Rig tune and snap weather helm

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There is a phenomenon called "snap" weather helm.  This happens when an otherwise well-behaved boat luffs violently into wind when a puff hits.  I began thinking that snap weather helm was a characteristic of hull shape, but realised I might be wrong when two different hull shapes snapped back at me, and I observed other hull shapes snapping as well, so I spent a little time investigating.  It now seems to me that the reason for snap weather helm is inadequate rig tune.

My thinking was changed by the wind tunnel tests which showed that, far from twisting off as wind speed increased, the jib of my IOM actually showed reduced twist with the rigging tensions I was using.

I routinely tension my rigging so that you can strum it.  Such tensioned rigging doesn't sag or give way when a puff hits, and if you are not ready, your sails develop more drive than the hull can handle.  Generally, the mainsail does give way in a puff, almost no matter what the tensions, and the head of the main twists off, due pretty much to simple stretch of the sail fabric.  But the jib behaves differently, and it'll only twist off significantly if the topping lift releases, and releases early.

The topping lift release depends upon two things, the jibstay tension and the amount of the jibstay tension fed into the topping lift by the pivot offset.  I managed to cure my snap weather helm by reducing my pivot offset, deliberately allowing the topping lift to begin releasing well before reaching the top of the rig's wind speed.  (Remember to have some way of restraining your released topping lift!  It spoils your race when it hangs up on a spreader...)


In the first draft of this page, I said that my understanding is that it is the jib which primarily holds the boat pointing to weather.  Ease the jib, and you may well increase speed and decrease heeling, but you'll not be able to point.  If a puff hits and the jib does not twist off, I said that the effect is to hold the boat so strongly to weather that she luffs up violently -- that is, she shows "snap" weather helm.

Will Gorgen and Scott Vernon have e-mailed to say they don't think the jib works this way.

The jib pulled in tighter does not cause the boat to steer up into the wind. In fact, as you trim the jib in tighter (leaving the main where it is), the boat will head away from the wind due to the increased power from the jib which is forward of the centre of resistance.

Looks like my understanding about the jib is quite wrong!  I'll have to scratch my head some more to explain what is going on...


Some e-mail correspondence has helped me understand "weather helm" in general, and what I'm trying to understand here is the "snap" part of the weather helm.  Let's take a nicely heeled boat beating to weather, and imagine her to be perfectly balanced.  We have a "lift" couple that tends (very strongly!) to make her round up. This lift couple is the result of the heeled boat's sails centre of effort being offset and to one side of the hull's centre of effort.  But the boat is sailing perfectly balanced -- so what is opposing this (very strong!) couple?  Something of exactly the right magnitude and direction which nullifies it.

In a perfectly well-behaved boat, as the boat heels and as the lift couple develops, there is this opposing couple that perfectly matches it and nullifies it. Now I understand that there are very few perfect boats (smile), and so in general a boat tends to develop weather helm. But the amount of weather helm usually developed is modest and manageable and develops gradually with heel, and my understanding is that we are seeing the slight discrepancy between two rather large couples which do not quite match. For example, if the lift couple was 10 N.m at 30 degrees heel, the opposing couple might be 9.9 N.m, and it is the rather small difference of 0.1 N.m that gives us the boat's moderate weather helm.

But some boats give "snap" weather helm. For some reason, the difference between the lift and the opposing couples of 0.1 N.m at 30 degrees leaps to 1 N.m (say) at 40 degrees with the puff. Why? What is it about the hull or the rig or whatever that allows the increase in the lift couple to so suddenly outpace the matching increase in the counteracting couple that normally so effectively (almost) nullifies it?

2005-12-18


2011 Lester Gilbert