Catamaran dinghy 10-11' long

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

  1. Boatguy30
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    Boatguy30 Senior Member

    I built this 11.5' hull with a much different hull shape for some more trials. I'm hoping it is stable enough to hold myself and my 8 y/o son. He will be in the stern working the motor! Hope to have it in the water tomorrow or Tuesday.
     

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  2. buzzman
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    buzzman Senior Member

    The thought just crossed my mind a few days ago, and lo and behold, here you guys are, already 'doing the numbers' on a catamaran dinghy....

    Keep up the good work BG, guess I'm not the only one watching this thread now...

    :)
     
  3. Manfred.pech
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    Manfred.pech Senior Member

    Hydrodynamics

    Can you please take photos of the wave patterns running under outboard motor? I´m very interested in the water flow of these ~3,5m long slender hulls with flat bottoms.
    If you are interested in results of canoe stern hulls: http://www.panix.com/~brosen/catamarans.html
     
    Last edited: Aug 26, 2015
  4. Boatguy30
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    Boatguy30 Senior Member

    I will check that out. At the moment, I'm a bit stuck. The slender hull needs either 2 small outriggers. I had a pair I was going to borrow from a friend if needed, but he's sold them!

    So I really don't know if I have the energy to build outriggers. Thinking of going with something like an O'Brien Transport 9.9.
     
  5. buzzman
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    buzzman Senior Member

    But the Headwater Transport 9.9 is a pram dinghy, not a catamaran...??

    And O'Brien rates it for a 10HP outboard in order to get it planing, which is double the 5HP you were originally contemplating for the catamaran dinghy.

    Is it just me who's confused..??

    I would have thought that either Roger Mann's Puddle-cat 9
    http://rogermann.org/puddlecat/designs/puddlecat-89/

    ...or Mark Gumprecht's L'il Nip would have been ideal
    http://rogermann.org/puddlecat/designs/puddlecat-89/

    Unless you want something longer to carry heavier loads, in which case Mark's 13' Glider would be the go.
    http://www.duckworksbbs.com/plans/gumprecht/cat12/index.htm

    None of the above would be any more difficult to build than the 11.5' hull you've already built, and I'm sure you could simply build another of those and adapt the two along the lines of the Glider.
     
  6. Boatguy30
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    Boatguy30 Senior Member

    Yes, indeed a sad state of affairs. Those other cat designs don't seem like they'd be as good a dinghy to me. At 11.5' this was getting a bit long anyway. I should point out that hull in the pics was made in around 8 hours for about $40 and was only intended for testing.

    I've got plenty of other projects going on. Will see what comes along. I'm am giving some thought to one of the spirited designs smaller say 2.8m designs.
     
  7. Jim Caldwell
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    Jim Caldwell Senior Member

    PVC pipe and the large diameter pool noddles will make a quick and cheap temp out riggers.
     
  8. Manfred.pech
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    Manfred.pech Senior Member

  9. buzzman
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    buzzman Senior Member

    I think the problem here is that you don't know what you want.

    If you want to carry a load at a reasonable speed, then a cat dinghy will do it better than a pram of the same length.

    If you want a planing dinghy with only a 5HP that's not going to happen, unless you use a pram dingy about 8ft long and fly solo without cargo.

    If you want a load carrier (you said 500kg...??) then the Schionning design already pointed out, with a 25HP on the back, might be the solution, or a 12' RIB ditto.

    But you also said you wanted light, and able to be left on the beach without any worries, and there aren't too many places in the world today - other than very remote islands where it's even possible to do that, especially if you leave the outboard on the back.

    Someone will want to steal it..!!

    But if it has to be small, and light, and carry a load, and do at least 8Kts, then either the Puddleduck or the Li'l Nip would do, and could also carry a load and/or 4 people in reasonable comfort.

    Perhaps not in a seaway, though.

    And a ply dingy made with 4mm and 6oz glass over will be way lighter than anything but an inflatable.

    Personally, if I was building either of these two, I'd lengthen the rear sponsons to make a well for the outboard, and to provide additional floatation and so carry a bigger outboard, maybe an 8HP or even 10HP - if you really want to get it to planing speed.

    But it's all relative, really. For example, the Wooden Widget folding dinghy, the Fliptail, weighs bugger all, and could be folded and taken away from the beach, and it is shown on their site planing with a 3HP and one person, but I wouldn't want to drag it over sharp coral or oyster beds....and you still have to carry the motor...

    You could also modify the O'Brien 'Transport' to have longer pods either side of the outboard. Or any other pram design for that matter.

    Actual builds of small dinghies have shown this to be a useful addition, especially with small underpowered motors. Or small motors and overloaded boats.

    The bottom line is, there's no such thing as a perfect dinghy. Cat dinghies will slam in a chop and send buckets of spray over the boat. Known fact. Inescapable design feature.

    Flat-fronted prams have a tendency to do the same. Known fact. Design feature.

    Getting any dinghy up on the plane requires HP. Bigger the boat, more load on board, the more HP required.

    So utilising a smaller motor, but carrying a big load means you won't get up on the plane. But with a decent length cat dingy you might get to 8-9kts with an 8HP.....

    But how often will you be carrying a big load?

    Cruisers do it often. Water jerries. Fuel jerries. Groceries. Cat food. Whatever.

    Look around a Caribbean anchorage at the cruising boats, and see what they carry.

    It's about 50/50 inflatables to tinnies.

    In the Med, where it's mostly sandy beaches or wharves you rock up to, people tend to use inflatables, and then usually RIBs.

    In Oz, where we have lots of sharp rocks, coral and even sharper oysters on our wharf pilings, we tend to go for aluminium dinghies (tinnies) and a 10HP or 15HP donk.

    The latter will weigh the arse end of the dinghy down, but it will still plane with 500kg aboard. And a tiller extension enables the driver to sit forward and thus get up on the plane easier when solo....

    But other than some remote island beaches, I wouldn't leave mine anywhere, unwatched and untended, for very long at all.

    It's amazing how many dinghies grow legs and go wandering off on their own!

    You'll have to decide on the set of compromises you are willing to live with, yourself, no-one else can do it for you.

    Good luck and keep us posted.
     
  10. buzzman
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    buzzman Senior Member

    Another option, if you're now thinking about pram dinghies, might be the Sherpa from John Welsford.

    Not the most handsome dinghy I've ever seen, but he claims it's designed to carry 6 passengers in calm waters and still be able to motor or row.

    He doesn't mention speed, but I guess not much with 6 pax aboard...

    But ought to get 8kts with a 5HP and only two.

    http://www.duckworksbbs.com/plans/jw/sherpa/index.htm

    You might also like to consider the Skylark range from Selway Fisher, as they would also suit the bill, especially with some strengthening of the transom to take a bigger outboard like 5-8HP.

    http://www.duckworksbbs.com/plans/sf/dinghies/gp/skylark.htm
     
  11. buzzman
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    buzzman Senior Member

    But of course, the above are not multihulls, and as you originally posited a multihull, catamaran, design, I presume there is some particular preference for such a design?

    If light weight is one, and this means "becasue it's harder for me to lug a heavy boat around" then what about simply designing it so it can be disassembled, making the components light enough to easily haul about, or load onto a vehicle etc.

    Lifting onto a bigger boat should not be an issue as davits or the boom and tackle can be used, as most crusing sailors already do.

    But frankly, I think the Li'l Nip with an extra foot on the pods at the back would be light enough and do what you want 90% of the time. Even stretched to 10 feet, with an extra foot in the bows as well (you specified 10-11ft originally) would still be light enough to haul easily.

    And if it isn't, then why not simply add a pair of the folding 'beaching wheels' or the clip on 'dinghy wheels' that are readily available at marine stores, to enable easier beaching and hauling of the dink?
     
  12. Boatguy30
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    Boatguy30 Senior Member

    I mainly wanted the cat for the efficiency. Did not expect it to be too wet but any small boat will be. Clearly the super fine hulls will be pretty wet.

    I've owned heaps of inflatables but was just hoping to do something a bit better this time. Its my thinking the spirited 2.8m will perform as good or better than a rib with the same horsepower. Was a bit surprised to see how narrow(4') the 9' version was. And my load goal was 500-600 pounds not kg.
     
  13. buzzman
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    buzzman Senior Member

    Yeah, sorry, the 500kg I referred to was the sort of load a 12ft tinnie with a 15HP can lug around, on the plane, not referring to your design load of 500lbs, although I suspect it's too low anyway.

    If an average person is 80kg (not an unreasonable assumption in these obesity-ridden days) then that's 175lbs, so 4 pax is 500lbs, no other load - ie: no motor, no fuel tank, no shopping bags....

    Heck, two jerries of water and two of fuel adds another 80kgs (approx) which is another 175lbs, so maybe around 700lbs would be a safer bet to aim for.

    Then if one or more of your pax is less than average (eg: slim woman) that would allow for the motor and tank and any shopping.

    Hadn't seen the Spirited Designs Ripple tenders before, that's a neat design.

    More 'boat shaped' than the Li'l Nip or Glider, but same principle, however I note that the Ripple 2.8 has a flat transom, whereas I was suggesting lengthening the Li'l Nip to 9' by adding a foot to the rear pods for additional buoyancy aft and to take a bigger motor.

    Not only does this add more buoyancy it makes it easier to start the motor, as the transom is not so depressed into the water, thus reducing expansion space in the muffler. Always easier to start a motor if it's at its design depth, not lower than this.

    Simply doubling the thickness of the transom and doubling the laminating in the joins, plus a couple of internal triangular 'skegs' from the transom inner face to the inner hull bottom, with maybe a decent set of 'knees' in the corners of the transom, ought to be more than enough added strength for an 8-9HP motor. Heck, double that and run a 15HP!

    Another thought on the Li'l Nip is to fill in the space between the bows and make a space under the thus formed deck for an anchor locker, and space to keep flares, mooring lines, etc.

    Also, despite the more 'boat shaped' lines on the Ripple compared to the Gumprecht designs, I think the narrower well each side of the hump might become tiresome, especially if you want to turn around, as you won't be able to spin on your ankle, you'll need to a lift a foot above the hump, turn it, and then place it back down again, or risk twisted ankles.

    The Li'l Nip or Glider with their vertical-sided centre humps will have a lot more flat foot space on the floor, making them more comfortable, and ought to give them a reduced loaded waterline too, as the flat-bottomed hull (opposed to the chines of the Ripple) ought to sink less for the same mass loaded, having a broader 'footprint'.

    I would probably also put a seat in the transom end, deep enough that you could sit forward and pass the tiller behind you when tight turning, and the fuel tank or spare fuel can for the on-motor fuel tank could live under there.

    A narrow board that could be dropped into place as a forward/central thwart, so passengers could face forwards, either side of the tunnel hump, could be fashioned so it temporarily velcro'd down to the hump for normal two up or solo operation, and then could be quickly 'ripped' up (no tools required) and dropped transversely onto lands in the hull sides that also had velcro on them should you wish to carry passengers.

    Would be fine for short trips and would avoid the 'tripping over it' factor of a permanent cross-thwart.

    My additional 2c worth...

    But I think your heading in the right direction.

    I like my ideas for *me*... :) :)
     
  14. Jim Caldwell
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    Jim Caldwell Senior Member


  15. Manfred.pech
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    Manfred.pech Senior Member

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