most efficient propulsion?

Discussion in 'Propulsion' started by JaredT, Aug 2, 2008.

  1. uggliozzi
    Joined: Mar 2009
    Posts: 9
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: South Australia

    uggliozzi Junior Member

    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.
     
  2. Guest625101138

    Guest625101138 Previous Member

    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
     
  3. Guest625101138

    Guest625101138 Previous Member

    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
     
  4. FAST FRED
    Joined: Oct 2002
    Posts: 4,519
    Likes: 109, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    "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
     

  5. Guest625101138

    Guest625101138 Previous Member

    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
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.