This past weekend I took the Sloop-a-palooza class at the Center for Wooden Boats.
It was a beautiful day. Warm (60 deg F) and sunny. We sat on the docks and talked about sail shape & hull hydrodynamics.
The big lesson for me was about overtrimming sails. Typically when you talk about Bernoulli & airfoils, lift is described as a single vector off the airfoil. However, that vector is the composite of the forces of the air on both sides of the airfoil, across its entire surface. The forces are perpendicular to the tangent of the sail surface at each point. So, the part of the sail near the luff produces lift forwards, while the middle of the sail produces lift more athwartships. If you overtrim the sail, bringing the leech around, then the sail surface will produce lift to stern, slowing the boat.
This is why, when trimming a sail, you slack the sheet until it luffs and then harden just enough to stop the luffing. In this shape, the maximum lift will be forwards.
(Sailing physics combines many forces in complex ways. My understanding & explanations will always be oversimplified.)
We then hopped on some small sloops (3 Blanchard Jr. Knockabouts and 2 Falcons) and cast off. There was next-to-no wind. We ghosted around the lake for a while and experimented with weight placement and sail trim in light airs. I spent most of the time lying down on the foreward part of the deck, to balance the weight of the helmsman and other crewman in the cockpit.
After an hour, a breeze picked up and we started sailing.
The course instructor had suggested a blindfolding exercise. One person takes the helm & mainsheet, blindfolded. Another takes the jibsheets, also blindfolded. The 3rd gets to look, but not touch. The 3rd gets to give instructions (“tiller hard to port” “watch out for that float plane!”) and describe the sail shape (“the jib is luffing slightly”). The sail shape instructions go away after a while, as the blindfolded folks get a better feel for the boat.
It was a great exercise. I was totally surprised to find that the job of the 3rd (with vision) was a lot of work! I had to pay attention to everything and describe it in detail. It was good practice for the jargon, too (“tiller to port. No, my port. Err, starboard.”)
In the afternoon, I got to take out the Yankee one-design Venture (See the bottom of the CWB Links page). Sarah + me + 2 other students. Airs were light again for most of the time. I again climbed up on the deck, this time carrying the large Genoa around the mast with each tack. It’s a beautiful boat, and she’s put a lot of work into it.
I was surprised how complex the rigging on the Venture is, after my experience on the simple Blanchard Jr. and the Petrel. Just figuring out which line was the Genoa’s halyard took a few minutes. I hope to get checked out on one of the larger sloops soon, so I can take friends & family out in relative comfort for a tour of Lake Union from the other side.