Replacing Ply Panels With Composite

[john_nohj]

Hi, I cant seem to find an answer to this question anywhere on the net so here seems like the best place to ask.
I've recently bought a woods strider catamaran built in ply and epoxy.
The ply is starting to develop rot in several places so is going to need to be replaced soon.
A thought had crossed my mind about the possibility of replacing the whole of the hull ply with grp / foam sandwich boards or something along those lines.
Is there a reason people don't do this?
I know that sandwich would obviously be thicker than ply but that's not insurmountable.

Has anyone done this before?
There must be a reason I cant find any info on this!

Any input appreciated,
John

 

[gonzo]

Firstly, you would have to re-engineer the whole structure. Secondly, foam panel are not likely to bend like plywood.

 

[john_nohj]

If the sandwich was the same strength or stronger than the ply it was replacing, why would it require re-engineering?

 

[TANSL]

If the sandwich was the same strength or stronger than the ply it was replacing, why would it require re-engineering?

If the sandwich was the same strength or stronger than the ply it was replacing, you don´t need re-engineering.
But to verify that the new sandwinch panel is equal to or stronger than the existing one, you will need to do some calculations. You can place several very resistant materials but the whole can be very weak. And if the resulting panel is not as strong as the existing one, as Gonzo says, you have to recalculate the entire structure.

 

[philSweet]

John, assuring that the panels are in fact as strong as the ply panels IS engineering. So is assuring that the bond patching in the new pieces will transfer all the loads without rerouting the stresses elsewhere. You are probably only thinking about the global sort of strength, but there is puncture and gouge resistance, dent resistance, rigidity, peel strength, buckling, thermal issues, corrosion compatability, the ability to hold fasteners and support hardware attachments, etc. Plywood is pretty good at most of these things. On a cost basis, it is very good at these things.

 

[gdavis]

hello john....was the plywood marine grade?? Was there damage to hulls where the rot started? How about leaking deck fittings and all? May be that the easiest way out is some good marine ply. I've seen meranti ply that was 25 years old still in perfect shape. You just have to be sure there is not water penetrating the glass epoxy sheathing. Sealed on both sides with say 6oz boat cloth in epoxy should last many moons if taken care of. Perhaps and extra layer on the bottom for a little more protection.You should also make sure you can ventilate the inside of the hulls when not in use. By the time your done buying foam core heavier cloth, more resin and probably more frustration and time plywood might be the easy way out for you. I've done boiling tests on marine ply and didn't see it delaminate. They say boiling it for 1 hour then letting it completely dry then more boiling is equal to like 30 years +,-.Some times I really don't know why i do tests like this, maybe i'm a bit odd. Any how, maybe this helps you,,,,,,,,,,peace..g

 

[john_nohj]

George, there doesn't appear to be any damage to the outside of the hull. The rot seems to have started from the inside. The inside paint is peeling away and I'm guessing ambient moisture has allowed rot to take hold on some of the inboard surfaces.
As it is a ~22 year old boat Id think it has probably held up pretty well. It will be a month before I can really take it apart and find the full extent of damage however I will be replacing the hull panels regardless (both for repair and for the "enjoyment" of the project).
I have been doing some very rough cost estimates and of course ply will be far cheaper and easier, however in the long run I envisage GRP sandwich would be far less maintenance, and possibly more attractive resale?
Gonzo, I have been thinking the bending problem through. What if one were to produce a panel of 1 side GRP and 1 side foam (possibly slitted to aid bending). Then bond this in place with the GRP side inboard. That would then leave the foam side exposed to be glassed over once all panels are in place?
And finally philSweet, I'm an electrical engineer by trade, and have access to mechanical designers to point me in the right direction as far as materials calculations go. I would be prepared to do physical tests aswell. Is this still unfeasible in your opinion?

And thank you all for your replies so far. Please excuse my ignorance/naivety on the subject. I spend far too much time reading about these things online, but sometimes I find a lot of conflicting views. I find nothing compares to asking advice advice from experienced people!
I'm fairly sure ply is the way to go, from this thread and others experiences, however I still want to explore the possibility of composites!

 

[gdavis]

Hello again john, here at the boatyard we use some foam core that is definitely bendable. I would say that it bends as easy as say 1/4 ply and is about a 1/2" thick. It also bends on a very fair curve. I'll check in the AM and get back to you tomorrow evening with the brand name and density and all that stuff. Stay tuned...........g

 

[gonzo]

The first question to answer is: how are you going to attach a foam sandwich panel to a wooden structure?

 

[gdavis]

hey john

Okay, got some info for you. The core you could use is Core Cell 1/2" A500 or if your hulls of a lot of shape you can get kerf cut a500, this will bend in both directions for you. You may be able to use the original hull as form for the new ones. Cut to shape, lay the core on the hull and epoxy the joints together,(1 side at a time) hold the core on with 2 sided tape or maybe hot melt glue or I have other ideas. Then put on 1 layer of 1208 +- then 1 layer 1808 0-90 with hopefully vinyl ester resin.(good stuff). This should be enough to hold the shape so you can take the layup off of the hull( form) and put a layer of 1208 +- on the inside. This will really stiffen up the shape so you'll have to be careful it maintains the shape you want. The new sides will have a slightly different shape due to the thickness of the foam but you should be able to coax them in. So now after taking a lot of measurements, beam,heights,rocker,lengths you can cut and shape then join the two halves together then fit the hull to the deck. That's a lot of work john! After the hulls are joined up you'll have a very ridged hull that you can add bulkheads to where needed before attaching the hull to the deck. If the deck is plywood it can be epoxied to the hull, core cell and wood both like epoxy. Where ever needed you can lay in g10 or teak blocking for hardware attachments.You should also run glass tape on the seams where the hull meets the deck Are we over budget yet?. Is the deck plywood? Is it in really good shape? If you have to replace the deck also maybe you should reconsider this project and go back to new ply wood hulls and get this bad girl together and go sailing. Know what I mean. As for resale value you'll never make all that cash back, so again don't worry about it and use the boat, have fun! Hope this helps or at least helps you to see the light, isn't working on boats fun?? I've been doing it for 32 yrs and still have my sanity, well maybe half of it.......................................g