Correct propeller for inboard??

[Keith7481]

We have a 1978 Columbia 9.6 that we bought recently. When we had it brought out of the water for the season, I was looking at the prop. In comparison to other boats of similar size in the yard, our prop looks dramatically smaller. Its the only 2 blade prop that I see amongst boats of similar size (but unknown engines). The engine is a Yanmar twin and it is a replacement engine. I believe the original was the same engine. Also, the prop is not the original, so no idea if the previous owner put a correct prop on the new Yanmar. Anyone have any suggestions on where I can find out if we are using the correct prop.

 

[steelespike]

Re: Correct propeller for inboard??

I would think Yanmar could help you out if you get them the particulars of your set up.
I'm not well acquainted with inboard sail boats;but prop selection should follow the same basic rules.
The motor needs to operate freely within its rpm range and reach top rpm at wot.
Your prop selection would depend on the gear ratio if any or direct drive. and rpm of your motor
Prop type would depend on the responce control of the boat in powered maneuvers.
My understanding is these boats are slow to respond to certain maneuvers so your results may need to be compared
relative to other similar boats.
The 2 blade prop may be an attempt reduce drag when under sail at the expense of top performance under power.
Prop size is usually marked near the base of the blades or on the end of the hub. diameter X pitch
If you get us your drivetrain particulars, prop size and wot rpm and speed we may be able to tell if your prop is right.
I would assume the boat maxes out at 6 or 8 mph.
Being a prop question you may do better in the prop forum.A moderator may move it if he feels it's needed.

 

[Keith7481]

Re: Correct propeller for inboard??

Thanks SS. Yes, max speed is about 8. Good idea to check with Yanmar, I don't have the engine particulars so will start with them to see if they have any recommendations. I agree that the smaller prop may have been a way of reducing drag. Appreciate the Yanmar idea.. K

 

[SolingSailor]

Re: Correct propeller for inboard??

Check out David Gerr's Propeller Handbook

 

[N_Young]

Re: Correct propeller for inboard??

There are a few prop calculators online usually as part of a prop suppliers website, but I would first find out what you have in terms of pitch and diameter and then talk to a prop supplier. The pitch and area swept are factors of hp rpm and displacement, so knowing these you should be able to get a good idea.

Seeing as you are getting 8 knts that's above hull speed so pretty good. Your max hull speed (31' displacement) is only 7.5knts

 

[N_Young]

Re: Correct propeller for inboard??

There are a few prop calculators online usually as part of a prop suppliers website, but I would first find out what you have in terms of pitch and diameter and then talk to a prop supplier. The pitch and area swept are factors of hp rpm and displacement, so knowing these you should be able to get a good idea.

Seeing as you are getting 8 knts that's above hull speed so pretty good. Your max hull speed (31' displacement) is only 7.5knts