Propeller Inertia

[trannyguy]

Is there a way to measure the inertia due to the propeller when the boat is idling as well as when running at part load/ WOT in the water? I have the polar MI of the propeller from the PRO-E model. However I was interseted in measuring the effective inertia that the propeller turning at a certain rpm would contribute to say at the input shaft of the drive. So its the dynamic inertia of the prop, (if there is something like that term!) that I am interested in.

 

[GregLocock]

The inertia won't change (apart from the coupled inertia of the water). I must admit I don't really understand what you are trying to measure.

Cheers

Greg Locock

 

[trannyguy]

Greg,
You are right. I would like to understand how to measure the contribution of the hydrodynamic forces on the propeller to its inertia. To make it simpler, consider a MerCruiser sterndrive. There is an upstream inertia of the engine and flywheel on one side of the clutch and there is a downstream inertia of the prop shaft, prop and hydrodynamics of water on the other side. When in neutral the prop is not rotating, however when a gear shift is made from Neutral to Forward this downstream inertia needs to be accelerated by the clutch mechanism. It is this hydrodynamic component of the downstream inertia that I want to calculate.
Any thoughts?

 

[GregLocock]

The general topic is similar to fluid/structural interactions, which aerodynamacists have some understanding of.I'm sorry, I don't have any specific references, you can google as well as I can!

Cheers

Greg Locock

 

[pko]

If you aren't doing it for a vibration analysis but a clutch load anlysis, wouldn't you be better off treating it as a damping factor, since it's velocity dependent?

In either case, it's probably going to be easier to measure than it is to calculate. The energy absorbed by the water when the propeller is accelerated will vary not only due to the propeller characteristics (pitch, diameter), but the rate of acceleration, inflow velocity, and depth of propeller.

If you are fortunate enough to have propeller performance characteristic (torque coefficient(KQ)/thrust coefficient(KT)/advance coefficient(J)) curves, you could get a good feel for non-transient loads and effective power(thrust) for a given input power. An input/output energy balance would tell you how much energy is being absorbed in the water without producing work and might give you a feel for the energy losses you're dealing with. However, I don't know of any body of data explicitly valid for transient loads such as a clutch engage cycle.