t500hps

This thread is one of the topics I planned on tackling this spring. I just bought a Formula 312 and was wondering about the props on the boat. It has 502's with what looks like 1.65 Bravo's and a labbed 30 pitch prop. Could that be right? It pulls 77mph on speedo, haven't run to GPS yet. If correct, what would that do to mid-range speed/RPM?

chromecat

30" props are suppose to be the most efficient... therefore you want to gear towards alway using a 30 if possible... at least that is what i was told

Don

aTX427

Chromecat, I ran a 30"p on my eliminator daytona and did not see any more efficiency verses a 26, 28 or 31. I tried a 32 and it was the most efficient, but I could not turn it as hard enough. I think it would have been about the same slip 14-16%.

Bob - I am shooting for mid to upper 80's with heads and shorties. I would rather go with 1.5's because the used prop selection is better, but have been finding more 1.36 lowers for sale than 1.5.

WETTE VETTE

Gear ratio doesn't really matter until you are running out of prop. I have heard 34 and 36 Bravo props are poor out of the hole so it may be better to run a 30 or 32 with 1.36 gears than a 34 or 36 with 1.5 gears. Aside from this it really is splitting hairs and may not have any difference on the performance of the boat. What is the difference between turning a 30 pitch 6000 RPM through 1.5 gears vs a 27 pitch 6000 RPM through 1.36 gears? Not much, and that is consistent throughout the RPM range. Many people say 1.5's have better acceleration than 1.36 and we typically select a prop for a desired WOT RPM. The 1.5 will accelerate better than the 1.36 if the same prop were used, but the 1.36 will require less pitch to give us the desired WOT RPM and thus it will accelerate the same or very close to the same as the 1.5.

throttleup

I would go with the 1.5's especially for a boat that is going to run in the mid 80's. Depending on your engines you should only need a 28-29 pitch prop in a V bottom boat to run low to mid 80's. Like GLH said this puts you close to a 30 pitch prop which is a great pitch in a 15" diameter prop.

There has been a trend lately to put the 1.65 ratio behind the 496HO motors. Some manufacturers have found the boats have a better top end speed than with the 1.5 ratio.

Like everything else in boating the only way to know which is the best for your particular set-up is to try it. But if you don't have the time or money to buy two sets of lowers, one with 1.5's and one with 1.36's, the 1.5's would be a safe bet on your application, John.

Matt

rbtnt

I have always heard that the 1.5 gears are stronger. Don't know how true that is.