Noah and I had a good talk on the phone today. His experience is very helpful.
Guys : I have tried to get a logical explanation to "no avail", of why Catalina's OEM 3 bladed prop is a 15"x 9" for a 21hp engine C34 and the SAME for a 35hp engine C34??? It's like HP is not a factor?!?
Here's a logical explanation why
lower pitch may be a better fit for the
more powerful M35B motor, although it admittedly fails to explain why Catalina didn't use a higher pitch on the older boats (which seem to be working very well with 15"x10"):
Horsepower
is a big factor, and that's why the 35 HP M35B/HBW100 has a more aggressive gear ratio (1.79:1) than the earlier 21 and 23 HP motors (2.05:1). At a given motor RPM, the M35B's transmission uses the motor's greater torque to rotate the prop 15% faster (1.79:1 reduction vs. 2.05:1 reduction). Since our boats all have the same 29.8' LWL and therefore the same 7.3 kt theoretical hull speed, a prop of a given diameter and blade shape that's spinning 15% faster with the M35B/HBW100 will "screw" the same distance/speed through the water with a 15% lower blade pitch. So the M35B powertrain is properly matched to the boat's hull speed with a 15% lower pitch prop. Attempts to put a more aggressively pitched prop on the M35B powertrain will accomplish little other than preventing attainment of full spec RPMs because the boat cannot plane - it will just dig its stern in the water as it tries to "climb out of the hole". So all you guys with older, less powerful motors can stop scratching your heads over this. Just because your 15"x10" or even 15"x11" prop works well for your powertrain does NOT mean that it will work on the MkII boats with more power motor and more aggressive gearbox. We spin our props a lot faster. As a first guess, I'd take any prop that the older motors are spinning and reduce its pitch by 15% for the M35B/HBW100.
I've read as many comments on this as I can, and can see that many older boats are seeing success with the higher pitched props. Some MkII owners are also happy with 15"x10" props. But it looks to me that among those who are not happy with 15"x10" props, virtually all seem to be with the M35B/HBW100 powertrain. I believe that my above explanation is why. I don't have time to go back and re-read everything I've seen over the last several days, but here is one example:
John Langford, MkII boat:
https://c34.org/bbs/index.php/topic,8715.msg62337.html#msg62337When I used the 15x10 configuration on my new flexofold the stern submerged at my normal cruising revs of 2200 rpm and I couldn't get to 2900 at all. For the MK II at least, 15x9 is the right combo. It's a great prop in forward and reverse.
I think there's another example that just posted while I was writing this.
There is a lot more to a prop's design than just the blade pitch. The Flexofold props, in particular, have been shown to generate significantly greater speed than other props at a given RPM (see the famous Yachting Monthly tests).
https://www.yachtingmonthly.com/gear/folding-and-feathering-propeller-test-29807 That just doesn't happen by magic. If a prop is giving more speed at the same RPM, it is going to have more resistance against the shaft's torque, and result in lower RPMs at full throttle. So in my case, where I'm planning to replace a "skinny blade" original Sailer 15"x9" prop with a Flexofold 15"x9", I think the Flexofold's blade design is going to lower my maximum RPMs even if I stay at 15"x9" blade pitch. I do believe that 15"x10" might fail to achieve full throttle RPM of 3000 per Universal spec, so I am gaining confidence in my choice.
