Posted by Brian Guptil (sailordude@…>)
Gentlemen
The reason sailboats, and powerboats as well, sail-about at anchor is the center of lateral effect of the hull, deck, masts and, well everything sticking it the airstreams is forward of the center of lateral effect of the hull, keel and rudder. This is desirable if you want the boat to “round up” when you release the helm, (a very important safety feature). In aircraft, they call this feature, “inherent stability”, If you trim the airplane to fly level, reduce engine speed, the aircraft will pitch down, seeking the SPEED. at which it was trimmed. If you advance the throttle, the nose will pitch up, again, to try and achieve the speed for which the aircraft was trimmed. Same is true on a boat, when you trim the boat for balance, and lock the rudder in place. The boat will maintain that balance until the wind speed changes, at which time, the boat will turn into or away from the wind, (slowly at first) to again achieve the same boat speed. As long as it can achieve that speed, it will sail on.
On a cat ketch, it is easy to see that the windage is well forward. The main is almost at the bow, forward of the keel. Where the mizzen has little effect on windage since it is very near the center of lateral resistance in water. You can achieve “inherent stability” by placing the center of the windage AFT of the pivot point. by Anchoring by the stern. Or fly a stabilizing sail well aft, or on ketches fly a small mizzens set the sail, sheeting to the centerline.
The test is to anchoring by the stern, in winds high enough where the boat would be expected to swing, if anchored “bow too.”
I have used this technique in an extreme, by anchoring in (by the stern) 180 feet of water so as to act as one end of a starting line. If your boat makes up one end of the start line, you can’t be wondering about, in the wind. IN the two days I did this, the wind typically increased to 20 on the afternoon and the boat was stable on the line.
Brian -