3 bladed prop for 1988 Catalina 30

[Brian West]

I have a 1988 Catalina 30. It is equipped withthe standard Universal M-25, three cylinder diesel.Right now I'm running a 2 bladed 13x10 right handprop, which I believe is the prop that came fromthe factory. In flat water with no wind I get about4 knots @ 2000 RPM, 5 knots @ 2500 RPM and 6 knots at 3000 RPM.My question is, with a 3 bladed prop could I get the equivalent speed at a lower RPM? My principal objective is to cut down on the noise level in thecabin while under power. For instance, could I get 6 knots @ 2500 RPM? If a three bladed prop wouldresult in a significant improvement, what would bemost appropriate size and shape. Also, what penaltyin sailing performance would I experience going to a three bladed prop. Any advice would be appreciated!

 

[Richard]

13 x14

I have the same set up as you but, at 2000 rpm I can do close to 6kts. I also have a 13 x14 prop. There are a lot of factors the effect motoring performance, like condition of the bottom, Are there barnacles on the prop or shaft? Is your transmission slipping in forward? Is your tach giving you the right reading? I would try to keep the RPMs below 3000 and check the above mentioned things. You can have your prop repitched to 14 for next to nothing but, check out the other things first. Also, invest in hour of a reliable marine mechanic's time, it will be well worth it.good luck

 

[Steve Dion]

Brian, hook it on down the road.

Brian:Hook it on down the road to San Leandro and talk to Kilian Prop. They will give you the real prop poop. Talk to these guys before you do anything. They are very knowlegable in this area. Obviously having a clean bottom is the ONLY way that you can measure your true performance.

 

[Kevin l. Woody]

three blade is better

Dear Brian,Thanks for the letter. In almost all the instances you describe in your letter, a three blade would be your best choice. A three blade, in most cases, is apx. 30% more efficient then a two blade. The result of that is decreased noise and increased ability of distributing what the motor is capable of producing. The item, which is very difficult to insure and predict, is the speed increase. If the current propeller is in poor condition or is very inefficient, then I would think you should see some increase in speed. The addition on one knot of speed is not out of the question if the engine is currently producing the power it takes to push the vessel at that given speed at that given amount of load. It often becomes a problem when we want the vessel to exceed a given speed when the engine is not capable of producing the required output. In determining the proper diameter and proper pitch I would first look at the current two blade and its load impact to the motor. If you are capable of obtaining maximum RPMs of the engine, on more or no less, then I would believe the current loading or current propeller configuration is correct. I would then cross over to the two blade dimensions to the new three blade. Most likely I would suggest that you reduce the pitch by one inch when adding the additional blade. Keeping in mind that we assume the two blades loading is sufficient. Please feel free to contact me directly and I can run the numbers through our computer for you. Hope this helps.Sincerely, Kevin