Fibreglass over Wood Construction/Reinforcement - Structural Question

[stevepamboukes]

Hello All,
I was going to put a hatch into my deck so I could install a bilge pump, next thing you knew I was tearing my entire deck out to rebuild the frame and deck.

Rewind a bit, I got the boat about a year ago - at the metal scrap yard . Its a 1960s Starcraft Riviera, or similar. It was cheap, so I bought it and the trailer it sat on and got it on the water. Restored the engine, most all systems really, and made it my own. I knew the deck was soft and rotten, last year I just put a piece of plywood down over it and called it good. I had poked an inspection hole thru the deck and knew it wasn't a pretty situation down there. Rotted out ribs and stringers. Like mud. So I just played naive and just ran it in the back channels for the summer and it was fine.

So this year when I caught a glimpse at that rot again, I ended up ripping the deck out and gutting all the rotted wood/fiberglass ribs and stringers. Its pretty well gutted, and ready to be built back up. The fiberglass that makes up the hull is actually in decent shape, and dry enough that I think I can reapply fiberglass to it.

My question is about the wooden structure. Does the majority of the strength and rigidity come out of the wood, the glass, or the combo of both? Seems like a simple question, and obviously it would be ideal to have a rugged, well built wood frame with several layers of hefty fiberglass draped over it. I'm wondering, if the majority of the strength/rigidity comes from the fiberglass, is the wood really just sort of a place holder to give shape to the glass? Of course where I'm going with this is, how much time/how good does the wood frame need to be? Ideally I can just lay in some fairly-tight fitting ribs and stringers, then just fiberglass the hell out of them and get a good bond to the existing hull. When they set, the strength will be in the glass itself... the wood really wouldn't be contributing much. My carpentry skills are limited, though I'm fairly experienced with fiberglass layup type work, so i'd rather shift the time and effort into glasswork than woodwork.

Keep in mind this thing is a little 16' runabout that I take out for a few hours at a time. Comes in and out of the water. Given the age and condition of the boat, and being a fairly inexperienced boater, I'd always take it pretty darn easy on her to try to keep the stress off the questionable hull/frame. So I know it needs to be good.... but this isn't something I'm racing or going to France with

Curious about your thoughts. Hope I get some good news. Expecting bad news...
-Steve

 

[Ned L]

As a real quick answer,.... The strength comes from a combination of the wood and the glass over it, though mostly from the wood on your typical production built cunsumer boat. Yes, you could rebuild without the wood (use a place holder as you called it, .. even if just cardboard forms.). Truth is though, that is more time consuming and expensive. You will use a good bit more glass and resin to achieve the same strength as built, which are both a lot more expensive than wood. The most cost effective was is 'as built'.

 

[JASinIL2006]

Hard to beat wood for it's combination of strength-to-weight ratio and cost. If only it wasn't susceptible to rot...

Lots of boats (most?) now are built without wood. You can rebuild yours that way, too, but you will use a lot more fiberglass and resin, and the cost will be much greater.

A reasonable approach is to rebuild using wood and fiberglass as was done originally, and make sure the boat is properly cared for. Do that, and your wooden-structured boat could outlive you...

 

[Scott Danforth]

your boat lasted 50 years with wood encapsulated by fiberglass. if you do it as good as the factory, it will last at least another 50.

 

[tpenfield]

The boat manufacturers who use less fiberglass to encapsulate the wood are relying mostly on the wood. By contrast, some manufacturers add almost 1/4" of fiberglass over the wood structure. They are relying on both wood and fiberglass.

I suspect that your boat was more towards the lesser amount of fiberglass. you have the option to use a bit more and most likely do a better job than the factory.

If you can, build it back up with a similar structure to what was there. Less guessing with that approach.

 

[Woodonglass]

I agree with all the guys. This should be informative...Fabricating Decks, Stringers, and Transoms

 

[stevepamboukes]

thanks for the input yall!

Well, there's no doubt in my mind that wood will be involved in this structure. Its just that due to my skill level and amount of time I can put into it, I can't piece together a decent wooden structure that will actually serve any useful function as a structural body. On the other hand, its very easy to build a sort of superficial wooden "place holder" to just glass over (yes I've heard guys just using cardboard before!).

The main stringer that runs dead center fore to aft will be one long continuous piece. The ribs are contoured to the hull, then cut in half or notched to straddle the main stringer. The rest of the stringers will simply be cut segments to run from rib-to-rib. This is 1"x4" PT by the way. Then I kinda marglass em into place to get the heights right and take up any slop between hull and bottom of stringer. Then glass the heck out of em.

Glass cloth is cheap, resin not so much. But i'm ok putting the money there because, again, thats where my skills are. Even if it costs a bit more, at least I know I can do a good job building up a rugged glass structure.

Not that I'm trying to half-do it, but I think this will be a suitable method for the vessel. Remember she's a junkyard beauty, so I can't spend too much time putting lipstick on a pig. Just needs to be "good enough". And I think with a solid stack of glass n resin on top of some wood, it'll get the job done.
-Steve

 

[JASinIL2006]

Be careful about using PT wood... in addition to often being very damp (which will interfere with the fiberglass adhering), you also have to worry about PT chemicals coming into contact with other materials in the boat and causing corrosion. The typical approach is to use a exterior grade plywood - it doesn't have to be marine plywood, which is quite costly.

If you do a good job glassing the wood, you shouldn't need to worry about rot. (This is especially true of you store the boat properly.

 

[gm280]

Don't use pressure treated wood. Use exterior grade plywood. It already has glue that is waterproof and won't delaminate if it gets wet. If you will read a few of the boat projects on these forums, you will quickly get the idea of how to tackle your project. It seriously isn't very hard to do and will offer you the best and cheapest way to rebuild yours. And post up a picture or three for us to see what you are working on. That way we can offer the best suggestions and ideas to get you back on the water the quickest. JMHO

 

[Ned L]

Keep in mind that strength doesn't come from resin, it comes from the glass. In a layup you want only enough resin to wet out the cloth. That is one of the main advantages of vacuum bagging (it uses a minimum of resin).

 

[Woodonglass]

The stingers and bulkheads of ANY boat are the most critical structures of the craft. If you read the link I provided it tells the proper way to glass them. Using to much resin is almost as bad as NOT using enuf. Search the forum for Stringers repair and you'll find a TON of info on how to do it properly. If you don't then you might as well not do it at all.