Ratchet

I'm sure this has been asked before, but after searching for a while, I find myself reading too much other stuff.

Anyways...............

Assuming you can keep a bravo cool, what is the max RPM you can run a bravo at (and in what weight boat). I thought I heard there there was a class that spun them up around 7200 rpms ? is this correct ?

Any examples with boat weights, RPM's, props would be great.

Currently I spin 21's at in a 31 scarab at 6000.

Thanks
RJ

P Offshore

I had a friend with a 42' Fountain that turned them 7400rpm and did it for several years.

Ratchet

That's quite a bit..........wonder how hot they ran

SteveDavid

Ratchet,

RPM has very little to do with outdrive survivability. It is load and the torque produced on the drive that will cause failure. Aside from torque induced failure, if the drive is run at an extreme X dimension and cooling is thusly reduced, you might experience lube failure via viscosity breakdown/ That is why you see so many folks using drive showers of one brand or another (I use simrek halo showers on mine). As for your Scarab at 6,000 RPM, nothing to worry about in terms of RPM.
If you're doing alot of "wake jumping" then you might blow your drive. It's the free wheeling in the air and then the shock load upon re-entry that breaks vertical shafts, pinion gears etc.

Good luck, have fun.

Vinny P

I was under the impression that the oiling system in B1 drives would tend to aerate at higher r.p.m.'s.

mcollinstn

The needle bearings in the top cap and under the bottom pinion are RPM rated. While their rating is not an absolute ceiling where they will disintegrate if run over that speed, it is indeed a speed rating. Lube choice and other factors can affect their life at elevated speeds.

RPM on a wetsump drive does tend to encourage aeration and slinging of lube off the critical areas.

Bearings operate most efficiently in an oil MIST. This is not how bearings in an outdrive run. The top needles work off splash, and the rest of them run submerged in lube.

A drysump 6 is the only Merc drive that has an optimized lube system. All the rest of them are running on luck.

Change your lube often.

Ratchet

Steve, not concerned at the moment, since I'm spinning a rather small prop at 6000 (21p), I think I'm pretty safe. I've also ran these engines in front of alphas for 4 years and haven't eaten up too many drives. I do have drive showers though.

Anyways, since I have 355's small blocks in the boat, and exhaust, and all the acessories (I know weird combo), I was just tossing around the idea of putting together some small blocks in the 383's that spin about 7000 -7200 rpm (since I have a couple of 350 blocks here) & I was afraid that the rpms / harmonics at some point may affect the drives.

jhnrckr

Why not change the gear ratio or prop pitch to lower the RPMs? From my unscientific testing I found out that the faster I spin my props past 5300 rpm the more slippage I created. From what I can gather 5252 is the magical rpm number where all motors stop building torque and start building rpm.

Monty

jhnrckr,

Horsepower is merely a theoretical number derived by the dyno based upon torque produced at a given rpm.

Horsepower = (Torque x RPM)/5252

That's why you will notice that the torque curve and horspower curve intersect on all dyno graphs at 5252 rpm. 5252 is merely a constant and therefore torque and horsepower are always equal at that rpm.

jhnrckr

Does it follow logic to assume that slippage will increase as torque decreases?