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.

You may want to do some research on that. Quickly off hand i read about a stock 28 sunsation and a 29 laveycraft that blew bravos with UNDER 100 hours.

Also research the past when bravo gears used to be able to handle extra power and/last longer. Then merc changed who forged the gears.

500hp and up just plain wear out the gears faster then they should with or without hole shots.

I agree with c-spray that some damage has started before the failure occurs and just hammering the throttle from a standstill causes it get to a point of failure.

I also believe, well best guess

With a holeshot the prop burns (boils) a hole in the water (like spinning tires) because it is turning too fast with no forward movement (the water boils as we all know which is cavation) as the boat starts to leap forward from water starting to move away from the prop it's goes from almost zero loading (when in a full cavation mode the load is almost nothing) to full loading in an instant and that shock is what causes stuff to twist or teeth to shear off.

Like has been expressed by others it is like reving the engine and dumping the clutch, stuff will get over stressed

mine is a real ass dragger i thought maybe i had bad clutches or something.. i feel slightly better but im going to install some tabs to help it out

Ok How many people on here have a boat 10 yrs or older with original drives??Let's see how they come on plane WOT or easy????That will be the correct answer truth pills before posting!!Thanks!!

I think most drives fail because of the slip when getting on plane. The prop is biting and slipping thousands of times a min. Boats with the drives in deep are less likely to blow then ones with a higher X. There is also a lot of side load at speed if you have your prop starting to surface that can cause your drive to start to deflect. This changes the gear lash... On my pontoon boat I roll on the gas fairly fast. It is the only fun you can have on the thing because it accelerates like a rocket. Once it hits it's top speed... Not like I can roll on slow if towing a person water skiing.

'88-Fountain 8.8 330's and TRS, drives are DEEP, but even then if I tried to push the sticks too far forward in 95 degree aerated water (trying to plane in the spot where another boat just climbed out), it would spin the props, no forward motion. So, come up slow to keep them biting. When the props are cavitating, it is easy to detect that there is little load on the drives because it stops climbing up and actually settles back some.

But what I do believe a lot are missing here is that the load on the drive while cruising at 4600 rpm is NOT full motor torque output, it is less, perhaps a lot less than full load. At WOT, when the boat is no longer accelerating but at steady state speed then the torque at that time is what the engine can produce at that specific engine speed.

If the torque output of the engine at 4600 rpm was matched by the torque required to push the boat 4600 rpm, then when you "mashed" the throttles forward from that point there would be little or no acceleration, it is the differential between what the engine can produce and what the boat requires at that specific speed that determines acceleration rate.

Therefore, the starting torque load on the drive is determined by what the engine can deliver at the rpm's as they increase only when the prop is hooked up and pulling, It could be suffering slippage and what not but it can't be fully cavitating. So, besides WOT, climbing to plane may actually be the most load on the drive components. If WOT is far beyond torque peak rpm, actual torque at lower rpm may actually exceed WOT torque output as well.

If you're trying to plane coming out of a spot where a boat just took off on your going to cavitate the props regardless because that is dirty/airated water:whistle::angry-smiley-038:

Motor torque on a dyno is totally different than the torque on the drive, post motor, in the water, spinning the wheel...

You don't dyno in neutral.....your torque and hp would be zero...or close enough to make you cry....

What no one is pointing at is the combined load of the vessel, passengers, and prop pitch + bite and diameter creating the load.

Your motor has a torque "potential"....and for all of the obvious reasons, this number should be well below the maximum load the drive is DESGNED for.....forgetting about prior use/abuse....

Not hypothetically, but in reality, your drive requires a tremendous amount of torque applied to move slowly at the fuel dock, or to you local mooring spot. Not because you are using available hp, but because you are counteracting the inertia created by movement.

Force to loading surfaces changes exponentially as the tourqe required to move the vessel ( weight, current, wind, x dim/slip, prop pitch and diam, fresh or salt) in any direction.

So...

Considering that the main shaft and prop shaft are connected to a torque potential on one side, and a load unknown on the other, why spend money for being dumb?:party-smiley-004:

mine is 12 years old with 500 efi's and bravo x drives. not share if they came on the boat i just got it at the beginning of this year. but like i said in my other post i dont slam them down but role them pretty quickly or my boat stands up and burns out. now my boat is in the shop getting top ends and the drives will be gone over as well. so we will see and will let people know if i was just f-in my self all summer :poopoo: lol

My boat has a high X. When getting on plane, the big cleavers cavitate a lot. 4000RPM on the tachs is common before she even begins to lay over. However, at 4000RPM, Im at about half throttle, and the engines are still in vacuum on the gauge (supercharged). Im less worried about the drives in that scenario, as my engines spinning 4000RPM with cold oil in them still.

Shock loads breaks stuff. Take a piece of glass, like a car window. You can stand on it and not break it. However tap it with a ball peen hammer gently and watch it shatter. When a street car bolts some slicks on and heads to the track and dumps the clutch at the line, he snaps the axles, u-joint, etc. It doesn't normally snap things when the engine is pulling hard say in 3rd gear making gobs of torque in high boost.

Same goes for my Semi truck. The engines in the trucks put out say 1500 ft lbs of torque. Day in and day out the truck weighs 80,000lbs. From standing starts to uphills. Light turns green let off the clutch, and its full throttle 35lbs of boost, until I get thru the gears up to speed. You know when the u-joints snap, or rear axles snap?? Its when the wheels lose traction and hop (mud/sand), or you dump the clutch at full throttle. Hence the hammering effect.

So, in my opinion, what eats up drives, isn't the planning aggressively(not suggesting go from idle to WOT planning), its the unloading and loading them (airing out) and what not. Race boats with mild power eat up #6's, because the drives are always being pushed hard in rough water. Of course this doesn't apply to the guy running 1000's in front of his XR's. The only way not to break the drive would be leave it on the trailer.

my boats a 1989 23 ft with the original bravo 1 it has 680 hp 540 in it for the last 4 years and a 500 hp 502 for 6 + years before the 540. go easy to get on plane except a few times for a couple jet ski guys who thought they could take me out of the hole from a dead stop!! i keep my fingers crossed every year!!!! I think the 3000lb weight is what saves it.

That is freaking HILARIOUS!

My Experience.

High X dimention, high rpm take offs break stuff because of vibration because the way it is slipping.

Low x (underwater) propellers can break stuff on take off because it is grabbing enough to put a torque strain on the drive.

Like said, if you like to break stuff then go for it. And thinking that boats are the only thing you cant firewall... I don't know about you all but I don't firewall any of my cars from a dead stop. Even back when I had hot rods all it takes is a twisted driveshaft or blown rear or tranny and you will only do it very select times. If you do it every joy ride you rides wont be very joyous long and the same goes with boats.

Simplest analogy I can give is; When we drive up hill and the car is in overdrive and we push the pedal down further it shifts out of overdrive into a lower gear taking strain off the engine and the transmission.

If you hammer it out of the hole and the prop is hooked up (low slip), you're just lugging the drive at low rpm. If the engine makes enough power you can even have low rpm detonation.

When we're up and running we have inertia on our side and the boat is on plane so when we give throttle input it translates into more speed pretty quickly. Even then, I don't hammer down with 875 hp a side. :daz:

Well put! In the offroad world we can control breakage by running hub fuses and controlling the breaking point in the drivetrain. In all cases the breakage was due to some kind of shockload. I roll on the sticks easy getting on plane in the 311 because I am doing a whole lot more than just looking straight ahead and throttling down. I too would not want to throw the engines under a tremendous load with cold oil.

makes sense Thunder man!

Millions?!!! I'm one of the "Millions" with stock HP that have blown Bravo 1, XZ/XR, and other Bravo crap.

And as far as the post you responded to I have dropped the hammer on my Ford stock rear end at my current 438 hp 445 tq that was designed for 280 HP many times and still going strong.