Weather Helm Demystified

[Raxarax]

Whoah whoah whoooooah, his CE thing is completely wrong. CLR is fine, but the force of the sails is transferred through the mast. The mast is actually where the boat feels the force, and that is well ahead of the CLR. So, even if he had calculated the centroid of the sail areas, the area where all the forces are equal distance from the edges, that means nothing, because all that force is being transferred through the mast.

[dressmeister]

@Raxarax I think there is significant force transmitted via the mainsheet.

[Raxarax]

@dressmeister OK, it doesn't look like you understood what I said. Here, the sails do NOT push the boat, the mast pushes the boat, the sails push the mast. Therefore, the boat only feels the force generated by the sails AT the mast.

If you're talking about torque, then it might make sense, as a mainsheet with a centroid further sternward would generate more torque and twist the boat, but comparing the centroid of the hull to the centroid of the mainsheet effectively does nothing without torque

[dressmeister]

@Raxarax "the boat only feels the force generated by the sails AT the mast?" So where does the force in the mainsheet come from?

[sushimoustache]

My bad! I do hope I can pay better attention when I'm driving my boat to the windward mark. You covered the helm generating force of the hull shape quite well - but since you left open the possibility that I might yet have something to add: one thing that occurs to me is that because when a boat is heeled the sail plan is no longer centered over the boat hull's cumulative resistance for going through the water - irrespective of the hull shape - that also might be a contributing factor.

[dressmeister]

@sushimoustache Don't forget the mainsheet! Last time I got my finger stuck in the main block, I experienced quite a bit of force. The mast, stays and mainsheet will add up to transfer the force from the sail directly under the CE.

[sushimoustache]

@dressmeister ... but upon re-reading your comment, it almost seems that you intended to reply to "Raxarax." In any case, you can rest assured I will never resort to written hooting and hollering to make a point. :)

[dressmeister]

@sushimoustache Don't forget the mainsheet. Quite a bit of force coming from that little rope.

[sushimoustache]

@dressmeister I'm not forgetting. The weather helm from - say - flying only a mainsail on a sloop is significant and affects the boat even when at a standstill.

On the other hand, When I fly only a small jib on my sloop I get plenty of lee helm... until I start moving. When going to weather in 20kts of wind - whitecaps all around on S.F. Bay - I get *weather* *helm* from flying my 90 percent jib by itself. My tiller is a bit up and the boat will round up if I let go. About 20 degrees heel.

[sushimoustache]

I read an account of how the old-time yacht designers would make paper cut-outs of both the intended sail plan and the hull profile and balance them on the end of a pencil to determine the CLR and CE.

This video accurately covers the weather/lee helm from the perspective of the CE and CLR forces, but there is also a weather helm-generating force which comes from the boat hull moving through the water as it is heeled over. (IMNSHO)

[dressmeister]

@sushimoustache There is absolutely a hull-trim aspect to weather helm. I always address this with the "imagine you are a fish looking up at the bottom of the hull" approach. I thought it is in this clip, perhaps it isn't. Good point though.

[dressmeister]

@sushimoustache Check the part right around 3:58 or so. I think I address weather helm pretty well. However, if you have anything to add.. PLEASE let me know. I'm always looking for better explanations.