jcap

ok guys 32 pitch bravo1 4 blade (box stock im told) 5000 rpm 82mph on gps. Several prop calculators on the net all say thats over 20 % slip and that is bad for a modern deep v with 3 small steps.I called merc racing prop shop for help and advice,they said those numbers calculate to 15 % slip and that it was acceptable .I purchased the boat used but was told that it ran 79mph with a 28p 4 blade ,then a stage 3 procharger was added and the prop was changed to a 32 and only 3 mph gain ? doesnt make sense ? The boat is a 30Y2K superboat with a hp500efi with a procharger.Any thoughts and or help would be appreciated ,any similiar setups out there ? any other 30y2k owners care to share there set ups and numbers?
thanks

cobra marty

What ratio drive do you have?

jcap

yess forgot 1.5 new Bravo XR top with an imco lower

Goodfella

jcap,
Here is the formula:
RPM divided by GEAR RATIO X PITCH divided by 1056 minus 10%

Vinny P

Sorry Goodfella but that formula is wrong. Below is the correct formula:

pitch X r.p.m. / ratio / 1056

timucin

here is a link from bam marine
propslip

hope this helps

jcap

thanks for the answers
i didnt know that i should be using 30p for a bravo1 4 blade 32p. So would another manufacturers 32p lower my rpms ?I mean is a different 32p going to give a higher mph if i can maintain the same rpm?

SteveDavid

jcap,

Anything in the 15% range with a std. Bravo 1 lower unit and you're doing fine. The nature of the lower unit is inherently higher slip the faster you go. You can enjoy some reduction in slip with a larger diameter prop and possibly a 5 blade prop as opposed to the 4 blade you're now running. Chances are your 32' is a 15.25 diameter. The Hydromotive P-5 are 15.50 std. diameter. And as others have recommended, the P-5's generally need 2 inch less pitch to produce the same rpm as the 4 blade Bravo.

A pricey alternative is going with the new Merc Sportsmaster lower unit. Your slip will drop sginificantly, in some cases by 50%. Same goes for the IMCO SC and equivalent. As slip decreases some folks experience what they think is HP loss. In fact what they are experiencing is better prop efficiency and a resulting stronger load on their existing prop.

You'll generally find that lighter Cats will have lower slip numbers than heavier V bottoms. Also, surface piercing props such as those on Merc #6's will have lower slip than our mostly submerged Bravo's, Hydromotives, etc.

Check with Merc Hi-Perf, Dennis Cavanaugh and see if they have their new 5 blades available. He's a straight up guy and will guide you honestly.

Good luck,

Steve

jcap

Thanks for the help !!I just might try that P5 and yes mr cavanaugh is a gentleman ,I have spoken to him ,He did not however explain why his (or mercs ) calculations produced such a different result . well thanks again !

fountain27ho496

Calculated my slip to be 25%, sounds like the high
end. Dont really understand why and what this means.
Any help? thanks. JC