Prop For '83 Cal 27

[Sean]

Kevin, I very much enjoyed your contributions to the Catalina owners website. Thanks. Now I have a Cal 27 with a two blade prop. I am only able to get four knots out of it. I would like to switch to a 3 blade and hopefully achieve the 6.3 knot hull speed. I have a Yanmar YSM8 for a motor. Do you have a suggestion for the proper diameter and Pitch? Thanks, Sean

 

[David Foster]

No way

to drive a 27 foot keel boat of that era over 6 knots with 8 horsepower.But you ought to be able to get to 5+ knots in flat water. The original two blade prop on our '77 h27 will drive the boat at 5.2 knots.How to get there is another question. A three blade prop will offer better acceleration, and more "punch" into waves, but no improvement in top end speed. Determining prop size, pitch, and configuration is an engineering problem best solved by an expert at a good prop shop.So I suggest that you choose a prop shop, and talk to them about their recommendation for your boat and power plant. I was pleased with the rebuild Bay Prop did for my propeller, but there are others who come highly recommended.Unless your prop is a serious mismatch, or damaged in some way, your proplem may very well be elsewhere in the drive train, with the engine the most likely candidate. Do you have a tach? How many RPMs are you getting at the high end? Are you getting a lot of vibration? Does the shaft turn freely with the tranny in neutral?DavidLady Lillie

 

[Kevin]

I won't be as harsh as "no way" (sorta)

Hello David,Your theoretical hull speed represents the point the boat pops from the water and begins to plane. I own a 40’ race boat similar to a C&C 41, it displaces 14,000 and moves at apx. 6.75-7 knots with a 3GMF30F. In order for your boat to achieve such a speed I would guess that it would require maybe as much as 15-20 Hp, if not more. The difference being your current install of a YSM8 achieves less then 5 Hp at your current 2,400 cruise Rpm’s, this is before you deduct Hp for pulleys and tired motor. A good way to confirm the current prop is set correctly is to motor away from the dock, rev up the motor in gear until it no longer climbs. If it maxis out at the rated 3,200 you know the propeller is producing the required load. If you change to a three blade you will see a slight speed increase at your cruise Rpm range. I show in our records the original three blade is a 12” x 7”. Before you run off to buy one though I would suggest that you look at the current two blade, witness the numbers and let me know them. We can help you to the right one. Hope this helps and if you need to further discuss it please feel free to call me or stop by. We are on Beverly Park Rd in South Everett, if your Everett is in WA. Take care. Kevin