Efficiency of Exhaust-Over-Hub Propeller?
 

Hello all, A question for you. I bought a used boat (30 foot with twin
200hp Merc Outboards)and the previous owner had put propellers on it
that are solid hub and which exhaust over the hub. It slips a lot
coming up onto plane, but once up, performs fine and tolerates a lot of
trim without without cavitating. My primary priority however is fuel
efficiency and my question is: does anyone know the physics of this
set-up and whether or not I would be better off with the typical
thru-hub props. The ones I have have a lot of rake which would have the
advantage of reducing hull friction thru greater trim, but I might get
that as well will the thru-hubs. Any knowledgeable advice would be
greatly appreciated. Thanks.