Bravo28 - Under-pitched...?
 

Guys,

This is, in no way, intended as to throw shade of any kind on the seller I got the prop from. Just sayin'....

So, I picked up a Bravo28 to be used as a backup. Truthfully, my intention was to use the "new" prop as the main use prop, and the slightly dinged one I've had for awhile as the backup, but the "new" one is considerably slower than the one I've had for a while, across the RPM range.

While I didn't document any of this, nor did I even do an ad hoc study of any kind, I can tell you there is a considerable difference between to seemingly identical props, aside from the minor dings in the one. For example, the primary prop runs about 40-42 GPS at 3500RPM, where the "new" one runs 32-34 GPS. I could also reach my conscious rev limit of 5K WAY easier with the "new" prop than I can with the older one. As in, like, 75% throttle and no stretched out trim, where I need it on the firewall and trimmed out/up about as far as I'm comfortable with the older prop. While I know that every prop is different, being cast and hand finished, I wouldn't have expected THAT much of a difference. Is there that much difference between props of same part numbers?

Moving forward, there are a number of guys that do prop work. If I were to decide to send these out for repair/rework/labbing, who do I send it to? I'm familiar with BBlades, but I've heard/read of a handful of others. Just want some options. Also, can any of them just CHECK and compare these two props, just for the understanding of the empirical difference in output results?

Thanks. Brad.

Damn dude! Sorry it's not working out! As you can see it's a box stock never been touched bravo....wondering if the old prop maybe has thinner blades? You mentioned it has a couple dings on it wondering if some of the blade thickness or overall diameter is different?

BBYSTWY,

All good. Aside from the fact that the paint drip did not "just wipe off". Dang. That was some serious gnarly paint......

I did not do a side-by-side comparison, as I did the prop swap on the water. I should probably do that, now that I have the boat on the trailer in my shop. I saw nothing that might indicate I should expect anything out of place. In fact, I was hoping to recover any prop thrust lost from the dings in my other prop. I'll let you know what I find.

At this point, I'm just scratching my head, as I would have never thought there would be that much of a difference between seemingly identical props.

Thanks. Brad.

Guys,

For the record, I eliminated any errors in RPM reading that some may be aware of from another thread I have asking about a tach acting funny. I had my TechMate connected, both verifying the tach (It is off a little bit. Don't know why), and verifying same RPM between runs with the two props.

Thanks. Brad.

Is it possible that your original prop may have been labbed or modified at some point prior to you owning it??

From what I have been told, Merc quality control on new Bravo props is pretty bad with lots of variance

33 at 3500 with 28 pitch sounds way low. Something is off.

Griff,

I don't think so. There are no outward signs of any work done to it. Still a perfect mirror finish, and not-so-sharp of a leading edge we all know them for. Plus, the speed seems reasonable for the hull/HP involved, and for the prop pitch. I will be getting up in the boat and pulling the prop in question out of the pocket it's in and getting the older one off the drive in the next day or two to visually compare. I don't have any means of accurately measuring pitch without the excessive cost of a full cmm inspection, and I won't be sending either of them off till the boating season is over.

Thanks. Brad.

Liberator,

Ed Zachery.

Hell.... According to the BBlades prop slip calculator, 42GPS@3500 is 38% slip, which seems pretty high. But 33GPS is 47%, which is just nutz.

Thanks. Brad.

I ran 42 @3500 with 27 pitch(Bravo pitch is over stated) in Mercury calculator and it came in at 30% which is still crazy. When ever i checked slip on my 270 Laser it is always high also. Dont know if its a Powerquest norm or what.

It can be really hard to tell if prop has had work done and it should be if the prop was done right. It could have just been balanced, trued up the pitch and polished.
I'd wait for BBlades winter special.

Deduct 1" of pitch for Bravo props for calculating slip. They are stamped heavy compared to actual pitch. Prop slip still way high. What are your WOT speeds/rpms??

Griff,

We are typically running 40-42GPS @ 3500RPM, and 75-76GPS at 5100RPM, WOT and trimmed to the ragged edge. With the new prop, we were in the 33-34 GPS range at 3500RPM, and high 50s GPS at 5000RPM, but WAY off the WOT position on the throttle arm. I did not stick it or bother trimming it out.

I've heard that a 28P Bravo, for example, will run about like a Hydromotive 26. Is this because Hydromotive is a bit more honest about their pitch declaration?

Thanks. Brad.

I ran 46-48 with a box stock bravo 26 at 3500rpm.

I ran a mercury labbed bravo 28 and i can't remember the actual mph was it was definitely higher than the 26.

Something is way off with your new 28.

Guys,

I think I found it: Trailing edge cup.

Trying my best to duplicate measuring method and target points…..https://cimg7.ibsrv.net/gimg/www.off...6be185fe6.jpeg
https://cimg8.ibsrv.net/gimg/www.off...49d95773e.jpeg
Can you tell which one is faster? The older prop has an “X” marked in Sharpie on the hub, right above the “Bravo”. You can also easily feel the distinct reduction in cup in the last half inch or so of the trailing edge of the newer, slower of the two props.

Also found this:
https://cimg9.ibsrv.net/gimg/www.off...98a5ee609.jpeg
Is that a BBlades documentation number?

Thanks. Brad.

Yes, that is a BBlades labbed serial number and they keep all the specs on file.

Propslip is in the ball park of where it should be with your original prop at about 15%. Mid teens is normal for larger single engine V's

Your cruise and WOT numbers with the original prop are very similar to what my 28 Pantera was, except I was turning a custom labbed 26 and was a couple mph slower across the board

Griff,

Kinda what I thought. On all points.

Thanks. Brad.

Griff, running his cruise numbers it shows about 30% slip but WOT is around 15% like you said. Is that typical? Less slip at WOT?

Liberator,

Makes sense to me. Less boat in the water, the faster you go. Don’t know it that’s the physics behind it or not, though….

Pretty normal, especially on larger single engine V bottoms. As speed increases, the boat lifts more, with less drag and slip% gets better.