boat tail shape

[InetRoadkill]

I'm wondering about the advantages and disadvantages of different boat stern shapes. Particularly, notched sterns (transoms) vs. fair sterns on a sailboat.

I also have a question regarding the two different styles of faired sterns. One type is faired to a point and curves up out of the water coming together usually about a foot out of the water. The other other kind fairs to a point very close to the water line -- maybe only an inch or two out of the water. The one closer to the water would seem to offer a slight speed advantage since it presents the longest hull length to the waterline. The disadvantage I imagine would be that the stern wave may poop it. Is it simply a matter of styling or is there more going here?

[PAR]

Much depends on what the hull is designed to do. On a displacement hull, you don't want the hull to drag it's transom in the water, which creates a lot of drag, so it's kept clear. As these hulls heel, you gain an increase in WL length, which can marginally increase speed, unless the overhang is substantial. These areas out of the water can also provide "bearing" area when the hull is pressed under sail, acting like reserve displacement after a certain amount of emersion.

On hull forms designed to plane, you need a clean, crisp edge so the water flow can "release" from the boat. How one accomplishes this can be handled a few different ways, but most place the transom edge close to the LWL or just at it. As the hull squats attempting to "climb the hump" the flow will separate (if the S/L is high enough) from this sharp edge, where it can trail behind forming up the now departed stern wave system.