Larger Boats Are Faster
I think one of the objectives is to have the boats arrive within a day or so of each other (or several days). A party and awards ceremony takes place and at time the boats under 40 feet have had difficulty arriving in time, which makes it more difficult to manage the fleet. Also, if the smaller boats can't stay within a reasonable distance of the fleet, they'll be out of range if they need assistance. I think those are several points that factor into why the fleet is kept at or above 40' in length; it's speed dependent.
If you donít know where you are going,
Any wind will get you there