Advice needed on boat reconstruction

[mstrader77]

I am redoing a 1973 Fiberform Malibu jet boat. I posted another thread similar to this, but am rethinking my idea. 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.

 

[jbcurt00]

Re: Advice needed on boat reconstruction

I saw your other thread, and although I have little to base this on, I'll give it a shot:

The 1/4" plywood could be a core material that was used instead of laying up thicker glass. 2 thin sheet of glass on either side of a 1/4" sheet of plywood would be stronger then just the 2 layers of glass alone and much lighter then the number of glass layers to build to the same strength.

Google plywood core boat hull

Or, and somewhat more likely:
The plywood was added at some point during designing & building the hull originally. Plywood or wood strips are often found laid & glassed down in what appears to be a random fashion.

If this hull will hit 60+mph, it may be to reduce harmonics or deflection/distortion of the hull acting as stiffeners.

To eliminate stress cracking along the hull bottom, along that stringer on either side of the central bilge...

Perhaps it was merely a design element that someone on the design team thought necessary.

Having never seen below decks on a 1973 Fiberform, it is not surprising that your query has yet to be resolved.....

I know I haven't proposed a solution yet, I'll suggest you try to put it back the way it was originally built.

 

[mstrader77]

Re: Advice needed on boat reconstruction

Original construction is what I'm leaning towards. It would have been easier for Fiberform to leave it out originally. There must be a good reason for it.

 

[mstrader77]

Re: Advice needed on boat reconstruction

Here are a couple of pictures of how the repair turned out.

 

[eggs712]

Re: Advice needed on boat reconstruction

Looks good! Any more pics of how ya did it all?

 

[jc55]

Re: Advice needed on boat reconstruction

That's beautiful, great looking work!

 

[mstrader77]

Re: Advice needed on boat reconstruction

A little more info.

I used 1/2 inch exterior plywood for the core, front & side floor. I went 1/2 inch because I found some 5 ply shop grade for 16.00 at a bargain lumber yard near me and figured the extra weight would be marginal. The center floor section is 7 ply ? inch exterior shop plywood. The stringers are Douglas fir that was straight, dry and almost knot-free.

PL premium was used for gluing the core to the hull. I wet the hull with a damp cloth then used a V-notched trowel to spread the glue over the plywood. After setting the panels, I placed concrete blocks on the plywood for clamping pressure.

The stringers were glued to the plywood with PL also. I then used polyester resin, roving and CSM to tab the stringers to the core. Resin, ? inch chopped strand fibers and CSM were used around the edges of the core.
The side floor was glued to the stringers and core with PL. Stainless screws were also used. The center floor was bedded into the center with chopped fiber putty. CSM was laid over everything as a final top coat.

This may not be popular, but I did not glass the bottom of the side & front floors or the middle area of the core. In the couple of boats I have worked on, the only wood that was rotten was completely encased and not allowed to dry out. By leaving a section of wood able to breathe, I hope to avoid wet wood and rot. I will be installing plastic access holes in the side and front floor areas so these areas can breathe while being stored. We will see?

I'll try to load some more pictures.

 

[jbcurt00]

Re: Advice needed on boat reconstruction

Did you replace any flotation foam when you removed during demo?

 

[mstrader77]

Re: Advice needed on boat reconstruction

I did not put foam back in the boat.

 

[mstrader77]

Re: Advice needed on boat reconstruction

Some more pictures...

 

[mstrader77]

Re: Advice needed on boat reconstruction

And some more...

 

[bigdirty]

Re: Advice needed on boat reconstruction

uh huh.. 'fiberform' aka the 'barely floating sponge' All I have seen/heard over the years is not very good regarding this manufacturer, as they tend to absorb water over time into basically the entire hull due to a combination of the nature of the construction, and quality of original build. They end up getting heavier and heavier, and ive heard many people blowing up perfectly good motors just trying to get the boat up on plane :lol: Might not be such an issue in your case, and looks as though you have done some good work so i'm sure it will be better than factory by the time you are done.

 

[mstrader77]

Re: Advice needed on boat reconstruction

The foam in this boat had not absorbed a lot of water. I think there was more water in the wood than the foam. Of course the combination creates the problems. If the people installing battery trays, tool boxes, seats and fuel tanks would have sealed the holes through the floor, it probably would still be sound. I wouldn't have even known if I didn't pull the motor to repair the leaking jet intake. When I saw the half corroded engine mount lag bolts, I knew I was in for some work. On a bright note, the transom was in great shape. This is probably because it doesn't flex with a jet and the only fiberglass was around the edges allowing the wood to breathe.

 

[eggs712]

Re: Advice needed on boat reconstruction

I have a later year Fiberform (1980) 16' and it's actually in great condition. The reason seems to be that it has a good drainage system. It has a center hollow channel that runs from the bilge to the bow, where it is slightly open (closed bow boat). I've peeked my head into the channel from the bow and the wood surprisingly looks good as new!