Simple power catamaran?

Discussion in 'Boat Design' started by YotaTruck, Jul 2, 2015.

  1. YotaTruck
    Joined: Jun 2013
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    YotaTruck Junior Member

    I've been batting around ideas for a low HP boat to be used on our local (10 HP restricted) lakes for a while now. A while back I had looked at pontoons (as in the kind with big aluminum tubes for buoyancy) and quickly discounted them as too inefficient given the HP restrictions. I do see them on the lakes from time to time, but they're painfully slow and probably eat a ton of gas. Recently I've been poking around looking for the same concept, but with two easily driven plywood hulls. A lot of the power catamaran designs I find are like this one from Glen-L:

    http://www.glen-l.com/designs/outboard/aquacat.html

    Cuddies, set up for remote steering, cockpit sitting low in between the hulls, etc... I don't need all that. Essentially what I'm looking for is a glorified "party barge" - the space and simplicity of a platform suspended between two logs-except the logs will be well designed plywood/glass/epoxy hulls rather than cylindrical aluminum tubes.

    These two designs come close:

    http://www.catamarandesigns.com/ecocat.htm

    It looks I could just omit the cuddy, but it's too small. I think 20' would be ideal. This one is even better at 22':

    http://www.clarkcraft.com/cgi-local...=933013271&cart_id=20000109163100207071003015

    Anyone have experience with Clark Craft plans? $16.00 for the set-they are scale only, but that's still dirt cheap. My only concern about this one is that she sits pretty low in the water, but it's mentioned in the caption that many builders add height to the hulls to make her sit higher and ride drier. Any thoughts? I plan on calling Clark Craft today and telling them about my power restrictions and planned crew load (4 people, about 550lbs) to see what they think. The description says "simple box hulls" so I'm not sure how efficient they would be.
     
  2. upchurchmr
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    upchurchmr Senior Member

    Buy an old Tornado catamaran.
    Narrow it to 8 feet so you can trailer it.
    Put on the lightest deck you can make, moving the rear beam fwd.

    The best think about the Tornado is that the hull has very high volume compared to other beach cats. And old one are not worth much.
    10 hp will work well if you don't submerge the transom very much.
     
  3. rasorinc
    Joined: Nov 2007
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    rasorinc Senior Member

    Look up customer build photos for the Aqua Cat on their site.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Does it matter, so long as it does what is says on the tin!

    How efficient is your car compared to say Prius??..does that mean you wont use your car because it is not as efficient?

    If the car goes from A to B...in your budget, time, requirements etc...what difference does it make?

    Chasing after rainbows is never the way to buy a product.
     
  5. YotaTruck
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    YotaTruck Junior Member

    I get what you're saying, but you're oversimplifying. 10HP is not a lot of power, but it's the most I have to work with so I have to try not to waste it. I could try to get an extra 5 by sneaking in a 15 under a 10HP cowl, but the rangers at the lake are wise to pretty much every trick in the book. Like I said, I've seen plenty of 20' or so pontoons chugging along at 6 knots (if that) with a 9.9 screaming near redline. Fuel costs aside, that's just annoying to have to listen to.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    YotaTruck

    Well, with respect, since I assume you're not a naval architect, again, you're missing the point.

    All you need to know about a hull of the SOR you are searching for is its Length-Displacement ratio.

    In simple language, how much does the boat weigh (full load) on its waterline length?

    Hull shape is a red-herring, please ignore the sales pitch hype. The lighter the boat for its length, in your language, the more efficient it shall be.

    Thus the more equipment and "stuff" you put onto the boat the heavier it shall be and thus, the bigger the engine required.

    If your objective is smallest engine possible...you need a boat that is as light as possible for its length compared to others. That's it, no magic.
     
  7. YotaTruck
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    YotaTruck Junior Member

    I was assuming that two (for lack of a better term) "boat shaped" hulls would be better than two aluminum cylinders-if I'm wrong about that, then I might as well find an old pontoon and fix it up. From what I've seen from various catamaran plans, they seem to need less power to achieve the same speed at the same weight.

    So does a 20' catamaran have a waterline length of 20' or 40'? Honest question-I really have no idea whether you count it as two shorter hulls or one long one.

    Probably about 2500lbs.

    So I could build two 24" x 12" x 20' plywood boxes and they're going to perform exactly the same as more "boat shaped" hulls?

    I can certainly understand that, but unless I go on a serious diet and tell the family to stay home, and build the entire boat from carbon fiber, 2500lbs is what I need to move.
     
  8. upchurchmr
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    upchurchmr Senior Member

    For $16 just buy the Clark Craft plans and study them.
    You basically can't get cheaper than that.

    It might help understand the situation.
     
  9. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Hmmmm....okay, i see where this is now.

    To put things into perspective for you. Firstly 20' can be either LOA or Lwl, but let us assume it is Lwl. And this si the same for either catamaran or monohull. If a monohull one hull of 20' if catamaran two hulls side by side also of 20'.

    I don't understand imperial units..so 20' = 6.10m

    For an efficient hull, you need a length-displacement ratio (LD) of at least 8.0. Any higher is much better. But, for starters let us assume a value of 8.0.

    Thus L/(D)^3 = 8.0, this gives a displacement of 443kg or 977lbs. That is total displacement.

    Now, looking at typical graphs of resistance and how the resistance changes with increasing or decreasing LD ratio, see below:

    L-D ratio-1.jpg

    So as you can see an LD ratio of 8.0 is very favourable in terms of resistance. But can you build a catamaran it engines outfitting and you and fuel etc etc..for a total of 977lbs?...well from your responses above...seems not as you are looking at 2500lbs or 1130kg.

    So what does 1130kg mean in LD terms? = 4.2.

    So looking at the graph above where does a value of 4.2 sit....OOOPPSS...not very favourable. In other words VERY inefficient.

    So what are your choices....build the boat with whatever you want how ever you want..and put the smallest least fuel consumption engine in it you can find the speed will be what it is and be happy with it. Since running at slow speed will be your most efficient means too as it shall be just skin friction that dominates, not wave making. But if you want to gain those 5knots....you'll be going into the big hump part of the curve above, not good!

    Otherwise, as noted earlier...you'll be chasing rainbows.
     
  10. YotaTruck
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    YotaTruck Junior Member

    Understanding is what I'm looking for-I've already ordered them. Like I said, if I can't build something better, I'll just look for a tired pontoon and rebuild it.
     
  11. rasorinc
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    rasorinc Senior Member

    Consider a RIB with a real light weight support hull. Also, Ad Hoc is very helpful and is light years ahead in boat knowledge then you or I. You could also fill the tubes with helium.
     
  12. gordanm
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    gordanm Junior Member

  13. Squidly-Diddly
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    Squidly-Diddly Senior Member

    I'd find a Hobie 17 with working trailer, good hulls but lacking good rigging and sails. Sooner or later one will show up on Craigslist for next to nothing. :) Have a tow vehicle with all known tow ball sizes and light connectors ready to roll at a moment's notice.

    http://www.hcana.hobieclass.com/default.asp?Page=9302

    It has the symmetrical hulls w/swinging dagger boards, and its more or less exactly 8' beam (probably designed for easy, legal trailing, THEN length determined from the 8').

    Says max payload is only 400lbs, but that must mean hiking one hull out of the water in wind and chop, with crew swinging about like monkeys.

    I'm guessing 10hp would drive her to limits of hull speed and beyond.

    Just keep the center pod build light, yet also independently semi-sea-worthy with positive floation. Maybe fabric over frame? Blue vinyl tarps over PVC tubing for first attempt/mock up? Maybe modify a camping tent? (chop one in half?)

    Bonus: A not-completely butchered set of Hobie 17 hulls and trailer will always be worth something, even sought after. Low end "custom build"? Not so much.
     
  14. garydierking
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    garydierking Senior Member

    [​IMG]

    I converted an old Tornado catamaran using a 9.9 hp Honda and it will do 13.8 knots. More info here.
    Gary
     

  15. Manfred.pech
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    Manfred.pech Senior Member

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