Flettner Rotor for small craft - Design and Build Prototype

Discussion in 'Projects & Proposals' started by rwatson, Nov 29, 2022.

  1. rwatson
    Joined: Aug 2007
    Posts: 5,920
    Likes: 320, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    After years of interest, I am finally able to think about building a working Flettner Rotor to try out.
    I've started buying the gear and planning the structure, so its time to get suggestions and ideas from anyone interested.

    I have a spare Canadian Canoe hull, which I will build an additional hull for as a stable platform, but FIRST, I am going to build the Mast Rotor structure, and test it on land.


    The aim is to test results along these theoretical figures

    If I can get 5 HP at 15 knots of wind, that will be terrific.

    I plan to build a truss from Steel Square Tube, 25 x 25 mm, 1.6 mm wall. Four edges to the truss. It will weigh about 26 kilos. If I used 20 x 20 mm, it would save ~ 10 kilos, but my gut feel is that it wouldn't be strong enough.

    I am going to have to disassemble to structure every launch, so maybe I will need to have the truss in two parts, to make it easier to erect.


    I found that I can buy GoCart axle and accessories quite cheaply
    It has the bearing blocks, sprocket, chain and brake system together, so this will form the basis of the mechanism.

    The batteries and motor will be at the bottom, and drive a shaft to an aluminium plate at the top of the truss.

    I plan to make the Cylinder in 2 sections, each section comprising 3 panels


    I speculate that I will have a 4mm Aluminium circular plate at the top, and 2 more "rings", that the fibreglass panels will bolt to.
    For the lower rings, I will need to attach some sort of bearing fitting to the Truss, to support and guide the Aluminium Rings inside the cylinder.

    I've ordered the batteries based on an old Mobile scooter I have. It has two older 12V AGM batteries I can experiment with and I can use the charging system on it to charge the 2 new batteries I've ordered.


    My logic is that if these two batteries give satisfactory longevity for Kart driving, then they should find running a small pedal assist motor pretty easy.
    If the pedal assist electric motor isn't strong enough to spin, say 10 kilos of fibreglass and aluminium rings, I can either get a bigger motor, or give up :)

    I've ordered a motor controller that I hope will work.

    As well as a Tachometer, it is reversible, and steady speed controllable.
    While I am waiting for it and the new batteries, I can start building the truss.

    That will be enough to be getting on with. The excitement begins.
    bajansailor likes this.
  2. rwatson
    Joined: Aug 2007
    Posts: 5,920
    Likes: 320, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    The "Mast"

    Doing some further analysis of the supporting Truss.

    After adding up the webbing components, the whole thing would have been getting up in to the 40-50 kilo mark, which for a relocatable item, is just too awkward.
    So, by reverting to a Tripod Truss, and using 20x20mm - 1.6 ,main lengths of square steel, and 15x15mm sq for the "webbing" , it should come in closer to 20 kg.

    By looking at various commercial trusses, which are all built in Aluminium, I found load limits between 250 and 1000 kilos for this configuration.
    If I can achieve minor deflection at a top load of say 130 kg, that would be adequate.

    I will test the finished truss, and if it has too much deflection as a freestanding mast , I will have to look at using stays.

    Applying stays seems quite straightforward. The plan was to have one end of a Kart axle spinning an aluminium "platter", so initially there was nothing to attach stays to.
    If I make sure the topmost axle protrudes past the top spinning "platter", and have the end run in a bearing inserted into a "crane", there will be a stationary point to affix stays to.
    I may have to make it a 3 or 4 armed crane, depending on what the whole system is attached to.
    For initial ground erected testing, that will be very useful, to. It will save me having to build a robust foundation pad.

    There is probably some software that could calculate the truss performance. I will have to do some research.
  3. rwatson
    Joined: Aug 2007
    Posts: 5,920
    Likes: 320, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    Note for Build Diary -
    Got quoted about a AU$1,000 to do an engineering case analysis for the steel tripod setup.
    That would be extra to the cost of buying the steel and fabricating it.

    Took a gamble, and followed my own advice from years a go, and bought a premade aluminium Stage Truss

    It comes pre-rated NewMast.jpg TRUSSTSpecs.png , rust proof and ready

    It comes pre-rated, rust prof and ready built for the same price, for the same weight.
    Trusst CT290-430S Box Truss 3m https://djcity.com.au/product/trusst-ct290-430s-box-truss-3m/
  4. mc_rash
    Joined: Aug 2020
    Posts: 96
    Likes: 29, Points: 18
    Location: Netherlands

    mc_rash Junior Member

    Hey @rwatson nice project, I would like to see it on the water.

    1. Have you thought about using thin Aluminium panels instead of GRP for the hull of the rotor? It might be lighter and cheaper to wrap an Aluminium sheet than producing Fibreglas panels with molds etc. Or are there prefab GRP panels for this purpose?

    2. How are you going to achieve enough stability for your canoe to prevent capsizing? Do you plan adding a keel?

  5. rwatson
    Joined: Aug 2007
    Posts: 5,920
    Likes: 320, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    Hi McRash
    Thanks for the input.
    Aluminium is a great contender, but has limitations if one can't Tig the light sheets.
    The other nuisance consideration is that thin Aluminium tends to Tin can and bend in use.
    My limitations mean I will be transporting the rig a lot, and having to reassemble at test locations.
    To my mind this means small easily handled sections, that are self supporting.
    Currently, I plan a small mould that will create 1/8 of the tower. That is, 4 sections make the lower half, 4 more sections make the top half.
    That way, one easy to create, accurate mold will produce precise replicas for joining. Say 1.5 m tall, and from memory, under .7 m of curved surface.
    Based on personal experience, I am way more confident attempting that with f glass.

    Re, the canoe. Yes, that would be a project balancing that rig :)

    In the fulness of time, I would create a 20ft outrigger for this canoe for testing, IF I can't come up with an easier solution.
    There are always plenty of half finished cheap hulls for sale, and it may be easier to convert one of them.
    My brother doesn't know if it, but his 26ft f glass yacht is in my sights :)

    Initially, I am going to mount the entire rig on my 20 ft canoe Trailer.
    This will allow me to do a huge amount of testing on land, and sort the bugs out there.

    The trailer is 2.3 metres wide, so it has a good righting moment :)

    The thing is, the testing will involve winds coming from the side, but that translates to fore and aft thrust vectors, so I can apply a lot of power to the land rig with zero chance of a capsize.

    Put it this way, I will either look like a mad engineer with a skinny spinning merry go round, or, at 400 rpm, in 15 knots, I will be able to push the trailer and towing vehicle in a straight line at slow speed.

    I may accept bets :)

    But, on land I can solve the main risk
    1. Excessive power use
    2 vibration
    3 mechanical loads
    4 tower anchoring loads

    So many mysteries to solve
    Last edited: Jan 20, 2023
    mc_rash likes this.
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