lightweight core for low load area composite panels?

Hi all - this forum looks a great place for tech advice on this sort of thing so I hope someone can help.
I have a 16' GRP fishing boat called a Teal, (a planing/semi planing hull), but has suffered extensive rot of all the wood structures inside the hull (including the wood of the keel).

Since I need to rebuild the deck/floor, and the rear wall of the cabin I want to make them as light as I realistically can do (as the 50hp Yamaha on the back is a 'tiny' bit under powered for the hull in my opinion, and all weight saved will help).

I want to use a sandwich type construction for these panels instead of just using treated marine ply, in order to make them very light (not sure yet whether to use fibreglass or carbon due to budget), but when I've read about stuff like closed cell foam I've seen it quoted as 2/3 the weight of plywood. If that's the case, its not a significant enough weight saving compared to ply to be worth me using.

Is there any sort of foam core I can use that will weigh next to nothing (like styrofoam, but more suitable for marine use) and be ok to use for these areas? In terms of load, the deck will never really have more than 2 people on board, and there are a series of ribs/braces beneath to support the deck area. It is allegedly not so much important in terms of outright strength in these boats, but does aid rigidity of the GRP hull I believe.

Second question is that the inside of the hull has strengthening/rigidity pieces attached in places. These are typically an inch or so tall and 2 inches wide, and run perpendicular to the keel, up the side of the hull in places. They are basically glassed over bits of foam, and I was wondering what kind of foam this would be?

The boat was built in the 70s or possibly 80s, and the foam looks rather like 'Expanding Foam' as used from an aerosol can, but I gather that is not at all suitable for marine use as it can absorb water.
As such am assuming it's some other kind of foam but have no idea what. Can anyone suggest what it would most likely be?

Thanks in advance for any useful info!
Jim

 

light weight and drurability dont go hand in hand normally !!!

any way of getting some pictures ??
Making glass foam panels is a tough choice , with soft foams the glass simply peels off very easly so dont go there !!. For a fishing type boat better to get back to plywood but epoxy coat and seal it all sides and edges!! specially edges !! . bouyancy type foam is a waste of time and yes it will soak water over time and make you boat progressivly heavyer and heavyer as time goes by .
Carbon fiber dont even consider it what so ever . the exspense is not worth considering and you need special epoxy resins , dont think any more about it !!wasting you time .

 

Hi tunnels

Thanks for the reply.

Yes, pics are no problem.
Here's the boat in general so you can see what it actually is,



Here's the current (horrible) state now I have gutted the inside of her.


These are the strengthening foam ribs I referred to (note one on the left is almost completely missing, and the nearer left one cut open). You can also see how the keel (or what's left of it) just stops and is missing for most of the area where the cabin is for some reason. The glassfibre support either side of it remains though, indicating there originally was wood there (removed by the previous owner I presume!).


Effectively, the layout is like this:



I have previously made panels from carbon but on a smaller scale so I'm familiar with the process for that. I'm happy enough to be making laminated panels and working with epoxy (I like the west system stuff) as I've done it before and know what to expect, it's just I don't know what's a suitable lightweight core suitable for marine use.

I'm trying to avoid the use of ply where possible because of its weight. Yes it's what was originally used (I presume), and is the defacto norm for this sort of thing, but I was hoping to save weight by using some sort of foam and composite instead.

I'm not at all keen on sealing the deck in place by glassing it onto the edges of the hull etc because of the paranoia I've now got about water intrusion.

The previous owner had done their own rebuild work where they had built a whole new deck themselves from ply (marine I presume), and sealed it round the edges.

Eventually after declining performance on the water and refusal of the boat to get up on the plane etc, I found the deck was utterly rotten beneath the fibreglass skin, and proceeded to rip it all out.

The compartments around the keel, and its ribs that you can see in the picture, were initially filled with fresh air, but gradually after he did this work (and continuing after I bought the boat), the area beneath the new deck was slowly filling with water. I don't know how it got in, but I do know that it couldn't get out.

Thus the accumulated weight of the water affected the boat's performance and behaviour in a highly negative way, and ultimately caused all the wood in question to rot to the point where it was black and crumbling to the touch!

When removing the remains of this 'handiwork' I have drilled drain holes between these compartments to assist drainage, but I really don't like the idea now of building a deck that is sealed again for the risk of anything like this occurring in future.

I've had to leave it for 4 years due to house & kids and stuff but am now ready to get busy renovating.

Now since I've witnessed first hand (the hard way), just how badly the extra weight of that water and rotten wood affected the boat's performance I am keen to rebuild in a manner that minimises weight in the new structure.

Hope that all makes sense.

Not questioning at all the cost of carbon etc, or that plywood is the norm etc, but even if I use fibreglass there must surely be a very lightweight core that I can use for this to make new deck and cabin panels that are way lighter than plywood, and still suitably strong?

Appreciate the input anyway.

Cheers
Jim

 

Very inappropriate boat name as you can see!

 

This is the sort of thing I was hoping to use as core material.
Poly Urethane Foam Sheets
A friend who has done a lot of work with composites/carbon says he has used this sort of foam with fantastic results for making incredibly strong laminate panels.

The only question mark is that it wasn't for marine use, but surely the only real caveat with that would be the issue of whether it's likely to absorb water if it's exposed to it, and from what I can find out online this kind of foam doesn't absorb water.

So wouldn't foam like this be perfect for what I need to do?

 

forget about carbon !!!!
So you have a glass hull with a timber frame and keel
becasue the timber will be soaking wet it will be heavy . and possibly rotten or very close to it ! If you are going to rip it out and replace it all make sure its 100% sealed up before the floor goes back in again . cut you floor to shape and fit but take it out and totally seal it with resin top ,bottom and all the edges . There nothing wrong about using glass and polyester resin just make sure any glassing you do is onto a ground completely clean surface so it will stick!!
Use a 24 grit disc on a grinder and have the surface 99% clean and scuffed !! Id also be inclinded to resin coat the glass hull under the floor as well and seal the glass to keep the water from getting into the fibres . So you will have a lot of grinding and sanding to be done !!

 

Saving weight with foam is fine, but you have to also match the physical attributes of the plywood also or your cored replacements will just buckle and break up. Generally, it's more costly and more time consuming to use a cored construction, but you do end up with initially inert materials when completed.

The foam you're intending to use isn't a structural foam, so all of your loads will have to be borne by the laminate (meaning yet more 'glass work). Which means you don't need the foam at all, you could use aunt Rosie's mash potato mix and save the foam money for something else.

I'm not convinced you need to do any of this extra reinforcement. Have you pulled a core sample and checked hull thickness? Most hulls from that era where heavy laminates.

 

Brief discussion of cored panels

http://www.westsystem.com/ss/comparing-cost-and-weight-of-flat-panels

Popular cored panel for cabin soles.

http://nida-core.com/english/nidaprod_laminated.htm


In the end plywood will be the best choice. Foam or cored panels are expensive and require substantial handwork, reinforcing, for a cabin sole that will take physical impact abuse.

Cored panels would be a great choice if you want to experiment with building lightweight furniture to fit into your skiff. Purchase a sheet and experiment.

The local boatbuilder uses much Nidacore in the 10 meter tour boats that he builds.

 

Thanks for the replies guys. I have a friend who's done a lot of laminating with urethane foam and has had great (ie very very strong and light) results so I thought it would be a good option since it's meant to be non absorbent.

I should clarify here that this work isn't for 'reinforcement' purposes per se. Nor for the hull itself. The actual hull is fine and is in solid condition.

What I need to build here is the deck/floor (ie what I'll be standing on when in the boat), and the rear wall of the cabin. Sorry if any of that sounds stupidly obvious but I just wanted to be clear to avoid confusion.

Appreciate all the advice, and the link to West System info on panels is particularly useful.

 

And what about interior walls that go off of structural bulkheads and have no loads? What about galley cabinets? What is the lightest, cheapest foam you can use there?

 

Dont know. You must ask a Professional Interior fitter for cost advantages.

The boat Im sailing uses foam core with 3mm plywood panels. Its not cheap but its versitile and easy to use.

Many yachts are being fit out with Goldcore lightply furniture.

http://www.goldcorelight.com/features.htm

Coosa is another popular product.

http://www.coosacomposites.com/

 

Soles and cabin bulkheads usually are structural and help in load transmission if not absorption.

 

Par, do you know of such panel material for making cabinets and such?

I have severally dummy walls, cabinets and seating and such to make that are not structural.

In fact, i'll be motoring my boat about 1000nm with no interior at all - just a shell and bulkheads.

 

Polyurethane foam on boats is only suitable for making an ice box. Nothing more.

Low load panels usually can be made with PVC foam of 60 kg/m3, or the new Airex T92.60 which is PET based. (cheaper and better fire properties).

But it is hard to give specific advise.

 

I made some panels that were maybe 2x3' out of the foil faced brown foam from Home Depot, 3/4" thick. I made a frame using 3/4" x 3/4" pine or cedar which the foam fit in snugly, more or less. About every 5-6" over the whole surface of the foam I used a rigged up wire to poke a 3" strand of roving (from some 24 oz woven roving) through, leaving equal amounts sticking out the sides of the foam, like a tufted quilt. I then vacuum bagged it with polyester and 1 1/2 oz mat on each side. They came out very lightweight and stiff. The pieces of wr acted in tension and compression, making small compression pillars while also directly connecting the two mats to prevent them from pulling apart. With epoxy you could do the same with any of the cheap blue or pink lumberyard styrofoam. It wouldn't cost hardly anything for you to make some test panels and then assault them to see if they'll work for anything on your boat. The wood frames or other inset pieces of wood give you something to put screws into.


As far as Cresters keel and ribs and floor replacement goes, I would replace everything as it was, probably substituting treated ply. There are a lot of different sorts of loads involved, one of the main ones is twisting of the whole boat, putting a bunch of shear load on the floor. Like putting your hands together flat and twisting them back and forth. Lightweight foam would just crumble with that load and pretty much any other type of load. The glass covered pieces of foam on the hull are probably fine, the foam just gives a shape for the glass and doesn't do anything else. Sometimes back then they used cardboard. A ply floor would let you screw things anywhere, a foam one, not so much. Make sure that when you start to put the boat back together, the hull is supported in it's original shape, with no bumps or dips or hollows or twists. Being on it's trailer might not be good enough. You also have to be careful that your own weight inside the boat doesn't do any of those things or cause it to flex and move as you are re-glassing it.