jadento

I have an 88 10 Meter Fountain with a newer transom and Bravos(1.50 ratio). I have a fresh last year set of GM 502 gen 6 blocks bored to 509, Stock internals, stock oval port heads, roller rockers and cams(basically 500hp specs). that dynoed at 500 and change HP(as expected). The boat came with stock Bravo 28 pitch props which I have been running. The boat runs right around 70 at 5000rpms with this setup. The boat hops right up on plane and accelerates fine. People have suggesting I need smaller props to get the RPMs up to 5500-5600(none have seen the dyno sheets) or I am lugging the motors and its not good for them. Using the prop slip calculator it seems if I go to 25 pitch 3 blades I should be in the 5500-5600 range at around 72 MPH. Obviously I will loose speed(alot) in the cruise RPM range. So question is. If the motors are not over heating (oil or water temp) is there a reason I should give up the cruise speed? Next question is...is 70-72 where I should be speed wise? I have read all the threads I can find on here and seems im missing several MPH. But looking at my dyno sheet peak Torque/HP, the fact that I quit gaining RPM at 5000 isnt a surprise because that is where things kind of start dropping off. I have tried 26 pitch bravos and did gain a little RPM but resulted in same MPH and loss of cruise speed. I am inclined to think the setup is fine the way it is. Especially when looking at typical Mercury factory rev limits. But looking for opinions?

phragle

what about getting your props labbed???

Griff

Like you said, "they" have not seen your dyno sheets. I'd get your current props labbed or keep your eye out for labbed set in the swap shop. Based on your dyno sheets, I would not drop the pitch and 25 3 blades would be way too small. You should have picked up 350-400 rpms on the top end with 26's.

You didn't really loose cruise speed going to 26's. You just increased the rpm to gain the same cruise speed. A pair of 28's at 3400rpms and a pair of 26's spinning at 3700rpms are probably using about the same amount fuel and your throttle is lever is probably very close to the same position. The reason being is that the 28's put slightly more load the engine. Kind of like needing to push the gas pedal on your truck down more when going up a hill to maintain speed and rpm.

Mbam

From a speed standpoint there would be no gain. Your power is essentially flat above 5000.

Griff is basically correct re the cruise speed.

However there is less power wasted just spinning the prop through the water at the lower RPM = more efficient.

Labbing your existing prop would give you the most benefit all around. If you simply drop the pitch to gain RPM you will slow down.

jadento

So in your opinions what RPM should I be looking for at WOT? I am thinking 5300 is where I need to be.

Mbam

5300 sounds fine as long as your valve train likes it. Just don't get there by dropping pitch
Phragle hit it - get your props labbed

Rookie

I wouldn't touch the props. I don't believe you will gain much on the top end. If you do you'll probably lose it in the middle. Most lab jobs I've had or friends had just turn a decent prop into a slippery prop with loss of mid range and some speed at higher RPM.

Griff

5200-5300 is what I'd shoot for, but I would want to see that in ideal conditions...........low fuel, 2 people, cool air and 1-2' chop.
A basic mild lab job with just balancing and truing the pitch should get you there.

ThisIsLivin

Something Tom told me from Hydro-Motive, prop based on your horsepower curve. That should give you your best overall efficiency and performance. Your so close to your target that labbing will get you there. Just don't let them change the prop dynamics. I've had a prop labbed and it made it garbage.

jadento

No personnel experience, but ive had several friends that did the "labbed" prop thing. None really had much success. Not saying it doesnt work. Just that they didnt gain much for the money.

John