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Old 14 January 2008, 12:50   #11
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Here's another one!

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!
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Old 14 January 2008, 12:51   #12
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Here's another one!

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!
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Old 14 January 2008, 18:14   #13
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Apparently one effect is to reduce vibration.



Quote:
Originally Posted by BassBoy View Post
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!
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Old 15 January 2008, 04:28   #14
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Quote:
Originally Posted by BassBoy View Post
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?
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Old 15 January 2008, 04:56   #15
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9D
Thanks for giving this so much thought. I'm reassured!
BB
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Old 15 January 2008, 05:40   #16
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As 9D280 says, deformation comes into play. Also the thinner the blades (ie stainless) the less slip you get.Therefore a 21" or 22" stainless prop will produce as much forward movement as a 23" ally one.
Not necessarily, your forgetting about diameter.
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Old 15 January 2008, 06:09   #17
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Not necessarily, your forgetting about diameter.
For the purpose of the explanation I'm assuming both SS and ally props are roughly equal in diameter and blade size-just a different pitch.

Pedant!
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Old 15 January 2008, 07:20   #18
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How about cupping and rake then?
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Old 15 January 2008, 08:28   #19
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How about cupping and rake then?
Yeah Ok, you do have to look at everything-point taken. It's only a rule of thumb after all though and will usually get you somewhere near if you use a close equivalent SS to ally prop.

Whether intentional or not though you do make obvious how much there is to be gained in using a good supplier who'll let you trial/exchange props and then keeping what works best-ie not the 'mail order and hope' method... Been a victim of that one myself-but also did quite well out of it too. AFAIK there isn't a Michigan Wheel dealer in the UK but the best ally prop I've found for my setup is a Michigan Wheel Match which isn't available here.
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Old 15 January 2008, 08:29   #20
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9D
Thanks for giving this so much thought. I'm reassured!
BB
What does an ADV instructor do exactly?
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