Is there a formula for prop size conversions?

[johnk86]

Don't know whether this even makes sense. I just bought a new Yamaha HPDI 200hp counter-rotating outboard for my boat. The place who shipped the motor guessed a 15 1/4 x 19 prop would be appropriate. The local marina that did the rigging said after "testing" that the prop should be notched down pitch and I said "like a 17 pitch?" and they said "yeah, that should do". So I got the old prop replaced with a 15 1/4 x 17 Saltwater Series counter rotating prop from Yamaha. My question is about a spare propeller. Seems everyone here thinks it is a good idea to get one, but another Saltwater Series (i.e. Stainless Steel") to be used as a spare would cost $400 - $500 - a lot for a spare. I noticed that aluminum propellers are cheaper; I know they don't hold up as well, but this is for a spare. Can't seem to find a 15 1/4 x 17 counter-rotating aluminum propeller or a used stainless steel prop (I would be happy with either). My question is whether there is some formula where a 15 1/4 x 17 might perform around the same as a 14 1/2 x 21 or something? Maybe its a real dumb question, but I would like to get a spare, and I know having a counter rotating engine limits my possibilities, but I don't want to sink a fortune into a spare. Thanks for any information you can provide.

 

[Mettaree]

Re: Is there a formula for prop size conversions?

Check EBay. Plenty,plenty props<br />

 

[DJ]

Re: Is there a formula for prop size conversions?

HokiePoq,<br /><br />Are there any aluminum counter rotating props available for that engine?<br /><br />If so, I would look for something anywhere between 15 and 19". Since this would be a spare, you can afford to pay more attention to RPM when the spare is on.<br /><br />Besides, aluminum props flex more than stainless. An Al. 19" may perform OK.

 

[johnk86]

Re: Is there a formula for prop size conversions?

I've been looking on EBay but can't find a 15 1/4 x 17 counter rotating prop for a Yamaha . That's why I'm wondering if another size would be comparable.

 

[Mettaree]

Re: Is there a formula for prop size conversions?

Check out Turning Point Props. Tho not sure if they make a CR prop chances are they do.<br /><br />

 

[johnk86]

Re: Is there a formula for prop size conversions?

DJ, do you know how a 14 1/2 x 19 or 14 1/2 x 21 would work compared with a 15 1/4 x 17? Seems that if the pitch increases and the diameter decreases, the net effect might be the same at some point. I'm no engineer, but intuition tells me that this might be the case. That would open up my options.<br /><br />BTW, looking on iBoats website, they don't have the size propeller I'm looking for in stock either.

 

[DJ]

Re: Is there a formula for prop size conversions?

HokiePoq,<br /><br />I wouldn't go any higher than 19", since the shop changed it to a 17". A 19 aluminum may behave like a 17 SS, because of the flex.<br /><br />Typically, the diameters stay very close to the same, regardless of pitch.<br /><br />I would go with the 14 1/2" by 19", if available in aluminum. The 17" in SS.<br /><br />Just be mindful of your wide open throttle (WOT) rpm's with the spare on.

 

[johnk86]

Re: Is there a formula for prop size conversions?

Thanks, DJ, that's good information. I have found 14 1/2 x 17 and 14 1/2 x 19 left hand propellers for my Yamaha outboard. I think I'll be ordering the 19. Thanks again!

 

[Mr.Ladyfish]

Re: Is there a formula for prop size conversions?

Did I miss something? Why not just keep the old 15 1/4 X 19 prop for a spare to be used in emergencies?

 

[johnk86]

Re: Is there a formula for prop size conversions?

I probably wasn't clear enough. I exchanged the 15 1/4 x 19 for the 15 1/4 x 17 straight up so the latter is the only one I have now. My old engine was a smaller Johnson so I'm not going to be using the prop from that.