Restoring a fiberglass hull

Discussion in 'Boat Design' started by David55cobra, Dec 8, 2008.

  1. David55cobra
    Joined: Nov 2008
    Posts: 18
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: California

    David55cobra Junior Member

    I have been reading as much here as time allows concerning restoration of an older fiberglass hull. The hull in question is an early 70's Dan Arena Runabout registered as a 20ft. It appears that replacement of the wood core transom is fairly straightforward - just a matter of cutting out the old one, replacing with the appropriate thickness of new plywood, then glassing it back in on the inside. My other two questions concern getting the bottom back into its original lines, as it has been sitting on a trailor for 3 years and is slightly ( looks like about 1 1/2" ) indented about midships on the port side, and the four longitudinal stringers are all completely rotten.
    I am wondering if the stringers must be replaced with correctly contoured solid wood ( pine?) , or if they could be a laminate of strips of wood glued to each other in place in the hull ( once the hull is put back into its proper lines of course ). I'm also wondering , if strips bonded to each other is something that makes sense, does the bonding agent need to be flexible to absorb flexing of the hull bottom, or rigid (perhaps epoxy) ?
    I already know that boats are a labor of love, not always logical. I have over 40 years of experience with wood boats, but NONE with fiberglass, so I'm definetly starting from scratch here. Thanks
     
  2. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 470, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    There are several ways to address the stringers you mention. Laminated layers, solid lumber, cored construction, etc. All will work.

    Place the boat on a cradle, one that supports it properly and force the hull down where it needs to be. Often with 'glass boats, the 'glass takes a "set" and will resist returning back to it's original position. No worries, just fill in the hollow area with thickened epoxy.

    You can laminate your stringers, though I think this is more work then necessary. Personally I'd just roughly cut a piece of solid lumber to fit (don't use PT). Scribe a line to match the bottom of the hull, trim the board and refit. When it's close (it doesn't have to be perfect), butter it up with thickened epoxy and bond it in place to the hull.

    Next you'll have to add some "tabbing" which is strips of fabric (cloth if using epoxy, which is strongly recommended), that "tie" the hull shell and stringer together. It should over lap considerably, say several inches. The old stringers should give you an idea of the bulk (thickness) necessary for the tabbing. With this done, the stringers are completed.

    Not knowing how much HP, how fast you're going to go, the hull thickness, etc. I'd be guessing at tabbing thickness, but 3 layers of six inch wide 8 ounce biax tape as tabbing will make a strong stringer. Stagger the layers to spread the loads and make less of a hump in the cured fabric.

    There are many previous threads on this site covering this type of repair. Use the search thingie (as you have been) and you'll get a fair idea how things work.

    Epoxy is easier to work with then the polyester your boat is made from. It's stronger and more waterproof too, so it's a logical choice, even if it does cost a little bit more.
     
  3. Ilan Voyager
    Joined: May 2004
    Posts: 1,160
    Likes: 94, Points: 48, Legacy Rep: 758
    Location: Cancun Mexico

    Ilan Voyager Senior Member

    Par has answered, I'll add only some evidence; the hull must be perfectly dry, and well sanded until the fiber where you'll glue. Use epoxy, which glues truly and has gap filling qualities; a putty of a bit of chopped mat, cabosil and talc for example will insure a perfect contact between the wood stringer and the hull.

    As you have a previous experience in wood, think of the polyester hull as a plywood one in need of a supporting structure. Some bulkheads are a litle luxury that improves the rigidity and strength of the boat. On a 20 feet it's difficult to go wrong.

    Old polyesters may become porous: a sealing with 2 coats of epoxy (three at the bottom) inside is a plus. For sanding the inside of the hull I favor a light sandblasting to take out the old paints and dirt. A bit messy with sand everywhere to vacuum but fast and efficient.

    A good epoxy primary on the gelcoat outside will keep the polyester dry from water intrusion
     
  4. David55cobra
    Joined: Nov 2008
    Posts: 18
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: California

    David55cobra Junior Member

    I've heard of applying a "barrier coat" to fiberglass, and am wondering if that might be the answer to waterproofing the old polyester resin ? A coat on the bottom/outside of the hull, and perhaps also inside to seal it, or is it better not to seal it inside so that moisture has a place to escape ? There has been alot of conversation about using cpes on the older wood hulls - some love it, and some say that it's the kiss of death as it tends to seal in moisture and start the rot process. If I remember, a fully soaked wood hull in the 17 - 20 ft range weighs around 700lbs more than dry - any idea how much more polyester hulls might weigh when wet? -- or am I just splitting hairs and looking for trouble where there is none?
     
  5. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 470, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    CPES has it's uses, though not nearly as many as once thought. It has no use on 'glass.

    A barrier coat on the outside of your hull is a wise investment. It's not necessary to do the same on the inside, though at the lowest point of the bilge, in the sump or drain pan, it would be a good idea to seal these areas also with neat epoxy. Moisture will collect in these area, so it's a fine idea.
     

  6. Ilan Voyager
    Joined: May 2004
    Posts: 1,160
    Likes: 94, Points: 48, Legacy Rep: 758
    Location: Cancun Mexico

    Ilan Voyager Senior Member

    700 lbs of water...it's a sponge not wood...
    Polyester and Wood are not the same thing.

    I seal the inside in old polyester boats for the following reason; as generally there are dirty, with falling paints, and a lot oh parts will be cut. I use sandblasting, a fast way to do the job. There will be some exposed fiber after the operation, maybe pinholes or some defects. So I seal with a 2 coats epoxy: the first one very thin, (with no more than 5 % of acetone -a very fast evaporating solvent, so none will remain in the epox- to break the superficial tension) to wet and a second generally with a charge like chalk-very little-with cabosil and tinted. A kind of gel-coat if you want.

    A third coat in the bilge. It facilitates the maintenance and protects the polyester from the water and often other products like oil.

    The polyester must be dry, that takes a few days if you ventilate well. Outside I use a classic system to seal any defect.
     
Loading...
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.