SoCal Jet Boats
Tech Talk => Hulls => Topic started by: mstrader77 on October 31, 2013, 09:12:29 AM
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I am redoing a 1973 Fiberform Malibu jet boat. This boat has a 1/4 inch layer of plywood under the stringers. It appears the plywood was tabbed to the hull, then the stringers were tabbed to the plywood. On top of the stringers, a plywood deck was attached and tabbed to the side of the hull. This created two decks that the seats mount to. In the center was a recessed plywood deck with open bilge beneath. The whole assembly was then coated with fiberglass and covered with carpet. The stringers and all the plywood were wet or rotten. Originally, I was going to replace the plywood and reconstruct it how the factory made it. I talked to a local boat shop about it and they said they would leave the plywood out and add CSM/Roving to reinforce the hull if necessary. This seems like it would be easier than gluing plywood down and would not rot in the future. The cost is about a wash either way. Is there strength concerns that could arise from leaving the plywood out? It seems that the foam would provide plenty of strength and render the plywood unnecessary. Does the plywood transmit the load on the hull to the stringers? Does the plywood act as a giant bedding compound? I am unfamiliar with this type of construction. Hopefully, someone can help me out. The previous owner said the boats top speed was around 65 mph, so I don't want to weaken the structure. Here are a couple of pictures. The red outline is where the plywood sits below the stringer. Thanks for any help.
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I am redoing a 1973 Fiberform Malibu jet boat. This boat has a 1/4 inch layer of plywood under the stringers. It appears the plywood was tabbed to the hull, then the stringers were tabbed to the plywood. On top of the stringers, a plywood deck was attached and tabbed to the side of the hull. This created two decks that the seats mount to. In the center was a recessed plywood deck with open bilge beneath. The whole assembly was then coated with fiberglass and covered with carpet. The stringers and all the plywood were wet or rotten. Originally, I was going to replace the plywood and reconstruct it how the factory made it. I talked to a local boat shop about it and they said they would leave the plywood out and add CSM/Roving to reinforce the hull if necessary. This seems like it would be easier than gluing plywood down and would not rot in the future. The cost is about a wash either way. Is there strength concerns that could arise from leaving the plywood out? It seems that the foam would provide plenty of strength and render the plywood unnecessary. Does the plywood transmit the load on the hull to the stringers? Does the plywood act as a giant bedding compound? I am unfamiliar with this type of construction. Hopefully, someone can help me out. The previous owner said the boats top speed was around 65 mph, so I don't want to weaken the structure. Here are a couple of pictures. The red outline is where the plywood sits below the stringer. Thanks for any help.
On my phone and didn't want to spend four hours posting, but in short, I am with the shop you spoke to.
The plywood that was in there was a shortcut in the manufacturing, they pre built the wooden components and it made assembly stupid fast. Unfortunately, never heard of a Fiberform that had not completely gone to pot a far as dry rot and wet plywood.
I will post again on this thread when I am I front of my laptop...
GT
Edit; btw 65mph my 240 pound ass.
Sent from my SGH-T989 using SoCalJetBoats mobile app
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OK, I'm on a real computer and not the damn phone
I am redoing a 1973 Fiberform Malibu jet boat. This boat has a 1/4 inch layer of plywood under the stringers. It appears the plywood was tabbed to the hull, then the stringers were tabbed to the plywood. On top of the stringers, a plywood deck was attached and tabbed to the side of the hull.
Pretty typical construction for that era.
This created two decks that the seats mount to. In the center was a recessed plywood deck with open bilge beneath. The whole assembly was then coated with fiberglass and covered with carpet.
Sounds about right.
The stringers and all the plywood were wet or rotten. Originally, I was going to replace the plywood and reconstruct it how the factory made it.
Nah... The factory did a hack job... This is very typical of Fiberform.
I talked to a local boat shop about it and they said they would leave the plywood out and add CSM/Roving to reinforce the hull if necessary.
The only thing is, what makes it necessary? Realistically, it should not need much strength added.
This seems like it would be easier than gluing plywood down and would not rot in the future.
Leave the plywood out.... just my opinion.
The cost is about a wash either way. Is there strength concerns that could arise from leaving the plywood out?
Not in my opinion. The fiberglass is several times stronger than plywood. If anything the plywood would create a weaker final product... Again just my opinion.
It seems that the foam would provide plenty of strength and render the plywood unnecessary.
The foam isn't even really necessary.
Does the plywood transmit the load on the hull to the stringers?
Does the plywood act as a giant bedding compound? I am unfamiliar with this type of construction. Hopefully, someone can help me out.
It was just a time saver for the manufacturer. No proof either way that it was better or not.
The previous owner said the boats top speed was around 65 mph, so I don't want to weaken the structure.
Betting it was not... Knowing that hull design pretty well I am going to go out on a limb and say 55 MPH. On a really good day.
Here are a couple of pictures. The red outline is where the plywood sits below the stringer. Thanks for any help.
I would take it down to bare, clean glass and throw down some matting or finish cloth and start building form there. Roving should be used for the structural reinforcing during the stringer install, it is imperative that the air bubbles get squeegeed out.
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Thanks for the advice. I pulled the boat to the shop near me and talked to the owner about the best course of action. He was really helpful and gave me a tour of the shop. After talking with him, I've decided to leave the plywood out and will put the foam back in. It sounds like the foam helps a lot with noise dampening and making the hull have a "solid" feel to it.
As far as speed, I only know what the previous owner told me. When he first got the boat (25 years ago), he had to replace the original block with a new one. The person he bought it from let the old one freeze. The engine has/had an Edelbrock OB4 manifold, Ka heads, and an 850DP Holley. It also has an Accel dual point distributor. I don't know what the cam is. The pump has a minor intake loader and 3.125 insert. A guy that used to work for Fiberform during the early 70's claimed the Malibu Jets were good for about 60 mph. So, it doesn't seem like 65 is TOO out of the question. The guy I bought it from said it would only do about 58 currently. I figure the pump and cam are a little tired. These are all taken from the indicated speedo, so they are probably slightly high.
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I decided to replace the plywood core and leave out the foam for now. I have the core installed and the stringers shaped and in place. Just need to break out the fiberglass and resin. Here is a picture of how the core turned out.
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im wondering how you expect to bolt a motor into that thing without even 1/4 stringers in it... you cant put mounts into plywood and expect it to hold...
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The stringers were not installed in the photos, just the core. The stringers are 2x6 that are about 13' long. Here are a couple of more photos with the stringers in place. I am rebuilding it like it left the factory. Not the best, but I figured it lasted 40 years, so it should be fine for my purposes.
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Hrmm not sure how I feel about stringers being bedded to plywood vs the bottom of the hull....
Maybe someone can chime in who has seen it done that way before. Good job on tackling it yourself.
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Hrmm not sure how I feel about stringers being bedded to plywood vs the bottom of the hull....
Maybe someone can chime in who has seen it done that way before. Good job on tackling it yourself.
Believe it or not, this is pretty much exactly like the factory did them....
As long as everything is laminated properly it will be plenty strong, just always thought this was a strange approach... Then again, odd never meant wrong...
GT
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Believe it or not, this is pretty much exactly like the factory did them....
As long as everything is laminated properly it will be plenty strong, just always thought this was a strange approach... Then again, odd never meant wrong...
GT
Not to side track or thread Jack but after the stringers are cut and before glassing over them, how are wood stringers attached/bonded to hull?
-beerjet-
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Not to side track or thread Jack but after the stringers are cut and before glassing over them, how are wood stringers attached/bonded to hull?
-beerjet-
I've done it a few ways. one way was to lay the stringer onto wet glass and put as much weight on it. the other way i've done it is to use a mixture of cabosil and resin and mix to a peanut butter consistency, lay the stringers down on that.
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I've done it a few ways. one way was to lay the stringer onto wet glass and put as much weight on it. the other way i've done it is to use a mixture of cabosil and resin and mix to a peanut butter consistency, lay the stringers down on that.
Sounds messy....
-beerjet-
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Sounds messy....
-beerjet-
Its not fun.... Went threw 2 pairs of pants, 3 shirts and a pair of shoes redoing the stringers and floor in the swtd.
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A couple of pictures of the repair. It should be good for another 40 years.