DavidM wrote:You are asking it to do something it was never designed to do, putting out peak torque at less than wot rpm.
And don't ever think about overpropping a modern, tubocharged, high output diesel. These are marginal enough at normal propping. Overprop and the exhaust gas temperature shoots up. That and the extra torque related mechanical stresses will kill them before their prime.
At great risk of starting something resembling the single vs twins debate, I have to make a couple of comments.
Unless you mean maximum torque, "peak torque"*is a moving target and is simply the highest torque obtained under a given condition. Maximum torque is always*produced well below WOT - whatever that means because a diesel engine doesn't have a throttle, it has a speed setting and a maximum fuel setting. If the engine reaches maximum speed the governor reduces fuel to maintain that speed. If it reaches maximum fuel flow, the governor reduces fuel*at that point. The lever on the*governor merely sets the balance point of a set of flyballs or an electronic null point, it doesn't do anything else.
A fixed pitch propeller's free running*torque demand is roughly the square of the rpm, as rpm increases, the torque required to spin it increases proportionally. That is why there is a difference between a propeller curve and an engine power curve.
An old mechanically governed NA diesel can easily be overloaded by "over propping"*or even*leaning a bit heavily on the power lever, or a sudden increase in current on the bow or even a sharp turn with a twin. The governor will deliver all the fuel it can up to the fuel stop in an attempt to reach the speed setting. The engine will smoke like crazy and will almost certainly run high exhaust temperatures. Even propped correctly this is often an issue because fuel is delivered more quickly than the engine can accelerate. In this case the overpropped powertrain won't necessarily wear due to higher mechanical stresses, it will suffer from high thermal stresses*and destroy the top end first.
The case described is very similar to tugboats or ice breakers*with engines that*are capable of producing far more torque than required for free running. They have a pair of propeller curves, free running in open water at full speed, and a second curve that reflects the much earlier rise (same torque but at a lower speed) that occurs when the boat speed is low due to heavy towing or restrained by ice or the prop turning in dense slush. They use a torque limiting governor that kicks in to*reduce fuel to limit torque to a predetermined relationship to rpm.
And this brings us up to the modern high strung turbocharged stallions that drive some*of the newest*boats. Because they are so well monitored by the engine control system the chance of overloading them due to a heavy hand on the power lever or overpropping is even less likely. The turbochargers are fitted with wastegates, the governor receives a signal from the inlet manifold so it knows how much air is available and if it is or is not within limits for the rpm or fuel flow that exist at that moment.
My take on this discussion is that, sure, you can overprop so as to bring the engine power curve more inline with the propeller torque curve but that will occur at a much lower horsepower output with a fixed pitch propeller so you will limit your top speed and power reserves. You will lose a broad band of speed and power options and increase the negative impact of headseas and winds. You will have to (or should)*reset your governor fuel stops and learn how to "ramp up" engine power like the big ships do.
As far as mechanical wear is concerned, a slower turning engine is far less prone to wear than a faster turning engine. The increased cylinder pressure and loads are easily handled by the bearings of an industrial or marine engine. I would concentrate on limiting thermal loads.
