Modular catamaran deck (Expandacraft)

Discussion in 'Fiberglass and Composite Boat Building' started by jtrosclair, Apr 25, 2017.

  1. jtrosclair
    Joined: Apr 2017
    Posts: 5
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: New Orleans

    jtrosclair Junior Member

    First off let me explain what on earth I am trying to do here.

    As someone who lives in the city, my only place to store a boat is under my house, and the only way to transport it is the roof rack of my car. I’ve been struggling for a solution for something that can accommodate my fiancé and myself.

    Salvation appears to come in the form of Expandacraft.

    It’s a modular boat system. What I am planning on buying is the 17 foot catamaran. I plan to use the five foot crossbars. This means I need a deck of at least 5X8 feet. The hulls and cross members would be the same as in the above video.

    I have a decent knowledge of fiberglass. I have restored a 27 foot Ericson and a 34 foot Hunter including structural hull repairs. I have not done much laminating from scratch. Anyway what I need to build is a 5X8 deck supported horizontally across the very ends and in the middle. So a maximum unsupported span of about 4 feet.

    I would like to build a one piece deck that is car top-able. Now, a 60 pound kayak is no problem for me to throw on the roof, but this thing will likely be unwieldy so I’d like to keep it as light as possible. It will likely have 2-3 people on it at most. I do plan to put one base plate for a fishing chair as well. It will be powered by a 3 HP outboard which will weigh 30 pounds or so.

    Here is my plan and you guys can tell me if it’s stupid :).

    The core will be ¾ inches thick. On both side edges and down the middle there will be 2 inch by ¾ inch solid stringers (fir, cypress maybe?) This will serve to add stiffness and sheer strength and where will be where the bolts attach it to the cross members. It will also allow for some higher density areas to attach mounts, fishing rod holders, etc. Between the stringers will be ¾ extruded (not expanded) polystyrene. The blue board from Home Despot. This is a 2 pound density foam which I understand may be a little light. However, this may also be a prototype version of the deck so I’m not sure I want to spring for Divnycell right now.

    As for the layup schedule I’m really not sure. I’m familiar with common layup schedules for canoes and kayaks, but I’m not sure what would be appropriate for this. I was thinking 3 layers of 10 ounce cloth on top and two on the bottom?


    1.) Is what I’m asking for realistically possible? Can a 5X8 deck that is reasonably durable be made for under 50 pounds?

    2.) Is my layup schedule even close?

    3.) I calculate the weight as: Stringers: .25 cubic feet of cypress/fir at 33LBs CF or 8.25 Foam: 2.25 cubic feet at 2 pound density for 4.5 pounds. Glass and resin: 5 total layers of 40 sq feet each at 0.14 pounds per square foot for a total of 28 pounds. This gives me a total of 8.25+4.5+28= 40.5 pounds. Does this seem about right? Once you add 2-3 pounds for the fishing seat base/backing plate and 2-3 pounds for a lightweight motor mount this would be knocking on the door of 50 pounds.

    4. Am I overbuilding in some aspects and underbuilding in others? Am I really gaining any stiffness with the stringers or could I eliminate them if I added some high-density core material of ¾ plywood in any place there are through bolts/ attachments? I would also assume that impact resistance on the edges of the deck is also important, but would this better be achieved by building up the laminate schedule around the edge of the deck?

    There is also the possibility of building this thing in sections, but I would prefer not to do that since I would like to keep assembly to a minimum.
  2. jorgepease
    Joined: Feb 2012
    Posts: 1,620
    Likes: 49, Points: 48, Legacy Rep: 75
    Location: Florida

    jorgepease Senior Member

    I think I would try to get some thick foam core 5 lb density and go easy on the glass. You would just need a good way to fasten it but I would also make the deck removable so you can load it easier. I think 17oz glass on each side would do it if you were careful.

    For core go as thick as you can find or use 3" by 3/4 in strips laid flat to act as stiffeners.

    Good luck
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