Gybing Plates

The reason (I think, just an opinion)it probably works better on two man boats particularly those 505 thingies that usually come loaded with half a ton of meat on the wire, is that there is enough power to drive it. it's been my experience using them on boards that whilst superb in light weather the moment the wind built it just scewed the board into a stall there just wasn't enough moment to overcome the tendency of the plate to that direction.

Now the reason I think it might be relevant to this little boat is because it is decidedly under boarded, it's a tiny plate and before I started in with the sander so flat as to probably not create any lift and to act just as a side slip prevention tool a la Finn. It made noticeable leeway, which it probably still might but with the plate angled and sailed free enough it might overcome, if not I have a cunning plan that might enable it to be switched off or adjusted without the usual knock at the back and having to retract it slightly.

As it is the plate is so loose in the slot I will have to build it up anyway, so I though I might be able to fashion an adjustable angled device, we'll see, all these ideas are great until you have to actually fabricate them.

Edited by iGRF - 27 Jan 16 at 10:19am

Originally posted by iGRF so flat as to probably not create any lift and to act just as a side slip prevention tool a la Finn

Its still lift. A flat plate will lift just fine, as we have all demonstrated with paper aeroplanes. And the amount of lift you end up with has to be the amount required to balance the sail. But, and its a very big but, the proper shaped foil will create that lift with a load less drag and at a much lower angle of attack (=leeway).
And yeah, I'm sure you're right: if the boat is spectacularly underboarded it will need to sail with a lot of angle of attack on the board, which means the hull will be crabbing sideways more than most, which means that the gains from angling the board to align the hull with water flow will be more than most.

Is it not the case flat only creates lift when an angle of attack is presented, whereas a shaped foil adds an element of bernoulli (not sure of the spelling) effect, which supports your argument of less drag and therefore less leeway, either way a decent naca section is always better than flat, unless of course your head is full of meat instead of brains and you sail a Finn.

Originally posted by iGRF Is it not the case flat only creates lift when an angle of attack is presented, whereas a shaped foil adds an element of bernoulli

You still need angle of attack with a shaped symmetrical board. Maybe less, I'm not sure in real terms how much this varies. Its all about coefficient of lift and I couldn't quickly find a reference on line of how much the coefficient of lift of a symmetrical foil varies between sections.

That's an interesting observation about fixed fins Dan, that hadn't occurred to me, but of course you are right. All part of the horrible mess sailors get into thinking about leeway, lee helm, weather helm and balance... Equilibrium between foils is a really good way of thinking about it.

Edited by JimC - 27 Jan 16 at 1:08pm

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.

Edited by JimC - 27 Jan 16 at 3:20pm