Catamaran dinghy 10-11' long

Discussion in 'Multihulls' started by Boatguy30, May 9, 2015.

  1. Boatguy30
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    Boatguy30 Senior Member

    2 horsepower, that's what I'm using and its a weak 2 at best. I will use a 4-6 hp on the 11' version.

    it also wasn't going to carry the load either now has done that. It will have 2x the wetted surface at 11' but I'll have 2.5x the horsepower and the longer waterline. Care to share your calculations? It would seem that relative hull speed must be linear?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    How accurate is the 5.5 knots, as in, how did you measure it ? You wanted 8-9 knots with a 5hp motor, loaded with 500 lbs "or so", you will get my vote if you manage that. Why restrict it to 5 hp, you should allow some latitude to increase that to at least 8hp. Also, have you experimented to see what happens as you move weight forward or aft, rather than carefully positioning yourself toward the centre ? I would expect a dramatic effect ! You will have a boat where you will need to be careful where the weight is placed, fore and aft, the opposite of a traditional dinghy, where care needs to taken to avoid shifting too much weight to one side.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Your frictional resistance, if we can assume that will be the greater part of resitance at your target speed, will increase considerably, given that a doubling of speed increases it four-fold, and your wetted area will also increase in the same way. The extra length isn't going to help unless you make the hulls slimmer than scaled-up size. You might get what you want with 10hp, given that you don't want to be flogging engines at full throttle all the time. But it will be very sensitive to fore and aft weight shifts.
     
  4. Boatguy30
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    Boatguy30 Senior Member

    It does go a bit faster with the bows pushed down via GPS, but the model bows re so low 7.5" that the bow wave starts to flood the hulls! The actual 11' dinghy will have 12" more freeboard and the front 30" of the hulls decked over.

    The 11' will be strong enough to take an 8 hp or 9.9 but you really don't gain anything over a RIB of the same lenght then. I think it will go well, maybe only 7 knots with the 5 hp. And have tons of space for lounging and fishing on the bridge deck.

    I don't plan to start the full sized model till August.
     
  5. Boatguy30
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    Boatguy30 Senior Member

    I added this foam fairing to flatten the run a bit aft. Amazingly it is now 10% slower so I guess ill go with the original hull shape. Wakes, etc seemed to be about the same.
     

    Attached Files:

  6. Manfred.pech
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    Manfred.pech Senior Member

    Your experiments are very interesting. May be the flattning is working better with higher speeds and sharper chines to reflect water down and to prevent it climbing up the sides for more wetted surface and thus more frictional resistance.

    [​IMG]

    Or you can modify the bottom to get it a bit concave as the photo shows on the hulls and the mould and gain some lift ? The boat on the picture is 3,30m.

    [​IMG]

    [​IMG]

    Discussion has been here:
    http://www.boatdesign.net/forums/boat-design/new-hobie-design-49494-2.html
    http://www.boatdesign.net/forums/multihulls/team-lalous-new-multi-50-trimaran-46659-2.html
    http://www.boatdesign.net/forums/boat-design/stern-extension-help-rowing-50924-5.html
     
    Last edited: May 21, 2015
  7. Boatguy30
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    Boatguy30 Senior Member

    Working on getting my neighbors 4 hp running to test hull with larger motor and foam fairings and knock them off and try to without as well.
     
  8. Manfred.pech
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    Manfred.pech Senior Member

    Good luck for your Tests. Nothing in my life with boats has been so exciting as testing hulls in a testank or in reality, trying to understand the results and then to optimize them as far as possible.
     
  9. Manfred.pech
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    Manfred.pech Senior Member

  10. Boatguy30
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    Boatguy30 Senior Member

    Don't think I'm up to figuring out the data on those links.

    I did get the 4ish horse running. It's about a 1969 motor that had been sitting for abut 20 years. Full power we achieved rarely as constant attention was needed to the high speed mixture needle.

    But I did manage 7.2 knots with the foam removed with little more wake than at 5.5 knots. So I'm going to go with the 11' model. Will start building in August or Sept, unless something else comes along.
     
  11. Manfred.pech
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    Manfred.pech Senior Member

    Thank you for sharing the results of your tests.
     
  12. Boatguy30
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    Boatguy30 Senior Member

  13. Manfred.pech
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    Manfred.pech Senior Member

    Wavewalk is a nice little multipurpose boat -- but with such a weighty, noisy bulk of motor ??? There are smaller and lighter four/two stroke motors on the market http://www.ebay.de/itm/Ausenbordmot...=100010&rk=15&rkt=24&mehot=lo&sd=261895212073 and http://www.ebay.de/itm/Ausenborder-...005&rk=1&rkt=6&mehot=pp&sd=261892661482&rt=nc .
    I think, Bg, your little Catamaran has more payload and for your purpose better hulls.

    [​IMG]

    https://www.youtube.com/watch?v=dmfSDUUpZkU (Sorry no english language)

    5 hp: http://www.ebay.de/itm/Ausenbordmot...7&pid=100005&rk=2&rkt=6&sd=261892661482&rt=nc

    https://www.youtube.com/watch?v=nl9cRaZWQh4
     
  14. Manfred.pech
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    Manfred.pech Senior Member


  15. Boatguy30
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    Boatguy30 Senior Member

    To update this thread, I just picked up 6 sheets or 1088 okoume some for the proposed dinghy and some for a few other interior details in my Vardo. I also have a direct drive 1967 Evinrude 5 hp which weighs only about 18kg. I think I will try the model one more time with the 5hp before going further. I'm sure I can find something else to built with 4 and 6mm okoume.

    Will post of pic of Homestead Hardwoods in NW Ohio. They have 2 horse in a pen at the back of the plywood warehouse!
     
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