Gybing Plates

Originally posted by JimC Originally posted by iGRF It then follows that this angle should be adjustable, because that leeway angle must surely alter with wind strength on some hull forms, I've not heard of it being done, but I agree, that would seem to be the logical approach. Indeed in an ideal world I suspect one would dial in a lot of angle coming out of a tack, and reduce it as the boat comes up to speed and the foil generates more lift. The ultimate expression would presumably be a daggerboard in which the leading portion was a gybing foil with adjustable angle of incidence, and the trailing portion was a flap so the foil was adjustable with the total camber able to be altered... I think I know a Canoe sailor in California who would build that, if indeed he hasn't already!

You can adjust the AoA of a gybing dagger. Done plenty on i14s and on Chris Maas breathtaking ICs. Bear in mind you can be doing say anything from 7kts to 12kts upwind in an i14 while twin stringing, so remembering Cl varies with V^2 thats a big Cl range and hence Alpha range.

You could 'spring' the diamond, so more pressure gives max angle and less pressure reduces it, but I'm not sure that's the way round you want it, then actually you could spring the other side for the opposite effect, wait a minute, this needs more dwelling on, so like a suspension ATB you springload with adjustable tension, I'm going to think further about this...

Originally posted by iGRF maybe its early onset altzeimers

So I think I've got it now, amazing learning something new when you think you know everything..
Although tbh the term as y'all use it never really came up much in our world, only the Original Windsurfer acted perceptibly as described and forced a crabbing technique to force the boat to weather to overcome 'leeway', hell, I still can't believe I've not used the term before, maybe its early onset altzeimers.

Anyway in essence the trick then must be to align the angle of the plate offset, to the percieved leeway angle of a given craft. It then follows that this angle should be adjustable, because that leeway angle must surely alter with wind strength on some hull forms, in fact logic then suggests the actual internals of the cb case should be constructed to vary that angle, has that been done I wonder..

My there's lots to think about, but thanks everyone for the input, it's been fascinating and thought provoking.

Edited by iGRF - 28 Jan 16 at 10:27am

Originally posted by JimC Originally posted by iGRF are you saying the helm is on or about the same angle as the plate and if so that would be constant weather helm? Yes, and then again no... There's a lot of muddled thinking about weather helm, and Dan's point about equilibrium and balance is a good way of thinking about it. Here's what I think: Sailing along in a straight line everything must be in balance. You've got a sideways load from the sails acting on a centre of effort somewhere between the two foils, and sideways load, normally on the both foils. Because there's sideways load on the rudder then because the blade is mostly or all behind the pivot it tugs on the tiller, and people think of it as weather helm, but its not as simple as that. Our first boat has a non gybing board, and in order to provide side force the board has to make an angle of say 4 degrees to the oncoming water, and we call that leeway, and again people think of it as a bad thing, but its not, its intrinsic to any boat with a symmettric foil fixed on the centreline. OK, so first think of the balance with all the load on the centreboard and none on the rudder. Mast raked well forward I expect. For the boat to be in equilibrium the rudder must contribute no side force. It won't be pulling on the hand in either direction, and it will be angled exactly to the water flow - so 4 degrees to leeward! That's what was traditionally thought of a balanced, but actually all the side load is on the board, so the rudder is being dragged through the water and doing nothing but contributing wetter surface drag. Next the rig is raked back a bit and so think of the boat with the rudder exactly on the centreline and going in a straight line. For this to balance then there must be proportionally equal side load on rudder and centreboard. The rudder blade is making the same angle of attack on the water as the cb, and this is probably the lowest drag configuration for a modern boat. The rudder is pulling on the helm's hand, so many people would say there is weather helm. So yes, but its *good* weather helm! Incidentally the 'leeway' will have reduced a bit, because with the rudder taking up some of the sideload the centreboard needs less angle of attack on the water. OK, so rake the rig back so its all over the rudder. There's near enough no sideload on the centreboard, first thing that we observe is that the boat has stopped making any leeway. No load on the centreboard, no angle of attack on the centreboard, no leeway. Next thing we observe is that because all the side load is on the rudder, its got to have a shed load of angle of attack to create the necessary lift. This means its got to be pulled well to windward, and we have shedloads of what was traditionally called weather helm, and we have loads of drag and having no leeway is definitely a bad thing! OK, so we now know that we want to have the rudder parallel to the centreboard (roughly). So now we fit a gybing board and the centreboard is angled to what the leeway angle was in the middle example. Boat makes no leeway. But to maintain the balance the rudder blade still needs to be parallel to the centreboard, so it has to be pulled to weather the same amount that the centreboard is angled, and it will pull on the hand by the same amount that it was. So on the one hand we could call it weather helm, but on the other hand its exactly what we want... So what we are doing with the rudder, in order to sail in a straight line, is actually to change the angle of attack on the rudder so the side load is shared correctly between the two foils. Dan, would you agree with the above description? Originally posted by iGRF Anyway, small boat under foiled, should I or shouldn't I? For what my opinion is worth, give it decent shape foils first and see if it still feels underfoiled and how much leeway you are making.

bang on Jim. In reality the rudder is about a degree or two further yawed than dagger for same Cl as its operating in the downwash. Also, in a non steady state you'll be running a variable side force off the rudder so may load it up a bit less to get a similar RMS value or Cl.
High aspect boards like to operate ~ 4 deg AoA (less and they are dragging too much WSA, more and too much induced. Low aspect foils best at sat 2 deg as induced drag more expensive. Even with high aspect foils, you wouldn't want to show the hull that much AoA (leeway) as its a 1:20 aspect ratio foil and will be picking up a lot of induced. Its easy to design / size foils for slow boats, as they go the same speed once up to speed. HP boats are generally massively over finned as they can go say 50% faster in wind than when just powered, for same RM i.e. side force. Mind you all that area is generally needed for getting out of tacks in most boats.
Dan