fiberglassing in new floor

[78 Thompson]

I have a 78 Thompson I am putting a new floor in, and want to put a layer of fiberglass over the top, so no water can get to the plywood. I'm thinking a layer, maybe 1/8 inch or less thick and tie it back into the sides of the boat, like it was originally.
I don't have much experience with fiberglass and am totally confused on what kind of resin to use epoxy or polyurethane. I know epoxy is more expensive, so do I really need it? I'm guessing they used poly when the boat was made? Is there any problems with adhesion to the plywood with poly? Many years ago I tried to fiberglass over pressure treated ply and found out that it didnt bond together.
So bottom line I need some suggestions on what to use, what brands are the best, what are the most economical, what to use for cloth or mat, not really sure what the difference is, if any?
also I'm guessing I have about 10'x2' sections on both sides of the engine and gas tank. So how much resin would I approximately need, to cover the complete floor?

 

[todhunter]

Have you looked at the conditions of the stringers and transom (by drilling core samples and evaluating the wood chips)? The experts on this site will tell you that the floor is the last thing to rot after the stringers and transom, and this was true on my boat. It would be a shame to put all that work and money into replacing the floor only to have the rest of the structure fail shortly after.

Your boat is probably made with polyester (not polyurethane) resin, so you're probably good to go with using that for the repair. You'll want to use a non-treated wood like exterior grade ACX plywood. You can do all your repairs with 1.5 oz chopped strand mat (CSM) and biaxial 1708 cloth.

My boat is a 18.5 ft ski boat and when rebuilding the transom, stringers, and deck, I figure I used about 30 gallons of resin, 10-15 yards of CSM, and 25-30 yards of 1708.

 

[78 Thompson]

I basically just looked at the stringers, and kinda felt for soft spots. Looking at them and walking on them they feel pretty solid, clean and dry looking, not sure about transom, I hate the thought of drilling holes, can I tell by tapping on it with light hammer or something. The boat has been in the family since 1980, it has always been covered and or stored inside for the winter, so no water ever sits in the bilge. But I suppose there is the chance the water could have gotten in around the outdrive or something? The floors really took a beating from waterskiing and tubing, coming in on the ladder in back, had the most rot back there, but I suppose the occasional leaky cover probably got to the rest of the floor.
Yeah sorry, polyester thats right.
I do have marine plywood for the repair, so good to go there.
So is mat different from cloth? do I need to use mat and cloth? which do I put down 1st and what does CSM mean? Do I need to use special mat or cloth for polyester or is that only when using epoxy?
Is that a typo? 30 gallons?

 

[GSPLures]

Use a 1/4 bit and don't go all the way through and check the shavings. You can then mix some thickened resin to fill the holes if it is good.
If not you have to replace it. Outside of the dangers of a rotten structure, your new repairs will prematurely fail.

 

[Emerger]

I have a 78 Thompson I am putting a new floor in, and want to put a layer of fiberglass over the top, so no water can get to the plywood. I'm thinking a layer, maybe 1/8 inch or less thick and tie it back into the sides of the boat, like it was originally.
I don't have much experience with fiberglass and am totally confused on what kind of resin to use epoxy or polyurethane. I know epoxy is more expensive, so do I really need it? I'm guessing they used poly when the boat was made? Is there any problems with adhesion to the plywood with poly? Many years ago I tried to fiberglass over pressure treated ply and found out that it didnt bond together.
So bottom line I need some suggestions on what to use, what brands are the best, what are the most economical, what to use for cloth or mat, not really sure what the difference is, if any?
also I'm guessing I have about 10'x2' sections on both sides of the engine and gas tank. So how much resin would I approximately need, to cover the complete floor?

For what you are doing, two layers of 1.5 ounce chopped strand mat will do. 1.5 ounce CSM weighs 1.5 ounces per square foot. You will need roughly 4 ounces of resin to wet out each square foot of mat. Measure the square footage of the work and use the numbers to estimate the materials. Remember to calculate twice, once for each layer.

Polyester resin will be fine for your project. It's cheaper and easier to use.

You mentioned that you didn't have success with fiberglass adhering to pressure treated plywood. It is likely that the plywood was still wet from the treatment when you glassed it. Let it dry for a couple of weeks first.

 

[todhunter]

You should drill the holes. They're easy to fill with resin or 5200 if the shavings come back dry, but believe me...I know the feeling. You're afraid of what you're going to find. Tapping with a hammer or any object is not going to tell you anything. A rotten stringer sounds like a new one.

Mat is different from cloth. Mat is used for waterproofing, cloth is used for structure. 1708 cloth has a 0.8 oz mat stitched to the back, so you don't need to use a separate mat if using 1708 for structural repairs. The mat should go down against the wood. For new decks I would do one layer of CSM on the bottom and either 2-3 layers of CSM on the top, or a layer of 1708 and a layer of CSM over that.

CSM = chopped strand mat

No special mat or cloth needed with polyester (that I'm aware of).

30 gallons is not a typo. 1708 drinks a lot of resin. Making peanut butter (thickened resin) for fillets takes a lot of resin. Being a newbie, there was a learning curve so I wasted some resin. So yeah, 30 gallons.

 

[Chris1956]

In addition to what Emerger said, Lay some strips of cloth over the tops of the stringers, and around the perimeter of the plywood deck and saturate them with poly resin. Now install the plywood deck and screw thru the deck into the stringers with SS wood screws.
On some boats, the edges of the deck needs to be tapered down, and to be held down with weights until the resin and glass strips cures to bond the deck to the glass of the hull. I simply used cements blocks for this. After the resin cures, the mat can be laid on the plywood deck, overlapping the perimeter to bond to the rest of the hull, and saturated.

 

[78 Thompson]

You should drill the holes. They're easy to fill with resin or 5200 if the shavings come back dry, but believe me...I know the feeling. You're afraid of what you're going to find. Tapping with a hammer or any object is not going to tell you anything. A rotten stringer sounds like a new one.

Mat is different from cloth. Mat is used for waterproofing, cloth is used for structure. 1708 cloth has a 0.8 oz mat stitched to the back, so you don't need to use a separate mat if using 1708 for structural repairs. The mat should go down against the wood. For new decks I would do one layer of CSM on the bottom and either 2-3 layers of CSM on the top, or a layer of 1708 and a layer of CSM over that.

CSM = chopped strand mat

No special mat or cloth needed with polyester (that I'm aware of).

30 gallons is not a typo. 1708 drinks a lot of resin. Making peanut butter (thickened resin) for fillets takes a lot of resin. Being a newbie, there was a learning curve so I wasted some resin. So yeah, 30 gallons.

Yeah you are partly right on, I don’t really want to know. What is the 5200?
What is the peanut butter? Something mixed in the resin? Where is the most likely spot for rot on the transom, down low probably? How about stringers on the tops?

 

[todhunter]

3M 5200 is a marine sealant.

Peanut butter is thickened polyester resin. You usually add Cabosil (aka fumed silica) to thicken the resin, and 1/4" chopped fiberglass fibers for strength. You mix it to a peanut butter-like consistency, and use it to fill gaps, holes, etc.

Drill the holes low...maybe 1 inch from the bottom of the stringers and transom. I used a 1/4" drill bit. Be careful not to drill all the way through to the other side - just through the fiberglass layer on the side your drilling, until the drill hits wood. You want light dry shavings that smell like fresh cut wood / saw dust. Dark or damp wood means rot. Black muck means advanced stages of rot. You can see good vs. bad here: https://forums.iboats.com/threads/1...ers-and-upholstery.739025/page-2#post-5615083

 

[Emerger]

Yeah you are partly right on, I don’t really want to know. What is the 5200?
What is the peanut butter? Something mixed in the resin? Where is the most likely spot for rot on the transom, down low probably? How about stringers on the tops?

Overwhelming at first isn't it? Peanut butter, 5200, cabosil, etc. Stepping back for a second, you said you wanted to replace your floor. I'm assuming there's some reason to replace the floor such as soft spots or something. You might want to start by removing the floor because it's fairly easy to replace, depending on how far you want to go. It will also give you access to examine the stringers and transom to get an idea of just how much will be involved, again, depending on how far you want to go with it.

I'm posting a link to part one of a series of videos on youtube posted buy a guy who rebuilt a boat with a floor similar to yours. I don't know the guy, but he's pretty thorough about explaining what's involved in a rebuild. You may not need to go as far as he did but the videos will be helpful.

 

[78 Thompson]

3M 5200 is a marine sealant.

Peanut butter is thickened polyester resin. You usually add Cabosil (aka fumed silica) to thicken the resin, and 1/4" chopped fiberglass fibers for strength. You mix it to a peanut butter-like consistency, and use it to fill gaps, holes, etc.

Drill the holes low...maybe 1 inch from the bottom of the stringers and transom. I used a 1/4" drill bit. Be careful not to drill all the way through to the other side - just through the fiberglass layer on the side your drilling, until the drill hits wood. You want light dry shavings that smell like fresh cut wood / saw dust. Dark or damp wood means rot. Black muck means advanced stages of rot. You can see good vs. bad here: https://forums.iboats.com/threads/1...ers-and-upholstery.739025/page-2#post-5615083

So I may be getting ahead of myself, but if I get to glassing in the floor. Is the process, to get flooring screwed down, probably coat bottom with a layer of CSM 1st, then fill the cracks around the edges with the PB, then tab it in with the 1708, then lay the CSM over the complete top of floor?
I have been reading your thread and have a couple questions, what is the reason you are putting radius on some of the pieces of wood, so you can get more of the PB in the corners?
the other question was, I think you were doing the motor mounts and it looked like you ran the 1708, I think it was, straight up and left it, to be cut off. Why didnt you fold it over the top of the floor? probably not a good idea to have the floor involved in the support of the engine?
Your project is looking really good!
Thanks for the help

 

[todhunter]

I agree with @Emerger - watch JMink's video series on YouTube where he rebuilds his boat. Also Friscoboater's Garage on YouTube - watch his Sea Ray restoration. Take notes. It's a lot of hours over video but these two channels gave me the confidence to do my boat.

Your order of operations for glassing the floor sounds pretty good to me.

 

[todhunter]

I routered the edges of some boards so the fiberglass would go around the edge. Like the tops of the stringers and the edges of the deck around the bilge. Fiberglass doesn't like to lay around sharp corners, which is why you fillet with PB on inside corners and round over the edges of the wood for outside corners. For your question on the mounts, I could have wrapped the glass around onto the floor and that would have been fine structurally, but there would have been a bump there from the layer of glass tabbing that wrapped over the edge.

Thanks for the kind words. I'm a first timer and had the same questions and reservations as you do, just about 7 months ago. Go watch those videos and take notes.

 

[78 Thompson]

Well you guy’s were right, the transom is completely rotted out. I don’t know what’s keeping it together. I must have been running this thing for years with it rotten. Has anyone ever seen one of these just give way and sink the boat? It’s an I/O so the front of the engine had good support, but not sure how the back is holding?

 

[JASinIL2006]

You have to fix it before running it. A boat with a rotten transom is unsafe for you and your passengers.

 

[briangcc]

If you're transom is shot, good bet your stringer and motor mounts are too. How confident are you that a good wave isn't going to break things apart?

I certainly wouldn't launch knowing that the transom is shot.

 

[78 Thompson]

Yeah I don't think I would chance it anymore. I am actually surprised something didn't happen to it, the last couple years that we were using it.
Guess we got lucky!

 

[GSX455]

Well you guy’s were right, the transom is completely rotted out. I don’t know what’s keeping it together. I must have been running this thing for years with it rotten. Has anyone ever seen one of these just give way and sink the boat? It’s an I/O so the front of the engine had good support, but not sure how the back is holding?

Just search youtube motor falls off, or broken transom.
Here what mine looked like, and it came out in wet chunks. If it flexes too much you could crack the fiberglass, especially if you have a 90 HP motor, that weighs 300 pounds.

 

[Scott Danforth]

if the floor was soft enough to need replacing, the transom and stingers rotted decades prior. the floor is ALWAYS the last thing to rot. ALWAYS.

 

[78 Thompson]

The outside of my transom has hairline cracks in it. Is this just the gelcoat cracking? Or do I have more severe problems? I can get a picture tomorrow, if that would tell you more.