Pete emailed me to explain the previous conundrum about the ropes round the boom that don't raise the sail. Apparently they are called "lazy-jacks" and mean that if one person has to lower the sail in a hurry, it falls down a channel and not all over the deck. He writes: "
These rope don't, indeed shouldn't, be taking any pressure when the boat is sailing. It would spoil the wing-shape of the sails and affect their performance and the rope breaking strain is usually too low for heavy work. Usually, the lazy-jacks will be loosened after hoisting the sail to ensure they don't interfere with the sail. Indeed, on some boats I've crewed on that have them, they are carried forward and lashed to the mast to ensure they have no affect on surface airflow (they have enough drag effect to affect performance of racing boats). When the sail is to be dropped, the lazy-jacks will be tightened up again before loosing the halliard holding the sail up." This made me realise that there is a lot about sailing that I don't understand. I occurred to me to ask why a sail needs to billow outwards and why it can't just be a solid board. One answer to that question can be found on
http://physicsbuzz.physicscentral.com/2015/05/the-physics-of-sailing-how-does.html where the sail shape is deflecting the wind and using the reaction force to push the boat forwards. I'm definitely going to use this idea with my classes.
