"Snap" Weather Helm

Hello Lester

I read with great interest the technical content of your website.
Lately my attention was attracted by what you called “snap” weather helm and some explanation you tried to give for it: Hull characteristic, rig tuning …

Although I’m don’t pretend to be an expert (my interest in IOM is only a few months old), I d’ like to submit another explanation for this “snap WH”: Fin stall.
As you know most top yacht builders are now using rather thin fin and rudder profile (6% t/c). As you stated in various articles thinner the fin, lower the drag , but sooner the stall…
Can we imagine that a sudden puff could increase the boat lee angle past the 5-6 degrees , making the fin stall, suddenly increasing heel angle and weather helm as a result.
what do you think ? Am I totaly off ?


P.S pardon my bad english, which is not my mother language

I thin k that snap weather helm (SWH) is a rather complex set of thing happening at once:

1.- A puff hits
2.- Apparent wind moves back, relative to the boat.
3.- As the boat was on a beat, sails are trimmed hard, mainsail usually trimmed closer amidships than the jib.
4.- Heel increases violently
5.- Mainsail, being trimmed harder and receiving the wind now from a more abreast direction, is not able to “release” enough wind and becomes more overpowered than the jib (which is more open than the main sail), so more weather helm is needed (and skipper is not able to react quick enough, all this happening within a second)
6.- As more heel is induced, the rudder stalls
7.- Without the rudder effectively balancing the boat, she pivots on the fin (which I dont think reaches stalling) and tacks violently…

Why some boats do not produce SWH? Dunno, maybe better sail trim, longer, more effective rudders, heavier bulbs, more “balanced” (rigs and hulls, that is)… Don’t have an answer on that…

Makes sense?