Ahoy, Snorkey. I didn''t see a reply to your question about broaching and the centerboards
contribution to it so I''ll comment.
First, recall that when a boat heels, it sails on a different underwater shape and this gives it a turning moment into the wind (producing weather helm).
Second, just as the centerboard resists side slip, the rudder resists any tendency to
push the stern sideways. Thus, if the boat heels dramatically, the rudder comes partially out of the water or is much shallower and more horizontal to the water.
Third, imagine you are pushing your boat with your hand on the back of the mast. It is
on its feet so the push is down the centerline, thus the boat goes straight. Now,
heel your boat and push with your hand on the same spot on the mast. Because of the heel, your hand is now outside the boat so that when you push there in not only a forward moment, but a turning moment as well.
Broaching is when the boat turns violently,
uncontrollably into the wind. It happens when
these three elements come together. That is,
excessive heeling, changed underwater shape, and no resistance to turning from the rudder coupled with the sail''s drive being outside the boat.Basically, the boat just spins around on its lee side.
A broach can happen on any point of sail,
but it usually happens on a reach or run. The main causes are rolling, having a mainsail area that is too large relative to the foresail or spinnaker, heeling under spinnaker, and following seas striking the boat on the quarter.
If you have too much sail up, roll largely, moving the push well outside the boat, getting over on the rounded portion of the hull, with the rudder coming out of the water, the windvane effect will cause her to spin around on her lee side.
If the main is too large relative to the spinnaker or foresail, it will aid and abet the broaching tendency.
Because the spinnaker is so large and powerful, and because it rides so high on the
mast, if you heel well over, there is a large and powerful turning moment out on the end of the mast well outboard. This happens in racing quite often when they are close reaching with spinnaker.
Finally, if you have large following seas and are flying a lot of canvas you can have a
problem. If you are boiling along, rolling your scuppers out, and you take a large swell
on your quarter, it can lift and push your stern, adding to and aiding the turning tendency that you already have from heeling.
This happens a lot in dinghys that can''t really reef and are running with large mains
in too much wind.
Finally, the solution is to keep the boat on her feet. You have less turning moment, and more rudder in the water. If you get hit by a broach, get all the weight you can outboard, ease sheets, and bear away a little. If you are fighting a tendency to broach, not just an isolated incident, reduce
The only contribution that the centerboard
makes to broaching is that the boat will trip
over it. The boat has strong forward momentum, then suddenly spins around broadside to the wind, tripping over the centerboard. A little centerboard to help tracking, but no more. About 1/3 is right.
I recommend my comments under General, to
Bob Ohler, regarding mast rake, for additional info. relative to windvaning.
I hope this helps.