What is the Mercruiser Max Prop RPM

[alldodge]

Working on a complete engine rebuild along with a new outdrive, Question is I currently turn a Bravo 3 at 5300 RPM max and the drive is a 2.0 ratio running 24 pitch props. Currently have 3 sets of B3 props and would like to use them to fine tune the WOT. When the engine comes back it will produce more power and therefore I if I producing 100 more HP I should over rev with using my current 24 pitch props. So the question is if I change my current ratio drive from 2.0 to 1.81 I will increase the rpm of the prop above 2500 rpm, is there an issue?

 

[Tail_Gunner]

That;s a offshore question, They do how ever build there motor's to turn 6000 and use very last drop. Now a b3 that a good question assuming your running a crusier I would keep my focus usable rpm band...where does the torque curve fall that is where I would maximize my efforts. Sounds different but my experience with a big block in heavier boat's is as soon as you fall below that they fall on there face.

Ive just read a bit of your build process you have a lot going on there and a lot of money. Go to the 1.8 that will allow you to use a smaller pitch aka less drag...A 100 hp increase may take you down more than what you have.

 

[alldodge]

Tried asking this question in my engine thread and hoped Rick would offer some input. He had a real good write up here on iboats on max rpm of the Bravo drives, but so far haven't heard anything. The offshore folks when your talking the higher rpms is with surface drives. First time I heard about max bravo rpm is from Rick and have been trying to find out if it will be a problem with what I'm wanting to do.

The issue is "IF" the max rpm is around 2500, then I cannot go to a 1.81 turning 5200 rpm. This would mean I would need to stick with the 2.0 and increase prop pitch.

 

[Tail_Gunner]

Tried asking this question in my engine thread and hoped Rick would offer some input. He had a real good write up here on iboats on max rpm of the Bravo drives, but so far haven't heard anything. The offshore folks when your talking the higher rpms is with surface drives. First time I heard about max bravo rpm is from Rick and have been trying to find out if it will be a problem with what I'm wanting to do.

The issue is "IF" the max rpm is around 2500, then I cannot go to a 1.81 turning 5200 rpm. This would mean I would need to stick with the 2.0 and increase prop pitch.

Yes I understand the limit I believe its due to high rpm cavitation and I may be off the beat'n path here but smaller props generally rule the day. Now finding out where they fall on there face is another thing...Recently I propped a big block and went up 4 pitches and a very aggressive 4 blade to find the sweet spot and then found out weight (people) had little to do with anything even surface tension was not a big factor.

Sweet spot: speed and different rpm levels...we settled at 4200 rpm vs 4600

 

[alldodge]

Yes I understand the limit I believe its due to high rpm cavitation and I may be off the beat'n path here but smaller props generally rule the day. Now finding out where they fall on there face is another thing...Recently I propped a big block and went up 4 pitches and a very aggressive 4 blade to find the sweet spot and then found out weight (people) had little to do with anything even surface tension was not a big factor.

Sweet spot: speed and different rpm levels...we settled at 4200 rpm vs 4600

If that's the issue then OK I have an issue which cannot be fixed by going to a 1.81, will also be the same issue if 2500rpm is where the prop starts coming unglued like an engine over revibg. Sure would like to know the answer

 

[Tail_Gunner]

https://www.mercurymarine.com/en/us/propellers/selector/#/step-one

There ya go you can select boat type weight power...you might have to jump yrs around to get the hp drive right...Good luck.....This approach to propping bring's tinkering to a whole new level.

 

[alldodge]

https://www.mercurymarine.com/en/us/propellers/selector/#/step-one

There ya go you can select boat type weight power...you might have to jump yrs around to get the hp drive right...Good luck.....This approach to propping bring's tinkering to a whole new level.

Thanks for looking but it won't provide me with what I'm looking for. I know that if I go to 1.81 ratio B3 drive that my props will fit. My engine rpm will be within acceptable range of around 5200. The issue is the prop will be spinning higher then 2500 rpm. I'm trying to find out what is the maximum rpm a B3 prop can spin in the drive. HT32 response in another thread said 2500 was max, so I need some info to clarify before I drop 8-10K in a new drive.

 

[Tail_Gunner]

Well call bb blades,for the time he has spent in merc he should have a handle on this blade rpm selection thing

 

[alldodge]

Well call bb blades,for the time he has spent in merc he should have a handle on this blade rpm selection thing

Cool , look forward to hearing

 

[Tail_Gunner]

Cool , look forward to hearing

No you call him he's a great guy.....I have done some looking around..........are you trying to predict prop rpm at 5200 rpm aka prop rpm 2700 @ 5200 engine rpm.

 

[alldodge]

No you call him he's a great guy.....I have done some looking around..........are you trying to predict prop rpm at 5200 rpm aka prop rpm 2700 @ 5200 engine rpm.

Don't know bb blades or find him/her looking on the net

No prediction necessary 5200 engine rpm with 2.0 drive, prop spins at 2600
Engine at 5200 rpm with 1.81 prop spins at 2873

Edit: just found
http://bblades.com/propeller-test-program/

If this is who you have in mind I call tomorrow

 

[Tail_Gunner]

Don't know bb blades or find him/her looking on the net

No prediction necessary 5200 engine rpm with 2.0 drive, prop spins at 2600
Engine at 5200 rpm with 1.81 prop spins at 2873

Edit: just found
http://bblades.com/propeller-test-program/

If this is who you have in mind I call tomorrow

That is him......Very interesting you are so sure of your outcome I never have played with duo props this is a first. Don once spoke to the subject I find it .....different to bad 45 auto is no longer posting. He had a very good grasp of duo prop's..

 

[alldodge]

That is him......Very interesting you are so sure of your outcome I never have played with duo props this is a first. Don once spoke to the subject I find it .....different to bad 45 auto is no longer posting. He had a very good grasp of duo prop's..

Thanks but this is arithmetic not a set out come.
5200 (rpm) / 2.0 (ratio) = 2600 (rpm prop)
5300 (rpm) / 1.81 (ratio) = 2873 (rpm prop)

Either way looked at bblades site more and found if you fill out his form he can provide answers. The form says give us engine speed, drive ratio, trim angle, type prop and we can tell you how to improve total out come. With this info every prop shop can provide better outcome.

Posted to OSO in drive section with same question, and it turns out there is no info out there on Merc max max prop rpm. So I was chasing a ghost, and what I thought was accurate info turns out was not. I now have a new lessoned learned

 

[Tail_Gunner]

Yes ive chased the ghost....found out a few thing's that cant be verified. I think what you are chasing is prop tip speed...there is a point where all prop's turn fast enough the tips begin to cavitate..in airplane's is called supersonic. I find it hard to believe a boat prop can go supersonic in water but who know's anyting is possible. This much I can tell you the smaller the prop you can turn at full wot the better the performace...Drag boat props are the shining example they turn huge rpm but at the end of the day there tiny and those drag notor's have torgue from hell a large prop would be nothing.

The lager the blade area or pitch the greater the disturbance at high rpm that simplifies it.and at the same time the smaller the pitch the hole shot is improved hence the 1.8 gearing. But on a large boat like yours the b3 has to be tops.