What is the proper 2 and 3 bladed prop for my boat

[Jack]

I have a Catalina 27 and feel that the prop may be undersized. I would like to realize more power while motoring. Even though I do not race I like to go as fast as possible while under sail, therefore drag is also a concern. Here are my boats particulars:Displacement 6,850 lbsWaterline Length 21' 9"Engine: Universal Diesel M18, 14 Hp @ 3200 RPM, 2:1 reductionCurrent Prop: 2 blade 13 x 10 RHPI performed some calculations following an article in this months Sail magazine and the results say that I should have a prop that is 21" in diameter (that is the figure not taking into account reduction. I would not be able to fix a 21" prop in any case).If, after taking into account my specs find that my current prop configuration is indeed inadequatecan you reccomend an appropriate 2 blade and 3 blade prop configuration. I would also be interested in your comments on pros and cons of both configurations.Thank you

 

[Mark]

I need the same exact help

Jack,I sounds that I have exactly the same boat, motor, prop, etc........ and the need for a little more power.Even at full throttle, and a very clean and slick bottom, Hunter, Capris, and other boats of similar size are always back in harbor before me.This gets important when bad weather is involved!Please let me know if you get any answers that work!!!

 

[Kevin l. Woody]

Right prop

Hello Jack and Mark,The propeller, which Jack listed in his e-mail, is the correct unit according to the original installed propeller on the Catalina 27. I would assume that what you are describing as a problem is the lack of punch or sustainable speed. ? . In almost all cases a three blade is a better choice over a two. You will see an apx. 30% increase in efficiency over the two blade. This will not necessarily increase your flat out speed but will insure that when heavy adverse conditions arise your boat speed will have a tendency not to fall off. Of coarse the down side in the increase of drag. Your current two blade produces apx. 130 pounds of pull at 5ish knots, the new three blade could produce 200 pounds of pull! Could reduce your average sailing speed by apx. ¼ more knots. Alternatives would be to purchase a larger disk ratio two blade, less cavitation possibilities, but greater drag. Best of both worlds would be to us a feathering style prop. (sorry for my own plug, what you are asking there is no other alternatives) You will have little to no drag with a large disk ratio. Both a two and three blade configuration is available. Your three blade needed is a 13” x 9” 1” shaft. Hope this helps you. Sincerely, Kevin L. Woody

 

[jack]

Proper prop size

Mark, Let me know your E-mail address and I will send you some other info I have gotten... You can reply to me off site at jkahn2@optonline.netJack