Light rowing/sailing/electric tender 10-11 foot of of styrofoam?

Discussion in 'Boat Design' started by DennisRB, Oct 16, 2014.

  1. DennisRB
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    DennisRB Senior Member

    Following my thread regarding a cat tender. It seems that a traditional mono would be better suited. SOR is a light weight tender for a 43 foot cat that is not too tippy with an easily driven hull. A wood floor Achilles with 10hp will be stored on the boat for long range and diving duties.

    The rowing tender will be used most of the time. I already have an electric motor and I am using it on the Achilles, but I assume performance will be much better on a better displacement hull shape.

    It will be a bit of an all rounder. My favorite design is the PT11. This should do all I want.

    http://www.ptwatercraft.com/ptwatercraft/PT11Home.html

    Are there any other similar designs offered in plan only? I have access to a CNC router. Would it be possible to make an MDF mould upside down then use 2 layers of flat panel 25mm styrofoam to make a 50mm thick foam tender shell then vacuum infuse the whole thing in one go with epoxy or a cheaper resin that wont melt it?

    Would this end up as a stiff, strong and light weight tender? I know styrofoam has low shear strength but it will be 50mm thick and its only a small boat. I was hoping the result would need zero stringers and would not require any floatation. The finish need only be to a workboat standard as it will be bashed around.
     
  2. rwatson
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    rwatson Senior Member

    Styrofoam will generally not be worth the effort.

    Proper sheets of hi-density foam for core will produce a better result, for little extra cost really, and a lot less work and unknown longevity.

    Something to consider, I spoke to a big cat owner some years ago, and he told me he was replacing his motor tender with one that would sail as as well.

    His experience was based sailing alone, and anchoring off some small, interesting islands, with a fair offshore wind. His outboard had been reliable, but the risk of being caught motor-less, in wind too strong to row against made him abandon the venture.

    There are a few designs that do all three without major side-effects.
     
  3. DennisRB
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    DennisRB Senior Member

    Cheers. I know I wont get one that will do all three well. That is why I would keep the Achilles. It motors well. This one is meant to sail and row well. And motor slowly with a electric. I think those 3 are compatible and the PT11 I linked would do them as well as I expect.

    I just thought XPS styrofoam would be OK in this application since its cheap and light enough to make a very thick hull, so thick the whole build will be greatly simplified by omitting floatation, stringers and bulkheads. This hull shell would also be strong and stiff enough to man handle pre glassing and the uncomplicated shape should lend itself well to infusion?

    Here is a good test of a special XPS core vs divinycell (which is what my cat is made out of)

    http://www.trinardo.com/y.a.t-yetanothertrinadofoamcompare3

    Yes it tests compressive strength only, but for such a small boat with a thick core surely the low shear strength will be fine if its OK for windsurfers and surf boards? In fact I would argue that IF the shear strength is adequate it will actually be BETTER since the compressive strength will make it more resistant to denting.

    Thoughts?
     
  4. DennisRB
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    DennisRB Senior Member

  5. DennisRB
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    DennisRB Senior Member

  6. rwatson
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    rwatson Senior Member

    Very nice - it has everything, and has a large fleet behind it. Nesting as well. Great link to have.

    Foam building is always a lot more problem than ply and glass, and you don't need to worry about the structural integrity issues of a tested design
     
  7. DennisRB
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    DennisRB Senior Member

    I spent literally 4 hours on duckworks to find that one. :D If you believe the add its very competitive at all 3 requirements (prob least at motoring but perfect for slow speed electric) Anyone else got a suggestion?

    I am on my way to pick up a walker bay 10 with sail kit and tubes for 1/4 of the new price. I would struggle to make anything for that price. However, I still want to make a foam tender in the way I describe one day, just for proof of concept and learning new techniques.
     
  8. gonzo
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    gonzo Senior Member

    That design is tender and not stiff. If you want large initial stability, a flat bottom is the way to go.
     
  9. SukiSolo
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    SukiSolo Senior Member

    The Puffin was produced in the UK in the very late 60s' and was basically a glass clad EPS dinghy/tender. Rare to see these days but very light. I have been in them, rowing and with small outboard etc, and they are OK.

    Try this link, it should give you some detail on it.


    http://messingaboutinboats.wordpress.com/previous-boats/puff/
     
  10. DennisRB
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    DennisRB Senior Member

    Are you referring to the spindrift? I dont mind if it gives up a bit of stability for performance.

    We have the walker bay 10 now. Only rowed it so far. This boat is bigger than I thought it would be. The length is fine but its very wide and probably too wide to use on the davits with the tubes on (I have not used the tubes yet). Longer with less beam would be good.

    Seems to row well with one person. Cant find a way balance it well with 2 people when rowing without causing bad draggy trim. Its very stable. I am going to try sailing it in the afternoon and I am going to pick up some batteries for the electric motor in a few minutes. There is no where to put the batteries so I will have to make a custom enclosure.
     
  11. waikikin
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    waikikin Senior Member


    You might row ok 2 up with the rower facing forward... from the aft seat even with a padded milk crate to sit on... just a thwart..
    hasa worked for me.

    Jeff.
     
  12. rwatson
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    rwatson Senior Member

    Most small dinghy are trim ... ' challenged'.

    The best solution is a perhaps removable, fore to aft bench seat for rowing. That way you can always fine tune the fore-aft weight.
     
  13. DennisRB
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    DennisRB Senior Member

    That would work, but I expect the loss in drag is probably negated by the loss in rowing power by facing forward. I need to row against strong current.

    I think if I make a battery box up forward (behind the original front seat) I will be sitting far enough forward for the forward most oar position to work with the passenger sitting at the back. Now I sit in the middle seat with the passenger at the back and the stern drags.

    Its wide enough that I may even be able to row side by side with the passenger.

    I have picked up the batteries and will see how she sails and motors with the electric today.

    All good research for my ultimate dingy solution I plan to build before our next big cruise. Nikki and I actually like rowing. The inflatable is just not going anywhere under oar unless there is no wind or current.
     
  14. DennisRB
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    DennisRB Senior Member

    Good idea and something to think about on the final solution, but that brings us right back to my original thread on the cat tender. It has a full length fore an aft bench by hull design alone.

    But I just a sturdy plank should be an easy adaption to the WB10
     

  15. DennisRB
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    DennisRB Senior Member

    I wonder if the walker bay tube system can be used on a more efficient narrow rowing hull? The WB10 does not even need it. It would be of more use on a narrower hull, I guess it would be designed so with 2 people and a small load the tubes would be just out of the water?
     
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