16' Semi displacement electric cat

Discussion in 'Boat Design' started by Brentmctigue, Jun 21, 2020.

  1. Brentmctigue
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    Brentmctigue Junior Member

    New design taking on board all your sage advice. This one is actually smaller than the last one. Ive got to get it in the garage. Would appreciate your comments.

    Ive included my excel sheet on resistance. Values are output from Mitchlet. I'm unsure if I'm interpreting them correctly to get get a power required.

    cheers
    Brent
     

    Attached Files:

  2. Alik
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    Alik Senior Member

    15kts at 6.62/0.5=13kW? Sounds like a joke. Maybe for empty boat?
     
  3. Ilan Voyager
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    Ilan Voyager Senior Member

    I'm going to ask some stupid questions about the design choices.
    On a very small boat 15 knots it's definitely planing, even 10 knots is already pretty fast. The distinction between semi planing and planing is indeed very academic on such a small boat.
    Why the complication of a catamaran on a such a small boat furthermore electric? That looks extremely expensive to build, and surely heavy because of the big surfaces and the batts.
    A 16' to 18' monohull will be easier to build, far cheaper, will be probably lighter and more tolerant about the repartition of weights (ie the passengers), and probably practically as fast with a small power, as you'll have more surface for planing sooner on flat water. Plus a bonus of more place for the batteries, I guess LiPo 12V 100Ah in series to get 48 to 60V and some amps hour. I say 12V 100 Ah Lipo because these batts although expensive are rather easy to find already made so with 10 batts you have 60V 200Ah. If you want a decent autonomy you'll need a lot of batteries needing lots of place plus a lot of electric circuitry.
    If you want a cata longer hulls are advised if you do not want to have a wet boat, with more place at the center and aft for the passengers, and more bow to cut the "waves" and to get a better centering of the weights.
    The choice of type of hull is simple for the same displacement using a small power: short and chubby mono able to plane soon or long and slim multi. The main advantage of the multi is the smoother curb of resistance with minimal hump, but the con is cost and complication, the main advantage of the mono is the simplicity and cost at the cost of the hump for planing. On so small sizes you use these motor boats only by nice weather.
    In my opinion there is bit much too rocker in your design too similar to a sailing boat hull. There is an enormous surface, and that's heavy. It lacks also cruelly of spray rails, the water will climb and spray at the flat topsides, a sort of Coanda effect. The spray rails maybe be very simple but they are mandatory. Small sailing cats are very wet, but there is not upholstery, nor delicate things, nor electricity, nor passengers who have being showered so the spray rails are not used.
     
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  4. Ad Hoc
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    Ad Hoc Naval Architect

    Or a boat filled with helium!

    The LD ratio is 1.97 ... I think he's missing several decimal places in his 6.62kW total EHP power!!..not to mention interference factor for 2 hulls has gone awol... etc etc...:rolleyes:
     
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  5. Brentmctigue
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    Brentmctigue Junior Member

    Helium would be good! What have I done wrong with the calcs? The resistance values are output from Mitchlet. Calculated for both hulls. The kW I have multiplied the speed by the resistance. Is this correct?
     
  6. Alik
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    Alik Senior Member

    Michlet is wrong calc method... I asked about calc method few pages ago, but there was no reply.
    Use Molland method.
     
  7. Brentmctigue
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    Brentmctigue Junior Member

    Ok. Thanks Alik.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    It is the most obvious one to use.
    But, sadly the hulls they used, are higher LD ratios.. So he'll have to "guesstimate" a pro-rata curve for the low LD ratio...which given his lack of understanding, as already noted by not even reading your posts, is probably a bridge too far for him.

    Im not aware of any series with such low LD ratio hull forms.
     
  9. Alik
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    Alik Senior Member

    They need to make real weigth estimate first. Including all payload, etc. Not a 'yacht designer' style where it should look attractively light, but half of weigths are omitted.
    After that, they might find Molland is OK for this LD ;)
     
  10. Brentmctigue
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    Brentmctigue Junior Member

    I did read his post I simply didn’t know how to respond. It is painfully obvious that this is beyond me. I have no training for it whatsoever excepting the reading I have done. It would be like you trying to design a building, say a 30 story apartment building, or a specialist medical facility. Both of which are on my board at the moment. I’de like to think I wouldn’t be so critical of you for giving it a good hard go.
    I’m learning a lot from you guys and appreciate your feedback. I know I’m pushing s*** uphill with a very sharp stick.
    I looked into the molland method. The equations are beyond my skill level. I used the Mitchlet software because it did all the heavy lifting for me. The Delftship software exports a Mitchlet file that I can use so it’s quite easy. Suggestions?
     
  11. Brentmctigue
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    Brentmctigue Junior Member

    At any rate I’ve accepted defeat (and all your advice) on the smaller cat and am now working on the larger 8m. Displacement figures I’ve used allow for 5 people at 75kg + 20kg of gear each. I’m finding it hard to get the displacement up and keep the Cp low. Trying for Cp 0.57.
     
  12. Richard Woods
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    Richard Woods Woods Designs

    you may find this article useful especially the bit about rocket science Sailing Catamarans - The Ideal Pacific NW Cruiser (a Skoota powercat!) https://www.sailingcatamarans.com/index.php/articles/43-power-catamarans/478-the-ideal-pnw-cruiser20

    why a low Cp? Tony Molland was my head of course when I was a design student
    Wave interaction between the hulls is a major deal, as is windage so both should be considered in any drag calculation

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
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  13. Brentmctigue
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    Brentmctigue Junior Member

    Hey Richard,
    That article was great. Some architects would argue the point about architectural practice being rocket science though I usually sit at the back of the office with a set of colour pencils so I wont. Particularly appreciate the picture of the asymmetric hull showing chines.
    My understanding is that a lower Cp produces a slimmer hull so less Rt. I’ve been trying to keep the bow quite fine too which keeps the centre of flotation back.
    My main problem is that as soon as the hull length gets big enough to cruise I cant put it on a trailer anymore.
     
  14. Dejay
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    Dejay Senior Newbie

    Is this in the book "Ship Resistance and Propulsion"? Or in some other paper?
    I don't find a reference to a specific "Molland method".

    PS: Also how does prelimina.com compare? Does it use an acceptable method for such a project?
     

  15. Brentmctigue
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    Brentmctigue Junior Member

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