Falcon

There is a disadvantage that is not often considered with a heavy flywheel;
Boat is propped for 120mph, rough water only going 80-90mph, prop breaks loose at lets say 4000rpm. It doesn't take but a split second for even a heavy flywheel engine to hit the rev limit. When reentering the water the water speed tries to slow down the drivetrain and engine back to the 4000rpm. This inertia event is huge. The less inertia in the engine the easier it is to slow down. Of course this assumes a missed throttle event and that the engine would have been limited by the rev limiter. A lighter flywheel engine would make this situation slightly less likely, but if the time is enough the lighter flywheel could actually be a benefit.

CcanDo

Falcon,Your statements are logical and I would agree. However,would you agree that the lighter flywheel primarily benefits components behind the flywheel?

Components ahead of the flywheel may be otherwise effected. The blower may see increased rotor slap,the valve train may see increased stress and harmonic ring may increase.

The heavier flywheel may serve as protective dampener for the engine. However, subject to drive model,the drive may pay the price.

cuda

All I can add, is the flywheel from an HP500 weighs 30 pounds. I just weighed one last week.

Falcon

CcanDo, there are definitely other considerations. As you mention too much or too little weight can affect the torsional vibration, also tuned out by the crank damper. You were correct that I believe less engine inertia (lighter flywheel) could mainly benefit the drivetrain, the component between the engine and the water! The flexplate (or drive coupler) can also come into play. The problem is that most of this stuff is just trial and error, hard to determine the best combination.

CcanDo

Falcon, For the case of comparison, lets use Bravos vs. #6's.
We know the 6's are rated @ higher torque. Therefore, 6's are thought to be more forgiving when subjected to shock load,regardless of input horsepower. Generally speaking, the Bravos are thought to be adequate when powered by a 500 EFI. However,the Bravo life expectancy must be more effected by variables, such as hull type and weight and X dimension.

We are in agreement the heavier flywheel may shorten power train life. However, is the decreased life expectancy primarily because of jumping the boat in and out the water? If yes, how about spacing the X down and using a heavier flywheel ? While that may be counter productive, theoretically the prop should see less time in mid air and less potential shock load. The deeper X will improve planing and subject to the flywheel weight not destroy top speed numbers. It just takes a little longer to get there.

Falcon

Flywheel weight shouldn't have anything to do with hp, so if a boat likes a certain x for speed and acceleration the flywheel shouldn't change things, except that a lighter flywheel should accelerate harder all other things being equal. I wouldn't necessarily claim that a heavier flywheel will shorten drive life, just that under certain conditions it could. Under the right conditions a lighter flywheel could be worse, such as what you first mentioned, short missed throttle events that the heavier flywheel could limit the rpm change enough to lessen the inertia impact. I know that a boat with high x's that chine walks can be very hard on drives, even 6's with enough power. Not sure what the flywheel weight would have done in that condition.
I do believe that many of the older supercat and open class engines did use aluminum flywheels, but they turned more rpm too. Inertia isn't just weight, it's the speed it is going.
Interesting things to think about.

Edward R. Cozzi

I've heard of re-inventing the wheel, but never re-inventing the FLYWHEEL!
All that rhetoric and still we're no smarter than we were before!
Mercury can order engines with any flywheel they want, yet they still keep delivering engines with the heaviest flywheel they can get. Do you think they are on to something here?
If you are so sold on an aluminum flywheel, just go buy one and try it out and stop all the masturbation!
If putting on lightweight flywheels would give MerCruiser an edge, don't you think with Brunswick's resources for research and development that this would have happened already?

CcanDo

Falcon, My comment about the x,HP,speed was with respect to shock load. logarithm from the heavier flywheel should counter balance some degree of lost efficiency found with a lowered x. In other words, The heavier flywheel inertia should maintain RPM longer or from bump to bump,while ultimately increasing MPH. The deep x may or may not cause MPH to suffer. However,would the deeper x be safer for the components ?

Falcon

CcanDo, I don't think that it would necessarily change any of the lost efficiency, but it is known that a deeper x is easier on components. Deeper keeps more of the blades in the water at once for smoothing out the impacts of each blade entering the water (surfacing), although the tendency toward 4-5-6 blade propellers also helps this. It also reduces the bending load and torque steer loads on the propshaft and gearcase. The current flywheels usually used are already on the heavy side, probably to reduce the chance of shift stall, but the newer computer controls also help this.

CcanDo

OK,Mr.Edward R. Cozzi,What is the ideal flywheel weight ? And,how much TIME will this take ?