Submerged transom drag

Among the threads I ran across a post containing the comment that "transom immersion can double the drag at displacement speeds...." -- Assuming this is true, I would like some explaination. Is this due to reduced pressure (toward bow) from turbulance of flow coming off the hull surface which ends at the transom?? Is there any way to get around this problem without extending hull to full plan-view point at rear or designing so full transom is above waterline??

 

Those over-encompassing claims are usually hard to prove. The submerged transom creates drag, but it will depend on what you compare that mode of operation to. For example, a 25 deep Vee hull at 3Kt will be creating a lot of turbulence and drag. The same hull at 20Kt will have a clean exit and less drag. If you were to modify the hull to have the transom above the waterline at 3Kt, it is very unlikely it will ever achieve any speeds above 7 or 8 Kt.

 

I agree with Gonzo's speed effect -- a speedboat transom is submerged at low speed but exposed at high speed. How does one calculate the drag produced at low speed?? and is there any way to help the situation at low speed?? I realize there are all kinds of movable flaps to help control flow at high speed but are there any design techniques to help reduce submerged transom drag at low speed?? I would think rounded (spherical) transom or short point would not help and might even make drag worse??

 

I have noticed some sailing designs with the transom submerged. These tend to have shallow buttocks lines. It always seamed counter intuitive to me. but these tend to be fast boats with some planing ability.

 

The transom stern reduces drag only above a certain speed, as the separation of the flow from the hull at the stern makes it appear w.r.t. the flow that a longer hull is traveling thru the water. At low speeds, the transom stern loses this effect, and produces a net increase in drag over say, a cruiser stern.