Lifting Spinnakers:does it lift the bow?

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.

 

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.

 

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.

 

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?

 

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

 

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/g...ndtunnel.pdf#f02dc9f31b501bfbb8b1a1b0d697d6f6
Advances in Wind Tunnel Analysis of Yacht Sails
http://www.friendship-systems.com/g...rds_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

 

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:

 

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.

 

Answers

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.

 

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

 

Resoloution of forces.

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