Volvo MS2L gearbox. OK to freewheel or not.

[lw395]

"Traditional practice was to have a two blade prop and lock it in line with the keel. "

But didn't this mainly apply to long keel boats where the keel formed a sort of shelter for the 2 bladed?

Yes.
When keels stopped being like that on racing boats, they invented the folding prop.

Has anyone with a 3 blade fixed prop ever been sailing along at a steady speed then noticed the boat go faster or slower by locking/unlocking the prop?
I suspect the real world difference with a typical gearbox is much less than with something that spins really freely, like an outboard.

 

[RichardS]

Yes.
When keels stopped being like that on racing boats, they invented the folding prop.

Has anyone with a 3 blade fixed prop ever been sailing along at a steady speed then noticed the boat go faster or slower by locking/unlocking the prop?
I suspect the real world difference with a typical gearbox is much less than with something that spins really freely, like an outboard.

Or, indeed, a saildrive as I can turn my props turn by hand with virtually no resistance.

Richard

 

[vyv_cox]

I don't detect any appreciable resistance when turning my shaft by hand, either with the one inch one, PSS seal, yanmar gearbox on the Sadler or the 30 mm shaft, packed gland, Hurth gearbox on the motor sailer. It is only turning a couple of bearings and unmeshed gears, so don't see why there should be.

 

[pcatterall]

The prop on my long keel Neptunian does not start to rotate until 4 knots sailing..
I am fitting a shaft brake as the noise is irritating and I think my Thames Marine box should not freewheel in any case.
When sailing at 5 or 6 knots I can easilly stop and brake the shaft with my hand so am goung to try a conventional bike brake rather than remove the shaft to fit a disk.

 

[AngusMcDoon]

This seems so obvious to me, but for some reason it is often questioned.

The theory that a spinning prop creates more drag than a freely rotating one stems from autorotation. This is the effect where a freely rotating rotor blade will create more drag than if it were fixed. It allows a helicopter to land with no engine power and a sycamore seed to float to earth slowly. In these cases the drag created by autorotation is hugely more than if the blades were fixed (not that they could be for a sycamore seed). However, there's a very good reason for this - they are specially designed to be able to do it. A helicopter will never be licensed to fly unless it can land safely unpowered.

The way autorotation works is by designing the pitch along the blade carefully. The inner part of the blade is moving through the air much more slowly that the outer part. By using this and varying radially the pitch the angle of attack of the air as it hits the blade is controlled. This variation in the angle of attack means that one section of the blade is being driven by the upcoming air, causing the blades to continue to rotate, and another section is driving the air, causing the upwards force on the blade that slows the helicopter's or seed's descent.

It's a difficult balance for the blade designer (or evolution) to achieve, and in a helicopter requires an immediate response by the pilot to set the blade pitch correctly when an engine fails. Once the moment is past and the blades slow to below 80% of their flying speed an autorotating state can never be attained and the helicopter will plummet. This is the reason why during helicopter pilot training the instructors make you practice simulated engine failure again and again.

A downside of a rotor system that can autorotate like this is less efficiency when powered, but the ability of a helicopter to land safely on engine failure is paramount. A boat propeller is designed to work only one way - to create thrust when powered, and is never required to autorotate. If allowed to run freely it will rotate, but not autorotate in the helicopter sense where it is creating significant drag. That's why the myth is just a myth and there will always be less drag allowing it to rotate.