dynamic effects of heeling
"So heeling moves CLR forward (by making the rudder less effective) and creates more weather helm." I'm fully in agreement that heeling shifts and makes dynamic the CLR and wouldn't argue with the above statement... but would suggest there may be other forces invovled, possibly much more influential than the loss of rudder force. My reason is simple, that almost all sailboats suffer increased weather helm from heeling whether it be the rudder issue outlined by David or the couple mentioned by Dave but some boats suffer much greater heeling induced forces causing weather helm than others. These hulls are now becoming predictable so that "lead" ratios are greatly increased for them in an effort to tame their behavior. However, not much is said regarding the why. Do these hulls experience some force that others don't? I for example have a boat that will experience adverse weather helm under jib alone if she is heeled excessively while there are certainly lots of boats that will suffer lee helm if pressed under jib alone. Why the difference? One theory could be hull lift forces caused by the foil of the hull footprint. A narrow beam hull with reasonable rocker when heeled may not see a significant hull form footprint change because the reasonable leeward hull curvature now immersed is offset by the generous curve of the boat bottom. However, on a wide beam hull with minimum rocker, the heeled hull yields a generous leeward immersed curvature and a much less curvature to windward, a significant asymmetrical form variance yielding a laminar flow variance with a longer path to leeward. If this form variance produces a lifting force, it will be to leeward and have a center of lift and if offset from the CLR will have a dynamic effect of shifting the CLR as its force is added to/subtracted from the CLR forces as CLR is the sum of all hull forces. If the center of the leeward lifting force were aft of the CLR, it would shift the CLR forward and reduce the "lead" and throw the boat out of balance. Interestingly, there is a model that seems to verify this theory. Most boats when trimming the stern down, cause a CLR move aft that reduces weather helm by lengthening the "lead". The Catalina 250 however benefits from trimming the bow down. The why seems to me to be that when trimming the bow down, the center of the asymmetrical form lift is trimmed forward closer to the normal CLR and thus doesn't impose as much forward shift of the CLR and therefore less "lead" reduction effect. Thought along this vane can also explain the great amount of leeway the wider/flatter hull form suffers if it is pressed over too far, that the hull form is lifting to leeward, opposing the windward lift generated by the keel and rudder. Inerestingly, there are some mono hull forms now designed that produce windward lift when heeled. They are forms with generous rocker and wide aft sections with fairly straight lines from the entry to the stern. When heeled, the windward section becomes longer than the leeward and thus low pressure is generated to windward rather than leeward as typcial to traditional mono hull forms. George... in conclusion, it means don't heel too much on a windward course. Check the VMG, it might tell that an upright 4.5 knots yields a better VMG than a 25 degree 5.5 knot, at least it does on my boat.
