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

Being on the rev limiter is not the fastest. It's also not good for the motor to be hung on the rev limiter for more than just an instant. It causes bearing failure and accelerates cylinder wall/ring wear if it's there often. Rev limiters should only be used to prevent accidental over revs.

As far as the RPM goes I've got a 23 Mirage that will go 65@5000rpm in light chop. I had a 24 Bravo that ran 68@4600rpm in light chop. Now I've got a 26 Bravo that runs 67@4350rpm on glass water. Have to wait till spring to see how it does in chop which should pick it up a couple. The 26 is about 4mph faster than the 24 @3500rpm. As far getting on plane their all pretty equal. The 26 carries the boat better at lower speeds than the other two. All of those are box stock props. I have a borrowed Billy Eaves Labbed 26 Trophy that runs 72@4600. That prop was labbed for use on an Allison with a 2.4 Merc. While it's the fastest thing I've had on it, it's hard to get on plane and falls off plane about 26-2800rpm. My BBC IO just doesn't like the smaller diameter at lower speeds. Eventually the 26 Bravo is heading down to Billy for a tune up.

Smitty275,

We aren’t on the rev limiter. We max out at about 5K RPM, running about 76MPH GPS. My hope is to keep it at that RPM, but with the lab work allowing for a little more pitch, rendering another couple MPH.

Thanks. Brad.
(947)545-8991

Interesting to hear from another model boater...though I have been out of the sport for many years. Point being is yes, I too used to spend hours on prop balancing and shaping, even had a CMM program made by one of my friends working in quality control to check the props (or more accurately, compare blade profiles to each other). It was pretty amazing the benefits one could realize in the r/c world, one would think similar benefits are there in the real world.

The rev limiter part was for a different post. But the prop info was to show how much difference there can be between props and how they make the boat ride. The right prop guy will make your existing prop do exactly what your wanting. I'm no prop guru but I know they thin and contour the blades along with making them all the same at a pitch that takes advantage of the rest of the work. They put cup in different areas of the trailing edge to add stern or now lift.
I'm fortunate to live close to Billy Eaves. He's top of the list for prop guys. Especially when it comes to the high performance/racing end of things.

I'm with J on this one. I would not mess up your current prop. I would have a different prop worked. What's probably going to happen is that the prop is labbed, RPM increases, Pitched up to lower max RPM, +0.3746 top speed increase and you wash out your cruise speed.

Rookie,

According to BBlades calculator, if I spin a 29P the same RPM as a 28P, with about the same slip, I should gain ~2.5MPH at WOT, and I'd assume proportional gains throughout the RPM range. I can't, for the life of me, find a disadvantage to a more efficient prop, assuming that's what I'd get.

Care to clarify "wash out your cruise speed"...? Just want to understand, as I definitely don't want to go backwards in total package performance.

Thanks. Brad.
(937)545-8991

Check with Bret at BBlades, he’s the pro,but I don’t think it’s always more efficient in all ranges. Sometimes there is a trade off. “No free lunch”

I agree with your assumption that there is no disadvantages for having a more efficient prop. But where do those efficiencies come from? From a static analysis the prop calculator is great, but when you look at it dynamically thinning blades make them weaker and more prone to deflection and other changes that need to be made to get desired outcome. The labbed props that I have usually are really slippery in the cruise area and some gains on top. When I raced our fastest labbed prop was awesome top end and on long courses, but slippery out of the corners and didn't perform as well as a non-labbed tuned prop. I have a pile of props I've tested over the years that kind of follow that.

I have a friend that sent off a set of 26" and a set 28" Bravo 1's to be labbed and tuned for his boat. They just never performed as the best overall prop. Would have been good money saved using each as a stock prop for what the lake conditions were that day. He was trying to build the best overall prop for his boat.

I have sets of stock Bravo 26", 28", 30" and Mercury labbed 28" for my boat. The stock 28's are probably the best overall and the Merc labbed 28's are the fastest, but they are loose and don't bite as hard under acceleration or throttling in bigger water.
I'm not saying don't send a prop out to get labbed, Just don't expect linear results.
Just my experience over the years.

If you lab the prop (making blades thinner) but add cup (I didn`t know this was possible) The end result would be the same in my opinion.
Would a car analogy work, if you go down in gear ration but get bigger tires you`ll still be at the same rpm so nothing changes at the end of the day ? Obviously you can run bigger tires so that the only change lol

Perhaps I am just unlucky. I have tried at least 3 labbed props on 3 different boats and not 1 of them was faster than the stock prop. Well one of them did run 1 mph faster but it had more slip, harder to come on the plane, didn't drive as well and certainly not worth the 1 mph at WOT. The other two ran the same speed as the unlabbed props but one of those drove as well and carried the bow higher but not increase in top speed. The last one was a Mercury labbed prop, the other two were done by "known prop experts". I tried these due to many good results that were posted by different individuals, but every boat is different and you never know until you try.

There are always trade offs. In your car example, most likely the larger tires would have more traction, more surface area. Would most likely hurt top end, more friction, more weight.

I had a couple of labbed props on my boat. For my usage, the stock otb bravo was so much better than any of them. I would not waste anymore of my money on labbing going forward. I have spoken to numerous boaters, quite a few on this site. Most of the results were disappointing. And that's with sending them back multiple times. Most won't post that on this site, because they feel they will be attacked. So they keep their mouth shut.

underpsi68, "Most won't post that on this site, because they feel they will be attacked. So they keep their mouth shut." Amen to that

Every boat is different and I wouldn’t discourage anyone from trying. But I used your same logic, bump up from a 28p to a labbed 30p.

Results were not good. Every aspect was worse, plaining, cruise, slip, top end. Reworked it, no difference. Sent that thing down the road. Can’t say I didn’t try. Stock B1 28p is back on.

For those saying labbing a prop didn't work for them and discouraging others, at least let them know who screwed up your prop so they don't go to them. Literally anyone can lab a prop. Only a handful are capable of getting it right.
My recommendation is to find someone that's highly recommended in your area or where your going to be vacationing. Then meet with them in person and have video of how YOUR boat currently handles, rides, attitude of the boat at different speeds, etc. with the prop you want modified. (Every boat is different, even with in the same model.) Then tell them what you'd like to be different. If they don't tell you what their basic approach will be and where they can meet your expectations and where they can't meet your expectations walk away. Don't expect an in-depth description, but just "we can add cup here" or "we can thin the blades, add cup on the tips, and correct the pitch" which will make "blah blah blah" happen. Otherwise it's like telling a guy to build you a motor for your car to go X mph in 1320' without telling them how heavy the car is, or what gears it has, etc.