most efficient propulsion?

Rick,

Thanks once again.

I don't live all that far from Bolly (same council area) and will make the journey. I haven't so far because the Bolly website doesn't come across all that friendly and I am unsure of how they would react to someone dropping in. One of the joys of a twisted chain drive is that the direction of rotation can be reversed merely by reversing the twist in the chain and so I can cope with a clockwise Bolly prop quite easily.

My own experiment with 2mm aluminium flat shows that, by putting a bend in it before shaping the profile, quite a lot of rigidity is produced. I must protect the propellor no matter what one I use due to the snaggy environment and my next experiment will be with 1.6mm flat steel. I think that the blade itself will be strong enough, the weak point should be at the root. I'm thinking that I might be able to finagle screw-in blades that can be replaced as individual blades and that might be able to have their angle of attack adjusted at the expense of a little drag at the hub.

The current drive leg is 25mm wide for most of its length with a small bulge at the bearing. I had intended to use a fairing on my drive leg to try and smooth out the flow around it and to minimise drag. I might just see if I can get away with two 2mm struts that follow the line of the chain and use a fairing on the bearing only.

You always set me thinking.

 

I have not visited Bolly but I have corresponded a few times and found Les very easy to deal with and responsive.

It costs quite a lot to get a prop made to spec but their boat prop is good enough for most circumstances. At AUD55 I do not hesitate recommending it knowing the effort it takes to make a prop and the scarcity of my time.

You can get good robust props from here in a few days:
http://www.hobbycity.com/hobbycity/...&Product_Name=Master_Airscrew_propeller_15x10
I have a few of them just to play with and test for strength but all the ones I have found have too much blade area for the pitch to achieve best efficiency. I have thought about cutting down a bigger, higher pitch one but the price of these are up around a the Bolly boat prop.

The prop is a key component and it pays to get an idea of what a good prop will do.

I only ever tried one twisted chain drive. It was hopeless but I did not spend much time developing it. I quickly moved onto the little right angle drives. They make life much simpler and I have found them to be very reliable if sized correctly.

Rick W

 

It is interesting to look at the cost of a drive leg. The Cd for a 20% NACA foil is about .02. So the drag for something 400mm deep and 100mm long is:
Drag = 500 * 2.4^2 * .04 * .02
=2.3N

Power required at 80% efficiency is:
P = 2.3 * 2.4 / .8
Say 7W.

So 7% of your effort will go into pushing the leg through the water.

This is based on it being only 20mm thick.

I made a very thin leg. It was 12mm thick and about 80mm long by 400mm deep. It was costing me about 10% of my power because my speed was a little higher. Proponents of drive legs never bother to do the calculations.

It is a lot to give away when you realise you can get away with an unsupported 1/4" shaft that costs about 1W. In basic terms the shaft is an order of magnitude better than a drive leg.

Rick W

 

"It is a lot to give away when you realise you can get away with an unsupported 1/4" shaft that costs about 1W. In basic terms the shaft is an order of magnitude better than a drive leg."

I have seen claims made by a company that their method of encasing the shaft , so the water the shaft goes thru does not get spun by the shaft.

Their claim was 10% or so more efficient for the incased shaft , even tho the diametrer is a bit higher.

Are they on to something besides Marketing Hype?

FF

 

FF
I have never calculated the viscous drag on a rotating shaft. I know it imparts some energy.

I do know that spinning a 1/4" shaft at 300rpm does not take any discernible torque as I know instantly if I drop a prop.

With the curved shaft I set the curvature for the fatigue limit and the prop as high as possible to minimise the wetted length. With a longer, larger high speed shaft the drag might be significant. I doubt that it ever gets to 10% of installed power. At some point though it is beneficial to eliminate all submerged appendages in favour of low efficiency surface prop.

Rick W