Let me take another whack at this one. If one plots a graph of force (vertical axis) vs. time (horizontal axis), it’s generally the shape of the curve that makes or breaks (literally) driveline parts. “Spiking” the curve – which is what happens when you sail majestically off of a 6 foot swell with the sticks pasted all the way forward while you steal a smooch from your sweetie – and then land in flat water, will almost always break something. And it was either the magnitude and / or short duration of the peak load achieved that did the damage.

On the other hand, even more gentle applications of more power than a particular component was designed for – over time – will eventually lead to failure. Said another way, everything will break at some point.

The same goes for the human body. For instance, when they test humans in a centrifuge to see how they react to g forces, they build up the g forces up slowly. Sure, everybody will pass out at some point, but they generally just pass out – without any long term damage. Now, if the human body is subjected to a sudden shock (g loading), one can really get hurt. (Remember concussions?) And the peak force involved may actually be less that what was applied in the centrifuge.

So back to the original question of why some stuff (drives, props, etc.) don’t always hold up to what some folks throw at them, the answer is actually pretty simple. Much of the stuff we’re breaking is stressed – or worn – past its breaking point. Why don’t manufacturers just build stuff strong enough so we can’t break it? Again, simple. The cost would be prohibitive and folks would just keep adding more power until it broke anyway. In fact if you look at what the Mercury Bravo drive was originally designed to handle and compare it to what some folks are throwing at it today, you’ll find it’s actually a pretty good drive.

GLH: Yep - bored, but also curious about the concept. In 15 years and 800 hours of boating, I've failed two DPX's. One was at 3800 rpm and only 2 hours on the drive. Obviously, an incorrectly assembled drive. The other one was a complete upper gear failure while cruising at 65 mph and part-throttle in a lightweight boat.

Let me re-phrase the question: If I peg the sticks coming on plane and my engines are at 2800 rpm, that means that I am feeding 535 lb-ft of torque to the drives. How is this harder on the drives than if I peg the sticks at 4600 and feed 630 lb-ft of torque to the drives, holding it wide open, until I reach top speed at 5200 rpm, at which point I am feeding 600 ft-lb to the drives?

I agree with BigYellowCat: The stress and shock caused by having a fully wound-up engine and drive re-engaging the water at a ssignificant speed differential has to be w-a-y worse.

On the other hand, if your boat's setup means that your engine spools up to peak torque at WOT while coming up on to plane, then the idea of taking it easy makes absolute sense. Could it be that drive failures while coming on plane are more a matter of a situation where the drive is ready to fail, and this is merely the first application of power since the damage occured?

Great conversation so far!

A better device would be a torque-limiting coupler that would allow slipping without damage. Then you wouldn't have to constantly replace a sheared pin (or a smoked coupler...)

This would be my guess. From the drive's perspective it's stress, absent a really rapid change due to inertia (coming out of the water and dropping back in) is limited to the amount of torque the motor can put into the drive. I don't see how the drive would care if it were just coming up on plane or already on plane. I've mostly messed around with ski boats/bowriders/runabouts and don't see many drive failures. The drive on my Cobalt 272, which is mostly used for running around to islands and pulling skiiers/tubers (so lots of quick starts) has 1,084 hours on it (Bravo 1 being twisted by a 454 Mag MPI) to date with no troubles. It's propped such that from a dead stop WOT yields 4,000 rpm so it comes out the hole pretty well for a fairly heavy boat (~6,000 lbs with fuel). It is somewhat rarely run WOT at top speed and pretty much never in situations where it's prop is coming out of the water on a regular basis.

Another thing that might do odd stuff is that if you are proped such that you can be WOT at 2,800 rpm or some rpm way below normal WOT rpm, maybe there's some weird harmonics or something going on. I know I've flown a few planes with rpm restrictions where you couldn't hang out at certain manifold pressure and rpm range combinations because it would induce some weird harmonics that could damage stuff. A lot less likely to get this on a V8 boat motor than with a small airplane motor (very long strong, large displacement cylinders and low rpm, so fairly large, irregular power pulses -- 360 CID flat 4 cyls mostly that I've messed with and one 540 cid flat six).

You aren't taking into account the weight of the boat and the resistance of the water, once you are cruising along the extra power required to go a little faster is a lot less than the power required to go from idling to planing. Taking your numbers for example how much torque at idle speed 50 lb-ft? to 535 lb-ft 1000% increase, from 535 to 630 18% increase.

Also the boat has a lot to do with it. A small light boat can handle full throttle planing all day long, a big heavy boat the drives may not survive.

You also asked why we put up with these drives if they keep breaking, if you have a 38 ft V with mild power that breaks drives you can step up to #6's and then you can hammer the hell out of them from a dead stop and they will live just fine.

The problem with everything related to boats is there is always a compromise. The bravo is light, inexpensive, they shift, props are cheap ect ect, but they can only handle so much power and weight.

A #6 is heavy, needs a transmission, sucks up a lot of power, costs a fortune ect ect. So you could build a boat with 525's and 6's and never have drive issues but who would do that, we would all say to ourselves, well I have these 6's that can handle a ton of power, put the 525's up for sale and find some big power for the boat, and then start breaking 6's.

I like to run hard out of the hole, acceleration is fun, but I am pretty much on plane as the diesels start to make boost so I think my drives are safe.

:lolhit::lolhit:

I agree. Good synopsis. You could mount a Bravo on an aircraft carrier, and it will never see more stress than the engine can provide, unless you start wave-jumping.

pstorti - I'm not sure that I get your point about "extra power" being the cause. Stress loads are directly proportional to the total torque being supplied by the engine, and it is this absolute load that breaks things. If 630 lb-ft doesn't break a drive, how would 535 lb-ft break it?

Here's another question for the racers: Have any of you ever broken a drive while coming up on plane? I'll bet that most (if not all) racing failures are in rough water where the throttles are not perfectly in sync with the boat's airborne excursions.

I believe that a drive failure while coming on plane is nothing more than a damaged drive failing the first time it is subjected to a heavy load. I believe that most drive damage occurs from cumulative over-stressing as a result of either mis-throttling or over-powering. Unfortunately, there is no way to prove that theory, as it's pretty hard to break something, restore it to it's state just before it broke, and then break it again.

Anyway - Great input from everyone. Thanks!

You aren't taking into account that the drive has two ends, the total load on the drive has a lot to do with what is happening at the propeller not just what is going into the input shaft.

Think of it another way if you are towing a heavy boat the load on the drivetrain would be higher starting from a dead stop going uphill than it would be driving up a hill if you were already rolling. The same thing somebody else mentioned about drag radials at the track vs regular tires on the street, same car same power but a lot more stress on the axle and half shafts with the drag radials on a sticky track.

IMO, this is the real issue- many (most?) here are running bravos well past thier design point. Millions of them run just fine with stock engines and holeshots. Even holeshots pulling up multiple skiers w/ alphas.

pstorti:
I get what you're trying to say, but I think that you're confusing "power" with "torque". They are both related to total load, but are different entities. I appreciate your perspective, though.

apollard:
That's exactly why I went with Volvo DPX's and Indy drives in my boats. Even if spare Bravos are "cheap", spending hours on a tow line is a price that I don't want to pay.