Repower; prop info

alynnellis · Post by **alynnellis** » Mon May 25, 2009 5:25 am

To All; My project info; Cummins 330 hp, ZF gears with 1.6:1; existing props 23x26 original Bertram issue from their 2.57 :1 BW gears. I can't get prop calculator to behave with my inputs. Biggest descrepancy is choice of hull type. What is conventional wisdom on this issue? 1987 FBC Thanks.

Rick · Post by **Rick** » Mon May 25, 2009 9:48 am

I'd look up Doug's repower in FBC section although I think he's using ZF 220's. But you'll walk knowing more about what you'll need. Going to be a beauty when you're done....nice engines. Rick Ticket 85 SF Merc 454 MPI's Falmouth, MA

IRGuy · Post by **IRGuy** » Mon May 25, 2009 5:17 pm

I have the Cummins 6Bs, 315 HP, and Twin Disk trans with 1.5:1 ratios.. and seem to have the same performance everyone else gets with 21 D by 23 P props.. and my engines rated at 2800 rpm will do WOT at 2950 rpm. Your old 23x26 props are way too much for your new engines.. We all get pretty much the same performance from the same setups with only minor differences in trans ratio and prop specs. With a slightly lower ratio you would probably do well with slightly more aggressive props. BTW.. what are your shaft diameters?

Post by **dougl33** » Mon May 25, 2009 5:43 pm

21X22 3 blade with a number 5 (aka med cup). Several guys are running these same props with 1.5:1 trans ratio. Cruise is 24 knots at 2400rpms with top speed of 28-29 knots at 2900+rpms. You'll need new props as you cannot change your current props enough to help you out. If you go with a trans ratio of 1.75:1 you can get by with your 23 inch diameter props.