Solar Trimaran concept - please review.

Discussion in 'Boat Design' started by kerosene, Oct 22, 2020.

  1. kerosene
    Joined: Jul 2006
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    Location: finland

    kerosene Senior Member

    El-tri-Imperial.jpg El-tri-Metric.jpg

    a 20ft'ish pylwood tri. Narrow waterline but wider "upper hull".
    2ft beam waterline, 7" draft. 1/4" plywood with 6oz glass (+ more on seams), deck and floors 3/8" ply
    Akas and reinforcing wood local pine.
    2 more bulkheads under front and rear decks.
    Straight hull section in cockpit area for modularity.

    600w Solar + 24V 100Ah LiFePO4 (100% cycle ok) + about 1000w (draw) 24V brushless trolling motor (2-3 options that are cheap in that range).

    Goals for design:
    -easy to make
    -family camp cruising on nearby lakes
    -30 nm range @ 5 knots
    -typical summer day solar would cover range requirements
     
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  2. kerosene
    Joined: Jul 2006
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    Location: finland

    kerosene Senior Member

    Resistance per freeship looks like this.
    To convert power requirement to electrical draw I multiplied by 2. Not sure if that is still too optimistic.

    cheers



    upload_2020-10-22_15-51-4.png
    upload_2020-10-22_15-53-6.png
     
  3. kerosene
    Joined: Jul 2006
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    kerosene Senior Member

    I used 50% efficiency assumption from electricity draw to propulsion. Seems that William Fraser had that kind of efficiency on his very purpose built solar kayak setup so I doubt it is a realistic level for trolling motor, even with better prop.
     
  4. kerosene
    Joined: Jul 2006
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    kerosene Senior Member

    I guess it is perfect then...
    :)
     
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  5. Clarkey
    Joined: Aug 2010
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    Clarkey Senior Member

    Well it certainly seems quite reasonable. Even if you find that you can't reach your targets at 5 knots it looks like a small speed reduction will bring the range within reach. I guess it will hinge on motor and prop selection but you knew that already.

    We must be pretty close to a solar electric setup competing favourably with a decent sailing rig on cost grounds now?
     
  6. Dejay
    Joined: Mar 2018
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    Location: Europe

    Dejay Senior Newbie

    Oh I just saw this, looks like a nice project! Did you start building this?

    I'd think about putting the solar panels out of the way and up on a bimini instead

    You could throw the hull into prelimina.com as a monohull and compare numbers. For a power trimaran it might also be better to have shorter amas but I'm not sure if that matters much on these scales. I've skimmed papers that suggest that placing the amas at the right position for the right speed basically negates any drag they cause.

    I'd check if you get lower resistance with more length. A low Length / displacement ratio seems to be most important but hard to satisfy with 4 people on board. And too long might not be practical or that important for a low power boat either.

    In the end I think it will work and you gain or loose maybe 0.5 to 1 knots from your preliminary calculations.

    And interesting presentation haha. I think as a video it would be better to not have the camera tied to the head though. But seeing the the virtual product together with the body language of an avatar could become a useful tool in the future.
     
  7. kerosene
    Joined: Jul 2006
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    kerosene Senior Member

    Have not started yet and quite unlikely that I would start this summer. Simply too much going on but if I got workspace cleaned before autumn that would be a good start.
    I have checked the resistance on freeship and the lates at 7m might knock on 7 knots in ideal conditions (round bilge version).
    The ama placement is a topic of its own. I think ideal would be closer to rear, most “stabilized” monos are like that but on my latest renders the idea was to allow option for rowing. The akas would also be less on the way if further forward. This is probably not ideal but Angus rowing trimaran setup has similar and he seemed pretty happy with the way it performed.

    The boat is not much more than a kayak so bimini is pushing it and would add a lot of weight.
     
  8. Dejay
    Joined: Mar 2018
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    Dejay Senior Newbie

    Not sure how heavy a bimini has to be, I was thinking a light tent-like structure. It certainly is not needed, but could be nice to have the vulnerable solar panels out of range of kids, be unshaded and provide some shade.

    But putting the solar panels up would then allow paddling between the hull and the amas.

    What could be interesting is to combine foot pedaling with an electric motor. You could then also use a bottom bracket e-bike motor then. That's not simple though. Best to keep it simple with a paddle :)
     

  9. SolGato
    Joined: May 2019
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    Location: Kauai

    SolGato Junior Member

    Provided your hulls are narrow and the boat not overloaded, a trolling motor will get you about 5-6MPH at best.

    The thrust rating of the motors will be important with three hulls as you’ll have a lot of windage, so current, swell and wind will all work against you. You’ll want a motor with a good amount of thrust. You might want to consider hanging a rudder and leaving the motor fixed for better control.

    With regard to your solar, 600W of multiple panels wired in series feeding a MPPT charge controller as much voltage as possible will work best.

    On the battery, yes most LiFePo4 can be cycled 100%, but using 80% is a better value to plan for. With your solar and an MPPT, you’ll be able to cruise off sun alone if you monitor your power. To do this, plan to install a Battery Monitor with a shunt. This will act as a “gas” gauge and tell you how much power you have left in the battery by counting every amp in and out, predicting how long you can continue to draw while taking into consideration any power being generated by your solar in real time. Using the gauge, you can find your zero point where you are not pulling from the batteries but running solely on the panels. This is very helpful when you know how far you need to travel.

    Lastly, you’ll want PWM (Pulse Width Modulated) controlled motors. Typically up to about 70% or so throttle, the controller will offer a big efficiency gain, but if you are running full throttle at all times, you don’t really gain much. So again, it’s best to go with a higher thrust motor that you can run with less throttle for best efficiency, then when you need it you’ll have more thrust on tap when you find yourself fighting wind, current, and swell. So you want enough thrust to move your boat at the cruising speed you envision, and you want that to be achieved at around 70% throttle. Planning with this in mind means you’ll be pulling less amps at cruising speed which will increase your range and allow your battery to recover as quickly as possible while under way.
     
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