Electric propusion like Thai "Long Tail Boat"

Discussion in 'Electric Propulsion' started by Bertil, Mar 18, 2020.

  1. Bertil
    Joined: Feb 2006
    Posts: 33
    Likes: 1, Points: 8, Legacy Rep: 25
    Location: Sweden

    Bertil Junior Member

    I have a trimaran length 8,50m weighing appr 1100 kg and like to get rid of the fuel outboard motor replacing it with a electric motor. The drive train should be that the shaft would go out of the stern a bit over the water. On the shaft inside theres the motor and some arrangements keeping the motor in line but so the motor can glide up and down. Through the hull their is a bearing tolerating the shaft to be turned down in the water up out of the water. On the shaft in a long tube is the propeller (2 bladed) as in the Thai long tail boats. So on the shaft their is the el.motor and the long tube (appr 1,2 m) and the propeller balancing each other around the "flexible" bearing.
    I counted on that and it will be lighter compared with what I have today a 6 HP motor, as I am satisfied with the el. propulsion giving me 2,5 hours at 5,5 knots.
    Now to my question:
    -Will that work? Vibration and so on? It works in Thailand with very long shafts, why not in Sweden?
    Bertil
     
  2. Artem Klochko
    Joined: Dec 2018
    Posts: 42
    Likes: 13, Points: 8
    Location: Ukraine, Dnipro river

    Artem Klochko Junior Member

    Hi Bertil,
    where do you want to get long and straight shaft? If lenght is about 2+ meters you need not usual turning lathe to make such shaft. You need a special lathe for long raw blanks. Such work will cost a bit more than make a short shaft. As for 2,5 hrs @ 5,5 knots - it is about 2 kW at the motor and 5-6 kWh usable energy at battery for your boat
     
  3. Bertil
    Joined: Feb 2006
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    Location: Sweden

    Bertil Junior Member

    The shaft will not be longer than appr 1,6 m. That could not be a problem. On the outside appr 1,2 m and on the inside appr 0,4 m.
    About the energy needed iI am using the new Samsung li-ion battery. 13 * 3,7 volt= 48 volt weighing about 26 kg.
     
  4. Rumars
    Joined: Mar 2013
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    Location: Germany

    Rumars Senior Member

    Yes it will work. Now weight reduction is another thing. First you only redistrubute the weight, the total system weight (motor+battery) is probably going to be more. Second your longtail solution will probably not going to be lighter or simpler then taking the engine of an outboard leg and mounting the electric motor instead.
    If you still want a longtail you must calculate propeller angle and motor speed so that the boat moves forward.
    For a small outboard you don't necessarily need a shaft in tube solution, a normal shaft supported at only one end is enough. Depending on the used motor you will need gear reduction.
     
  5. Bertil
    Joined: Feb 2006
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    Location: Sweden

    Bertil Junior Member

    -About the weight:
    With fuel outborder: Motor 24,5 kg, Petrol 7 kg, Outborder fastening on the stern 5 kg, Motorequipment, cablels, batt.charger etc 4,8kg, Battery(LiFePo 13,5 kg= Sum 54,5 kg.
    With el.motor and thai solution: MotorLynch 200-D135rags 11 kg, Battery -Li-Ion 48 v, 94 Ah, 26,1 kg, shaft, bearing,inverter, charger etc 15,9kg= Sum=53kg.
    -About the calculation of shaft angle and motor speed: I have done those calculations.
    About the gear: You do not need a gear with the motor above as it only runs with RPM 1914 (=32 turns/second) at speed 5,5 knots.
     
  6. Bertil
    Joined: Feb 2006
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    Location: Sweden

    Bertil Junior Member

    Another thing: You move appr half of the weight to the bottom of the boat. Thats good for the VCG.
     
  7. Zilver
    Joined: Nov 2007
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    Location: Amterdam the Netherlands

    Zilver Junior Member

    Hi Bertil,


    The pancake motor is a good find !
    If I understand correctly that the motor is inside the tube, there might be problems cooling ? Also make sure that the motor has a bearing that can handle the axial forces.
    Using the longtail solution you cannot steer with the motor. I would think that that would be unpractical on a multihull.
    Another idea might be to mount the motor (tiltable) under the front beam , and fix a shaft bearing under the aft beam. Then you can hoist the propeller out of the water when not in use. In Holland the sailing barges had this kind of arrangment in the early days of motorisation. It was called a "lamme arm" (Limb/paralyzed arm) because the shaft was hanging from the side of the boat.


    Greetings, Hans
     

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  8. Bertil
    Joined: Feb 2006
    Posts: 33
    Likes: 1, Points: 8, Legacy Rep: 25
    Location: Sweden

    Bertil Junior Member

    The picture: Really cool, doesn't it vibrate and why not. Do you know a kind of equation for that?
    About your text:
    -The motor is not in the tube. it's sitting on the shaft on the other side opposite to the propeller.
    -The motor and the propeller balancing on the shaft, and the shaft sitting only on the flexible bearing.
    -Yes, I must take care of the axial force. I must steer by the rudder as I cannot move the shaft in vertical direction only horizontal (Up and down the water).
     
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