View Full Version : Prop size for a plastic boat

12-11-2003, 02:57 PM
KATY my 32' 1967 full keeled yawl sailed like it was on rails. The motor and drive train functioned better than I had expected. The year of work had paid off. The second time out in winds brisk enough to send spray blistering into my face the 35 year old sails gave up. New sails will be here next week. Problem ...... I gotta pay for the darn things!
I own an O'day 23' that I need to put on the market. The current prop just isn't cutting it!
23', LWL 19' 6", Beam 7' 11", 3085#, 10 HP inboard diesel, 1 -1.62 gear box. The prop that the machine shop fixed me up with is a 2 blade 12" and it for sure is too much prop. Help please.
Gerald Niffenegger

Bob Cleek
12-11-2003, 07:05 PM
Too much prop for the engine or too much drag on the boat, or what? Prop selection is one of those things best left to experts. Check the manufacturer's specs for your engine. They will likely give you recommended pitch and diameter specs. Beyond that, try to find a propeller specialty shop to advise you. You might also connect through the class owner's association with others who have your boat and find out what is working for them.

12-11-2003, 10:47 PM
Thanks Bob. There is too much prop for the boat. O'day 23's with inboards would be few and far between. They came from the factory with a transom mount. I installed the inboard and used it for a short time before a fiberglass specialist looking for work conveniently cut my boat loose and sank it. I can get the boat to hull speed but punching into heavy seas it plugs along and an increase in throttle only increases the amount of black smoke. I am sure the prop expert, if there were one around this area, would start with a computer calculation and then fine tune from there.

12-11-2003, 11:50 PM
Check with these folks Michigan Wheel Propellers (http://www.miwheel.com/) .

Click on "Prop-it-Right" on the top right - you can fill in all the specs and they'll e-mail you back with just what you need. Or keep interacting with you until you both figure it out.

Super nice folks.

Bob Cleek
12-12-2003, 06:27 PM
Check with somebody around you who knows a lot about marine engines and have them look at it doing what you describe. I am familiar with O'Days and realize few have inboard power. You probably have more weight in her ass end than you need, but that's not likely the problem. What you are describing sounds simply like an overpowered hull that is going as fast as it can. When you give her more power, the prop just cavitates and your engine runs too fast with too light a load and gets hot, which starts burning oil quick.

On flat calm water, a standard displacement hull, like an O'Day, theoretically will top out speedwise at somewhere around 1.5 times the square root of her waterline length. It is generally impossible to make it go any faster, no matter how much power you put in it. In a head sea, her hull speed will drop dramatically, since there's that countervailing element. Until somebody who knows a lot more than I do about matching up hulls, props and engines (which shouldn't be hard to find) tells you differently, I'd just be happy with hull speed and just plugging along in rough water and not push the engine beyond the limits of the boat.

12-12-2003, 07:34 PM
Bob Sorry I didn't explain the whole thing. The motor is air cooled Toyama diesel with not much of a flywheel and would probably be best suited for a big lawn mower. It doesn't have much low end but is strong at higher Rpm's. On super flat water and no wind I could get 5 - 6 knots. The motor sounded like it was running about 1,000 to 1,500 RPM. I don't think you could get the 12" prop to cavitate. When I said chug into a head wind, I am talking 2 knots or less. In other words almost stopped. Opening the throttle more doesn't help, just lugs the engine down and as a result black smoke from un burned fuel. I have sent info. to the Michigan supplier and am waiting for their recommendation.
As for weight distribution ..... the motor is under the step in the area where the ice box was located. That put most of the engine weight below the water line, over the keel and helped the performance when under sail. The boat feels and handles much better than when I had the 15 hp Merc. hanging on the back. The air cooled motor is also super light for a diesel. Must admit that it makes more noise than the 10 hp Yanmar setting in my shop.
I could just pass the problem on to the buyer but I won't.
Thank You

12-13-2003, 07:13 AM
It looks to me that your gear box doesn't have the right reduction. Your engine should deliver it's 10hp at 3000-4000 RPM (I don't know the exact specifications of your engine) If you reach your hull speed at 1600 RPM(engine),1000 RPM shaft, this is way to soon, to get it to 1400RPM you need 10 hp of your engine and it can't deliver it at this RPM. You have to change your gear reduction to somewhere between 1:2/1:2,5. But you need much more info to make an exact calculation. Buy the book of Dave Gerr, Propellor handbook.