Hardtop support

Discussion in 'Fiberglass and Composite Boat Building' started by fallguy, Oct 26, 2019.

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  1. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    I am planning to add a hardtop to the Skoota.

    The hardtop would be build foam sandwich. It will allow the following..

    1. Location for mounting flir, radar, antenna, lights.
    2. Provide shelter from sun or weather for the helm and aft bench area.
    3. Allow for a weather enclosure.
    4. Provide a windscreen made from lexan. I am sensitive to wind and it affects my eyesight after about 30 minutes at the helm.

    challenges-

    1. In order to build, marine haulers have told me it must detach or I will be too high
    2. I must account for the weight on the roof so my cabin roof does not sag; there is no bulkhead under.
    3. I want it to be about 42-48" for and aft. It will require an aft support. The forward supports are all cabin. The width is 8' 6".
    4. My aft supports would interfere with the cockpit unless I pushed them to the outer edge of hull. If so, they would be about 14 feet wide and need to be arched to avoid being head dingers, but they would be great rocket launchers.

    How do I determine scantlings to support the beast so my roof won't sag?

    How do we spec the archway or vertical support for the back of the thing?

    The thing will be curved a bit. Can I build it from 12mm corecell or is that going to be too thin?
     
  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    I think that yours is a Skoota 32?
    Sailing Catamarans - Skoota 32 live aboard cruiser http://sailingcatamarans.com/index.php/designs-2/6-powercats/469-skoota-32
    Will your hardtop be similar in shape to the roof shown in the side profile view in the link above?

    My pal Roy at A & T Marine here in Barbados is an ace fabricator of T-Tops and bimini hardtops -
    A & T Marine Ltd. https://www.facebook.com/atmarineltd/

    Re being able to detach it for transporting the boat, Roy has the same problem, and he uses these 'breakaways' in the aluminium tubing structure
    https://www.easternmetal.com/pdfs/ems_catalog_complete_MARINE.pdf
    Scroll down to page Marine 14.

    Could the forward supports be on the edge of the cabin roof to avoid any risk of the cabin roof sagging?
    Re the aft support for the hard top, are you thinking of something like a goalpost approx 14' wide?
    Putting camber in it will stiffen it up considerably - however that is still quite a long span. Would it be possible to have a pillar on the centreline to help?
    I would think that using 12 mm corecell should be plenty strong enough - it does not have to support much weight, and you could have locally extra aluminium structure in way of the radar scanner (which is probably the heaviest item on the roof).
    The hard tops built by A & T Marine are much thinner than 12 mm, and they seem to work well. They are not designed though for people to climb up on them.
     
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  3. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Yes, that is the structure.

    Yes, 14 is the span. A center vertical support is not ideal as it would be installed onto a floating panel.

    I could install a vertical support about 32" over and behind the helm station. That would reduce the span to say 12'. I could do the same on the other side, but it is a stripper pole, and not desirable in the middle of a nice space.

    Thank you for the reply and the links to hardware.
     
  4. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    DSCN2963.JPG DSCN2964.JPG
    Re Richard's side profile drawing of the Skoota 32, there appears to be about 12" of camber (ie quite a lot) over the 14' of width.
    The more camber that you have, the stiffer the structure will become, up until you have a circular arch - the Romans sussed this out when they were building bridges a long time ago.
    I have attached a couple of photos of the 49' ally powercat shown in my avatar.
    Re the bow view, the bimini structure over the flying bridge was built using 1.5" diameter tube - it only has a slight camber (approx 3") over approx 7'6" width. It is still pretty stiff though. If you have about 12" of camber that will be much stiffer, even over 14'.
    If need be you could perhaps convert the tube into an effective 'tee' section by adding a plate flange on the underside - this would be deepest at midspan, and tapering as it gets further outboard.
    Or even into an effective 'I beam' section using another tube lower down, with vertical tubes forming the 'web', as per the stern view on Reef Runner. Although this might be a bit of an overkill. We built it this way on Reef Runner because the overhead beams had to support kayaks hung sideways in slings.
    Maybe build a basic goalpost, and then test it by applying a load in the middle, and then stiffen it up to suit?
     

  5. fallguy
    Joined: Dec 2016
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    Likes: 481, Points: 93, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    I am realizing there is no reason to build it 14' wide.

    It will provide a framework for my helm enclosure at 8' 6" wide.
     
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