Prop Lab Work
 

Guys,

I'm heavily considering taking advantage of the winter special and send my prop to BBlades. I've got a couple questions first, though.

I've previously discussed with Brett the advantages of the lab finish. My primary interest is in the increased RPM that can be expected. But, seeing that we are pretty much at our rev limiter at WOT and peak trim, I'd think adding a bit of pitch would bring the RPM back down, resulting in a better top end speed and overall efficiency through the RPM range. Am I on the right track here? If so, how much increase in pitch would be recommended?

Some specifics: PQ280 @4800# dry, Merc496HOw /Stg2 Whipple flash and M1 Procharger (advertised 615HP+Whipple gains), B1 drive 1:1.5 ratio. We are currently running about 76MPH @5K RPM with half tank and 2-3 ppl.

Thanks. Brad.
(937)545-8991

I had a set of Mirage props done by a local guy a number of years back....i was told at that time that for every pitch you move its about 300rpm assuming all the rest remains the same. So if you are at 4700 with a 23P prop and you re-pitch it up to a 24P prop then rpm would drop down to 4400 rpm
Im sure the experts that work on props daily have a better understanding of where it would put you based on what you have in the boat. :)
In my case i went from stock 330 tired motors to some mild 450hp bbc and went from a 23 to a 25P props and it all worked great :)

Wally,

Yup. I've read that very thing: 1" in pitch will translate to 300 RPM up or down. I'm going to guess it's more of a percentage thing, but we are almost always dealing with pretty much the same approximate RPM range, so it's always right about the same delta. That said...

Interestingly enough, Brett suggests that the lab work should yield about a 300 RPM gain, given no other changes. I'm assuming this is from either less friction, due to the surface finish, or matched blade pitch/progression, allowing the blades to not "fight" each other, a more efficient pitch progression, thinner/sharper leading edge, etc.... My thinking is that, since we are right at 5K RPM with the prop in it's current state, if the lab work yields a 300 RPM gain, and an additional 1" in pitch takes it away, we should be 1" in pitch's worth in top end speed gains, and better overall efficiency throughout the entire RPM range.

Am I on the right track here, or am I out in right field?

Thanks. Brad.
(937)545-8991

Sounds reasonable! But i dont work on props! :D That being said i have taken new props right out of the box and put dial indicators on them and they are WAY out of tolerance between the blades! When i had my23's done the guy actually scanned them and showed me on the screen how far apart they were! Only one blade out of the 6 between the two props was actually at a true 23pitch! One was over 24pitch! the other were +/- 3/4 pitch.
And if its a true labbing they will balance the prop so you dont have heavier blades then the rest...ive watched the guys at Merc racing put them on a balancer....find the heavy blade....measure the thickness of the blade and decide how much they need to take off material wise to help bring into spec....

BBlades can dial in the rpm increase from about none to 300rpms. He can increase cup, pitch, etc. to compensate for rpm gained by balancing and thinning the blades

Wally,

Yup. I've sharpened and balanced at least a hundred props for my model boats. I've got a jeweled balancing tool that will make you pull your hair out, trying to get a prop balanced. It is so precise and friction free, any out-of-balance at all shows up. I've also noticed, being cast and hand finished, these props will almost always have variance in the blades, not only in pitch, but in shape. It has always been my process to get the shape symmetrical, then the pitch, then balance. Something pretty cool that some of us do, in an effort to take advantage of the once-per-revolution power pulse that is inherent to single cylinder two strokes, is to actually pitch one blade a bit higher than the other, then clock the prop so that the heavier pitched blade is entering the water as that power pulse is set off. The trick is to determine the amount of twist in our flexible prop shafts (speedometer cable) to get this clocking at its most beneficial.

Pitch progression is also a major factor in engine load that doesn't translate into final product speed. In our model boats, it's pretty easy to get too much progression, so that the trailing edge pitch will actually push the back side, or non-working face, of the prop blade against the oncoming water. This results in the higher pitch at the trailing edge only serving to load the engine, and not actually increasing speed at all. I'm very sure our full-sized boats are WAY too heavy for the prop slip to be so minimal as to allow this to happen, but I'm also very sure there is an optimum pitch progression, based on HP, weight and effective pitch of the prop. And that's where Brett's expertise and experience/wisdom comes into play. He's undoubtedly got acres of notes and records for the various configurations he works props up for. I've emailed Brett on this very topic. While I await his reply, I thought I'd toss it out here for us laymen to discuss.

Thanks. Brad.
(937545-8991

you lost me at Wally....lol ;)

It sounds like you already have the perfect prop on there. On the rev limiter at WOT if what everyone is always looking for. What are your goals? Are your slip numbers high? Do you struggle getting on plane? I would buy another prop and have them work it for what you’re looking for. Prop work isn’t an exact science and I would hate for you to screw up a good working prop if the changes don’t work out

Double post

JBraun,

I am seeking to maximize the output of my available HP. As has been discussed, stock factory props are typically well short of perfect. While I would agree that prop work is not an exact science, I do think, from my readings, that Brett has this "black art" about as figured out as one can have it. Matching the blade pitches, at the very least, can't do anything but help.

I've gotten my reply from Brett, and it sounds like I'm on the right track. Most everything he's conveyed matches what I've read here and elsewhere.

I have not made a final decision, as I am still discussing with Brett.

Thanks. Brad.
(937)545-8991