We spotted two catamarans in the harbour at Ravenglass. I don't know a lot about sailing but I do use these boats as an example of stability. The double hull increases the base area. A boat will capsize when the line of its weight acting vertically downwards goes down beyond the edge of the base area. The wider the base area, the harder it is for this to happen. We talk about there being a resultant moment - an overall turning force - acting. If the boat tips, so long as the weight continues to act within the base area, this resultant moment will pull the boat back to its original position. Single hull boats have a keel under the water to produce a moment that opposes the turning force of the wind acting on the sail. I did look on Wikipedia and found that the lack of keel on the catamarans means that there is less drag from the water. Catamarans can go faster. They are not likely to tip over sideways, for the reasons given above, but a very strong wind can tip them over forwards - they sort of go head-over-heels.
