|Double reefed, 15 - 25 mph|
|No reefs, drifting along|
I don't want to spend a whole weekend sewing just to find out the boat won't point so I've done some searching on the web, some asking around (the watertribe forum was very helpful) and some calculating.
Will a third reef result in too little sail?
Adding a third reef will reduce sail from 96 sq ft to 31.5 sq ft. Is that enough sail to make headway in 30 mph winds? The force exerted by a sail is proportional to the square of the wind velocity. Jim Michalak has given a simple formula:
Max sail pressure (lbs/sq ft) = .0034 * V(kts)2 * C
|Force on sail (lbs) for different sail areas (sqf)|
and wind conditions (mph).
Another possible issue is related to force of the wind on the hull. As you reef down the sail, the force of the wind on the hull has a greater effect on the total force.
|Wind force on exposed hull (frontal area) as wind increases.|
Will there be too much lee helm?
I am also concerned about lee helm, an unsafe situation in high winds. Without any reefs, the centroid of the sail is even with the trailing edge of the leeboard. With two reefs in, the centroid is 13" farther forward (see picture below). I would have expected to experience lee helm with the second reef in, but instead I get significant weather helm, especially during gusts above 25 mph. I have no idea why this happens, but I can think of a couple of possibilities:
- If you heel her over to port she wants to round up to starboard - just like steering a surf board. Perhaps the turning forces due to the hull are a lot more powerful than the turning forces due to the sail center of effort..
- As you heel the boat, the center of effort moves to lee because the mast is angled over the water and the center of resistance (leeboard) moves to windward. Perhaps the geometry of this situation lessens the imbalances in force (there is still a tendancy toward lee helm, but the force vectors are smaller).
- An imbalanced rudder can make the problem more pronounced if the center of resistance of the rudder blade is too far behind the rudder pivot (thanks to SOS for that suggestion)
Jim Michalak's articles on sail math: