location of prop on Inboard

[k2panman]

Does anyone know about or where to find info on location of a propeller on an inboard boat? I have a 34' silverton converting to twin diesels. The prop shafts and struts are bad and need to be chaged, the rudders are broken also. The boat was originally designed with gas engines and 18 x 18" props I believe. My questions are: what distance should the props be from the bottom of the boat? What distance from the rudders?

[woodboat]

I am not a boat designer. With that said every inboard I have every owned you can get the prop off with the rudder on but just barely. It takes some effort and spinning moves Also my rudder is slightly offset to facilitate this. The props need at least 1" clearance from the bottom 2" is better. Since it had inboards why not put the parts back in the same location?

[Eric W. Sponberg]

K2,

You first have to determine what the new propeller diameter will be. With the change to diesels, the power and drive line might be different. You should also determine the size of the new rudders. Once you know the prop diameter and the rudder size, the top of the prop should be at least 15% to 20% of the prop diameter below the hull. The leading edge of the rudder at the top forward corner should be at least 20% of the prop diameter behind the top of the prop. The back edge of the rudder should be at least 10% of the prop diameter forward of the bottom corner of the transom. Also, it is preferred that the propellers are opposite turning, when viewed from astern looking forward, with the starboard propeller turning clockwise, and the port propeller turning counter-clockwise. Finally, the rudders should not be in the same line as the prop shaft centerline. The rudders should be placed outboard of their respective propeller shaft centerlines by 10% of the propeller diameter.

Eric

[k2panman]

Thanks for the help! The boat is a project boat that someone else started. The running gear that was there is all wrong - the props were 5" behind the cutless bearing, the struts were two different sizes, the engine oil pans were resting on the bottom of the bilge, etc. So I'm trying to get things in where they should be.

Does anyone know of a website with information on prop placement? I visited a website a while back and lost the address that had a lot of information on props, with a lot of the information that Eric posted. Thanks again for the information!

[Eric W. Sponberg]

K2,

I don't know of a website for you, but in Professional Boatbuilder magazine #78, Aug/Sep 2002, there is a very fine article by Don Blount (one of the foremost powerboat designers in the world) called "Rudder Design for High-Performance Boats", page 72. Therein lies a wealth of information, everything you need to know about rudders, their design, and their placement in relation to the propeller. You might be able to get a copy from Professional Boatbuilder at www.proboat.com.

Eric

[kapnD]

K-2, You should read David Gerr's "Propeller Handbook". Also perhaps "Inboard Motor Installations". Its an old book, cant remember the author's name, but the information you need is all there in a very understandable style. You'll find some useful articles on Boatdiesel.com too. Good luck, Don

[cyclops]

Eric. The offsetting of the rudder helps in pulling of a shaft. Does it also reduce the blade WHOMP as it pass the rudder ?

[k2panman]

Thanks for the tips on the books, KapnD - I've ordered both of them from Amazon.com - the Inboard Motor Installations is out of print (last printing about 1978) but a used copy was available. I'll post info on what I found and did in case anyone is interested.