Join Date: Oct 2009
Location: Gloucester, MA
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You have to treat the boat as a system so the sail plan is part of it and the hull form and steering are another part. In moderate winds, there is nothing wrong with sailing under genoa alone. In fact, it is advisable in some scenarios where it will balance your helm better(for example running before the wind on many boats). Sometime when you have a nice steady wind around 10 knots or so, try broad reaching with just the genoa up and then try it with just the main up. With just the main up, the boat will try to round up and you will have to work hard at steering (which stresses your steering gear if taken to the extreme) whereas with the genoa up, it will likely be quite easy to steer.
The reason to not sail under genoa alone is related to stress in high winds. By concentrating all of your load on one sail, it will put greater forces on the sail fabric, sheets, halyards, blocks, winches, etc. If you put up the same sail area split between two sails, the forces would be much less. Modern equipment can take a lot of force so you don't need to worry about this until the wind really pipes up. As for the old salt telling you that you will lose your rig, that isn't necessarily the case. The force from the jib is transmitted to the forestay and sheets (your halyard will be stressed some too but it does not provide a driving or heeling force). The force that this stay applies to the mast is by definition in line with the stay because it is not a rigid body. Since this stay deflects very little, the force of the sail can greatly multiply the force transmitted to the fitting on the mast. Since the top of the most is stayed, this force is turned into compression of the mast and more tension in the backstay. It would take a tremendous force to cause the mast to buckle. On most boats, you would have to be carrying a tremendous amount of sail in really windy conditions to have to worry about this at all. However, you do have to worry about shock loading. If you accidentally gybe, you could well bring down your rig but you probably are going to anyways if you main is up. The main does provide a steady (hopefully) force that will keep the mast from pumping for and aft but most masts have large enough sections that this is not an issue.