Removing sail logo
If the sail is dacron and the sail logo is actually stenciled on with ink or paint, I don't think you will have much success removing it without damaging the sail fabric. You can try using a fabric paint to cover up the old logo and then apply a new logo on top. The new logo can be made of an adhesive backed colored material specifically made for applying sail numbers and logos. If the previous logo was adhesive backed fabric, after it is removed, it will leave a slightly sticky residue on the sail. If your sail is white dacron, you can sprinkle the sticky area with baby power. The powder will stick to the sail and will prevent the dark shadow which will eventually form from the accumulation of dirt.