Catamaran Dinghy

Discussion in 'Boat Design' started by Jolly Mon, Jun 30, 2019.

  1. Jolly Mon
    Joined: Jun 2019
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    Location: East Coast, USA

    Jolly Mon Junior Member

    Hi Everybody,

    I’m interested in building another catamaran dinghy 10-11’ long. I’m interested in minimizing the use of wood and targeting a finished weight of 100 lbs or so.

    Is there a foam core layup that would be strong enough, puncture resistant... I’d like to run it at 14 knots thru a good 2’ chop.

    I’ll try to post some pictures of my first one. It was built with 1/8” luan door skins for hull sides and 1/4” luan for hull bottoms.

    Thanks for any ideas

    Rob

    04F31EBD-517C-4BE6-8EB8-B0A1449EEF4C.jpeg 8DC98C56-805C-4985-AE0A-6032B47B89FA.jpeg B8CB6A0D-E26F-47B8-8F5D-431D746B04BB.png
     
  2. jehardiman
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    jehardiman Senior Member

    For something that small it is not a strength issue, it is a stiffness to weight issue. And for that, it is hard to beat wood without spending a small fortune in an optimized carbon/epoxy/foam one-off where the engineering alone will take more time than the total wood build. Build it in wood, use it a decade, burn it, build another, and you would still come out ahead in time, cost, and environmental friendliness.
     
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  3. scape
    Joined: Jan 2012
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    scape Junior Member

    Contact Rob Denney of HarryProa.com he can probably adjust one of his tender designs to be as small. He prefers foam infusion, which is pretty specific and might not be what your looking for. I doubt you'll get the performance against 2 ft chop in anything that small though.
     
  4. Manfred.pech
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    Manfred.pech Senior Member

  5. Ilan Voyager
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    Ilan Voyager Senior Member

    I do agree totally with jehardiman. It's useless in such a boat to spend a fortune in overengineering; infusion, foams, epoxy and exotic fibers. The gain of weight will be minimal and at 14 knots jumping 2 feet waves you do not risk to break a 11 feet plywood catamaran correctly built.
    Nice pics of a sweet catamaran. Compliments.
     
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  6. Manfred.pech
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    Manfred.pech Senior Member

    [​IMG]

    Simple, light and effective design of SAILHAND
     
  7. HJS
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    HJS Member

    Eleven feet, 3.4 meters, and 14 knots means that it is a fully planing boat. Therefore, the boat must be shaped as a planing boat. This means that the planing bottom surface must have the right size and that the center of gravity must be in the right place. In addition, the tunnel sides should be parallel so that no standing wave is formed in the tunnel.


    Mell Cat e 0154  engine pod.JPG Mell Cat e 0154 up.JPG
     

    Attached Files:

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  8. Jolly Mon
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    Jolly Mon Junior Member

    Manfred, thank you for the link and picture. Sailhand is very inspiring. Do you have any other pictures of the build or information on the layup schedule?

    Thanks, Rob
     
  9. Manfred.pech
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    Manfred.pech Senior Member

    Sorry, Jolly Mon, I do not have more about the build or the lay up of sailhands small catamarans.
    Here you can find an E-Mail adress:
     
    Last edited: Jul 3, 2019
  10. jehardiman
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    jehardiman Senior Member

    Rob, if you read all the posts by Sailhands (who is in Oz BTW) in the thread linked by Manfred you'll find the layup and also the facts that the hull is resin infused (I assume by vacuum bagging to get it that light) in a mold using divinycell cored glass. This is what I and Ilan were alluding to...the cost of the materials and the time and cost to make the mold are excessive unless you are contemplating series production rather than a one-off.
     
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  11. Ilan Voyager
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    Ilan Voyager Senior Member

    Again I agree with Jehardiman. Over engineering is the disease of our days, we are in a period which loves complications, gadgets and useless features.
    I laughed a lot seeing the video of the open deck high tech GRP foam infused catamaran with a 3.5 HP on a mirror lake. There is a problem of centering as the guy needs a tiller extension to stay on the center and keep the cat in its waterlines. No seats...A pontoon. Visibly there is a lack of volumes behind while running. I would like to see this cat in a 2 feet chop...
    All that complication for a small boat with a 3 HP on a lake. A simple plywood flat bottom dinghy will do exactly the same job seen on the video for a fraction of the cat cost. It would be lighter
    If a more able boat is needed; higher speed, stability, capacity to go in a good chop a catamaran is a good option if a bit longer. And if it's a summer boat used mostly in nice weather and kept out of the weather in winter, a simple plywood will do the job.
     
  12. Jolly Mon
    Joined: Jun 2019
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    Jolly Mon Junior Member

    Thank you everyone for your thoughts, feedback and suggestions. You all are very kind and welcoming.

    Having the right tender or dinghy for a cruising sailboat is invaluable. I’ve owned a 12’ RIB, a 12’ Livingston catamaran and now the 12’ self-built catamaran. I prefer the catamaran style dinghy for its comfortable and dry ride. On a catamaran dinghy when at speed the wind funnels down the tunnel and creates lift. It feels like you’re riding on a cloud compared to a flat bottomed John boat or RIB. How many times have I seen the typical cruising couple make it to shore in their RIB and have a wet butt or completely soaked.

    The problem with wood is the weight. Okume plywood has a density of 27 lbs/ft3 compared to divinycell h80 at 5lbs/ft3. Having previously built with 1/8” luan doorskins for side hulls and 1/4” for the bottoms, well I’m not going any thinner. Sure I can redesign me current boat to take some weight out instead of considering an alternate core, but that’s another discussion. Now, before somebody jumps onto the idea that the divinycell h80 is going to take so much more glass than the wood, well, that is the point of my query. How much will it take? How much is enough?

    The Gudgeon bros suggested a layer of 10 oz cloth on each side of the foam.

    I’d love to find some tables showing parameters for different cores and cloths.

    Btw, sailhand talks of using divinycell, but the picture looks like he’s using predominantly honeycomb with divinycell for transom and foredeck. Or maybe that’s plywood...hard to tell.

    The pvc tiller extension is quite common among dinghies. When the operator weighs more than twice the boat what’s in the boat and where it’s placed is what’s important.

    Thank you all again for your help,

    Rob
     
  13. HJS
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    HJS Member

    Watch the video carefully and watch what happens around the engine. Very great drag is created there. It is the result of the standing wave between symmetrical hulls and the lack of pod in front of the engine.
    JS
     
  14. Jolly Mon
    Joined: Jun 2019
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    Jolly Mon Junior Member

    JS, thank you for that. I had this on the Livingston catamaran too. They had a little skeg (or pod never heard that term) in front of outboard lower unit but it wasn’t sufficient to prevent water rising up the lower unit and flooding into the back of the boat. It didn’t do it all the time. Maybe it was worsened by where we sat?
    You’re suggesting the tunnel sides be straight and parallel will alleviate this standing wave.

    Rob
     

  15. HJS
    Joined: Nov 2008
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    Location: 59 45 51 N 019 02 15 E

    HJS Member

    The pod should have a flat horizontal underside and lie just above the water surface. The hulls should have a little toe in to be sure that no standing wave is formed. Note that the spray strips are important for keeping the tunnel dry.
    JS
     
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