Inclined Prop Shaft Question

[Guest625101138]

I have been making high efficiency propellers for low power applications. It means the blades are high aspect, low slip and preferably 2-bladed.

One of my observations in the early days was thet the props vibrated badly at higher speed despite efforts to statically balance them.

I eventually realised the vibration was due to unbalanced forces on the blades due to the shaft inclination. The upgoing blade had a negative angle of attack while the downgoing blade had a high positive angle of attack. In effect, when the blades are horizontal one blade has reverse thrust while the other blade has high forward thrust. This puts very high cyclic bending loads in the shafy and I have actually bent shafts as a result.

I eventually learnt how to put the knowledge to advantage (with the help of a colleague) by using a flexible shaft that is unsuported at the outboard end. The forces on the prop cause it to align with the flow providing it is pushing. I have included a photo set of an 8mm aluminium shaft that is 1.2m long that is quite able to push the yellow boat to 8kts with a power input of 500W. I have also included a video of another prop on a longer shaft at slow speed in dead calm conditions and you can see the way the prop aligns to the flow as I change direction - look for the silver flash.

What I am wondering is if anyone using an inclined shaft with a more conventional prop has noticed vibration that they cannot correct? The problem would be more noticeable with a high aspect 2-bladed prop.

The beauty of this design is that I can reduce drag be eliminating the strut. I have used various shaft materials but 2011 T8 aluminium wprks well. Another advantage is that you can just bend the shaft up to inspect the prop. Would be good on a sailing boat to reduce drag under sail.

Rick W.

[PI Design]

Hi Rick,

Sounds a neat solution (and nice looking boat). Have you considered just using a three blade prop instead. Incidentally, doesn't the prop in photo's have 4 blades?

Without wishing to hijack your thread, do you get the small craft transactions from RINA? The latest one contains a paper about pedal powered boats like yours, by an Oxford don. They advocate using a forward mounted rudder to help stabilse the boat, mush like the front wheel on a bike. This enables them to just use one slim hull, i.e. no ama or cat configuration - reducing drag.

[Guest625101138]

Yes it is 4-bladed prop in the photo. The one in the water is a 2-bladed. THe 4-bladed prop aligns at lower rpm than the two bladed prop it tends to drop down a bit when the blades are vertical until you get a few rpm on.

I have seen dynamically stabilised boats but using a stern hung rudder. It does not seem to make sense that a bow rudder would work as the lateral forces are generated by the control surfaces in the water and not the grip on the road. I would be interested to see the paper.

My view on using bigger control surfacess is that you end up with more drag from them than what is caused by the weight and drag of balanced outriggers just kissing the water. I now use two outriggers for this reason. My latest rudder is just 8" deep and 2" chord so it only costs a few watts.

Rick W.

[PI Design]

Its copyrighted so can't post the paper, but I'll send you a PM - I'm sure RINA won't mind, after all this is your thing. The author argues specifically for a bow rudder rather than a stern one so the balance and steering elements of the rudder force are working together rather than against each other, but I admit I haven't read the paper in that much detail.