Composite Kayak Bench Seats

Discussion in 'Fiberglass and Composite Boat Building' started by gypsy28, Jul 30, 2013.

  1. gypsy28
    Joined: Mar 2010
    Posts: 218
    Likes: 26, Points: 28, Legacy Rep: 120
    Location: NSW Australia

    gypsy28 Senior Member

    I have a Hobie Tandem Island sailing trimaran/kayak. I would like to build a set of "Haka" bench seats. For those not familiar with Hobie TI's I have added a photo with wooden "Hakas".

    The Haka bench's I want to build are 1600mm long by 350mm wide. They sit on the two aluminium beams that join the outriggers to the kayak hull. The distance between the two aluminium beams is 1550mm. The seat needs to be strong enough to support one persons weight (say 90kg/180lbs) A little flex would not be an issue.

    I have some offcuts of 20mm Polycore Honeycomb that are a little oversize that I could trim down to use as the core. Can anyone suggest a suitable laminate. I will be using Epoxy resin (West System) for lamination. I would like to keep the weight of the seats to the barest minimum. The timber ones pictured are around 6 to 8 kg. I am aiming for 4kg at the most.

    I have thought 600 gsm Biaxial on both sides of the honeycomb would be sufficient? Could I go lighter? Maybe 450gsm. Is there any benefit in using say Unidirectional on the bottom and Biaxial on the top (or vice versa)? What if I built it with a slight upwards curve over the 1600mm length, would that add much stiffness?

    I am trying to keep away from Carbon, but depending on the weight of the glass I may consider using carbon. Would 200gsm carbon each side be sufficient?

    Sorry for the long story, but any advice would be much appreciated.

    Cheers, DAVE
     

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  2. rwatson
    Joined: Aug 2007
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    Location: Tasmania,Australia

    rwatson Senior Member

    Flat boards are uncomfortable to sit on.

    I once made 'seats' on a trimaran by stretching nylon fabric between the two beams 1.8 metres apart, and the hull and a 75mm aluminium beam close to the outer hulls ( about .5 metre ) . I laid 300 gsm bi-ax onto the slightly concave fabric, with another narrow layer close to the edges for re-inforcement.

    If that was too lightweight, it wouldn't be hard to lay some foam strips underneath like ribs, and glass those in, until it felt substantial enough.

    It made a nice curved, comfortable seat, with the beams supporting the two ends. I made some scuppers at the bottom of the 'curve' to drain off any spray.

    I was concerned that it might form a bit of a 'wing' for wind across the boat, but it never gave me any trouble.
     
  3. gypsy28
    Joined: Mar 2010
    Posts: 218
    Likes: 26, Points: 28, Legacy Rep: 120
    Location: NSW Australia

    gypsy28 Senior Member

    Thanks rwatson for your advice, sounds like an interesting method. However, I should have been more clear with my description of the intended use.

    The main use of the "seats" is really for camping/expedition gear storage in lightweight waterproof bags. The statement that they needed to hold the weight of a human was merely incase I needed to get to the edge of the boat temporarily for any reason. I am much to comfortable in my padded skipper seat to hike out on the amas :cool:

    Cheers, DAVE
     

  4. rwatson
    Joined: Aug 2007
    Posts: 5,875
    Likes: 311, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    Fair enough. Maybe the advantage of a 'draped' construction would be that anything tied to it would be better held in the curve, and stronger than a flat panel.
     
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