Plywood epoxy Catarmaran Bouyancy

Discussion in 'Multihulls' started by nickvonw, Apr 11, 2011.

  1. nickvonw
    Joined: Oct 2009
    Posts: 22
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: on the move

    nickvonw Junior Member

    This might seem like a stupid question, but as i have been told the only stupid question is the one you dont't ask..... so here goes

    hyperthetically would a 40 foot catarmaran built from plywood and glassed with expoxy float if both hulls where severally damaged??? would the bouyancy of the plywood keep the thing afloat??? Do thes boats need foam bulit on for bouyancy

    just wondering

  2. bernd1972
    Joined: Mar 2011
    Posts: 67
    Likes: 7, Points: 8, Legacy Rep: 88
    Location: Laboe, Germany

    bernd1972 Holzwurm

    It depends on all the other stuff aboard like engine, batteries and so on. Of course the amont of glass applied to the plywood has an influence on it as well. Considering all that stuff thant normally goes into a boat I would say no, it will sink whr wrecked.
  3. Richard Woods
    Joined: Jun 2006
    Posts: 2,209
    Likes: 175, Points: 63, Legacy Rep: 1244
    Location: Back full time in the UK

    Richard Woods Woods Designs

    A well designed multihull will have each hull subdivided with watertight compartments and crash bulkheads. So no, it should not sink.

    Please don't add pour in foam to airtight compartments. It makes any leak finding/repairs impossible and also absorbs water from condensation and so gets heavy and breaks down.

    Richard Woods of Woods Designs
  4. bernd1972
    Joined: Mar 2011
    Posts: 67
    Likes: 7, Points: 8, Legacy Rep: 88
    Location: Laboe, Germany

    bernd1972 Holzwurm

    Hopefully you are right Richard. But based on your description it will not stay afloat due to material properties as I understood the question but because of adequate design.
  5. rayaldridge
    Joined: Jun 2006
    Posts: 581
    Likes: 26, Points: 0, Legacy Rep: 322
    Location: USA

    rayaldridge Senior Member

    It's very difficult to put enough glass and resin on a piece of ply to make it sink. This is an easy experiment to make. Cut a square foot of ply out and start layering on the glass.

    A ply and glass boat should not sink, even without adequate crash bulkheads and flotation boxes, unless it's full of really heavy stuff, which should not be the case for any good multi. I think the reason for adequate flotation and careful division of spaces is not so much to prevent absolute sinking as to allow the boat to float high enough after holing to allow repair and recovery.

    It's the case that several ply and glass boats have survived encounters with reefs. The reefs chewed the bottoms completely out of the boats, but the boats held together and floated ashore.
  6. Paul B

    Paul B Previous Member

    When my cockpit was rebuilt we used 6mm marine ply, coated both sides with WEST epoxy and added one layer of BIAX to the outside after assembly. Then the surfaces were prepped and painted.

    We cut holes for 6" inspection ports in the cockpit sides. The sections we cut out did not float. I was surprised.
  7. rayaldridge
    Joined: Jun 2006
    Posts: 581
    Likes: 26, Points: 0, Legacy Rep: 322
    Location: USA

    rayaldridge Senior Member

    What kind of ply was that? I guess I'll have to do the experiment I proposed, because I'm very surprised, too.
  8. Autodafe
    Joined: Jun 2008
    Posts: 137
    Likes: 6, Points: 0, Legacy Rep: 112
    Location: Australia

    Autodafe Senior Member

    As Ray points out it depends heavily on the the type of wood in the ply.

    Ply available in my area varies in density from 0.3 to 0.8.

    In australia "Marine" ply does have some density constraint written in the standard, from memory it says a density of at least ~0.45.

    Most ply made from tropical hardwood and pine will fall in the range 0.4 to 0.6.

    Sheathed in glass (glass/epoxy density is around 1.5 to 2) inside and out it will usually float, however as Richard points out design floatation is better than intrinsic floatation.
    If you rely on material buoyancy then the holed vessel will end up floating with just the cabin top above the surface, and is only slightly better than totally sinking. It's nice to have but should not be "Plan A" for dealing with watertight damage.

    Design buoyancy compartments can be placed low in the hulls and crossbeams to ensure a live-able compartment is above water whichever way up a holed vessel is floating. If the compartments are well designed and built then foam is just extra weight and cost.
  9. oldsailor7
    Joined: May 2008
    Posts: 2,097
    Likes: 42, Points: 48, Legacy Rep: 436
    Location: Sydney Australia

    oldsailor7 Senior Member

    It's not a stupid question.
    I have an example from personal experience.
    My friend Kark Uthoff (RIP), designed and built a 40ft Catamaran in 1/4" ply/epoxy, with steel truss crossarms designed by a civil engineer.
    It had fine L/B ratio hulls and was a very fast and attractive boat. .
    On a filthy rough night in a race on Lake Ontario, one of his daggerboards dropped right down in the case and could not be retrieved. The racking forces on the hull cracked the hull structure and the hull flooded. The cat canted over to an acute angle and Karl hoisted a distress flare up the mast and was safely rescued. Next day Karl rented a light plane to search for the boat ---but nothing was found. It was assumed that the other hull filled with water during the rough night and the steel crossarm structure dragged the remainder to the bottom.
    In the same race we also took on a lot of water in the floats due to leaking hatches in our Bucanneer 28, but the end 1/3rds of our floats were filled with air bags in the form of large soft plastic acid bags which were blown up with a vacuum cleaner. So apart from some unwanted water ballast, (LOL), we were OK.
    Lesson learned. :eek:
  10. CatBuilder

    CatBuilder Previous Member

    Hmm... good vote for Okoume.

    Being one of the lighter weight plywoods for its strength (and in this case, volume), it would float better than other more dense woods.
  11. DarthCluin
    Joined: Mar 2009
    Posts: 131
    Likes: 3, Points: 0, Legacy Rep: 47
    Location: Florida

    DarthCluin Senior Member

    Weight of water from Wiki Answers:

    Seawater per cubic foot - 63.6 to 64.3 pounds

    Fresh water per cubic foot - 62.4 pounds

    Plywood weights from Boat Builder Central site:

    Okume BS1088 plywood per cubic foot - 31.8 pounds

    Meranti BS1088 plywood per cubic foot - 39.7 pounds

    Plywood weight from Boulter Plywood:

    Douglas Fir marine grade per cubic foot - 37.5 pounds

    Epoxy information from the WEST website:

    WEST System Epoxy, Resin 105-A (1 quart) with 205-A or 206-A Hardener (0.43 pint), mixed weight 2.87 pounds:
    Saturation coat, porous surfaces - 90 to 105 square feet
    Buildup coat, non-porous surfaces - 120 to 135 square feet

    Weight of glass fiber fabrics is per square yard (per Clark Craft).

    Per the Fiber Glast website, resin is calculated at 1.75 times the fabric weight

    Here are the links:

    I enjoyed the materials calculator at Fiber Glast's website. Plywood weights were calculated from 1/4" or 6 mm sheets, depending on the species. Douglas Fir 1/4" was 3 ply, not 5 (5 ply was not offered in 1/4").
  12. bruceb
    Joined: Nov 2008
    Posts: 1,271
    Likes: 50, Points: 48, Legacy Rep: 214
    Location: atlanta,ga

    bruceb Senior Member


    Since I just removed some 3/8 epoxied and one side glassed ply,(3 ply pine) and had some just finished 6mm (real 1/4") meranti, I also "floated" some samples for a test. Both did float, but just slightly. I only expect a thin ply boat structure to be neutral, and any positive flotation has to be added or designed in as tanks or compartments. On my 24' about 1500lbs tri, the metal and equipment is between 375-400lbs. I have weighed almost all parts and equipment- a lot more than the wood could be expected to float. I carry dingy air bags with around 800lbs volume as reserve flotation and I have just added bulkheads to divide the floats into three compartments each. Larger/thicker boats might fair better if they are not over-loaded with equipment, but I would not count on it. B
  13. catsketcher
    Joined: Mar 2006
    Posts: 1,315
    Likes: 165, Points: 63, Legacy Rep: 790
    Location: Australia

    catsketcher Senior Member

    There are a couple of cases of trimarans getting their bottoms ripped out on rocks and floating with their wing decks awash.

    It would be a mistake to use the material properties of any composite for flotation. Assume my cat is built from 10mm foam. Surface area of hulls is approx 12 x 3 (gunwale to gunwale) x 2 hulls. Area of foam = 72m squared. Volume = 72 x .01 = 0.72 m cubed. This is 720 litres. My boat weighs 4 tonnes. So even with foam and no laminate ( and you need a laminate ) you won't float a multi with its hulls material.

    As Richard says put in lots of crash bulkheads. There is no reason to put a single berth up the front of a cat. Put a full bulkhead at the start of the bridgedeck and have a proper sail and fender bin that self drains in each hull.

    At the aft end don't put a single berth in the back. At the end of the bridgedeck put a full bulkhead in and have spot for garbage, dinghy gear, waste tank etc accessible from on deck. You need to seal off the wet and odourous spots from the accommodation anyway.

    On Kankama with in built water tankage and built in ice box as well as above you have about a 50% chance of getting a hole in a compartment if you pierce the hull.


  14. oldsailor7
    Joined: May 2008
    Posts: 2,097
    Likes: 42, Points: 48, Legacy Rep: 436
    Location: Sydney Australia

    oldsailor7 Senior Member

    Heres a case in point where we were glad we were sailing a trimaran.
    We were sailing home from the western shore of lake Simcoe, in our Piver Nugget, across to Sutton, when we felt a thump as we hit a deadhead in the water.
    For those who are not familiar with deadheads, they are logs or the trunks of trees which have become waterlogged with only one end still at the surface. Thus they are like battering rams if hit end on.
    It wasn't a big thump, but within 60 seconds the floorboards in the hull began to float. We had been holed in the fore section of the main hull.
    There was nothing we could do about it, the floats took up the support of the boat and we simply sailed on to our dock at Keswick.
    We were able to hoist up the bow of the boat by using the bollards on both sides of the dock to bail out the water. The hole was an impact crush as big as a fist and we were able to patch it up from the inside with underwater curing epoxy putty. This surficed until haul out time (it was near the end of the sailing season) when we did a proper glasscloth and epoxy repair.
    It might not have ended so conveniently if we had been sailing a catamaran.:eek:

  15. Steve W
    Joined: Jul 2004
    Posts: 1,835
    Likes: 72, Points: 48, Legacy Rep: 608
    Location: Duluth, Minnesota

    Steve W Senior Member

    Even without built in floatation it will most likley float due to trapped air pockets as well as the small bouyancy of the structure,remember much of the plywood in the boat as well as the solid stock is sealed but not glassed such as bulkheads,berth tops,cabinetry etc so provides more flo atation. It is a rather moot point though because its not a lot of use if its not habitable so you need to design in floatation to keep it floating high and level. The use of foam is fine as long as its in the form of block styrofoam which does not absorb moisture, not the pour in 2 part foam which does. I used to own a Macgregor 36 cat which had block foam everywhere,so much in fact that there were plugs in the hulls which could be removed to drain water by the power of the bouyancy alone, you would do this after righting from a capsize should it ever happen,then replace the plugs and bail the rest,"sunk" was just an inconvenience, the downside of course was no storage space.
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.