Originally Posted by BassBoy
I am about to change/test 22" Stainless from 23" alloy. The stainless prop must weigh at least twice that of the alloy if not a it more.
My questions are about inertia.
How much extra load will the extra weight of he prop put on the final drive and gearbox? Will the rubber bushing cope with the extra load? What effect will the extra weight have on constant engine speed i.e. will it produce more of a flywheel effect and therefore take less energy to turn it once running at desired revs?
Over to you hydrodynamicists!
Ignoring the hydrodynamics momentarily, Here's a mechanical view:
The extra weight will make next to naff all diference - IF it's properly balnced! The weight (which will reduce in water anyway thanks to Mr Archimedes' principles) will act vertically down - I bet you could lift either prop using a large bent paper clip to support the dead weight. however the shaft on your 140 is sized to take a LOT more force than that - think of the effects of bouncing through waves etc etc, and for that matter keeping the powertrain together as you shove 140Hp through it! So the weight will have no more effect than not taking a 2l bottle of your favouritre fizz with you on board.
The bushing in the prop will have been designed to cope with the forces it will see. The biggest of which is a circular shear between the hub and the body of the prop. You'll know if that's dead 'coz you'll get all the revs, but not a lot of action! Also you'll see a kind of extruded ring of solidified ex molten rubber round the back end of the bush if it's slipping.
The "flywheel" effect on paper will I guess be there, but again, on a 140Horse engine, it's going to be next to negligible difference -the flywheel up under the lid will have a lot more effect on the engine than the spinning mass of the prop. As Nos says, the blades are thinner, so the angular momentum probably won't be too far away from the Ally anyway. But with an engine that size the difference will be pretty negligible compared to the forces created by the drag of moving the blades & the boat through the water.
So yeah, on paper you'd probably see a miniscule fraction of a difference, but nothing you're likely to notice when you're out there. Getting a suitable pitch for the metal of your choice is the best bet.
Over to the Hydrodynamicists?