Of course they really work. A better question is, do they make the boat faster over a wide enough range of conditions that they are worth the extra trouble.... and that is a question not so easily answered.I've got an older 505 and a few old classic Moths. Both allow gybing boards, but do they really work?
What I do not understand is that it seems there are two benefits:The basic idea behind the gybing board is to construct the board/trunk so that it can assume a slight angle to windward of centerline; after all it is a symmetrical foil and they need an angle of attack to generate lift.
If the foil is aligned with the centerline of the hull, the angle of attack becomes the angle of the boat's leeway and the hull is sliding sideways at that same angle...... hulls are generally better at going straight forward thru the water, or at least, that's the assumption.
Right, I was going to use the phrase "net angle" but that sounds like something to do with fishing. Basically, a gybing board does not change the angle between the apparent wind and the boat's track.Point 1 is a bit poorly worded.
In physics terms, your sails are sheeted wrt to the track of the boat.
If you sheet the main and jib to same angle wrt cl and suddenly you switch gyber on and your heading remains roughly the same you are actually sheeting your sails 4 deg more with respect t track, so possibly oversheeting a little.
You are right re overpower at least in fast boats. If you are doing 4 deg leeway once fully hiked at say 5 kts, then with more wind and twisting sails and going say, 7kts for same sf then the cl and Aoa will reduce a fair bit to say 2 deg, so the benefits of removing 2 deg of leeway from hull are about a quarter of the benefit of removing 4 deg of leeway.
Worse, if you leave it gybeing at 4 deg when doing 7kts you will be making positive leeway so adding induced drag back on, and likely making a mess of helm balance.
Hulls are not good at generating lift. This is why a horizontal foil to lift them out of the water can go so much faster. Some hulls don't mind a small skew angle, like scows, but others really hate it..... or rather, the water going by them does... ...
So with no experience of gybing boards my gut feeling is that point 1 would be good when you're looking for power. And maybe point 2 would be good when hull drag isn't worth the lift it provides due to hull shape.
Mine stops when raised part way as well. We tend not to raise the board anymore when wire running. Hence gybe stopper.I don't think a gybing board on a 505 is any kind of problem. The old simple way of stopping it was to pull up the board a bit so that the flat is engaging the trunk.
My old boat has one. 4000 series. I'll go take some photos.
Starting from this assumption, you might conclude that the board should be angled at whatever angle makes the hull go straight through the water.If the foil is aligned with the centerline of the hull, the angle of attack becomes the angle of the boat's leeway and the hull is sliding sideways at that same angle... hulls are generally better at going straight forward thru the water, or at least, that's the assumption...
Interesting points, thanks.Starting from this assumption, you might conclude that the board should be angled at whatever angle makes the hull go straight through the water.
That's not generally correct; the situation is much more complicated.
For one thing, if the board has any lift (i.e. side force) on it, it will induce a curvature in the flow over the hull, so there is no angle that will allow the hull to go straight through the water.
For another, if it were possible to have zero side force on the hull, the loading on the board would be zero at the point it intersected the hull. This would more-or-less eliminate any beneficial end-plating effect that the hull has on the board, so the board's induced drag would be higher.
The situation is analogous to the aircraft design problem of determining the optimum incidence for mounting the wing on the fuselage. This requires CFD calculations or wind-tunnel testing to determine, and invariably finds that it is best for the fuselage to carry some of the loading.
Sorry, I was being a bit sloppy with that part because the physical situation is a little unrealistic. (That's why I added the "more-or-less"). Maybe a better way to look at it would be that having the hull carry some of the load gives the board a larger effective span & therefore smaller induced drag.Getting to the italized bit.... I don't quite get this. Why would there be zero load, or zero dP, on the gybing board at the hull or root? It's taking an angle to the centerline of the hull, so if the flow is perfectly aligned with the hull, there is still the same angle of attack for the board and the same flow/dP regime.