PROP CHANGE

[RJSOLACE]

I have a 2013 gemini legacy with a 2 bladed prop and want to change it to a 3 blade. I would like input from others that have changed as to what prop they used and any other info that might be helpful. Thanks Ron

[Wolfe10]

Welcome!


Please tell us what engine you have and what size/pitch your current prop is.


Is your current two blade prop properly sized (at WOT within 150-200 RPM of governed RPM).

[RJSOLACE]

I have a yanmar 2ym15, and have been "Told" that the prop is a 13 x 12 x 1 bore. I do not know if the existing original prop was sized properly.

[Wolfe10]

Proper sized prop:


At WOT, engine will go to within 150-200 RPM of governed RPM.


If it won't go that high you have "too much prop".


If it quickly goes to governed RPM you have "too little prop".


Think of prop size as the rear axle ratio on a truck.

[MicHughV]

Well, as usual, it is not quite that simple.

What are you looking for ?...more speed?....more power ?...what ??

Prop blades come in all sorts of figurations...wide blade..narrow blade...etc....etc..etc...

Your request to " change it to a 3 blade" is very vague.

Recognize that a 3 blade will be more drag under sail and so on.

I could spend all night discussing this with you, but instead would like to refer you to:
"The propellor Handbook" authored by Dave Gerr.

It is a very technical read and will strain your brain cells, but at the end of the day, you will better understand what it is that you are looking for.

The book will show you how to figure out the best prop for your use. This will entail about 4 pages of calculations, but the book will lead you thru' it.

As stated above, the engine should reach it's WOT (wide open throttle) with your selected prop, while underway, not tied up to the dock.

However, most diesel are not designed to run at WOT for any extended time, so what you are looking for is adequate performance at a suitable engine rpm.

Don't expect your small engine to perform miracles. It can only do what it can do. Going to a bigger or 3 blade prop is not going to provide you a burst of speed or power against a 6' wave.

If you want increased performance, you will likely need increased power, ie, a bigger engine..

As stated above, your request "to change it to a 3 blade" is very vague. Your little engine can only do what it can and no more.
I doubt very much if the prop is mismatched. Most boatbuilders and engine suppliers have a pretty good handle on what to expect from an marine diesel.

Furthermore, it goes without saying, that clean hull is paramount. Don't expect performance with a fouled hull or fouled prop.

Overloading your boat below it's lines is another detriment to prop performance, etc...

I told you it would not be that simple.

[MicHughV]

As a footnote, I looked up the spec sheet for the Yanmar 2ym15.

That is a very small engine...around 570cc displacement. The prop you stated is the one recommended for that engine. You can opt to go for a slightly larger diameter, but less pitch, 15x9, 2 blade prop, but I don't think you will notice much difference, you'll likely have a tad more power but less speed.

Leave it alone and learn to live with what you have is my advice.