Originally Posted by northoceanbeach
I was cruising downwind a little today and left only the genoa up. If this fine practice?
practice? No, but most of us do it. A sloop is designed to perform best with a mainsail and a jib, but sometimes you're sailing downwind for a long ways, and you're tired or lazy, and don't want to raise both sails. You can use either
sail alone downwind. Heck, you don't even need a sail to go downwind. A boat with no sails will drift downwind, as will a leaf on a pond.
As others have said, a boat sails best if there is a balance of sail pressure fore and aft of the mast. If you'll experiment with it, you'll find that you can hardly sail to windward at all with a small jib, but you can get by fairly well with a big overlapping jib, like a 150% genoa. The reason is because with the small jib, all the sail pressure is forward of the mast, and it pulls the boat's bow off to leeward with such force that the keel can't resist it. A big genoa overlaps the mast, so that some of it's surface is aft of the mast, and that helps balance the forces on the sail and keel just enough so that the boat can still sail fairly well to windward. The problem with a big sail is that, when you tack, it's much more difficult to tack in very light air than a smaller, non-overlapping sail, because it tends to snag on the rigging as the boat swings across the eye of the wind. So, if you want to sail with only a jib, and you want to be able to work to windward a little, you should use an overlapping jib. Everything I've said assumes that you're sailing in moderate winds. When the wind is blowing hard, if you need to work to windward at all, you want two sails up, because you need the boat to perform its best in those conditions. The only time you should sail in high winds with only a jib is when you will be sailing downwind exclusively.
SOmeone told me not to do it upwind because it puts bad pressure on the mast. I'm not sure if he knows what he's talking about or not.
I've heard that, and think it's plain nonsense. A properly supported, well-tuned mast is held erect and in column by it's supporting stays and shrouds, and, unless you're carrying way too much sail area for the windstrength, it shouldn't begin to overstress your rig.
If your rig is not reasonably well-tuned, then the answer is not to avoid doing things that might stress it. The answer is to tune it.