Jolene said..

Donk107 said..
Hi Jolene

In the article it mentions that there was preload applied to the test rigs shaft bearings to create a drag on the shaft similar to the authors own yacht shaft in neutral


Regards Don

I did read that Don and like I said they proved that a freewheeling prop has less drag with their set up. I seem to think that the key word here is energy and not so much the word drag which probably gets used incorrectly as I did. Drag may not be the issue, It may be possible that a rotating prop creates equal or less drag but it dose take energy to turn it. Did they measure the energy required to drive the test boat. That energy must come from the wind via the sails.Your not going to turn that prop for nothing. I wonder if it is possible that it can take more energy to spin the prop than what it takes to overcome the drag? Its a long time ago that I went to school.

Hi Jolene

Not sure on that

It has been 36 years since I went to school and I was not that clever back then

I let mine spin based on the advice from Yanmar but for the sailing I do on mine ultimate speed is not really a issue

Perhaps someone on here might be able to do some meaningful testing if they are interested

Regards Don

Don, I accept that the author quotes other studies but it all comes down to what the studies were testing. It is totally dependent on the prop being tested and the conditions used for the test.

A low pitch prop with broad blades will have more drag than one with high pitch and narrow blades. A three blade prop will have more drag than a two blader. Without testing it I can be sure that a spinning two blade prop will have much more drag than when stopped. The drag of a spinning prop equates to the surface area of the disc it creates.

On the Yanmar advice, I would like to know how a stationary shaft can damage any part of the drive train. How do they explain the time the boat is on a mooring which I would have thought is the same situation except that water isn't flowing past the hull.

I have a two blade folding which folds with water flow over the blades and stops.

Thanks LMY. That might answer it.

The catamaran experiment would still only apply to the particular props on the boat.

LMY said..
Morningbird,

regarding damage to the gearbox from a stationary shaft. It could be a brinelling issue. The shaft is locked, then the prop trailing through the water could conceivably transmit a level of vibration from turbulent water flow. Vibration in non rotating bearings is a bad thing.

I have tried to determine if there is more or less drag with the prop locked, and cannot find a detectable speed difference. Others may have a different experience, but I suspect that we are seeing such a range of opinions because the difference is minor, if there was a major difference we would all agree.

What we need is a catamaran, without folding props. Set the boat on a balanced work with the props unlocked. Then lock the windward prop. If the boat turns down wind, and the locked gearbox explodes we have the answer!

This would only work if several tacks were carried out. The leeward hull would be lower in the water and there would be greater pressure that side.