-2006, 10:31 PM

This subject was so off topic under the "Planing Trimarans" thread I moved part of it here to try to get more input. Other than my rc models I've had very little spinnaker experience but from my reading I'm convinced that certain spinnakers will lift the bow-particularly on skiffs. I'd like to hear anyone's experience and any references on the subject.....

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from the "Planing Trimarans thread:
AP, Dave Culp is dead wrong at least on RC spnnakers such as these:
s50witha1.jpg
Address:http://www.microsail.com/images/s50witha1.jpg Changed:8:47 PM on Friday, November 24, 2006
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And I strongly suspect that on several of the skiff classes the spins lift the boat.
When I develped these models they were the first fully gybable rc production spinnakers and in development we made hundreds of tests to ASSURE that the spin would lift the bow.In fact, the boats can sail in stronger wind dead downwind with the spin up than they can with just the main and jib! The spins are relatively small and flat but add tremendous power to these models; when a gust hits you can physically see the bow lift.
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105 see para 5:
J/Difference
Address:http://www.jboats.com/sailingj.htm Changed:12:24 PM on Wednesday, September 20, 2006
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505
"The power in the big spinnaker is incredible and the remarkable thing is it lifts the bow and actually make the boat easier to steer in big seas and big...."
Correct URL is-scroll down to box-"What the 505 is"-this is an english site:
The "Five-Oh" has verve!
Address:http://hem.passagen.se/waterat/505_has_verve.htm Changed:4:53 AM on Thursday, April 1, 2004
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Dr.Ian Ward 07 March 1999 (his second letter 3.0 Longitudinal Stability) Spinnaker provides Lift
Discussion
Address:http://culnane.navidat.com/dc/sailing/moth/wshpaper/discussion.htm Changed:10:46 AM on Monday, January 30, 2006
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I e-mailed Dave Culp and invited him to participate in this thread.(11/25/06)

RHough

11-25-2006, 12:46 AM

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:confused:

I'm not sure but I'm pretty sure this doesn't say anything about lifting bows?

I hope Dave Culp jumps in here ...

It ought to be fun.

I'm making popcorn ... :cool:

usa2

11-25-2006, 02:06 AM

Spinnakers almost always like to drive the bow down, not up.

gggGuest

11-25-2006, 07:33 AM

http://www.sailingsource.com/cherub/aero.htm
http://www.wb-sails.fi/news/95_12_Cherub/Cherub.html

Lifts the boat, but presses down the bow. Show me a video of the crew scrambling forward to keep the bows down in a gust and I might believe it lifts the bows!

I suppose it might just lift the bow if the dimensions were extreme with the luff averaging less that 45 degrees to horizontal.

With more angled luffs then the sail will have a considerable effect of stabilising the boat in pitch - as the bow pitches down the lift from the sail increases and the total pitch down force decreases. as the bow goes up the reverse. This effect would appear to be apparent in twelve footer videos.

I think that, purely with emprical observation, it is very dificult to separate what happens purely due to the sail pressing down or not on the boat from the hydrodynamic effects caused by increased speed and the effect of the sail lifting the boat.

Kiteship

11-25-2006, 01:40 PM

RHough

11-25-2006, 03:05 PM

"always" not "almost always." We settled this, believe it or not, in Sept 1995 in a long discussioin on rec.boats.sailing. The thread is here: http://groups.google.com/group/rec.boats.racing/browse_thread/thread/bb19d37380914ff2/35bcb5012600471a

Thanks for finding that link. I was pretty sure that I'd been through this discussion at least once before (in space and time long ago and far away).

It boils down to any driving force acting higher than the drag force of the hull pitches the bow down. Since the drag is at or below the waterline and the sails are above it, the drive force cannot lift the bow.

If the sail is rotated so the force that was heeling the boat is now lifting the boat out of the water (heave), the whole boat lifts, but the drive force is still pitching the bow down.

Reducing bow down pitch is not the same as creating bow up pitch.

Look at the fore and aft crew position of a boat that is sailing with the bow out of the water with a spinnaker up. Try placing the crew in the same position fore and aft at the dock. Will the bow be higher or lower?

Doug thinks that you and I are wrong, let's see if he can prove it. :)

foilr

11-25-2006, 06:02 PM

Doug,

I'd say the spinnaker does... but in a roundabout sort of way.

In a skiff (18, 49er, 29er etc), the addition of the kite would provide the extra speed required to bring the apparent wind forward, thereby reducing the breeze that would've been pushing the rig forward/down.

The bow-up aspect that you see on skiffs has a lot to do with hull shape and crew-position as well.

Tim B

11-25-2006, 06:32 PM

ummm, ahhh, welll errrr, yes, sort of, maybe.

In theory, if you can get the lift vector from the spinnaker to point directly away from the CB, yes, it will lift the bow.

In normal circumstances, the lift from the spinnaker is more or less forward, and so you have a considerable force and lever. This generates a massive bow-down pitching moment.

You can certainly reduce the moment, but what's it doing to the efficiency, and what was the trim angle anyway?

The only way to find out is to run it through a VPP which will predict pitch, based on a pitching lever. Even a back of the envelope sum will show that the change in trim is very small.

Tim B.

Kiteship

11-25-2006, 06:54 PM

You do not need anything as complex as a VPP program. Draw a sailboat. Note where its center of buoyancy is (very approximately is fine). Guess (within 10' and 30 degrees) where the resultant of the spinnaker is (a resultant is a line of force). Unless said resultant passes under the CB, the boat will pitch bow down. This is a physical certainty, not subject to reader opinion (not being snotty; just pointing out which part of the post is actually mathematics. ;-)

Try every boat you can think of. Any combination of sails. Feel free to cheat. Unless you use a free-flying kite, you will get a bow-down pitching moment. Period. This isn't a close thing, not by a very long shot.

Dave

ummm, ahhh, welll errrr, yes, sort of, maybe.

In theory, if you can get the lift vector from the spinnaker to point directly away from the CB, yes, it will lift the bow.

In normal circumstances, the lift from the spinnaker is more or less forward, and so you have a considerable force and lever. This generates a massive bow-down pitching moment.

You can certainly reduce the moment, but what's it doing to the efficiency, and what was the trim angle anyway?

The only way to find out is to run it through a VPP which will predict pitch, based on a pitching lever. Even a back of the envelope sum will show that the change in trim is very small.

Tim B.

Tim B

11-26-2006, 04:42 AM

You missed my point. The bow-down pitching moment is due to the driving force of the rig multiplied by a lever. So, unless there is no driving force, the bow goes down.

However, you can change the amount of bow-down moment, but it's unlikely to get the most out of the rig, and therefore the boat.

Do you know what the typical trim angles are upwind/downwind? except for very small boats I think you'll find it's a very small number.

Tim B.

QC3

11-26-2006, 11:14 AM

It seems any affect on the bow would depend on the force the chute applies to the vessel itself. The spinnaker only has three attachement points where it could influence the boat. 1.) The halyard - it would seem on most sloop rigs with the mast mostly forward that any force outward from the top of the rig would tend to drive the bow down. 2.) The sheet - which is tyically lead way aft. It would seem any force applied way aft would also tend to drive the bow down. 3.) The afterguy/foreguy (tack posisiton). Afterguys are typically led midships which would not help "lift" the bow so that leaves just the foreguy which is typically led to the foredeck. Can the lift of the foreguy alone counteract those other aftorementioned forces to lift the bow? I have usually found the opposite to be the case which is why you need to get all of your weight aft when off the wind.
Just my layperson's opinion...

Chris Ostlind

11-26-2006, 12:24 PM

Thanks for the very direct mechanical description, Dave. I agree.

I posted this note back on the other thread where this argument first came about. This has to do with the practical realities, but follows closely on the description as provided by Dave.

"If you have access to an F-boat, here's a simple experiment you can run to get a better idea as to the dynamic forces involved.

Take her out on nice breezy day and observe the placement of the crew on the boat for best performance upwind. Now, turn offwind and fly an assy. chute for optimal trim and apparent wind. Leave the crew at the same location on the boat and see how long you can fly the chute nicely sheeted-in without pushing the bow of the leeward ama under water and scaring the crap out of everyone onboard, much less the owner of the boat.

Is that a lifting rig when parts start to go submarine and the crew runs for the aft parts of the boat?"

The above quote was subsequently supplemented by Mike Leneman's quote below:

"The majority of Farrier designed boats that have gone over have done so while sailing downwind with a spinnaker........I would say that over 90% of the "capsizes" have been pitchpoles with spinnakers up. Spinnaker that looked, by the way, very similar to the ones on Doug's link.

If they provide so much lift, then why did these boats pitchpole? Why didn't their bows come up? They even have pretty big bowsprits, which should help the "upward" angle of the spin."

I know it sounds racy to get your head wrapped around the potential of your bow being lifted in this application, but no amount of dream scheming is going to get you over the hump of the physics when coupled with the experiential reality.

high on carbon

11-26-2006, 02:20 PM

Sail a Skiff of any kind, hoist kite, trim the kite, watch the end of the pole bend skyward. Hmmmm, What was that Newton second law thingy???

Yes the mast is bending forward too, Net combined force? About 30 to 45 degrees up from the surface of the water, net effect = lifting the boat.

I am old enought that I was around and designing and building boats in the I-14 class when we went to A-sails. even with a kite twice the size of the symetric kite, it most certainly provided more lift than it did pitching moment.

It is certainly influenced by geometry but for sure it provides more lift than the old kind of kites. If we had a bigger A-sail and it produced more picthing moment, we would not have kept doing it. That's why we could all of a sudden start developing finer bows again, we spent a lot less time going down the mine.

A-sails, set up right, lift the bow, period.

RHough

11-26-2006, 02:24 PM

A-sails, set up right, lift the bow, period.

And that is why when a gust hits, you have to shift weight foreward to prevent the bow from skying? :)

How about the CE is foreward and down compared to the old kites? Less pitch down does not equal pitch up.

gggGuest

11-26-2006, 02:38 PM

-sails, set up right, lift the bow, period.

I suggest you read the papers I quoted above. Lift the *boat*, sure thing, absolutely they do. Lift the *bow*? Not on a 14.
Another thing you might try is to let the kitesheet go at speed...

Doug Lord

11-26-2006, 03:27 PM

Kiteship

11-26-2006, 03:49 PM

Anecdotes versus mathematics, Doug. Show me the math and we can talk. (When you refer to "America One" and to "S50" are we talking about scale RC models or full size boats? I'm not knocking models, as you know I have no prejudice against models; just need to know the scale fo the experiment)

Dave

The info provided by Jim and Dave of past discussions is interesting and I appreciate it.
I have 16 years development in rc spinnakers and there is no question that the microSAIL! symetrical spinnaker lifts the bow-it's clearly visible to anyone that's ever sailed one in any breeze above 10 kts.. There may even be video on the site. RC boats lose a lot of pitch stability in comparison to full size boats and if these sails didn't lift the bow they simply wouldn't work. Further, we've done hundreds of demonstrations in strong winds- with America One and the S50 to illustrate that without the spin in winds over 15 both these boats would nosedive and with the spin they don't. A specfic "set angle " has been determined that allows this kind of symetrical spin to lift. These sails are set at a fairly big angle off of twin poles and in lighter wind it is possible to sail as high as a beam reach since the sail behaves a bit like an asymetrical because the windward pole tightens the luff and leeward pole opens the leach-and tacking downwind pays in racing. In stronger winds(over 12-15) tacking downwind does not pay because the boat tends to get overpowered and lift from the spin is observable approx. 30° either side of dead downwind In over 15- 20k the boat can be sailed dead downwind WITHOUT REEFING only because of the lift from the spinnaker; dousing the spinnaker in those conditions results in almost an immediate pitchpole.Sailing higher than dead downwind with the spin set in those conditions knocks the boat flat until the sail is doused.
There are very few that have experienced this -maybe 55 worldwide+ 14 or so prototypes but it is an absolute fact.
In trying to come to a realistic appraisal of what is happening on full size boats your theory should be able to explain what is happening on these models as well.
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a1a.jpg
Address:http://www.microsail.com/images/a1a.jpg Changed:3:32 PM on Sunday, November 26, 2006

RHough

11-26-2006, 04:10 PM

The info provided by Jim and Dave of past discussions is interesting and I appreciate it.
I have 16 years development in rc spinnakers and there is no question that the microSAIL! symetrical spinnaker lifts the bow-it's clearly visible to anyone that's ever sailed one in any breeze above 10 kts.. There may even be video on the site. RC boats lose a lot of pitch stability in comparison to full size boats and if these sails didn't lift the bow they simply wouldn't work. Further, we've done hundreds of demonstrations in strong winds- with America One and the S50 to illustrate that without the spin in winds over 15 both these boats would nosedive and with the spin they don't. A specfic "set angle " has been determined that allows this kind of symetrical spin to lift. These sails are set at a fairly big angle off of twin poles and in lighter wind it is possible to sail as high as a beam reach since the sail behaves a bit like an asymetrical because the windward pole tightens the luff and leeward pole opens the leach-and tacking downwind pays in racing. In stronger winds(over 12-15) tacking downwind does not pay because the boat tends to get overpowered and lift from the spin is observable approx. 30° either side of dead downwind In over 15- 20k the boat can be sailed dead downwind WITHOUT REEFING only because of the lift from the spinnaker; dousing the spinnaker in those conditions results in almost an immediate pitchpole.Sailing higher than dead downwind with the spin set in those conditions knocks the boat flat until the sail is doused.
There are very few that have experienced this -maybe 55 worldwide+ 14 or so prototypes but it is an absolute fact.
In trying to come to a realistic appraisal of what is happening on full size boats your theory should be able to explain what is happening on these models as well.

It absolutely does Doug.

Find the CE of the two sail configuration and the CE of the three sail configuration. I'll bet that the CE of the of the 3 sails is forward and possibly lower (probably in the case of fractional hoist spinnakers). No argument that there may be LESS nose down pitch from the forward/lower CE. Thus compared to the same hull with 2 sails in the same conditions the bow will be higher. That is not lift, that is less pitch down.

The reduced pitch down and greater power may even allow the boats to sail fast enough to plane, on most rockered hulls, that is a bow up attitude.

Here is a simple model sized proof:

At wind speed 15 the boat appears to be bow up.

If the bow up trim is due to lift from the spinnaker, at wind speed 21 the lift from the spinnaker should be double. Is the bow lifted further?

At wind speed 30 there is 4 times the lift, is the bow higher still?

At some point the "lift" from the spinnaker will exceed the displacement of the boat and it will be lifted clear of the water by the bow.

If the sail is lifting the bow, a video of the boat sailing down wind should show the bow rising before the boat speed increases. From what I have observed this is not the case.

What happens to the models in very high wind? Do they fly, broach, nosedive, pitchpole or plane?

Doug Lord

11-26-2006, 04:35 PM

rayk

11-26-2006, 04:53 PM

Kiteship

11-26-2006, 07:08 PM

I can imagine a boat getting some lift out of a spinnaker. The stern in particular.
Until the hull begins to plane the bows are dragging in the bow wave.
Scampering aft in a boat unlikely to plane is instinctive for the crew.
In a boat that is planing under spinnaker sitting aft doesnt seem to help trim and the crew could probably shuffle forward little.(Back to work you scurvy dogs!)

This is not the case, in actual sailing. Fortunately for us, there are a number of over-powered sportboats these days, and a plethora of over-testosteroned crews with which to over-sail them. The limit of stability is when the bow is driven under--typically a monohull will broach before this happens, but not always (cf: going down the mine). The crew always stays back--and wishes for more "back" to go to--as the boat is more and more overpowered. I've been sailing for a long time, and I've yet to see a boat pitchpoled over the stern, due to too much lift forward from the spi.

Despite assurances of the crew(s), *every* photo of a skiff running fast under asym has the entire crew standing on--or behind--the transom, out on the racks. Do this at the dock sometime, and see where the boat trims.

Multihull power, thus speed, is *always* limited by burying the lee bow. Both upwind and down. Again, where crew weight is sufficient to do any good, you *always* see the crew trying to get further back--and going to depower when the lee bow buries.

But try this as a mental excersise: if this is actually happening on these boats what does it mean for your theory?

It's not a theory, Doug, but simple geometry. Draw the picture, chart the lines of force, calculate the moments. It holds for *all* waterborne structures. From your statements, you only see this "phenomena" on model boats, never full size? If so, I strongly suspect a scaling or froude number factor. I don't know what it is, but as *you* are the only guy who's replicated it, I'm thinking it's up to you to demonstrate why the geometry--and physics of the thing says it cannot be done--yet you can do it. (FWIW, it is fairly well known that skiffs with 3 sails up are more stable off-wind than identical boats with only two--it's been said here why--the effective center of drive is lower with the asym up. Lower the C of E and you will definitely reduce the pitching moment. Reduce, not reverse. I'd also note that removing the asym reduces the brute power of the system, resulting in lower boatspeed--and a marked shift aft of the apparent wind. Is this where you're getting your nosediving, without the asym up?)

Dave

Doug Lord

11-26-2006, 07:43 PM

Despite assurances of the crew(s), *every* photo of a skiff running fast under asym has the entire crew standing on--or behind--the transom, out on the racks. Do this at the dock sometime, and see where the boat trims.

I'd also note that removing the asym reduces the brute power of the system, resulting in lower boatspeed--and a marked shift aft of the apparent wind. Is this where you're getting your nosediving, without the asym up?)
Dave

See the Skiff picture on the cover of High Performance Sailing-crew about midship.
I'm not sure but you may have something about the increase(not shift of) in apparent wind from behind when the sym spin is dropped on the model. But that doesn't explain the obvious rise of the bow when the boat is hit by a gust.
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PS-and when the chute is dropped the CE rises-that and the apparent increase may be an answer...

RHough

11-26-2006, 07:45 PM

In strong winds -20-30- you can only sail dead downwind with the spinnaker. If you do so without reefing the main and then douse the spinnaker the boat nosedives; if the spin is left up no nose dive period. One other thing: in rc sailing it's easy to see gusts moving and watch as they hit a boat: without a doubt and with no speed change a gust will cause a noticable lifting of the bow.
I don't know how much rc sailing you've done but I'm 100% sure you've never sailed one of my boats in 10-12 and above.
But try this as a mental excersise: if this is actually happening on these boats what does it mean for your theory?
I'm not trying to be smartass- I'm curious about what you think could explain this behaviour.

Here are a few more ways to test the theory of lifting spinnakers.

Take the boat with spinnaker rigged and gimble just above the CG it so it is level, but free to pitch up or down. Mount the gimble on the hood of your car and drive down the road. If the spinnaker lifts the bow as you claim, the boat will pitch up in the gimbles.

Take the boat and set it in a pond and attach a spring scale to the bow. Note how much tension it takes to lift the stem 1", now double the tension, then double it again. The pitch change curve you get is exactly what to expect if the spinnaker is lifting the bow. See if that trim comes close to what you have observed.

As I asked before, when the "lifting spinnaker" boat finally crashes, What is the nature of the crash? Does the spinnaker lift the boat out of the water? Are you saying that no matter what the wind speed the boat never lifts out of the water and never nosedives? That would mean that the spinnaker produces less lift at higher wind speeds, but continues to produce enough lift to prevent a nosedive? That would be a very neat trick.

You know very well that the lift force from the spinnaker will increase exponentially with wind speed. Therefore the boat should lift exponentially higher with each gust. Are you claiming that is what happens?

You designed it Doug. You tell us how it is possible for the spinnaker to lift the bow at 10 knots yet not pull the boat out of the water at 30 knots.

I once made the mistake of giving serious consideration to an idea that I know won't work. I won't waste my time thinking that spinnakers lift bows while driving the boat, it cannot happen.

gggGuest

11-26-2006, 08:04 PM

I won't waste my time thinking that spinnakers lift bows while driving the boat, it cannot happen.

I'd be wary about cannot: given a long enough pole and a short enough hoist I imagine it could happen, but I doubt such a rig would be very practical or desirable! Best to use one of Dave's kites - I bet they lift the bows like crazy!

Chris Ostlind

11-26-2006, 08:07 PM

How free of shoreline obstructions are these ponds where you observe these "lifting bow" phenomona?

Due to the close proxitimity of the sail to the water, and the likelihood of shore clutter to make the surface turbulence worse, how do you know that you are not actually seeing the sails reacting to swirling surface turbulence?

Since you don't mention it, I assume you have not considered the potential.

DSmith

11-27-2006, 05:28 AM

Crag Cay

11-27-2006, 06:44 AM

All the 'definitive' examples for the lack of lift in a spinnaker have been on boats with mainsails up as well, ie the total combined effect has been quoted. I think the improvised 'car top' wind tunnel, or even a real wind tunnel test, would be needed to isolate the component forces.

However some observations:
The 'everyone aft - we're going to nose dive!' cry was more common when deep running on non spinnaker boats such as Albacores, Enterprises, etc. Right from early Cadet racing as a kid it was noticeable how much more longitudinally stability the spinnaker gave in a blow.

I have always been surprised how much more stable even large keel boats are when running if you reef their mains first. It seems to dramatically reduce bow down pitch and therefore bow rudder. This has helped the handling of all sorts of over pressed boats from big IOR through to more recent big boats.

I think long bowsprits and asymetrics have altered the dynamic again. There must be an some uplift component because it bends the long carbon 'lever' upwards in excess of the deflection I can get from two of us hanging on it, when back on the trailer. (400lbs). The tack to Centre of Flotation (rotation) distance (lift lever) is about 80 percent of the height to the small heavy weather spin hoist. This must be a different picture to that of a conventional spinnaker where, as was said earlier, only the foreguy has any attatchment forward of the mast.

PI Design

11-27-2006, 07:38 AM

For what its worth I agree with Doug, Crag etc on this one. Imagine a boat with a vertical mast and no heel. How can the manisail produce a force down (or up)? It can't, but due to the CofE being above the Centre of Flotation a moment is produced, which does trim the bow down. Please note, this is a moment, not a force. Now, any sail with an aft raked luff e.g. jib, asymmetric etc will produce a suction lift force with a forward component and an upwards component. If I could post pictures I would draw it for you, but its fairly easy to imagine and at any rate the upward pull on the bow sprit demontrates it. However, two opposing moments are produced. One is the result of the upward component being forward of the LCF, the other is the forward component being above the VCF (as per the mainsail example). The two moments oppose each other. Which ever is greater will determine whether the net moment is bow down or bow up. But, either way, the force component is upwards, not downwards - hence a reduction in displacement (even if the bow is trimmed down).

water addict

11-27-2006, 08:03 AM

Totally dependent on the specific geometry of whatever boat. Sum the forces and moments created by where the spinnaker attaches to the boat (halyard, pole, sheet) and the CG + hydro drag. If the summed total moment causes pitch up or pitch down, then the boat will do that until forces and moments are in equilibrium.
Of course you would need some pretty sophisticated sensors to determine this and a lot of data analysis. Better to just go sailing and have fun!

PI Design

11-27-2006, 08:04 AM

And another thing...
The only way a spinnaker can reduce the bow down pitching moment is if it acts in the opposite direction to the moment from the main and jib. Raising the kite may lower the lever, but it increases the force, so that the net result would be to trim the bow further down (if you're statement was correct). Therefore, if you admit that hoisting a spinnaker lifts the bow (compared to not ahaving a spinnaker), you must be agreeing that the spinnaker has a lifting moment.

gggGuest

11-27-2006, 09:59 AM

Now, any sail with an aft raked luff e.g. jib, asymmetric etc will produce a suction lift force with a forward component and an upwards component.

Not necessarilly no. Just because the luff is raked down't mean it will produce upward force in any significant quantities. To grossly over simplify, imagine looking at the sail from above. Only the area of cloth you can see from above can be contributing to a vertical force...

PI Design

11-27-2006, 10:13 AM

Not necessarilly no. Just because the luff is raked down't mean it will produce upward force in any significant quantities. To grossly over simplify, imagine looking at the sail from above. Only the area of cloth you can see from above can be contributing to a vertical force...

Absolutely, but there is still some upforce, and none down (if the boat is upright). Therefore the net force is up.

Kiteship

11-27-2006, 12:40 PM

Sound thinking, but you're hoist on your own petard. You've got the main's moment, yes, but you then compare it with the asym's force, which is mistaken. The asym's *moment* is bow down, even though it's force reduces the boat's displacement. The two moments add to each other, they are not opposed. Sound impossible? No; imagine a lifting force, *straight up* which is applied at the stern. Force is up (vertical); but moment is bow-down. It's the same with the asym--DRAW THE PICTURE.

Dave

For what its worth I agree with Doug, Crag etc on this one. Imagine a boat with a vertical mast and no heel. How can the manisail produce a force down (or up)? It can't, but due to the CofE being above the Centre of Flotation a moment is produced, which does trim the bow down. Please note, this is a moment, not a force. Now, any sail with an aft raked luff e.g. jib, asymmetric etc will produce a suction lift force with a forward component and an upwards component. If I could post pictures I would draw it for you, but its fairly easy to imagine and at any rate the upward pull on the bow sprit demontrates it. However, two opposing moments are produced. One is the result of the upward component being forward of the LCF, the other is the forward component being above the VCF (as per the mainsail example). The two moments oppose each other. Which ever is greater will determine whether the net moment is bow down or bow up. But, either way, the force component is upwards, not downwards - hence a reduction in displacement (even if the bow is trimmed down).

Kiteship

11-27-2006, 12:50 PM

It is not necessary to sum the forces at each the attachment point (though you could certainly do it that way--if you had enough money!). It is only necessary to find (or assume) the center of each sail's applied force, and find (or assume) the direction of that force. Do this for each sail; take an origin (I suggest the Center of Buoyancy, but you can use any Origin), and measure the moments around that origin. you can work in just two dimensions if you like, since we are only investigating bow down pitch and not roll or yaw. FWIW, this is precisely how every yacht designer and navel architect designs and investigates forces from his rig.

I say "assume" above, because not very much accuracy is needed to prove the point. All the sails on all sailboats pitch the bow down--except free-flying kites. Period. You'd need a very (very!) long bowsprit and a very short mast to make this any different (I mean, bowsprit to mast base on the order of twice the mast height). Only kites fly sufficiently far from the boat to pitch the bow up.

Please, stop imagining and "feeling" for the answer and just draw the picture. This isn't rocket science. (well, actually, it is, but then rocket science ain't that difficult, either)

Cheers,

Dave

Totally dependent on the specific geometry of whatever boat. Sum the forces and moments created by where the spinnaker attaches to the boat (halyard, pole, sheet) and the CG + hydro drag. If the summed total moment causes pitch up or pitch down, then the boat will do that until forces and moments are in equilibrium.
Of course you would need some pretty sophisticated sensors to determine this and a lot of data analysis. Better to just go sailing and have fun!

Kiteship

11-27-2006, 12:55 PM

Oops! Sorry for the "navel architect" crack, Water Addict. It was unintentional (though one of my favorite jokes--get it? "navel" architect? Sorry, a little design humor. Very little.)

Dave

Chris Ostlind

11-27-2006, 01:04 PM

Since this discussion originated in the Planing Trimarans thread, I'm kinda amazed that Doug hasn't plugged an email to Ian Farrier to get him to chime in here in support of his lifting bows concept.

I remember a conversation between a friend of mine and Kami Richards at Pineapple sails in the San Francisco Bay area some years ago. My buddy wanted to build a really fast, small trimaran of 15'. He planned to have smallish amas and drive the boat "very actively". He described an assy. spinnaker for the boat to Kami and Kami said that would be fine if he liked sailing submarines.

Crag Cay

11-27-2006, 03:24 PM

Interesting article on the Cherub UK website:
www.sailingsource.com/cherub/test/doku.php/tech/spinnakerlifts

Kiteship

11-27-2006, 03:48 PM

Yes, Jim Champ did that study in response to the 1996 thread on rec.kites. Pretty clearly lays out the whole argument. There are other links to the same essential work, earlier in this thread

Dave
Interesting article on the Cherub UK website:
www.sailingsource.com/cherub/test/doku.php/tech/spinnakerlifts

PI Design

11-28-2006, 03:22 AM

Sound thinking, but you're hoist on your own petard. You've got the main's moment, yes, but you then compare it with the asym's force, which is mistaken. The asym's *moment* is bow down, even though it's force reduces the boat's displacement. The two moments add to each other, they are not opposed. Sound impossible? No; imagine a lifting force, *straight up* which is applied at the stern. Force is up (vertical); but moment is bow-down. It's the same with the asym--DRAW THE PICTURE.

Dave

I probably didn't make myself clear enough and I think you may have misunderstood me. The asym will produce a forcethat has both a forward and (to a lesser extent) an upward component. The centre of this force acts forward of thr LCF and above the VCF. Therefore the forward component produces a bow down pitching moment whilst the upward component produces a bow up pitching moment. Whichever m,oment is greater will determine whether the bow goes up or down. This depends on the geometry of the specific rig (luff angle, sprit length etc), but for most boats the bow down moment is probably bigger. However, this thread originated from the planing trimaran thread, where it was claimed that the sails produced a downward FORCE. This is not the case (for an upright boat). There is no downward force, just a moment that (may) push the bow down.

water addict

11-28-2006, 06:27 AM

Oops! Sorry for the "navel architect" crack, Water Addict. It was unintentional (though one of my favorite jokes--get it? "navel" architect? Sorry, a little design humor. Very little.)

Dave

Yes I know the joke. Been hearing it for a quite few years now. And I've created a couple of my own navels, they're in elementary school and sucking my wallet dry! So I guess I am of the navel and naval arch variety.

I am familiar with how rig forces are developed and boats are designed. The spin will develop an upward force component. How the total forces from spin are coupled with the hydro and mass forces will determine the total pitch attitude of the boat while sailing. If the spin is forward on the boat, the geometry can be such that the effect of spin forces could be lifting the bow. It depends on the specifics of the boat. For most cases it is probably likely that the chute causes a bow down moment, I'll give you that.

I sent you an email. Care to correspond or do you already have something in the works on this?

PI Design

11-28-2006, 07:09 AM

RHough, you are mistaken about the effect of hoisting the spinny. If, by raisng the kite, the bow down trim decreases, it MUST be due to an opposing moment. If I understand you right, you are saying that because the CofE of the spinny is lower than the main/jib combo, it reduces the overall lever effect. In this you are correct. BUT, the total force has been increased (due to the addition of the spinny), and therefore so has the moment (mmt = lever * force). If the spinny moment were acting in the same direction as the main//jib moment, the bow would trim further down. Take an example. A boat has a main/jib of area = 10sqm, with a CofE 4m above the vertical centre of flotation. Assuming that drive force acts purely horizontal and is proportional to sail area we have a bow down pitching moment proportional to 10*4=40m^3. Now hoist a spinnaker of area 10sqm, with a CofE 3m above the VCF. Its moment is proportional to 30m^3. The total moment is 40+30=70m^3. This is an increase in pitching moment, so the bow MUST go further down. But you have agreed that it does not. The fact that the lever of the combined main/jib/spinny is 70/(10+10)=3.5m (and therefore less than the 4m of the main and jib alone) is immaterial. The moment has increased from 40m^3 to 70m^3. Therefore there must be something else at play. This something else is the fact that the angled luff of the spinny produces an upward force in addition to the forward drive force. Let us say that this upward force acts 4m forward of the longitudinal centre of flotation. Let’s also assume that the upward force is the same magnitude as the forward force (from the spinny). This gives a moment of 4*10=40m^3 acting in the opposite direction. So the total moment is now 40+30-40=30m^3 – which is less than the main/jib combo – hence the bow will rise. This is the only way the bow can rise. If the spinny does not produce a net upward force, the bow can not rise relative to where it is without it.
So, there we have it. There is the maths that us non-believers were asked to produce. Please note, I fully agree that an upward force can produce a ‘downward’ moment. In fact, it definitely does in the case of a sailboat. But it can also produce an upward moment – and it must do if the bow is to rise. But there is no downward force. For those that don’t like maths, surely the upward bend on the end of the bowsprit is all the evidence you need that there is an upward force?

PI Design

11-28-2006, 10:05 AM

Some more numbers…
Imagine a 14ft boat with a 6ft pole for an asymmetric kite. The boat is sailing left to right. When planing, its LCF is 2ft from the stern (as the forward part of the hull is out of the water). Its mast is 20ft long, the main is 100sqft, with a CofE 8ft above the water, the jib is 50sqft (CofE 5ft) and the kite is 150sqft (CofE 6ft). The kite foot goes from the end of the pole to 3ft behind the bow, which means that longitudinally the centre of effort of the kite is about level with the bow (12ft forward of the LCF). In standard displacement mode the LCF is 6ft from the stern. Let’s say that the resultant force from the main and jib are horizontal, whilst the force from the kite acts slightly upwards (I think we’re mostly in agreement on this) at an angle of 30 degrees. Let’s also say that the kite is 1.5 times more efficient (ie force per square foot of sail area) than the main and jib. For now, assume that under main and jib alone the boat does not plane. When the kite goes up, the boat planes. I believe that all these figures and assumptions are quite reasonable for a quick and dirty estimate.
With two sails:
Moment =
100*8 + 50*5
= 1050 units, clockwise

The kite alone:
Moment =
150*6*1.5*cos30 - 150*12*1.5*sin30
= -181 units, clockwise
=181 units anticlockwise

Total moment, sailing with all three sails =
1050-181 = 869 units, clockwise
This means that the bow is still pitching down, but less than if the kite were not being flown. In other words, the kite is lifting the bow, but not enough to overcome the pitching moment. You are better off flying the kite, but you will still nose dive when a gust hits.
However, now lengthen the pole to 10ft. This increases the kite’s distance from the LCF to 16ft and increase the kite’s sail area to 300sqft. Now the kite’s moment is:
300*6*1.5*cos30-300*15*1.5*sin30
= -1262 units, clockwise
=1262 units anticlockwise
The total moment with all three sails is now:
1050-1262 = -212 units, clockwise
=212 units anticlockwise
The net moment is anticlockwise. In other words, the bow is now being lifted in an absolute sense. It is coming out of the water. Nose diving will not occur.
In both cases I would describe the kite as lifting the bow. I can see that there could be some semantic confusion over the first case, where the lift is relative not absolute. In the second case, the situation is clearer. Even if you nit-pick some of my numbers, hopefully you will agree that there is not *always* a strong bow down pitch from the kite as you claim.

Kiteship

11-28-2006, 03:13 PM

I probably didn't make myself clear enough and I think you may have misunderstood me. The asym will produce a forcethat has both a forward and (to a lesser extent) an upward component. The centre of this force acts forward of thr LCF and above the VCF. Therefore the forward component produces a bow down pitching moment whilst the upward component produces a bow up pitching moment. Whichever m,oment is greater will determine whether the bow goes up or down. This depends on the geometry of the specific rig (luff angle, sprit length etc), but for most boats the bow down moment is probably bigger. However, this thread originated from the planing trimaran thread, where it was claimed that the sails produced a downward FORCE. This is not the case (for an upright boat). There is no downward force, just a moment that (may) push the bow down.

Boy, I don't know where to start, PI. First, there is only one resultant force from a sail*--devolving that one force into two forces (for instance, "horizontal" and "vertical") is a human construct. Sometimes doing so serves a purpose, but in this case it makes it more complicated than it needs to be. If we can define a resultant force as having both a direction and a magnitude, we don't need to devolve it into vertical versus horizontal, thus we won't become confused about which may be the larger or which will "win" our little tug of war.

Now, in another post you suggest that your "vertical" and "horizontal" force components can be resolved as a 30 degree upwards angle. I dispute this, but am happy to concede it for the argument. And can we agree that this force passes through the aerodynamic center of the sail (not the same as the geometric center, but not too far forward of that)?

Can we further agree to take moments about a common point, and for the sake of this illustration, take that point to be the three-dimensional center of buoyancy of the hull? This is a point on the hull's centerline, somewhere *below* the waterline, where we already have a balance of both buoyancy and gravity forces.

Can we agree that, if we introduce a new force, and a lever arm back to this point, that we will have introduced a moment, therefore a rotation, and that such moment will either, for instance, pitch the bow up or down? Can we also try to stop taking moments about other random points on the boat's structure, and can we stop talking about things like "upwards" and "downward" moments, since moments are torques, not forces, thus cause rotation, not "upwards" or "downwards" movement?

Last, can we agree that any resultant force which passes above the CB will result in a bow-down pitching moment (rotation) and that any resultant force which passes below the CB will result in a bow-up pitching moment (rotation)?

OK, all set (I think): Now, let's look only at the asymmetric spinnaker. Let's leave the main alone, the jib alone, even the mast and bowsprit alone. We agreed that the spi's net resultant force was angled up at 45 degrees (which, again, I maintain is not possible, but surely is a working limit) and that the spi is located somewhere further above the hull's center of buoyancy than forward of same (for instance, the center of effort of this sail may be--on a 14' skiff--perhaps 8' forward of, and 10-12' above the hull's CB. Sound OK? Can we agree that such a spi's resultant force will pass above the hull's CB, thus does--and forevermore will--result in a bow down pitching moment?

The sail would either need to be carried far enough forward to bring the resultant below the CB, or the resultant will need to be "pitched up" enough to increase the resultant angle with the horizontal to achieve the same end, before there is any bow-up pitching moment. There is never "some" bow-up plus "some" bow down from a single sail--we took care of that when we presumed a single resultant force.

Regarding the resultant force from a spi, please do not be confused by the angle of the forestay; a sail's resultant force is *not* perpendicular to the forestay (in profile view); a simple top-view of the situation makes this crystal clear--unless you want to presume the asym sits at 90 degrees to the boat's centerline. Even then, the resultant *cannot* be brought under the hull's CB with any sane combination of sails. The only reason kites can do this (and kites *only*) is that they are flown sufficiently far from the boat that their resultant force can be made to pass below the CB, resulting in bow-up pitch.

Enough?

Dave

*For the real purists, and fans of FEA, there may be as many as an infinite number of "resultant" forces from a sail, but for argument's sake, can we please resolve all of these down into a single force? And, can that force be a vector, with both a direction and a magnitude? Thanks

PS; I find I need to respond to PI's next post, too. Sorry.

Kiteship

11-28-2006, 04:02 PM

Sorry, PI, but almost *all* of these presumptions are in error, IMO.

The LCF of a planing hull is near the front of the waterline. A 14'er sometimes gets all but a couple of feet of her hull out, but this is *never* a steady state condition (even though all those photographs might make it look so). LCF is nearer the daggerboard; perhaps 5' forward of the stern, when planing. It's nearer amidships when displacing, ie; 7' forward of the stern. You've got your COE's much to low; the boom itself on a 14'er is at least 3' above the waterline (often 4' as the boat pitches up--*not* due to the spi!); you've got your main's resultant just 5' above the boom.

The Intl 14' specs call for a 25' stick and 200' of working sail, not 150. Also 350 sq ft of spi, not 150 (cf Wikipedia). Your 6' sprit looks a bit short; the photos of 14's I've seen look more like 8'., but a 140 sf main's gonna have its center more like 10' above the boom, thus 13' above water, not 8'. Jib's gonna have its center perhaps 8' above the deck, but the deck is 1.5-2' above water (again due to bow-up position of the boat), so jib CE's gonna be closer to 10' above water, not 5. The spi's gonna reach from the end of the pole to pretty near the end of the boom, not a paltry 3' back of the bow; a 350 sq ft triangle on a 25' stick's gonna have a foot of something like 28', which uses literally all the space there is. then again, the sail is sheeted out, so perhaps your assertion that its center of effort falls near the bow isn't too far off. It's CE is easily 12-14' off the water, however, not 6'--the tip of the bowsprit is 3-4' up, just to begin with.

Second, a mainsail will always have a higher lift coefficient than a spinnaker (you suggest the spi is 1.5 X as powerful as a main, area for area). A main is actually about 25-30% more powerful than the spi, foot for foot--sometimes much more than this, but remember, the main is depowered on a run--it's just too tall not to.

You can re-run your numbers if you wish; the above as a little more than "nit picking" since all your underestimates result in bow-up pitching moments.

Last, you've mis-done your maths, at least once. In your last example (300' kite), you first show anti clockwise-and clockwise- moments from the spi to equal each other= +1282 ft lbs minus 1282 ft lbs (no net rotation from the kite at all--which simply isn't possible, but let's leave that alone), then when you add the bow-down pitch of the main and jib, you get a result in pitch up. Zero plus clockwise cannot result in anti-clockwise.

Here are some photos, for scale:
http://www.yachtsandyachting.com/photos/skiff/2005sanfrancisco14.jpg
http://www.yachtsandyachting.com/photos/skiff/2005sanfrancisco15.jpg
http://www.yachtsandyachting.com/photos/international14/2005eastlothian1.jpg
https://host220.ipowerweb.com/~underthe/undersail/Int'l14ClassWorldChamp2006/Int'l14Worlds06-6066.html
https://host220.ipowerweb.com/~underthe/undersail/Int'l14ClassWorldChamp2006/Int'l14Worlds06-7031.html
https://host220.ipowerweb.com/~underthe/undersail/Int'l14ClassWorldChamp2006/Int'l14Worlds06-8052.html

Dave

Some more numbers…
Imagine a 14ft boat with a 6ft pole for an asymmetric kite. The boat is sailing left to right. When planing, its LCF is 2ft from the stern (as the forward part of the hull is out of the water). Its mast is 20ft long, the main is 100sqft, with a CofE 8ft above the water, the jib is 50sqft (CofE 5ft) and the kite is 150sqft (CofE 6ft). The kite foot goes from the end of the pole to 3ft behind the bow, which means that longitudinally the centre of effort of the kite is about level with the bow (12ft forward of the LCF). In standard displacement mode the LCF is 6ft from the stern. Let’s say that the resultant force from the main and jib are horizontal, whilst the force from the kite acts slightly upwards (I think we’re mostly in agreement on this) at an angle of 30 degrees. Let’s also say that the kite is 1.5 times more efficient (ie force per square foot of sail area) than the main and jib. For now, assume that under main and jib alone the boat does not plane. When the kite goes up, the boat planes. I believe that all these figures and assumptions are quite reasonable for a quick and dirty estimate.
With two sails:
Moment =
100*8 + 50*5
= 1050 units, clockwise

The kite alone:
Moment =
150*6*1.5*cos30 - 150*12*1.5*sin30
= -181 units, clockwise
=181 units anticlockwise

Total moment, sailing with all three sails =
1050-181 = 869 units, clockwise
This means that the bow is still pitching down, but less than if the kite were not being flown. In other words, the kite is lifting the bow, but not enough to overcome the pitching moment. You are better off flying the kite, but you will still nose dive when a gust hits.
However, now lengthen the pole to 10ft. This increases the kite’s distance from the LCF to 16ft and increase the kite’s sail area to 300sqft. Now the kite’s moment is:
300*6*1.5*cos30-300*15*1.5*sin30
= -1262 units, clockwise
=1262 units anticlockwise
The total moment with all three sails is now:
1050-1262 = -212 units, clockwise
=212 units anticlockwise
The net moment is anticlockwise. In other words, the bow is now being lifted in an absolute sense. It is coming out of the water. Nose diving will not occur.
In both cases I would describe the kite as lifting the bow. I can see that there could be some semantic confusion over the first case, where the lift is relative not absolute. In the second case, the situation is clearer. Even if you nit-pick some of my numbers, hopefully you will agree that there is not *always* a strong bow down pitch from the kite as you claim.

water addict

11-28-2006, 04:17 PM

.......(stuff)

OK, all set (I think): Now, let's look only at the asymmetric spinnaker. Let's leave the main alone, the jib alone, even the mast and bowsprit alone. We agreed that the spi's net resultant force was angled up at 45 degrees (which, again, I maintain is not possible, but surely is a working limit) and that the spi is located somewhere further above the hull's center of buoyancy than forward of same (for instance, the center of effort of this sail may be--on a 14' skiff--perhaps 8' forward of, and 10-12' above the hull's CB. Sound OK? Can we agree that such a spi's resultant force will pass above the hull's CB, thus does--and forevermore will--result in a bow down pitching moment?

....(other stuff)
.

If the skiff is planing then the sum of vertical forces will be well aft of hydrostatic LCB, perhaps like 2 feet fwd of transom. Assuming x=0 at stem then LCB/F is at x=12. Using your coordinates LCE of spin is at x=0 and if VCE of spin is at z=10 then uplifting moment wins for 45 degree aero force that you list.

The CE on the spin will in reality be much closer to the luff of the sail than the leech.

I see nothing wrong with PIs reasoning. He conceded that in most cases it would likely be a bow down effect. But it is specific to the geometry of whatever the boat is. And also if planing or displacement mode.

Kiteship

11-28-2006, 05:08 PM

If the skiff is planing then the sum of vertical forces will be well aft of hydrostatic LCB, perhaps like 2 feet fwd of transom. Assuming x=0 at stem then LCB/F is at x=12. Using your coordinates LCE of spin is at x=0 and if VCE of spin is at z=10 then uplifting moment wins for 45 degree aero force that you list.

The CE on the spin will in reality be much closer to the luff of the sail than the leech.

I see nothing wrong with PIs reasoning. He conceded that in most cases it would likely be a bow down effect. But it is specific to the geometry of whatever the boat is. And also if planing or displacement mode.

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.

PI Design

11-29-2006, 04:49 AM

Thankyou WaterAddict.

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.

water addict

11-29-2006, 07:02 AM

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.

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.

PI Design

11-29-2006, 07:14 AM

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.

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?

PI Design

11-29-2006, 07:35 AM

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).

Kiteship

11-29-2006, 01:20 PM

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

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.

water addict

11-29-2006, 02:26 PM

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

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.

Kiteship

11-29-2006, 05:58 PM

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.
Maybe.

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.
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?

Your comment about CF and CB is wrong. LCF and LCB are nearly always in different places. Check basic nav arch texts.
Yeah, sometimes by as much as 1-2% of LWL. So we've answered the above question; we're splitting hairs. Enjoy.

Dave

Kiteship

11-29-2006, 06:02 PM

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).

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

water addict

11-29-2006, 10:38 PM

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

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.

Doug Lord

11-30-2006, 07:56 AM

Check this out:
Flying Jib as Spinnaker ? - Forums
Address:http://www.rcsailing.net/forum1/showthread.php?p=37519

Chris Ostlind

11-30-2006, 09:22 AM

Flying Jib as Spinnaker ? - Forums
Address:http://www.rcsailing.net/forum1/showthread.php?p=37519

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

water addict

11-30-2006, 11:53 AM

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.

rayk

11-30-2006, 12:56 PM

This is how I see it....

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

Chris Ostlind

11-30-2006, 01:17 PM

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?

http://www.northsails.dk/UK/Gradient_WindtunnelStory.asp
http://syr.stanford.edu/SAILFLOW.HTM
http://www.quantumsails.com/pdf/Experts-WindTunnel.pdf

Doesn't it make sense that they would have the most accurate take on all this? Don't get me wrong, I love the discussion and all that it has revealed, but Wow!, there are some reasonably well configured, state-of-the-art operations that do on-going work in the field and nobody has touched on an email to discover the potential.

What does that say?

rayk

11-30-2006, 02:23 PM

Stanford Yacht Research says on their website:

Limitations of Modern Sail Research

In computer and wind tunnel simulations, simplifications are made to the sail's environment. First of all, the sail is analyzed independently of hull and water. A rigid sail is used in an upright position. To completely model a real sail it would be necessary to use a flexible 3-dimensional sail in a heeled position, and to consider the effects of the hull and sea state. However, modeling something like this would be far too computationally expensive.

...so maybe we can carry on talking. Talk is cheap. ;)

Chris Ostlind

11-30-2006, 02:38 PM

Rayk,

Did you get that response from the copy on their website or from an email returned in answer to a sincere query?

This says you don't really want to know, you just want to endlessly blab about it with no outcome.

Andy P

11-30-2006, 04:26 PM

I think ggguest asked : what happens if you let the asymmetric sheet flap?

If whilst you are blatting along at high speed, crew well back in the boat, hanging off the transom even, then a kite big flap will result in :

a) if there is pitch up from the kite, - a nosedive

or b)
if there's pitch down from the kite, - the bow will go up

The result from real world ( 29er and UK Cherub ) is that the bow does go UP in a flap.
A big nosedive can be made less terminal by just such a flap of the kite.

As discussed, the boat does ride much higher with the kite up, so there's lift, but pitch down moment.

CT 249

11-30-2006, 06:10 PM

This is how I see it....

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

Yes, but isn't a vector (even a vector with some lifting component) that acts high up on the rig, going to create a twisting moment that forces the bow down?

It's been noted that a bowsprit pulls up; but then again, the force on a halyard is down, so the direction of the pull on the individual lines may not be an indication of the overall forces. The bowsprit probably isn't pulling up in response to lifting forces, just in response to the force on the catenary (?) between the masthead and the bowsprit.

Then again, people say a windsurfer rig creates a lifting force (and it certainly can, although we generally try to minimise the lifting effect) and yet if you slide the mast track on a Raceboard back at too low a speed, the stern will sink. And moving the mast foot forward prevents the board lifting at high speed.

Then yet again, it's easy to see why people stand so far back on skiffs at speed. Moving back gives you a higher-aspect planing surface; that's low drag. The higher speed creates more dynamic lift, so you can move further back.

Is the static C of B really so close to the high-speed planing C of B? Looking at the pressure "spike" near the stagnation point in the tests of Savitsky etc, and at the point where the stagnation point is on most dinghies or skiffs, isn't there a considerable distance between the two?

The fact that the lift has moved back accentuates the angle of waves you're going over, doesn't it? The stagnation point, in a normal chop, is less likely to be "bridging" the hollow; it's likely that the point developing dynamic lift will be down in the trough with the stern suspended on the crest behind? Are the crew perhaps just trying to prevent the porpoising that happens in a powerboat? Therefore, couldn't the fact that the crew are further aft be related to factors unrelated to whether the spinnaker is lifting or twisting?

I can see a lot of merit in the idea that spinnakers don't lift the bow, but if they don't, what are the reasons for the extremely common conception among skiffies etc that they do? Are they just mislead by the extra dynamic lift available due to the extra speed created by the spinnaker? If so, why do Tornado crews (where dynamic lift is not a significant factor) also feel that the spinnaker lifts the bows?

gggGuest

12-02-2006, 05:36 PM

what are the reasons for the extremely common conception among skiffies etc that they do? Are they just mislead by the extra dynamic lift available due to the extra speed created by the spinnaker? If so, why do Tornado crews (where dynamic lift is not a significant factor) also feel that the spinnaker lifts the bows?

Odd isn't it? Never really got to the bottom ofthat... I am in no doubt that the kite depresses the bow (now), I learned as a result of the '95 dialogue and especially Mikko's math model. Both the math model and the empicical evidence (sail flap) agree. You've got to be determinedly pig headed to ignore both.

I suspect secondary effects... I now believe that the kite stabilises in pitch, this is evident on twelve footers where the boat appears to "hang" on the kite. The change in the flow as the boat pictches must have a big effect on the various flows round the kite. Then also we know that the heavier the crew them more likely to pitchpole on similar boat, so any displacement reduction must reduce tendency to pitchpole, which must feel like an upwards force. But, writing this, I'm suspecting its "hang time" on the kite makes us think it lifts.

Kiteship

12-03-2006, 06:09 PM

Odd isn't it? Never really got to the bottom ofthat... I am in no doubt that the kite depresses the bow (now), I learned as a result of the '95 dialogue and especially Mikko's math model. Both the math model and the empicical evidence (sail flap) agree. You've got to be determinedly pig headed to ignore both.

I suspect secondary effects... I now believe that the kite stabilises in pitch, this is evident on twelve footers where the boat appears to "hang" on the kite. The change in the flow as the boat pictches must have a big effect on the various flows round the kite. Then also we know that the heavier the crew them more likely to pitchpole on similar boat, so any displacement reduction must reduce tendency to pitchpole, which must feel like an upwards force. But, writing this, I'm suspecting its "hang time" on the kite makes us think it lifts.

I suspect you're on the right track here. The "conventional" wisdom back in the 1996 thread was that the spi allowed the crew to unload the main and power up the spi, reducing the total height of the sail area's combined effort--and move it significantly forward, assisting in directional stability. I think there are several things working here--taking a lot of power from a tall main has it's problems, esp. on a small monohull. The center of effort of the sail moves around a good bit (as the sail twists or untwists) and, as the main's a "better" airfoil, changes in magnitude pretty radically as well. (You'll recall, Chris, that wingsails never really caught on with windsurfers, chiefly because of the very fast reaction of the wing to small changes in boom angle) Reacting to these quick changes on a small, agile and not very stable boat is a job. Replacing some of the main's power--and adding significantly to it--with a more stable, lower spi--which doesn't "come on" anywhere as capriciously as the main--likely contributes a great deal to overall stability.

BTW, Chris, generally porpoising of a planing hull is a result of having the weight too far aft, not too far forward. The hull keeps trying to self-trim (an admirable--and lucky--property of single-surface, stepless planing hulls), then gets repeatedly pitched back by the weight imbalance. The "self" part of the self-trim is overpowered by the weight trim.

Dave

Man Overboard

12-04-2006, 01:29 AM

Observations from a bystander: I have had some difficulties following this topic as it seems that there is not a consensus among those parties to the debate about how to visualize the dynamics of whether or not a spinnaker lifts the bow. Nor does there seem to be a consensus on physics formulas, and principles concerning the subject at hand.

Doug makes two primary observations that are the topic of this debate

“I'm convinced that certain spinnakers will lift the bow-particularly on skiffs”
“When a gust hits you can physically see the bow lift”

The questions that should be addressed are as follow:

In general, do spinnakers lift the bow of sailboats?
What characteristics of skiffs make them more prone to the bow lifting?
Is the lifting do to spinnakers, or some other condition?
What role do gusty conditions play in the lifting dynamics that have been observed?

Have these questions been answered? Let me point out some points of confusion for your consideration.

It is apparent that there is not agreement on an illustrative concept to apply physics principles. Case in point:

Tim B is the first to introduce the concept of a lift vector acting on a lever to generate a moment around the center of buoyancy in post #8 Dave expounds on this in post #9 by emphasizing a single vector passing under the CB. Both of these illustrations are confusing to me because a moment produces a torque around the C of F, which is not mentioned and it changes the C of B, which is not addressed. In addition it doesn’t address the moments generated by where the sail attaches.

The concepts as stated in post 8 and 9 are two dimensional concepts. QC3 introduces a 3 dimensional concept in post #11 that emphasizes not only the force generated by the sails, but also the moments generated by that force.

RHough is the first to introduce a plausible explanation, other than the spinnaker, for the bow of skiffs lifting in post #18

“The reduced pitch down and greater power may even allow the boats to sail fast enough to plane, on most rockered hulls, that is a bow up attitude.”

RHough is also the first to introduce the concept of center of flotation (indirectly) and its relationship to center of buoyancy in post #23

“Take the boat with spinnaker rigged and gimble just above the CG it so it is level, but free to pitch up or down. Mount the gimble on the hood of your car and drive down the road. If the spinnaker lifts the bow as you claim, the boat will pitch up in the gimbles.”

Crag Guy makes a coment in post #27 about bow sprits and the moment generated by them that deserves some attention. The models presented up to this point don’t address quantitatively His observations.

Water Addict makes a suggestion that diverges from the single vector concept. In post #29

“Sum the forces and moments created by where the spinnaker attaches to the boat (halyard, pole, sheet) and the CG + hydro drag. If the summed total moment causes pitch up or pitch down, then the boat will do that until forces and moments are in equilibrium.”

A model that illustrates these dynamics could also be used to include the bow sprit as noted by crag guy.

Dave argues against this concept in post #34 and makes a plea for a 2 dimensional model, resolving all forces two 2 dimensional vectors and calculating the moments they generate; restricting the diagram to just bow pitching.

This is a sound and practical suggestion especially for a form, yet it meets with resistance, to the discredit of those opposing him. Dave also suggests drawing a picture, yet we will see that that is not always helpful in proving a point.

In post #42 PI Design presents an example that has several variables generating many types of forces and moments in multiple planes; a statement restricts the example to just a single plane:

“Let’s say that the resultant force from the main and jib are horizontal”

This is most likely not the case, and it ignores the moment generated by drag

The terms clockwise and anticlockwise, absent a third dimensional vector suggest a two dimensional model

In addition, the example covers both planing and non planing states. The center of flotation is not stated as an absolute, and math involving moments about the C of F are either incorrect, or absent. There are assumptions about forces, and the direction of forces that are purely for illustrative reasons. Another concern is that this illustrative approach does not consider all of the moments that are present in real life, and although the math appears to be reasonable at first glance, the fact is that this is most certainly not a true, real life scenario.

As Dave suggests, Water Addict has drawn a picture and presents it in post #58. Notice that the C of E is clearly marked and a single resultant force drawn through the center of gravity. How do we come by the C of E? Is it not just the central location of the area of the sail where we assume all force is applied? Where does it show the magnitude of the vector? Is it known? If not why has there not been unknown ‘x’ values assigned? Where is the center of buoyancy? We are discussing trim of the bow are we not? Maybe one of the experts in this forum could explain to me how the force of the sail is being transferred to the vessel. It looks to me like two explicit attachment points, and one that is inferred. What is the effect of the moments caused by these levers? Is it possible to accurately represent those moments in a single vector? How much lift does the sail generate? And more important what is the drag produced do to the generation of lift? Are we going to use a single vector for that also? (a corollary: is drag, or lift the predominant down wind driving force? If it is drag, then is a C of E 1/3 up from the foot of the sail really appropriate?) I take it we are to assume that this is a free standing mast? Simple enough we don’t have to divide the force transferred to the backstay. There is a force on the main mast, isn’t there?

I am interested in the various views put forth, even if the logical assumptions are a less than accurate; we can consider it a work in progress. But I do get annoyed at pious statements such as this:

“Insulting a professionals' judgment when they spent years in training and have made a career doing this stuff is not the way to promote your product.”

Quite frankly, there has not been one clear, concise explanation of the observations made by Doug; neither has there been an accurate mathematical analysis of such. I am not impressed by “professionals” insulting one another.

I have made some revisions to Water addicts drawing; you can add or subtract elements to support rationale. Maybe one of you experts can further illustrate proper vector analysis, showing appropriate lines of force and resultant angles of trim, yaw, and heal. I suggest taking Dave’s advice and consider spinnaker forces and moments alone. Of course for experts, a 3 dimensional model should not be a problem, and in fact might prove insightful.

In the absence of empirical data, I suggest some of the following reading as a basis for any claims you make.

The Wind Tunnel Teaches Key Downwind Trim Lessons
http://www.quantumsails.com/pdf/Experts-WindTunnel.pdf
TWO-DIMENSIONAL CFD-BASED PARAMETRIC ANALYSIS OF DOWNWIND
SAIL DESIGNS
http://syr.stanford.edu/RINA_Steve.pdf
Downwind Performance of Yachts in Waves
http://www.cmst.curtin.edu.au/publicat/sailingscience99.pdf
COMPARISON OF WIND TUNNEL AND FULL-SCALE AERODYNAMIC SAIL FORCE
MEASUREMENTS
http://www.friendship-systems.com/getDocument.php?file=Dyna/2002_HPYDC_Windtunnel.pdf#f02dc9f31b501bfbb8b1a1b0d697d6f6
Advances in Wind Tunnel Analysis of Yacht Sails
http://www.friendship-systems.com/getDocument.php?file=sailing/HansenHochkirchRichards_HH05.pdf#e1200fbc01a6d2f7013948fcafb8d731

Special attention should be given to this particular article.
America’s Cup downwind sails: vertical wings or horizontal parachutes?

http://mapp1.de.unifi.it/persone/Allotta/ICAD/Richards2001.pdf

PI Design

12-04-2006, 06:04 AM

ManOverboard - I think you are overcomplicating the issue.

A quick summary: (apologies if I misrepresent anyone's view)

Dave(Kiteship), Water Addict and I are in agreement on the basic method for solving this problem. We describe it differently, but basically use the same approach. A 2d approach. We also agree that a bow down trim is a perfectly likely result. WaterAddict and I believe a bow up result is also possible. Dave agrees it is theoretically possible, but extrememly unlikely on any practical design. This is becasue we plug different numbers into the model.

We all agree that flying the spinny produces a force that acts forward and (to an extent that we didn't reach agreement on) upwards. What matters is the direction of the force, and the distance of the longitudinal and vertical centres of effort of the spinny from the centre of rotation of the hull (the LCF and VCF).

In more detail:

[QUOTE=Man Overboard;116592]
In post #42 PI Design presents an example that has several variables generating many types of forces and moments in multiple planes; a statement restricts the example to just a single plane:

“Let’s say that the resultant force from the main and jib are horizontal”

This is most likely not the case, and it ignores the moment generated by drag.

The terms clockwise and anticlockwise, absent a third dimensional vector suggest a two dimensional model
[QUOTE]
My mathematical model includes exactly the same forces and levers as the illustrative example that Dave asked for and WA provided. Its just that I split the force into vertical and horizontal components. I have only worked in one plane, as far as I know. You appear to critisize my approach for working in multiple planes (which I haven't) and then disapprove again when you realize that I have just worked in 2d - suggesting the approach is too simplistic and uses arbitrary values. If we can assume that the boat is upright at all times(a big assumption, but one that everyone has been making so far) then the problem is essentially a 2d one. You are the first person to suggest that a 3d model is required.
[QUOTE=Man Overboard;116592]
In addition, the example covers both planing and non planing states. The center of flotation is not stated as an absolute, and math involving moments about the C of F are either incorrect, or absent. There are assumptions about forces, and the direction of forces that are purely for illustrative reasons. Another concern is that this illustrative approach does not consider all of the moments that are present in real life, and although the math appears to be reasonable at first glance, the fact is that this is most certainly not a true, real life scenario.
QUOTE]

LCF is not an absolute. It moves with trim. As trim is the central theme of this debate, there is not a single LCF value that we can use. The simplest reasonable approach is to assume one LCF when displacement sailing and one when planing. For what its worth, as I assumed that the main/jib force is horizontal, I have not needed to use the displacement LCF (no upward force meaning there can be no moment).

In what way is the maths incorrect or absent? I don't understand this comment.

Yes, I have made assumptions about the direction of the force. I assumed 30 degrees. Dave does not believe that this is a reaonable assumption. That is fair enough, but the Cherub data provided by ggg suggests I could have used 40 degrees.

My maths model includes everyting that Dave's approach includes. In my opinion (and presumably Dave's), this is enough to demonstrate the point. All models are simplifications, they only need to contain sufficient detail and adequate data, not every last detail.

I had assumed that the boat is travelling with the wind forward of the beam so that the drive force is provided primarily by lift. This is not likely to be the case with Doug's RC boats, but I was arguing the general case, not a specific one. You can just as readily assume that the wind is aft of the beam and is primarily drag driven. It does not affect the problem.

To loosely answer some of your questions:

Bow lifting (or otherwise) is not just a result of aerodynamic forces/moments. As the hull speeds up, it will produce more dynamic lift and will begin to plane. The bow must be pitched up for planing to occur. That is to say, if you were to tow a hull (with all rigging removed) at increasing speed, it would eventually trim bow up. I don't want to get too off topic here, but the point I am trying to make is that Doug's real life trim is affected by more than just sail forces. I regret writing this last paragraph already, because Doug's models probably don't plane and I don't really like the planing/bow up thing I describd. However, I will keep it here, just to show that there are other factors besides aerodynamic forces and moment at play. RHough put it better.

water addict

12-04-2006, 07:30 AM

......stuff

I am interested in the various views put forth, even if the logical assumptions are a less than accurate; we can consider it a work in progress. But I do get annoyed at pious statements such as this:

“Insulting a professionals' judgment when they spent years in training and have made a career doing this stuff is not the way to promote your product.”

Quite frankly, there has not been one clear, concise explanation of the observations made by Doug; neither has there been an accurate mathematical analysis of such. I am not impressed by “professionals” insulting one another.

.....stuff.....

My premise in this whole topic has been that given the specific geometry, it may be possible that a spinnaker can produce a moment tending to lift the bow. I concede, and have stated or at least implied that usually this is NOT the case, and that spins push the bow down on most boats.

My impression from the manner of Dave's statements was that anyone who thinks that on any boat a spin lifts the bow is an idiot. So I reacted to that impression. Perhaps my interpretation was not warranted, and if so, my mistake.

I will try to limit my statements next time to ones that are less pious and will impress you Man Overboard.

Chris Ostlind

12-04-2006, 11:21 AM

Mike Golding's Ecover with super secret, reduced rig main that lifts the bow. And that with Alex Thompson on the foredeck.

Kiteship

12-04-2006, 01:01 PM

Mike Golding's Ecover with super secret, reduced rig main that lifts the bow. And that with Alex Thompson on the foredeck.

Man shoulda had a kite aboard. I know a guy who sells these...

D.

Paul B

12-04-2006, 05:36 PM

Man shoulda had a kite aboard. I know a guy who sells these...

D.

I don't agree wih some of the things you claim about kites, but in this case I think you are spot on. You should lobby for every shorthanded distance race to require a kite as an emergency back up system. That can't add more than a couple of hundred pounds to boats of this size. It will help make them more self sufficient if they have a rig problem.

How close winded can something like an Open 60 or a mini sail using one of your kites? I imagine at least a beam reach, if not higher?

They would be a perfect safety feature for the Transpac as well.

Kiteship

12-05-2006, 07:41 PM

I don't agree wih some of the things you claim about kites, but in this case I think you are spot on. You should lobby for every shorthanded distance race to require a kite as an emergency back up system. That can't add more than a couple of hundred pounds to boats of this size. It will help make them more self sufficient if they have a rig problem.

How close winded can something like an Open 60 or a mini sail using one of your kites? I imagine at least a beam reach, if not higher?

They would be a perfect safety feature for the Transpac as well.

Not surprisingly, I agree with you. An emergency kite, call it 2/3 the size of the boat's working sail area, would weigh, with all associated lines, gear and bag, less than 30 lbs for any boat under 50' We could put it at 15 lbs if we repurposed existing lines and took the performance penalty. Cost, all tolled, would run between $2-4k and yes, performance can be counted on from reach-to-reach, ie; through 180 degrees.

Interestingly, we pitched exactly two boats from the last Vendee. Both passed, both later lost their keels (!); very fortunately near land (in both cases) In mid-ocean kites might have saved lives, certainly would have saved boats--since there's no heeling moment, you can fly them without the keel. Can't recall whom, but wasn't a sracing kipper recently rescued mid-ocean from a boat with a failed canting keel; resulting in another competitor having to pull out of the race to assist, and wasn't the boat abandoned?

Cheers,

Dave

rayk

12-13-2006, 09:11 PM

Kiteship

12-13-2006, 10:01 PM

The drag of the hull is cancelled by the thrust of the sail.

The CE is forward of the CG and will lift the bow until CB moves aft to balance lift.

Illustration shows a wee boat planing under spinnaker.
Style of spinnaker set as shown does have a lift component.

It surely does have a lift component, rayk, but (unlikely as your spinnaker resultant is), this boat as drawn will still pitch down by the bow.

Cheers,

Dave

rayk

12-13-2006, 10:23 PM

There is no force forward of CG depressing bow in my illustration.

Bow rises until lift of spin + lift of bouyancy = weight of wee boat.

RHough

12-14-2006, 12:14 AM

There is no force forward of CG depressing bow in my illustration.

Bow rises until lift of spin + lift of buoyancy = weight of wee boat.

Your illustration is depressing. It is wrong and incomplete. Find a moment to think about. :)

The lift of buoyancy must = weight of wee boat, else wee boat would sink.

rayk

12-14-2006, 12:23 AM

RHough

12-14-2006, 12:36 AM

If you lift the bow with a crane by 50kg the CB will be CG-50kg.
Use Newtons to make it fancy. :)

(Re illustrations: I am using a Japanese language computer and struggle along just using MS Paint)無化し　欄簿

Still haven't found that moment to think eh?

The CG is a location. Lifting the wee boat with a crane won't make the CG move relative to the boat. You can move mass to alter the CG's location. The CG does not have mass, so you cannot remove 50kg from it. You can figure it out in a matter of moments. :) You just have to find the moment to figure it out.

rayk

12-14-2006, 12:54 AM

Together, spinnaker lift and bouyancy resolve against gravity. CG doesnt move in my illustration or explanation.
Losing lift from the spin would 'drop' the bow, until CB aligns vertically with CG.
The moment youve been waiting for starts when spin lift is lost and ends when CB and CG realign vertically.

RHough

12-14-2006, 02:18 AM

Together, spinnaker lift and bouyancy resolve against gravity. CG doesnt move in my illustration or explanation.
Losing lift from the spin would 'drop' the bow, until CB aligned with CG.
The moment youve all been waiting for starts when spin lift is lost and ends when CB and CG realign vertically.

If the line you have drawn between the CB and the CE of the sail is the total force vector, You can see that both the upward component (aft of the CG) and the Drive component (above the CG) act to pitch the bow down.

The is no uplifting bow force in the diagram, it is depressing. :)

You didn't happen to actually read the thread did you? :?:

rayk

12-14-2006, 02:39 AM

I did not draw the line connecting CE and CB.

It is not the force vector.

But you can resolve my red lines from CE and CB on it if you like.

I read the thread with relish.

rayk

12-14-2006, 03:03 AM

RHough

12-14-2006, 03:17 AM

The small amount of spin lift is some meters ahead of the CG.
A long lever.

Opposing that is the greater amount of bouyancy lift only a little aft of the CG.
A short lever.

When spin lift is lost bouyancy moves forward as the bow comes down.

Remember its a planing skiff sort of boat thingy some one else drew, but it is good enough to use for this discussion.

You forget that the drive is acting on a lever that is nearly as long as the "lift" lever. Even if the drive lever was half the length of the "lift" lever, the Drive is much more than twice the lift. The large amount of drive is some large distance above the cg, it is a large force on a large lever acting to pitch the bow DOWN. It is countered to some small extent by the bow up pitch moment you describe. The net is bow down.

Granted, the spin has some lift so it drives the bow down less than other sail combinations, but it does not cause bow lift or pitch up.

The only way you can "prove" that the spinnaker lifts the bow is by ignoring the drive moment, as you have done. :)

PI Design

12-14-2006, 04:14 AM

rayk

12-14-2006, 05:43 AM

Lift is acting about the CG. When lift forward and aft equal weight at CG all is well. Vertical forces in equilibrium.

Thrust and drag are in equilibrium, when the wee thing is planing along.

There is no moment.

water addict

12-14-2006, 07:11 AM

PI Design

12-14-2006, 07:29 AM

Ah, I see! The 'world is flat' idea always struck me as rather daft - wouldn't the sea fall over the edge?

P.S. What time is it in Maryland - shouldn't you be asleep???

water addict

12-14-2006, 07:41 AM

Ah, I see! The 'world is flat' idea always struck me as rather daft - wouldn't the sea fall over the edge?

P.S. What time is it in Maryland - shouldn't you be asleep???

Sleep? 4 hours a night on this side of the pond- I've been at my desk since 6am.

RHough

12-14-2006, 10:11 AM

I thought I'd managed to show that, with the right sail plan geometry, it IS possible for the bow to lift? If anyone still disagrees with my demonstration, can they explain why in a clear manner, rather than just "you've made a mistake". I fully agree, that for the majority of boats, the pitch moment depresses the bow, but this is not an 'always and forever' situation. There are designs where a bow up moment can exist - a long pole, high speed (hence planingon aft sections), perhaps a slightly bow up attitude etc, will all help to achieve this.
The Cherub analysis (referenced earlier) showed the lift force acting at 41 degrees. At this angle, the seperation of the CE of the kite longitudinally from the LCF only needs to be fractionally bigger than the vertical height of the CE of the kite above the waterline. In my opinion (and I haven't done any analysis to show this), a 12' Skiff is one class where a bow up moment is quite likely, especially if the bow is already slightly lifted due to other forces/moments (hence altering the angle of the kite's drive force).

The drawing that rayk is using shows a bow down pitch moment.

Since you have not done any analysis to show it, why not do some and share your thoughts? I think you will find that in the x - y plane the wind is flowing on the x axis, the force from the sail will be on the z axis (heel) and the x axis (drive). I would like to see where the y axis component comes from when the mast is upright. Every time I place a force vector at the CE of a spinnaker I can't prove a positive y axis force.

The one certainty is the drive acting on the x axis is above the CG. This creates a bow down moment. To lift the bow there must exist a force or moment that is greater than the bow down pitch moment. I've tried to find it off and on since the 1995 discussion and have never been able to prove it.

PI Design

12-14-2006, 11:04 AM

PI Design

12-14-2006, 11:52 AM

RHough, a mainsail on a vertical mast won't produce an upward force. I am talking about spinnaker in my example. Although the wind flow may be horizontal, the spinnaker will produce a force with some upward component, due to the angle of the luff. The Cherub data shows that the force from the kite is 68kg forward and 60kg up. These are not my figures, they have been derived by a well respected sail maker from a computer simulation. As they are the only figures anyone on this forum has presented, I have used them in my calcs. Even if you find the figures suspect, the general principal is that the kite will produce a resultant force with some upward component. Dave Culp (Kiteship) does not dispute this, he merely refutes the amount of upward component. Kiteship, WaterAddict and I are in agreement on the form of the free body diagram, but not the numbers to plug in. Kiteships original assertion was that the moment was ALWAYS bow down. I hope I have been able to show that this is not necessarily the case.

Kiteship wrote:

"Try every boat you can think of. Any combination of sails. Feel free to cheat. Unless you use a free-flying kite, you will get a bow-down pitching moment. Period. This isn't a close thing, not by a very long shot."

I have done this. I have posted my findings. Without cheating, I can find a sail that produces abow up moment.

Man Overboard

12-14-2006, 02:26 PM

PI Design states.
“I thought I'd managed to show that, with the right sail plan geometry, it IS possible for the bow to lift? If anyone still disagrees with my demonstration, can they explain why in a clear manner…?”

Theoretically, you certainly can imagine setting a spinnaker at a 45 degree angle or more; or another way of stating it - is to set the sail on a longer bow sprit, and lower on the mast so the moment generated from the sprit is greater than the moment generated from the mast. You certainly could show a situation where the spinnaker would lift the bow. I am not sure that this would be beneficial, as the whole point of a sail is to generate forward thrust.

In real life there are some things to consider that have not been discussed. Spinnakers generate force by two methods dependent on their angle of attack.
First: a force produced by parasitic drag, similar to the way a parachute operates. I am uncertain if it is correct to state that there is a leading edge in this situation, but for the sake of illustration, we would say that there is full separated flow on both the leading and trailing edge. Because of the wind gradient, there tends to be greater flow, and hence more violent separation towards the top of the mast, therefore greater driving force higher up on the mast. (When sailing down wind of course) There is no lift component in this type of scenario; simply a drag force parallel to the wind. The second way a spinnaker generates force is due to lift. This generates a much more powerful force than in the first scenario, but not without a high penalty. (Pun intended) Remember that with the component of lift, also comes the component of induced drag. You of course have heard it said of spinnakers ‘no big deal, the drag aids in driving the boat’. But it is a big deal, because like with most airfoils, the greatest portion of drag occurs towards the tip of the airfoil, which is again high up on the mast. The total force component on a sail, especially a spinnaker of any type is most certainly not near the geometric center of the sail, but quite a bit higher up, contributing to the moment generated by the mast.

gggGuest

12-14-2006, 06:23 PM

I hope I have been able to show that this is not necessarily the case.

You're grossly over simplifying. Go to the table at the end of the paper and look at the actual pitch moments. That's the critical point.

rayk

12-14-2006, 08:29 PM

A moment is the change from one stable state to another.
Using a moment to describe lift of the bow or depression of the bow means it must start from an equilibrium and end in an equilibrium, of forces.

Only four forces act on the 2D illustration. Lift>Gravity, and Thrust>Drag.(Up>Down Forward>Back...whatever)
All vectors are made up of these forces.

A moment starts when one of these pairs is out of balance.

Thrust>Drag
If the wee boat is tied to the wharf with a rope, when it reaches the end of its tether, drag becomes infinite.
Depending on where the rope is tied (fulcrum) a moment will occur rather quickly.
If the wee boat is tied to the wharf with a bungy cord, drag will increase to infinity over time with an increasing moment.

Lift>Gravity
The small amount of spin lift is some meters ahead of the CG.
Spin lift is the sum of all up/down forces around that point roughly.
When spin lift is lost bouyancy moves forward as the bow comes down.
Greater amount of bouyancy lift only a little aft of the CG.
Crew move forward and CG moves forward. Spin lift stays in CE but CB moves forward.
CB is the center of bouyancy lift. LCF goes out the window with two (CB CE) lift forces.

Thrust and drag for the illustration are balanced. In a dynamic sort of way.
Vertical forces are balanced in a static kind of way.

MalSmith

12-14-2006, 08:56 PM

At the end of the day, if the boat is sailing at a roughly constatant angle of trim, the pitching moment of the boat/sail system is zero. For simplicity, the pitching forces can be broken into horizontal and vertical components. The horizontal thrust of the sails reacts against the hull resistance force to create a bow down pitching moment. The vertical thrust of the sails reacts against the weight of the boat, through the centre of gravity to produce(usually) a bow up pitching moment. The sum of these two moments is the net pitching moment of the sails. This moment must be balanced by something, and that something is an equal and opposite moment which is usually created by offsetting the centre of gravity with the centre of lift of the hull. The centre of hull lift is the sum of of the buoyancy force and the net vertical component of any hydrodynamic forces.

To determine qualitatively the sense of the sail pitching moment (Ms), estimate the location of both the centre of gravity (CG) and the centre of hull lift (CL). If CG is aft of CL then Ms is bow down. If CG is forward of CL then Ms is bow up. Put more simply, if you pile the crew aft (usually the case), the pitching force is bow down. If you pile the crew forward (not often seen) the pitching force is bow up.

To answer then question, when sailing without the spinnaker, the vertical thrust component of the rig is fairly negligable and hence the bown up pitching moment will be insignificant compared to the bown down pitching moment. When the spinnacker is raised, a significant vertical thrust component may added well forward of the CG in addition to the added horizontal thrust of the spinnaker. The sum of these moments may result in a net reduction of the bow down pitching, giving rise to the impression that the spinnacker is lifting the bow.

Mal.

rayk

12-14-2006, 10:18 PM

Bold is mine
At the end of the day, if the boat is sailing at a roughly constatant angle of trim, the pitching moment of the boat/sail system is zero.Dead right For simplicity, the vector can be broken into horizontal and vertical components. The horizontal thrust of the sails reacts against the hull resistance force if it is tied to the wharf to create a bow down pitching moment, for about one second when the rope goes tight.
The vertical forces of the sails reacts against the weight of the boat, through the centre of gravity to produce(usually) a bow up force which is balanced by CB moving aft or forward. The sum of these three forces are not the net pitching moment of the sails. This force must be balanced by something, and that something is an equal and opposite force which is usually created by offsetting the centre of gravity with the centre of bouyancy of the hull. The vertical centre of hull lift is in line with the CG, sum of of the buoyancy force and the net vertical component of any aerodynamic CE forces.

To determine qualitatively the sense of the sails vertical forces, estimate the location of both the centre of gravity (CG) and the centre of hull lift CB. If CG is aft of CB then vertical force is bow down. If CG is forward of CB then vertical force is bow up.

Put more simply, if you pile the crew aft (usually the case), the vertical force is bow down. If you pile the crew forward (not often seen) the vertical force is bow up.
With net up force at CE,
pile the crew aft, the CG and CB moves aft....CEup CGdown CBup, helps you plane　
With net down force at CE
pile the crew aft, the CB and CG moves aft....CEdown CBup CGdown , keeps the bow up
The order of those forces is important.

Forces not moments, that is my only change.

By the way I quoted MalSmiths post because it was the most reasonable one yet!

Please reread this post because I had to do some editing :)

RHough

12-15-2006, 12:23 AM

You're grossly over simplifying. Go to the table at the end of the paper and look at the actual pitch moments. That's the critical point.

From the article:
"The third column, SinkF, is the sail force component perpendicular to the sea surface. Negative sink is LIFT, so the kite is lifting the boat at a force of 586 N, or about 59 kilos. The asymmetrical is driving the boat at 67 kilos, so the sail is lifting the boat nearly as much as it is thrusting it forward."

"lifting the boat" does not mean lifting the bow.

"Even all sections of the main are lifting (negative SinkF). I have assumed a realistic bow-up trim of the boat (from photos) - the mast leans so heavily aft that the main is actually working like your book cover opened. Nevertheless, both sails are pitching the bow down (positive pitching moment), the asymmetrical even more so than the main.

Together, the lift of the main and the asymmetrical corresponds to approximately 30% of the boat's displacement."

Here are the forces:

....................HeadSail.....MainSail .....Combined

Driving force......0.650........0.357........1.007 kN
Heeling force......0.520........0.274.........0.793 kN

Heeling moment...1.265........0.781........2.046 kNm
Yaw moment.......0.076.......-0.450.......-0.374 kNm
Pitching moment..1.682........1.146........2.827 kNm

I'm not sure how it is possible to take some of the data, cite it as a reputable source, then go on to ignore the conclusion. If you want to think the the A-Sail lifts the bow on a Cherub ... fine ... where is the crew weight compared to the CG or LCF? fore or aft? Do they run forward in gusts?

The information was posted in post #3 ... over 90 posts later, people are still arguing about it, yet no one has refuted the data.

Making statements like: "A moment is the change from one stable state to another.
Using a moment to describe lift of the bow or depression of the bow means it must start from an equilibrium and end in an equilibrium, of forces.

Only four forces act on the 2D illustration. Lift>Gravity, and Thrust>Drag.(Up>Down Forward>Back...whatever)
All vectors are made up of these forces.

A moment starts when one of these pairs is out of balance."

Indicates no understanding of what a moment is: "a turning force produced by an object acting at a distance (or a measure of that force)" Yes to be in equilibrium the forces and moments must balance, to state that moments only exist until equilibrium is reached is wrong.

Are we here to debate the premise, dispute the facts or argue with Dave Culp? :confused:

Now if you set out to create a sail the lifts boat and provides very little drive, I'm sure you could do it. What would be the point? (As Tim B pointed out in post #8)

rayk

12-15-2006, 12:56 AM

Yes to be in equilibrium the forces and moments must balance, to state that moments only exist until equilibrium is reached is wrong.

A moment occurs when two opposing forces become unequal.
A moment is the difference between two forces.
There are no moments when forces are equal.
A moment is not opposed by another moment.
A force is opposed by another force.

If you want to think the the A-Sail lifts the bow on a Cherub ... fine ... where is the crew weight compared to the CG or LCF? fore or aft? Do they run forward in gusts?

CB not LCF.
In a gust the CB moves aft as spin lift increases.
Moving forward during a gust is not on the cards, boat wont go faster.
When the gust passes lift decreases, residual speed plus CG forward increases the chance of submarining, on a wee planing skiff thingy.

Making statements like: "
A moment is the change from one stable state to another.
Using a moment to describe lift of the bow or depression of the bow means it must start from an equilibrium and end in an equilibrium, of forces.
Only four forces act on the 2D illustration. Lift>Gravity, and Thrust>Drag.(Up>Down Forward>Back...whatever)
All vectors are made up of these forces.
A moment starts when one of these pairs is out of balance
."

RHough

12-15-2006, 01:27 AM

A moment occurs when two opposing forces become unequal.
A moment is the difference between two forces.
There are no moments when forces are equal.
A moment is not opposed by another moment.
A force is opposed by another force.

Please, read a physics text.

Search definition of moment on the net. You are completely wrong.

The force of the sails act on a lever arm to heel the boat, the boat rotates until the righting moment is equal to the heeling moment. The boat stops rotating. The moments do not disappear, they balance each other. Only the motion resulting from the moments stops.

rayk

12-15-2006, 01:56 AM

RHough

12-15-2006, 02:37 AM

Equilibrium of Forces and Components of a Force. (http://www.saburchill.com/physics/chapters/0017.html)
Conditions for the Equilibrium of Three Non-Parallel Forces (http://www.saburchill.com/physics/chapters/0019.html)
Moment of a force. (http://www.saburchill.com/physics/chapters/0018.html)

To hold the door open you need to apply the same force as the door closer. Zero sum.
To open the door wider you need to apply moment(torque). Unequal force.

Zero sum does not mean zero moments, It means just what it says, the sums equal zero.

To open the door wider the opening moment (force times distance) must be greater than the closing moment.

Not all forces create moments. The force of gravity on your monitor creates a force that is opposed by your desk. Move your monitor on a plank hanging off your desk and the same force creates a moment. The magnitude of the moment is equal to the force times the distance.

Just because something is not moving is no indication that there are no moments.

A moment is a force acting through a lever. A moment is the result of force and the length of the arm. 10 pound on a 1 foot arm creates the same moment as 1 pound on a 10 foot arm.

A moment occurs when two opposing forces become unequal.

No, a Moment exists when a force acts on an arm

A moment is the difference between two forces.

No, a Moment is a force acting on an arm.

There are no moments when forces are equal.

No, two equal forces acting on different length arms both have moments.

A moment is not opposed by another moment.

No, heeling moment is opposed by righting moment.

A force is opposed by another force.

??? Scotty ... beam me up!

I'd say that you are not clear on the concept. :)

rayk

12-15-2006, 03:32 AM

??? Scotty ... beam me up!
Fair enough.
I cant argue with Star Trek. You have trumped me.
I was hoping you wouldnt raise to see me.

How about I draw wee planing skiff things which tear down wind dragging their wide arse through the water.
The competition is pretty tough for me. I havent really had any thing new to offer in this thread.

You can draw for the crowd that need a wide spoonlike planing bow to compliment the moment of the sail plan.
I think the racers are ready for a revoloution and your time has come.
Start now while no one else is doing it.

PI Design

12-15-2006, 04:09 AM

From the article:
"The third column, SinkF, is the sail force component perpendicular to the sea surface. Negative sink is LIFT, so the kite is lifting the boat at a force of 586 N, or about 59 kilos. The asymmetrical is driving the boat at 67 kilos, so the sail is lifting the boat nearly as much as it is thrusting it forward."

"lifting the boat" does not mean lifting the bow.

Absolutely. But this is telling us that the kite produces a forward component and an upward component which are almost equal (for this particular sail). Whether the upward component produces a bow up or bow down moment depends on the relative vertical and longitudinal distance of the centre of pressure of the kite form the trimming point (the centre of flotation).
"Even all sections of the main are lifting (negative SinkF). I have assumed a realistic bow-up trim of the boat (from photos) - the mast leans so heavily aft that the main is actually working like your book cover opened. [SIZE="3"]Nevertheless, both sails are pitching the bow down (positive pitching moment), the asymmetrical even more so than the main.[/SIZE

For this boat the moment is bow down. I beleive that for the 12' Skiff it would be bow up (see earlier calcs).

Together, the lift of the main and the asymmetrical corresponds to approximately 30% of the boat's displacement."

Here are the forces:

....................HeadSail.....MainSail .....Combined

Driving force......0.650........0.357........1.007 kN
Heeling force......0.520........0.274.........0.793 kN

Heeling moment...1.265........0.781........2.046 kNm
Yaw moment.......0.076.......-0.450.......-0.374 kNm
Pitching moment..1.682........1.146........2.827 kNm

You've missed off the sinking (lifting) force, which is vital.

I'm not sure how it is possible to take some of the data, cite it as a reputable source, then go on to ignore the conclusion. If you want to think the the A-Sail lifts the bow on a Cherub ... fine ... where is the crew weight compared to the CG or LCF? fore or aft? Do they run forward in gusts?

The conclusion is only valid for that particular boat. One point I have tried to make repeatedly, is that although the kite may be lifting the bow, the main sail will certainly be depressing it. If the main is depressing the bow more than the kite is lifting it, then the bow is still going down, therefore you still need to move back. This does not alter the fact that the kite (as a single sail) may be lifting the bow.

The information was posted in post #3 ... over 90 posts later, people are still arguing about it, yet no one has refuted the data.

Making statements like: "A moment is the change from one stable state to another.
Using a moment to describe lift of the bow or depression of the bow means it must start from an equilibrium and end in an equilibrium, of forces.

Only four forces act on the 2D illustration. Lift>Gravity, and Thrust>Drag.(Up>Down Forward>Back...whatever)
All vectors are made up of these forces.

A moment starts when one of these pairs is out of balance."

Indicates no understanding of what a moment is: "a turning force produced by an object acting at a distance (or a measure of that force)" Yes to be in equilibrium the forces and moments must balance, to state that moments only exist until equilibrium is reached is wrong.

Are we here to debate the premise, dispute the facts or argue with Dave Culp? :confused:

I am here aonly to point out that there are circumstances when a spinnaker CAN lift the bow, which some have said can NEVER happen.

Now if you set out to create a sail the lifts boat and provides very little drive, I'm sure you could do it. What would be the point? (As Tim B pointed out in post #8)

The 12' Skiffs are pretty fast, and as far as I'm concerned their kite can lift the bow. The point would be that nosediving is reduced, a worthwhile goal many would argue.

I agree with you about the moment definition. A moment is a force applied on a lever. Nothing more, nothing less. You do not need a pair of forces and you will still have a moment when forces are equal and opposite if they have different lever lengths.

PS Sorry about the bold text, I'm not trying to shout, just trying to differentiate my text from quoted text.:)

PPS Anyone know what a typical moment to change trim (MCT) is for a skiff type boat?

gggGuest

12-15-2006, 04:15 AM

From the article:
I'm not sure how it is possible to take some of the data, cite it as a reputable source, then go on to ignore the conclusion.

Depressingly it appears to be very easy to do so. We appear to have several budding politicians in this thread:-)

PI Design

12-15-2006, 04:25 AM

Depressingly it appears to be very easy to do so. We appear to have several budding politicians in this thread:-)

The conclusion is only valid for that particular boat. Okay, so I used the spinny lift data which is also only applicable to that boat :eek:, but I'm using it to make a point, not present absolute accurate calcs. The fact of the matter is that the force from the Cherub kite is acting at and angle of 41% to the horizontal. If you use that angle on the 12' Skiff dimensions, you get a bow up moment. If you use them on a Cherub (which has a shorter pole, I think) you get a bow down moment. I've only read the Cherub precis of the analysis, not the WB report itself as I can't access it. Any assumption on the location of the LCF is critical - can anyone who can access the WB site tell me what LCF value was used?

water addict

12-15-2006, 07:00 AM

The world is Flat!!!! The world is Flat!!! Why don't you understand!!!

PI Design

12-15-2006, 07:11 AM

The world is Flat!!!! The world is Flat!!! Why don't you understand!!!

You're right of course. I'm going to give up now and move on to proving that the earth rotates around the sun. It should be easier...:rolleyes:

Baronvonrort

12-15-2006, 09:13 AM

Okay, here are some fairly crude calcs for a 12' Skiff. I don't have the time, inclination or information to do a more refined calc.

A quick flick through the rules shows a mast height of 6.8m. Assume a mast head kite and that the mast is deck stepped 0.5m above the water. Assume also that the vertical CE of the kite is 40% of the way between the foot and the head (ie something between a triangular and square sail). Thus the drive force has a lever of 0.5+(0.4*6.8) = 3.22m.
Also from the rules: Hull length is 3.7m. Bow extension = 0.7m. Pole length = 2.5m. The end of the pole is therefore 6.9m from the stern. Assume that the foot of the kite reaches from the end of the pole to the mast, which is 1.5m from the bow. The foot length (clew to tack) is therefore 1.5+0.7+2.5=4.7m. Assume that the longitudinal CE of the kite is one-third of the distance from the clew to the tack = 4.7/3 = 1.56m from the end of the pole, or 6.9-1.57 = 5.33m from the stern. Assume, whilst planing, that the LCF (the point about which the hull trims) is 1.0m from the stern. Thus the lever for any vertical (lift) component of the kite force is 5.33-1.0 = 4.33m.
Using the Cherub data supplied earlier, the drive (horizontal) force from the kite is 680N, whilst the lift (vertical) force is 600N. It is reasonable to use the Cherub data here because the two classes are very similar (a Cherub measures as a 12' Skiff).
So, the bow down moment is 3.22*680 = 2190Nm.
The bow up moment is 4.33*600= 2598Nm.

Therefore the net moment is bow up.

Note however, that the two values are quite close. It is quite likely that in a real world, dynamic, environment that the moment will switch between bow up and bow down as things like sheeting angle, boat trim, wind strength etc change. There is a great video clip (go to the UK class website) of a 12' Skiff sailing along which seems to show this happening.

I dont know where you got your 12 ft skiff data and the mast heights,pole lengths and other dimensions are a bit out.Try >8m for #1 mast.
In all fairness they do have 4 rigs of varying size to cover the 2-37 knot windrange.
Sail area along with mast height and pole lengths are UNRESTRICTED in the 12 ft skiff class.
A Cherub does measure as a 12 ft skiff although it will be much slower and i do not agree that they are very similar and many other dinghy's that are 12 ft long will also measure as a 12ft skiff.Not many rules with the 12ft skiffs!

The skipper trapezes with both feet aft of the transom on a 12ft skiff when both are on the wire under kite.The crew would have his back foot in line with the transom.Depending on how fat the skipper is this gives a pretty large bow up moment.

Carry on lads...lol

PI Design

12-15-2006, 09:32 AM

Okay, in my haste I didn't mention the data is for the Restricted 12' Skiff. Details in Part 4 of:http://209.85.135.104/search?q=cache:QKM8YEcLLJsJ:www.skiff.org.au/Misc/Association%2520constitution%2520-%2520Aug%252005.pdf+12+skiff+pole&hl=en&gl=uk&ct=clnk&cd=7

Glad you're enjoying it. Bet you're enjoying the cricket as well...:mad:

Baronvonrort

12-15-2006, 10:02 AM

PI
There was absolutely no interest in the restricted 12 ft skiff down here and it was originally an idea to reduce costs for beginners.

There were 0 boats registered as restricted 12 ft skiffs.

The cricket is going well for us...lol

Andy P

12-15-2006, 03:54 PM

In the real world, in a cherub, the kite does stabilise the boat ( ie it's easier to get downwind in a blow with the kite up - apart from the hoist and drop) - there is noticeable lift from the kite so the whole boat is lifted, and it damps out the pitch.... it feels like up.. up ...up...up then DOWN then up.. up etc.
2-sailing there is much more frequent down pressure.- with the ups and downs reversed.
But the test is the flap whilst whizzing along - a flappy kite means the bow goes up ( and the boat sinks - the transom definitely goes down - well it would with both the crew at the transom. )
Some AUS cherubs have footloops along the transom - not the gunwales ( ie the crew stands right over the back )

With mods to the UK boats for bigger kites / masthead kites a few yrs ago, it was the feeling that masthead kites were faster, but more nosedivey, and longer pole short hoist kites were slower but easier to sail without nosing in.

CT 249

12-19-2006, 06:17 AM

I have no idea what this pic means, but I have never seen a Heron this bow up at such speeds.

The skipper was sailing alone and is a generously sized lad.

http://s08.picshome.com/e4e/img_0166.jpg

(For the benefit of most readers, I should say that the Heron - a '40s (?) cruising dinghy - doesn't normally carry any spinnaker here in Oz, much less something that looks like it was found in the beer trough after a 12 Foot Skiff party).

Doug Lord

11-17-2009, 07:25 PM

It's been a while but I don't think this is settled.....(don't ignore the humor):

bgulari

11-18-2009, 10:26 AM

that is a computer generated image

Paul B

11-18-2009, 12:29 PM

that is a computer generated image

Why even reply to his nonsense?

ancient kayaker

11-18-2009, 01:10 PM

Chris Ostlind

11-18-2009, 01:14 PM

Terry,

I'm assuming, then, that you have read the entire thread to this point and still have some questions that have not already been totally mashed into the ground?

Doug Lord

11-18-2009, 02:45 PM

I missed this thread earlier so it's good to have it come to life however briefly to allow those like myself another opportunity to take a shot at it.

Doug asks "lifting spinnakers: does it lift the bow?" That a spinnaker lifts the boat seems beyond question, and it is, after all attached well forward. No doubt a kite sail can lift the bow if attached sufficiently far forward and flown with the tether at a sufficient angle. But a spinnaker is also attached high at the mast which would simultaneously depress the bow.

I'm sure a spinnaker can be designed to lift the bow in the absolute sense like a kite. It would have to be attached well forward certainly using a bowsprit and have a great deal of rake. Those shown in Doug’s latest pics do have those features; if they are real photos that is, but it’s still a maybe.

Maybe the question should be reworded: “when the spinnaker is deployed is the bow higher than it would be motoring at the same speed?”

Does that give us a testable theory?
==================
Terry, they are real photos except the Moth, of course-which is why I said: "Don't ignore the humor".... On the models mentioned at the beginning of the thread there was no question that the bows were lifted-you could watch a gust hit the boat and see the bow lift up. And there is no question that the right spinnaker can reduce the displacement of the boat(as can a main working with a spinnaker)-see below. I hoped that by bringing this up three years down the line there might be some fresh thinking added.......
http://www.wb-sails.fi/news/95_12_Cherub/Cherub.html

Spin 50 and America One racing(first production gybable RC spin boats):

Doug Lord

11-18-2009, 02:53 PM

Heres a very interesting comment from 2002 on the multihulls list:

"I haven't followed the thread Dave mentioned in his post but I _did_ have
enough Math and Physics experience to say the following:
- a boat sailing at a constant speed under whatever headsail , be it spi,
genoa, blade jib or reacher fulfills 2 basic (vector) equations, 1)- the
sum of all forces is ZERO and 2)- the sum of all moments is zero . That's
it, neither more nor less .

The relative difficulty comes from quantifying the said forces and moments.
The forces are also interdependent . First is the sail lift vector (which is
projected along the horizontal x axis - thrust- and its magnitude is equal
but has opposite direction with drag , and the vertical component plus the
boat displacement plus boat buoyancy equals zero) . Lift unfortunately is
dependent on apparent wind , which is dependent on true wind and boat speed
which is dependent on boat drag. Hard to put into equation and solve .

The forces that generate moments are the sail lift, boat drag, boat buoyancy
and boat weight . The sum of all these moments w.r.t. an arbitrary point is
zero. Some of the moments are acting as follows:

-boat drag tends to rotate the bows down .
-headsail lift vector would tend to rotate the bows either up or down,
depending on the vector's angle and sail center of lift position. The moment
is the distance between the sail center of lift and the hull center of
rotation (this would be the arm) times the lift vector. The only thing that
matters here is if the lift angle is more or less than the hull center to
sail center line and horizontal . If it's above, the sail lift vector has a
net effect of lifting the bows.

For Dave, who tends to treat kites as a very special case, be assured that
mathematically the same above mentioned equations don't give a rat's a*s
about what specific numeric values some of the variables might have. For
instance, gradually lower the sail center , gradually enlarge the sail area,
increase its tilt angle and the forces resemble your kite boats. Increase
the sail center hieght, reduce its tilt angle and gradually you will get a
picture which will represent a traditional rig. It's not good kites vs. bad
rigs, it's just a matter of quantity . Take a large enough headsail, a low
enough aspect rig, a long bowsprit, and you'll see the bows lifting. Take a
small enough kite, attach it too high above deck and you'll see the bows
bury . The main thing is the same equations apply. If you want to optimize
things, you may do so with the equations (provided you wrote them first
} ). There's absolutely no reason for which headsails would be prohibited
from lifting the bows and kites would be allowed to do so . If your goal was
to lift the bows you could optimize for that. I'm pretty sure the overall
drag wouldn't be optimized at the same time, or the thrust would be
maximized , so your optimization might be local . Sailboat desing seems to
be today at about the stage airlpanes were before WW1 , a lot of gut feeling
and seat of the pants and very little science , a little voodoo if you
wish.

Best Regards,

Matt Tudor , MSEE
http://www.gigahertzelectronics.com "

ancient kayaker

11-18-2009, 05:10 PM

Terry,

I'm assuming, then, that you have read the entire thread to this point and still have some questions that have not already been totally mashed into the ground?

Yep. The first few pages had posts with substantive arguments and after that it became repetitive, personal or just plain quibbling. I skimmed through every single one of the other posts and read completely any that seemed to offer something new to the argument.

This question cannot be resolved by repetitive opinions, calculations based on assumed values, artists’ impressions of sail lift vectors pointing in arbitrary directions and photos of boats with their bows pointing to the heavens at the moment the photo was taken. If this was something being proposed for the first time I would invoke the rights conferred by my honorary Missouri citizenship*.

There is a considerable weight of opinion but it falls on both sides of the argument and feathers outweigh opinion that is not accompanied by data or observation. So I gave MHO of where we were at and asked if an observation could be made, proposing a method. Of course an on-board data logging system with load cells would be the thing to use and probably exists on Volvo boats, l’Hydroptere and boats of that ilk, but they are unlikely to release their data for all to see.

Don’t take this personally, any of you. Just pointing out that none of this could be admitted into court. I’m not saying anyone is wrong and least of all do I claim Doug is right. Come to think of it, he just asked the question...

* “show me” for the non-US folk ...

Chris Ostlind

11-18-2009, 05:29 PM

Terry,

Over the years, I have spoken with so many sailmakers, designers and sailors who use the sails, about this topic and the answers would make your head spin. These were really smart folks with vast collections of knowledge and experience in the sport. The responses have come from all quarters, based on a hundred different variables and active sources of knowledge. Some had charts and numbers up the wazoo. Others could tell you, very precisely, what they feel when sailing in dozens of different conditions. Some of them guessed and some of them will swear to you that they just inherently know.

You know what the sum total of all that data gathering reveals?

There's no real concensus... it depends on the boat and the rig... it's all about sea state and wind conditions... it's all about how the boat is sailed... and on and on it has gone. This thread haggled it to death and reached the same conclusion.

I know that some folks like to feel that there are soild, data driven explanations for the subject. Unfortunately, you're not going to be able to put numbers on it and call it analysed to completion. The moment they do, some one will come along and casually debunk the numbers with real world demonstrations.

Now, if you want to open a real stinky can of worms, which has also been haggled until it bled, show some courage and post a thread about the description of the planing state for monohulls and multihulls. You'll soon find out that this current topic is complete child's play by comparison.

Anyone?

ancient kayaker

11-18-2009, 06:54 PM

Terry,

... Now, if you want to open a real stinky can of worms, which has also been haggled until it bled, show some courage and post a thread about the description of the planing state for monohulls and multihulls ...

- God, no! I don't have that kind of courage. I would rather tell sail-boarders their craft were not truly boats!

Doug Lord

12-02-2009, 09:11 PM

bistros

12-03-2009, 06:48 AM

Why couldn't a spinnaker for a high performance boat be designed similarly to a kiteboarding kite with mods to make it as easy to launch as a "normal" spinnaker? In essence, it would have a more or less rectangular planform and be launched with a halyard ; the control might be with a combination of twin very light, folding poles and a retracting bow sprit. The goal would be to develop more power than a normal spinnaker... Could something along these lines work?

No.

You are proposing to make Florida orange juice from a bushel of Michigan apples.

Have you looked at the control systems for kites? They don't use multiple attachment points, all lines return to one point. Even large scale kite implementations (like on cargo ships) the kite is the only "sail" in the air, as the kite needs a clear area without obstructions like masts, shrouds etc. to get in the way.

Since I know it is impossible for you to agree with me, I now fully expect an onslaught of pictures, quotes from the dark corners of the Internet and obscure references from other credible people taken out of context. Tally ho!

--
Bill

Doug Lord

12-03-2009, 07:05 AM

I don't know enough about kite/spinnaker design to know whether this is possible or not.

RHough

12-03-2009, 09:53 AM

I don't know enough about kite/spinnaker design to know whether this is possible or not.

Is the point of the sail to drive the boat or lift it out of the water?

If you want to drive the boat, you design the sails to do that. It so happens that excess power not used to drive the boat can be used to lift the entire boat, thus lowering it's dynamic displacement and making it easier to drive through the water. There is an optimum relationship between Drive and Lift within the limits of righting moment.

How you get to this is an interesting puzzle. Kites can have very low heeling effect on the boat, and for water born craft where the total drag is very high a kite can operate at an L/D that makes them fast. If you want to sail at high multiples of true wind speed, look at the rigs ice boats and land sailors use ... not kites or spinnakers ... wings.

Doug Lord

12-03-2009, 04:22 PM

RHough

12-03-2009, 11:38 PM

The point would be to make the boat faster. I'm thinking that a "spinnaker" using some of the technology available for kites might be a whole new kind of sail for high performance offwind.

Just found this: http://www.ralfgroeseldesign.com/parasailor.html

http://www.seateach.com/Parasailor2.asp

Think about it Doug.

This is a sail for slow boats. Boats that sail with the apparent wind aft of the beam. Boats that sail slower than the true wind.

Can you imagine the conversation I had with these ****** at the last boat show I worked? :)

They could not answer any technical questions, they were selling snake oil to idiots. They were very happy when I left. :)

How does it set at high VMG angles?
Huh?
What is the range of AWA's that the design is superior to conventional sails?
Huh?
How high can you reach with it?
Huh?

Crap for idiots. an easy to trim sail for sailing DDW ... just fine for mom and pa kettle that can't figure out how to trim.

This decreases the horizontal energy that attacks the head of the spinnaker as well as the resulting torque on the mast. The bow is also considerably relieved. Rolling and swerving are drastically reduced.

Gosh ... horizontal energy is what drives the boat and this wonder produces LESS? :D

Doug Lord

12-04-2009, 02:24 PM

bistros

12-04-2009, 03:11 PM

The thing that intrigues me is that with all the research done on kiteboard kites there might be someway to incorporate some of those elements in a sail that could be set and doused on a boat-not flown like a kite. The biggest problem I see,so far, is the asymetrical shape of the kite which would make it very difficult to use.

Doug:

Rather than spend the time to outline the many problems with this theory, perhaps you find time within your busy schedule to actually walk the two blocks to the beach and watch someone using a kiteboard.

The kite can't fly until the shrouds are fully extended, the kite oriented properly to inflate and the control lines and frame are held in the right positions. None of these issues have anything to do with asymmetrical shapes. Kites are bilaterally symmetrical (like Victoria's Secret models & 505 spinnakers).

This discussion is pointless without you taking the time to observe the issues firsthand. Since you live about 45 seconds from the beach in Florida, go fly a kite! Five minutes of real-world first hand data collection sometimes is worth far more than days & weeks of Internet speculation.

--
Bill in Ottawa

Doug Lord

12-04-2009, 04:33 PM

While the planform of a wing could be said to be bi-laterally symmetrical each side of a line running fore and aft thru the center of the wing, convention more frequently defines a wing/foil as asymmetrical or symmetrical based on the wing section. That is the definition I used in defining kites as asymmetrical because the leading edge and trailing edge are different and the top and bottom of the kite(and the kite section) are different. In considering whether a modern kite section might work for a settable and douseable sail it seems that either the section would have to set on each gybe asymmetrically like an asy spin does(using a flexible symmetrical section?) or the sail, as I envision it, would be good on only one gybe. Best I can tell now, a symmetrical section that set asymmetrically would probably not be much of an improvement except perhaps with a relatively high aspect planform requiring a unique attachment to the boat at the top and bottom of the sail.
A more or less rectangular planform asymetrical might be a simpler solution but not by much-it would still need a new method of attachment/support top and bottom.
----------
Some spinnaker design ideas(see the referenced patents): http://www.freepatentsonline.com/5095837.pdf

Chris Ostlind

12-04-2009, 04:37 PM

Kites most certainly are symmetrical on a bi-lateral axis, Doug. You may have confused the term with longitudinal symmetry.

Help yourself to this page of definitions and get back to us...

http://www.google.com/search?hl=en&source=hp&q=bi-lateral+symmetry&aq=f&oq=&aqi=

If one puts a matching set of control lines on each side of one's field of vision,(it does take the opposing sets to control a modern kite such as those used by a kite surfer) and one then looks at the kite, it is decidedly bi-lateral in its symmetry.

Here's a photo, in case all this is confusing.

bistros

12-04-2009, 08:26 PM

While the planform of a wing could be said to be bi-laterally symmetrical each .... as asymmetrical or symmetrical based on the .... asymmetrical because ... settable and douseable sail it seems that either the section would .... asymmetrically like an asy spin does(using a flexible symmetrical section?) .... a symmetrical section that set asymmetrically would probably not ... rectangular planform asymetrical ...

Jesus Doug, when you go back and re-write your posts to retroactively make yourself look less wrong, you could be more honest and just say "I was wrong", instead of incoherent babbling ad nauseum.

When you caught me in a calculation error last year, I owned up to it publicly, didn't change the mistake in my post, and apologized while acknowledging that you were correct on that one issue. I'm an adult and I accept responsibility for what I write.

Getting things back on topic, I think the idea of using a kite-style airfoil on a boat in combination with a standard rig to decrease displacement and increase speed downwind is impractical.

How is your build going? From your hints it seemed like an early 2010 launch was the target. I'm done for the season working on Tom's boat - it is now too cold in the garage to work and epoxy cure times are unworkable.

Seasons Greetings,

--
Bill

Doug Lord

12-04-2009, 08:40 PM

Well there were two mistakes that I see: your misunderstanding of what I meant by asymetrical kite and my misunderstanding of what you meant by "bilaterally symetrical". I trust that is cleared up.

Doug Lord

12-04-2009, 08:43 PM

bistros

12-04-2009, 08:45 PM

This is an asymetrical section(from skywalk site) kite shown in the orientation it would have on a performance boat.The top and bottom of the kite as shown would be supported entirely differently than shown and the lines going to the center of the kite wouldn't be necessary. The problem I mentioned above remains:

Again, Doug you are completely off base and substituting conjecture for facts. The middle suspension lines are critical to maintaining the airfoil. I've been suspended under a similar inflatable foil section hundreds of times and if the middle lines were removed, I would not be walking (or alive) today. An inflatable airfoil section section works because of evenly spaced lateral shroud lines, even in orientations such as in the picture you show. Each vertical cell wall in the inflated foil needs a suspension line to control it's placement.

In a inflated foil section, the full width of shroud lines provide the same function as mast track or luff wire in a sail - without a luff rope, a main sail would not work, and without a rigid forestay and hanks or internal luff wire, a jib would not be effective. Just attaching head and foot (tack & clew) would not work.

If you doubt this, please refer to Dan Gardiner's reference work "The Parachute Rig", to understand the issues with inflatable wing sections. I was a FAI-licensed Rigger "A" for parachute reserve packing and equipment design and maintenance during my years of skydiving.

--
Bill

PS

If you want to learn more about this, either go across Florida to Zephyrhills (north of Tampa - a major skydiving Mecca) or up the coast to Deland (the other major dropzone in Florida. I've spent weeks there and there are people there who will happily explain inflatable wing theory to you.