Gybing Plates

Yeah, pretty much.

The board must create lift to resist the push from the sails. In order to create lift it must be at an angle to the water.

So think about the pointing too high thing.

Point too high and you still have loads of side load from the sails, so you still need a lot of lift from the board to balance it. However you have less forward drive from the sails, so you go slower. As you're going slower lift from the board reduces, so to get the same amount of lift you need more angle between board and water, so when the boat is in equilibrium its crabbing sideways more and creating loads of drag.

Now suppose you have an an adjustable gybing centreboard. Lets say the boat is making a course good of 45 degrees, and has 8 degrees of leeway, or angle of attack, with the board on the centreline. So the bow is going to be pointing 37 degrees, and say, for arguments sake, the sails are at 5 degrees to the centreline, so pointing 32 degrees.

Now dial on 8 degrees on the board. The boat is still making good 45 degrees, but the bow is now pointing 45 degrees too, so no leeway, and to keep the telltales right you've sheeted out that same 8 degrees, so the sails are at 13 degrees to the centreline. But the essential relationship between sails at 32 degrees and foils at 37 degrees is unchanged.

Hey, I've just thought of a demon tweak for a Moth. Give it an adjustable gybing daggerboard and the sailor could align the hull with the apparent wind in order to minimise aero drag!

Anyway, to summarise, like a lot of things on boats, leeway and weather helm have to be like little Bear's porridge, just right, and too much and too little are both faults.

Edited by JimC - 27 Jan 16 at 4:22pm

For a gybing board, the board is the same angle to the blue lines as above, but it's the hull that is rotated ( anticlockwise here )  by the leeway angle, so the blue lines ( track ) line up with the fore / aft lines of the hull.
Which can mean less drag off the hull.
So you make better height to windward because you're not slipping sideways as much, or you can sail a bit faster until the drag catches up again.

I've made boards for 5o5s, with gybe angle of 2degrees, with angles of up to 6 degrees for Hornets that have thick, short-chord small foils.

Hornets have buttons in the case, so the board locks straight when lifted a little, 5o5s use bumps on the board that lock the board sideways in the case when lifted a bit. A little forward rake to gybe the board, vertical and raked to lock.

Gybing boards work best in flat water, in waves you don't get the drag reduction as the water is moving about, so boats lock the board for wavy conditions

Gybing the board turns the hull parallel to its true track through the water, removing large drag from hull to smaller drag off gybing board, is that correct ?

Fuller rule number 1. Never trust anything prepared by an advocate of Cat sailing.

That diagram without context is convoluted.

I need something simpler and more relevant to displacement speeds, those true/apparent wind angles are off any scale we'll likely experience.

oh and 2547 not much tidal lee bow effect going on at our lake unsurprisingly.

Edited by iGRF - 27 Jan 16 at 8:21pm