gybing center boards

<<yes it can because you don't need to change lift to point higher. Preferably, one would like to reduce lift. It's just one of those tradoffs that you end up where you do.>>

Yes reducing lift reduces the induced drag component of the foil(s). Reducing drag will allow the boat to sail a course closer to the wind. Since lift from the rig is what drives the boat, there is always an induce drag penalty to produce forward motion when sailing upwind.

Reducing sources of drag other than induced drag will allow a boat to sail closer to the wind, or sail faster for a given amount of heeling force.

When a boat is trimmed and sailing upwind at a constant speed, you cannot simply change the angle of the board or foil to get more lift.

<< lift isn't to windward, it is perp to the course sailed. And it's not the same. You are transferring sideforce from the hull to the board and lowering the VCR in the process. This is a big deal on most craft.>>

"Sideforce"? and my terminology is a horror?

<<But you just said the lift won't change, so it must only be about drag. Lift/drag ratios are all but useless as bulk quantities out in the wild. They need to stay in the lab. What's the lift/drag of my Catalina? Is there a table somewhere? What is the meaning of a combined L/D when the items being combined are in different fluids traveling at different speeds over the hull in different directions????>>

Yes, in steady state trim the forces are constant and balanced. They don't change. When one of the forces changes, the boat either changes velocity or heading.

The L/D of my Catalina 30 is less than 1:1 below 10 knots true and a bit better than 1:1 at true wind speeds over 10 knots. The table is a Polar. The optimum speed and angle upwind allows you to figure the total L/D. BTW over 45° means the L/D is lower than 1:1, BTW less than 45° means the L/D is higher than 1:1. In and of itself L/D is not used much in defining sailing performance. It is handy to point out that reducing drag does two things. It allows the boat to sail faster at a given angle off the wind or it allows a boat to sail higher at the same speed.

<Sure it will (the physics). If I suddenly have less drag, I can point higher at the same speed, and still higher at the same VMG, but my absolute highest pinch won't change very much because there isn't drag (of the flavor being affected) worth mentioning at that speed. >>

My error ... by "appear to sail at a higher pointing angle" I meant that gybing the board will not bring the apparent wind forward. The boat will not "appear to be sailing closer to the wind". The bow will rotate away from the wind and the boat will appear to sail lower. No argument that *IF* gybing the board means you "suddenly have less drag" you can indeed sail higher.

I have not yet found a reference that shows the drag curves for sailboat hulls when they are moving through the water at some leeway angle. If changing the angle the hull moves through the water by 2-3° suddenly reduces drag I have not seen data to support it.

<< Viscosity is different and it is an imortant difference if you are trying to compare apples to apples via Re's.>>

Yes the Formula is L = .5 p v^2 A CL

p is density, hence my use of the term.

I don't think we disagree. I think terms need to be defined.

The point I was trying to make is that substituting a cambered board for a symmetric board will not "increase lift". It might reduce drag and allow a boat to sail higher, but the lift will always balance the forces that create it.

 

Thanks RH. I think the main difference between our two posts was what you captured here-

I tend to try to work the speed change business first since thats what I'm trying to do- go faster.

I think "sideforce" is the correct term, but I may be wrong.

 

There are a great number of similarities between sailing dynamics and soaring. Since I was doing glider design before I got into what makes boats faster I start from the forces involved. Thrust must equal Drag, Lift must equal Weight. Glide slope is almost directly comparable to sailing upwind in concept.

As you quite correctly point out reducing drag increases speed. Technically when the boat has accelerated to her steady state speed Drag = Thrust again. The velocity where that happens is higher.

What many sailors don't seem to get is that things like windage and hull surface prep have a very large effect on both speed and VMG. How many times have you seen new sails on a boat with bottom paint that has been applied with a roller?

On topic for this thread is the only way a gybing board can improve upwind performance is by reducing drag. Specifically the drag caused by the hull going through the water at the leeway angle.

In flat water the angle of the flow in relation to the hull is nearly constant from bow to stern. In waves or while planning this is not the case. In waves the local velocity varies between the trough and crest of each wave. A boat that is sailing through a wave pattern at an angle will see the water flowing across the hull at various angles as she goes through the wave pattern. When planning upwind I suspect the effect of leeway on hull drag is very small.

These conclusions are consistent with the reports of gybing boards being effective in flat water but no change in performance noted in waves or at higher speeds. Bethwaite played with flapped boards in to identical Moths years ago and could not find any advantage to using a cambered board.

My conclusion is that a gybing board may increase performance upwind on some hulls in some conditions. However to my knowledge there is no work published that shows that the benefit should apply to all boats or how to calculate and predict the effects. Thus IMO gybing boards add a design challenge that does not guarantee positive results and design and build time can be better spent pursuing drag reduction in areas that are known to get measurable results.

 

Increasing camber while keeping thickness and thickness distribution constant will generally increase maximum lift. But changing thickness and/or thickness distribtuion while keeping camber constant can also increase maximum lift.

As far as lift at a given angle of incidence goes that depends in part on how the angle of incidence is defined. Keep in mind that when sailing upwind "angle of incidence" is not fixed but as others have discussed the angle of incidence adjusts so that the amount of lift is provided which is needed to offset the transverse force of the sails.

 

And one major difference between a sailboat and a glider. Weight which Lift must equal in a glider in steady state flight is constant. But in a sailboat the lift of the underwater portion of the boat must equal the transverse aerodynamic forces, and the transverse aerodynamic forces are not constant. Rather they vary with:
- wind speed
- course sailed
- sail trim
- and a number of other factors.
So the analogy of a sailboat with a glider is useful up to a point, but can be confusing and misleading if the variation in transverse aerodynamic forces and thus lift of the underwater portion of the boat is not considered.

 

So you agree that the ever changing forces on the underwater portion of the boat make it less likely for a gybing board to have positive effect?

FWIW This is what Frank Bethwaite has to say about the glider, boat analogy:

Bethwaite's High Performance Sailing (ch 20.6) ISBN 0-87742-419-5

From my experience with glider design I had come to much the same conclusion and that background helped me to understand sailing in technical terms. I was pleasantly surprised to read this in Bethwaite's book.

You are correct about a constantly changing environment. As sail forces change the boat changes velocity, heel and leeway. The more unsettled the conditions are the less likely it is for a gybing board to have positive effect since the forces that combine to produce the leeway angle are always changing.

 

Again: nice thread,
Reading it I think there is still a lot to learn and discover.
To sail a boat as efficient as possible, I also try to look at nature; nothing beats nature when it comes to efficiency.
When I Look at how natural birds and fish change there forms dealing with the ever changing conditions they cope with I can only think that there is still a lot to improve and develop sailing boats.
Making foils more adjustable is only part of be able to better tune in the ever changing circumstances you encounter.

Cheers,

 

I don't agree or disagree. I don't see why the changing forces would make it more or less likely for a gybing board to have a positive effect.

The sail forces also change as sail shape and trim is changed, and the course relative to wind direction changes. These are choices made by the crew, not "constantly changing environment".

"Sail forces substitute for gravity, and exactly the same principles apply" only if gravity changes as the gliders speed and rate of descent/climb changes, which of course it doesn't.

When analyzing a glider the weight of the sailplane and therefore lift of the wing is fixed irregardless of glide slope or glider speed. For a given airspeed the angle of attack of the wing is that which produces the fixed amount of lift. The drag at that angle of attack is then determined, and the resulting glide slope calculated from the ratio of lift/weight to drag.

For a sailboat the situation is different since the aerodynamic forces and therefore the "weight" depend for a given wind speed depend on the boat's heading and course relative to the wind , and the sail shape and trim. So iteration is needed to determine the best combination of these factors in conjunction with the hydrodynamic "lift" and drag characteristics of the board(s)/keel/rudder/hull, etc. Not including this iteration in analysis may lead to faulty conclusions.

Bethwaite's comments about moving the CG aft on a glider to transfer load from the wing to the tail are incomplete at best. Moving CG aft also decreases stability margin and eventually aft movement of the CG in an aircraft will result in a pitch instability (which an active control system may be able to compensate for).

"The more unsettled the conditions are the less likely it is for a gybing board to have positive effect since the forces that combine to produce the leeway angle are always changing." Could you elaborate on why you believe this to be true, or is it intuition?

 

I have not heard of anyone doing this with boards either. I imagine the extra drag would probably outweigh the advantage of soaking.

For the 1984 OTC Jeremy Rogers built a boat (using the old 1979 Contessa 39 molds) with a keel that rotated (or "gybed"). The engineering design was done by David Alan Williams. I think you can find an anticle about it if you check Seahorse issues from '84.

Anyway, they experimented with turning the keel "opposite lock" when sailing downwind, and claimed it helped. I don't know how much it helped, since that old style boat ended up uncompetitive at the OTC.

 

I'm not sure how close winded those boats are. All I can go by is watching the inshore races. The crossing angles don't look too flash. We get to see the races on Fox Sports Net here. Not sure if you get that channel. Anyway, they don't have the nice graphics to show the angles and VMG like the AC races did.

 

You would probably end up with this sort of result.

 

Needs DSS, Paul?

 

I guess that would give you another handhold to grab while trying to right the thing after the wipeout.

 

I'm trying to remember what Jim Young told me about the gybing board/lift keel he had on Heatwave, his One Tonner. With the board right down, the foil could be gybed across 2-3 degrees but once lifted, even a little, was fixed. He said the system worked well but later it was outlawed by IOR so he didn't pursue it any further. Heatwave was good to windward in hard conditions anyway (with non-gybing board) but he thought there was still more to be gained but was halted in doing so. This was back in 1976-77. I'll give him a call and get more details.

 

As I understand it, the theory is that the board's zero lift line being offset to windward allows the hull to yaw to leeward and this reduces the drag of the hull through the water. In a steady state environment it may be possible show this. However in less than a steady state other factors come into play. Most notably wave resistance on the hull. Not the wave making resistance the hull itself creates as a function of speed, but the resistance of moving through the wave system. It is well documented that the power requirement for a given speed is higher in waves than in flat water. In general the wave pattern is perpendicular to the wind. Thus a hull yawed to leeward will be moving through the wave pattern at and angle that produces more drag rather than less.

The changing forces from the rig will increase the range of angles at which the hull moves through the water. I do not see it as very likely that a hull yawed into higher drag angle to waves and wind will benefit from reduced drag due to reduced average leeway angle.

Again, I have not been able to find any documented study that proves this one way or the other.

Now you are splitting hairs. If the boat is sailing at the optimum speed and angle the crew only makes changes in response to the changing environment. Unless they are using kinetics to drive the boat with repeated sail trim adjustments or crew movement. These are prohibited actions under the rules so are not a concern.

However anyone that has flown through turbulence knows that the apparent gravity and loading on the airframe does change, and it does so exactly like the varying forces of wind and waves on a sailboat.

This is why we we use average values for targets and calculation knowing full well that neither the air or the water are perfect. At best these are snapshots of forces at an instant. Not news.

This is obvious. The subject of stability was not addressed. Sailboats in good upwind balance would indeed be unstable as gliders. The helmsman is the active control system that allows it to work. It is easy to think that this is not so, since many boats can be trimmed to be positively stable in yaw. What demonstrations of a boat sailing upwind and holding could with the rudder locked fail to consider are the stabilizing forces that are at work on sailboats that do not exist in a simple CG placement wing tailplane glider example. This stability gets into the weather helm / lee helm where boats that sail at higher angles of heel require more lead than boats that sail flat.

It is somewhat educated intuition based on my experience with sailboats and gliders. The idea of gybing boards is far from new. To check if my grasp of the forces involved and their interaction is sound I have at various times looked for published results of testing. I have not been successful in finding anything backed by data and science. I have found several stories that claim that adding a flap or trim tab or a gybing board gives "more lift" and causes the boat to sail higher. I find it very hard to believe that a 2-3° reduction in the leeway angle of a hull will result in a drag reduction that provides a 5-6° higher sailing angle. I do keep looking for reference work however, I don't mind being wrong even a little bit.