This article is from the original Go-Fast site, first published in 1998
Question of the day
Why do my Bravo Drives keep blowing up?
The Bravo drive as supplied by Mercury Marine was originally designed for about 375 HP. In its current production form it is pretty good up to 500 horsepower or so. You are probably asking the drive to do something it was never intended to do. Do to the physical size of the drive, there is only so much that can be done to strengthen it. Having said that, I will try and give you some insight as to what is breaking, why and what you can do to help.
First, a short explanation of the various types of failure that apply to the parts in your drive. Just so you know, there are people with doctorates in the part of engineering called "Strength of Materials" I don’t have one of those, and this won’t give you one, but it is a start.
Fatigue is caused by repeatedly applying enough load to a part to cause it to deform (bend or twist). As long as the strain caused by the load is below the part’s elastic limit the part will return to its original shape once the load is removed. The amount of fatigue is related to the amount of deformation (how far the part twists or bends). For the same load, a bigger shaft (or gear tooth or other part) will twist less and be subject to less fatigue and stress. In this case, bigger is better! The exception is when you want part of a shaft to act as a shock absorber. Softer or more ductile materials will deform permanently (plastic deformation) and some will work harden. Extremely hard materials such as high carbon steel (like your gear teeth) do not deform very much when overloaded, but break do to brittle fracture or fatigue. After enough load cycles and the resulting bending or twisting a crack will start at some weak point on the part (usually on the surface) and grow with each repetition of the cycle. Shot peening and other surface treatments can help prevent fatigue cracks from starting on gears and other parts that are breaking from fatigue but won't do anything for brittle fracture or yield failure.
Twisted prop shafts are an example of plastic deformation. A hardened shaft will not twist, but is more likely to break from fatigue. However, there is a whole family of fatigue resistant steel alloys that make great shafts. These are currently available as aftermarket parts and are highly recommended. A billet lower bearing carrier adds additional support and helps eliminate fatigue from the bending caused by the prop. A bigger shaft is the best answer, and Mercury now has one available in the new XR and XZ drives.
The problem is that the weak areas on Bravo gears just aren't big enough.
Shifting in the Bravo is accomplished by the use of cone clutch. This ingenious mechanism uses the engine rotation and torque to engage the forward and reverse gears. As more torque is applied, the force applied to engage the clutch increases. Unfortunately, this force pulls on the floor of the gears in the Bravo drive.
Many repeated stress's, including the impact of each blade as the prop rotates (usually do to catching a little air) subjects this area to fatigue failure. Of course there is always that one giant wave that sends you to the moon when all you can think about is holding on to the wheel and BAM - broken gear - that is a yield failure. The same applies to the vertical and prop shaft. Repeated loading below the yield point will result in fatigue.
How you drive your boat has a lot to do with the longevity of the drive (along with the engine and the rest of you expensive parts). The single biggest cause of drive failure is poor throttling. To put it in perspective, a factory stock boat today can run in the 90 MPH range. This is faster than a world champion Open class boat of the 70's and 80's. Boats in Offshore competition today (and for the last 30 years) have at least a two man crew. With very few exceptions one of them is responsible for throttling.
Proper control of engine RPM when leaving and entering the water will do a lot to make your parts live. Timing is everything. The goal is to keep the engine RPM constant, regardless of whether the prop is in the water or not. This means that just as the props leave the water the throttles should be pulled back enough to keep the rpm from surging. It is not humanly possible to react quickly enough once the boat is up, and the tachs lag as much as 1500 RPM behind the engines.
Ideally what really happens is that you learn to anticipate and pull back just enough to maintain rpm. If you pull back too early the boat will trip (pull the nose down). Too late and you will over rev. All the way back to idle is not a good idea either. Even though that is ok for the engines, the props should really be turning close to the same speed as when they left the water.
Think of the tires on an airliner when it first hits the runway. As far as the prop and rest of the boat is concerned the water is non compressible, and the impact on water is as bad (or worse) as if it were concrete. The key to a graceful reentry is the perfectly timed application of the throttles to keep the engines at a constant speed. Too little or too many RPM can both damage the drive.
Most failures in the Bravo are prop shaft, floor of forward gear, tower in upper gear case, vertical drive shaft, or upper gear tooth failure. Mercury has finally addressed all these problems with the new X drives
The parts described above will break from either fatigue, brittle or yield failure assuming the drive is assembled properly
An RPM limiter is a must have. No matter how good you are, there is always that one time when you sneeze or are distracted. If your drive does blow, it could save your engine from total destruction.