I think Opher's comment was the key. No fixed start, no start line. Just tell everybody to gather at point A, leave at about X o'clock and dinner will be ready at point B when they get there. Sure some people will push to stay ahead of their buddies, but few will want to stray too far away from the pack when there is no incentive to do so.

One idea I had was a kite procession. Everybody who enters is given a cheep kite that they must fly while sailing. The course would be along a public beach or something to make it interesting for bystanders, and the boats would be expected to stay in a line ahead formation. I think it would be fun and generate a lot of interest among non-sailers. A recruiting tool.