Conversion: rowing boat to jet propelld trimaran

Discussion in 'Boatbuilding' started by Gecko_Island, Jan 3, 2015.

  1. Gecko_Island
    Joined: Jan 2015
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    Gecko_Island New Member

    Last summer I got an old (maybe 1950's) wooden 4+ rowingboat. Time passed by and I decided to rebuild it into a jet propelled trimaran for daytrips.

    So I removed all the rowing gear and started the construction of the sidehulls.
    Due to the old style craftmanship I'm not able to change anything in the main hull, so I have to put the propulsion system into the outer hulls.
    And because of the restrictions on my favourite lake, it's necessary to install an electric system.

    About the projected result:
    - LoA : 11,00 m
    - Width: 0,90 m main hull (~ 2,00 m including side-hulls)
    - Mass: up to 1.000 kg

    I will need a thrust of 300-500 N per jet to provide the power to accelerate properly and to achieve the topspeed of 8 Kn in rough conditions.
    I'm aware that I never need that thrust to cruise in better conditions or even with more load, but I think it's better to have some standby power if necessary.

    Maybe it's possible to build a dual inlet system (becaus of the very slim outer hulls) with 50 mm diameter each, an impeller of about 70 mm and an outlet of 50 mm again.
    Please tell me what comes in mind regarding power, propulsion system and construction.

    Do you have any tips on installing an electric powered water jet system?
    - I never did this before
    Where do I get all the parts I need?
    - medium-prized, precise and long lasting

    Thanks ahead for every positive feedback
     
  2. essenmein
    Joined: Dec 2014
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    essenmein Junior Member

    I guess first question is are you fixed on a water jet?

    For electric power its probably not the best drive, water jets are not as efficient as prop drive, this is an issue since you are relying on battery power.

    If you want water jet (and I've toyed with the idea a lot), the easiest way to get started is with electric centrifugal pumps, eg bilge pumps etc. You would then provide some restriction at the output to find the maximum thrust. As you restrict flow pressure goes up as does exit velocity, but flow rate decreases. Thrust being the reaction force of accelerating a given mass. So as you reduce exit port area, speed, and therefore acceleration increases, but ejected mass goes down, this will be a curve with a maximum for a given pump operating point.

    How ever I would suggest getting a couple of electric trolling motors and retro fitting those to your boat, much easier, and more efficient. If you want to make them better build a PWM motor drive so at part throttle you are not burning the difference as heat in resistors.
     
  3. essenmein
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    essenmein Junior Member

  4. Gecko_Island
    Joined: Jan 2015
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    Gecko_Island New Member

    Thanks alot for these answers.

    Yes, unfortunately I am fixed on water jets because of two reasons.
    At first some of the waters where I will opperate are very shallow or have a huge amount of plants and second my boat was build about 70 years ago, and I want to keep the design as clear as possible.

    Relying on battery power isn't such an issue for me, I may use some of the advanced batterys from an old formular student race car.

    The DIY water jet in the link just have a quarter of the thrust I need. I thought about something like this too, but haven't found a suitable pump yet.
     
  5. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Accelerate properly ? I'd be more concerned about endurance, as in how much fun before the batteries run down.
     
  6. Gecko_Island
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    Gecko_Island New Member

    I need the thrust for acceleration only. Cruising on hull speed only needs a third.
    Calculating with 4 regular car batterys I can run over 6 hours @ 8 knots.
    With the race car equipment it'll be even better.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Maybe a language barrier here, but you don't need acceleration unless you are going to stop every few metres. It is a resistance figure at a given, constant speed, that determines your range.
     
  8. Gecko_Island
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    Gecko_Island New Member

    that's clear for me.

    that's why I need just about 300 N to get over the hull resistance.
    in rough seas and a strong wind I need about 600 N
    and to stop the boat weighting 1 ton in about 15 meters I need 1000 N.

    and I guess it's the hull resistance at a given speed leading to determine the range
     
  9. essenmein
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    essenmein Junior Member


  10. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    I dont know much about jets ..or anything else for that matter...but these Seabob gizmos have a powerful battery powered water jet .

    Might be a good idea to fly down to Ibiza and look for someone zipping around on one. Once you have found a Seabob in operation , club the driver with a piece of timber, feed him to the sharks , then bring the seabob back to your shop and investigate its systems.

    [​IMG]
    subir fotos

    [​IMG]
    imagenes gratis
     
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