12v71 Natural

Dear sir I am interested in buying a 60' motor yacht built of steel. This yacht was built in 1970 by a crew boat builder using a crew boat hull. The boat has a pair of 12-71 natural's I am nthinking the boat will use about 30 gal per haour and go 15 kts.I write to ask what type of engine one might do a repower with and what the H.P. fuel consumption would be of the suggested replacement. I believe a 12 71 is worth roughly 400 H.P with scads of torque. This yacht also has 3 blade props, I am thinkng of going with high tech 4-5 blade props, also interested in your thoughts. thanks

I would rebuild the engines you have. Also with that much power you should be using at least four bladed props. The trick to great performance is mostly prop match to the engine horse power. The rule of thumb is take a reading of the engines no load RPM and subtract the full load rpm and there should be no more than a 150 RPM drop. This should be done with the engines in good tune, deep water on a calm day in opposite directions. Give your reading to a good prop shop and tell him the rpm you need to adjust to. I have seen boats gain 3-5 knots with far better fuel economy.

Thanks for your thoughts, the boat comes with a big set of 4 blade, maybe I will try them. Second question have you ever heard of running a 12 cyl. detroit 71 series on 6 cyl...is it possible. How do you release compression and shut off fuel to half the engine. does it make weird noises and damge engine??? I can't beleive it would be possible, I do know the MTU yachts I have run have a switch to shut down half the engine for manuvering. Appreciate your imput.

If you like I'd be glad to talk to you directly 978-265-7879. I'm a 43 year vet. of Detroit Diesel engines Infielddiesel.com