Plastic Classic Forum

Are there any viable alternatives to plywood for bulkheads in fiberglass sailboats?

I ask because plywood, at least in older boats, seems to be prone to rot.

The age of the boat is irrelevant. It rots if it gets wet. Maybe older boats leak more, although I have had brand new boats leak. There is really no alternative. I guess foam with heavy glass could do the trick but would be a bear to install, heavy if strong enough and expensive. The only other thing I can think of is to make sure you use the best ply you can find. Some manufacturers cut corners there, and those bulkheads rot even "better" (when wet of course).

Just focus on sealing the plywood. Get a nice good quality plywood and saturate it with epoxy, at least for areas likely to get wet. Saturating entire plywood panels would be terribly inefficient cost-wise. Paint would be better for areas not likely to get wet.

From lots of reading, its my impression that plywood quality nowadays are... not as good as those back in the 1960s. So hoping for better quality plywood today is just dreaming.

Plywood is a great material. It just has to be sealed and most importantly... stop all leaks. If there's any leaks still going on, fix that before replacing any plywood! Otherwise, it would be a waste of time and money.

- Case

The word quality is one of the most deceiving words in the English language, it is also one of the most misused. Using the word quality and its relationship to rot in regards to plywood means very little. When someone says one plywood is better quality than another what if anything are they actually saying? Are the characteristics that make it a higher quality of any value for the purpose it will be used for? For the sake of technical discussions I think it would be best if we never used the word quality at all.

Plywood is made of wood. Some woods are more rot resistant to others. Plywood like all materials has characteristics. You should choose the plywood that has the characteristics needed to do the job required. How you finish the plywood and your attention to detail in doing so will have more effect than anything on the rot resistance of any piece of plywood installed on a boat.

Probably the most expensive plywood used on boats is teak veneer plywood. Does this mean it is the best quality? It is usually poplar core, and poplar has terrible rot resistance as a characteristic. But if it is sealed well it makes great bulkheads (assuming you want teak and are willing to pay for it) and using a more expensive core will do very little to make it a better bulkhead plywood.

The most structural plywood made for boats is probable BS 1088 okoume marine. When I build a small boat it is the only plywood I will use. It is almost as expensive as teak veneer, and if you were to veneer it with teak yourself it would become outrageously expensive. It would make a wonderful bulkhead material, but there are far less expensive ways to make a bulkhead that would meet every characteristic required of it. Paying considerably extra for characteristics you don't need is very simply a waste.

Pound for pound and dollar for dollar the best plywood for any job, that is the plywood that meets every characteristic required and none that are not, is the most cost effective bulkhead IMO. You need to determine the required characteristics and those that are important to you but not required (cosmetic etc). For your own sake, leave the word quality out of it, more often than not it merely serves to deceive.

There are alternatives to plywood: composite materials like Coosa Board, foam or honeycomb-cored laminated panels, and that sort of thing. These tend to be expensive, but can be viable alternatives if one feels the price is worth whatever benefits these sorts of panels can offer. They'll require veneers or other covering for the sake of appearance, at least in visible areas. Some panels come with veneered surfaces pre-installed (for a price). Foam-cored panels can be very lightweight, which is good if that's the goal of the installation. Coosa board is not much lighter (if at all) than plywood of similar thickness/strength.

Wood rot is not a guarantee. Nor is leakage that tends to cause it. In almost all cases, improper installation, improper maintenance, and pure neglect lead to these issues for which people love to blame the wood after the fact, whether it's within a deck, in a planked hull, or in a plywood bulkhead.

It's not the wood that's the problem. Personally, I think that in 40 years we're going to find out just how well all this foam holds up after it's been neglected for decades. It's all going to come down to installation, maintenance, and proper care. It always has and always will, whatever the material might be.

Don't try and fix the symptoms: cure the problem instead. Keep the water out. Protect the wood or other material. All will be well.

The word quality is one of the most deceiving words in the English language, it is also one of the most misused. Using the word quality and its relationship to rot in regards to plywood means very little. When someone says one plywood is better quality than another what if anything are they actually saying? Are the characteristics that make it a higher quality of any value for the purpose it will be used for? For the sake of technical discussions I think it would be best if we never used the word quality at all.

Plywood is made of wood. Some woods are more rot resistant to others. Plywood like all materials has characteristics. You should choose the plywood that has the characteristics needed to do the job required. How you finish the plywood and your attention to detail in doing so will have more effect than anything on the rot resistance of any piece of plywood installed on a boat.

Probably the most expensive plywood used on boats is teak veneer plywood. Does this mean it is the best quality? It is usually poplar core, and poplar has terrible rot resistance as a characteristic. But if it is sealed well it makes great bulkheads (assuming you want teak and are willing to pay for it) and using a more expensive core will do very little to make it a better bulkhead plywood.

The most structural plywood made for boats is probable BS 1088 okoume marine. When I build a small boat it is the only plywood I will use. It is almost as expensive as teak veneer, and if you were to veneer it with teak yourself it would become outrageously expensive. It would make a wonderful bulkhead material, but there are far less expensive ways to make a bulkhead that would meet every characteristic required of it. Paying considerably extra for characteristics you don't need is very simply a waste.

Pound for pound and dollar for dollar the best plywood for any job, that is the plywood that meets every characteristic required and none that are not, is the most cost effective bulkhead IMO. You need to determine the required characteristics and those that are important to you but not required (cosmetic etc). For your own sake, leave the word quality out of it, more often than not it merely serves to deceive.

Well said!

John,

Google 'lightweight structural panel' and you will get a surprising number of relevant hits. Fiberglass-faced foam- or honeycomb-cored. Some with perfectly flat faces, others suitable for veneering or being concealed.

Grab a copy of Professional BoatBuilder Magazine and you'll see ads for such panels that are offered for boats, usually for weight reduction reasons. It's common to see cored panels on fancy yachts that have been custom veneered for the manufacturer. (I particularly like the fiberglass-faced-aluminum honeycomb-cored panels with 1/4" granite or marble veneer; these are used on building facades as well as countertops in fancy yachts.)

There are reasons that plywood panel products are still most common on boats: money, appearance, ease of use, and adequacy.

I ditto the previous posters' observations about edge sealing and keeping things dry for adequate performance of plywood aboard. I suggest that paying attention to rot resistance of the species is important but ameliorated by 'sealing and dry'. Quality of glue and layup is even more important, but these days even cheap plywood is likely to have good glues even if the core veneers are not high enough quality.

1 inch G-10.

That should hold and it certainly will never rot. ;-)

I was just reading this article about replacing bulkheads with G-10 panels http://www.westsystem.com/ss/replacing- ... bulkheads/.

Other than cost, does plywood have any advantages over G-10 panels?

1 inch G-10.

That should hold and it certainly will never rot. ;-)

Maybe I should add that I was kidding. It would work I guess but I think it would be too heavy and expensive to be a truly viable option. I haven't read the article though so maybe I stumbled onto the newest trend. ;-)

Britton,

The article makes a good point. I was chuckling as I opened the article but in this case, using G-10 is not a bad idea, (not cheap either) if your chain plates are mounted to that bulkhead. I have no idea how much stress a bulkhead with chain plate undergoes but it has to be pretty big. The chain plate is commonly a source of leaks, with plywood makes a case for potential rot, particularly if there is any working of the bulkhead. So while I am not saying stop using plywood for bulkheads because we all know that when done correctly will last at least 40 years but there is an argument for using G-10 for bulkheads. Although, I am not sure I would go with the 1".

When reviewing this article and considering the feasibility of G-10 for bulkhead replacement, consider the scale of the bulkhead in the article: it is quite small. Probably a good use for G-10 as the bulkhead is small enough to remain comparable in weight to plywood and not so expensive as to be ridiculous.



But I am sure those small chainplate bulkheads in the article are extremely strong now. But for a full-width structural bulkhead, the weight and cost and challenges of cutting/workability would quickly exceed the potential benefits for most mere mortal boats, I think.

At this point I have a couple of lingering questions:

1) What is considered "good" plywood for bulkheads, leaving cosmetics aside as one can always veneer the plywood with whatever wood one likes. As others have pointed out, there is no point in paying for features which are of no use in a bulkhead application, so what is the type of plywood with the best cost/benefit ratio and how hard is it to find?

2) I understand that when properly maintained wood is an excellent boatbuilding material. The rub of course is "properly maintained" which is why we see lots of old "classic plastic" sailboats worth rehabbing and far fewer old wooden sailboats worth repairing for general use. From my experence, labor costs generally far exceed material costs (even if you do it yourself and price your labor at minimum wage), so if an alternative material offers some advantages and costs even twice as much it might be worth considering.

Does anyone have experience with the Coosa board for bulkheads?

It is fine to use fir plywood. AB grade if you can get it. It comes with both sides smooth and will receive veneer fairly well. Even better though would be baltic birch or cabinet grade birch. These will have better layers and an even smoother harder finish to receive the veneer. All of these will have exterior glue, but will need a very good sealing job. Although even if you used marine plywood you would want to seal it well.

Jonnyboats wrote:What is considered "good" plywood for bulkheads, leaving cosmetics aside as one can always veneer the plywood with whatever wood one likes.

I think what's more important than the overall classification or species of any plywood is the construction of the inner plies, how many plies there are, and whether or not the plywood is void-free.

To me, "good" plywood is any that features more, rather than fewer, plies, has a known construction and quality to the inner ply material, and features void-free construction. These criteria actually cover a wide range of species, classification, and appearances, including standard-certified brands (i.e. BS 1088), many cabinet-type veneer plywoods, Baltic birch, and others.

Beyond these basic indicators, one has to make the external appearance choices that fit the project and expectations one has at any given time.

I realize that this is an older thread but thought I'd chip in anyway. I was under the impression that Baltic birch was made with exterior grade adhesives and have used it in previous boats. When I was about to purchase a sheet I was told by the dealer (Westwind Hardwoods in Sidney, B.C.) that it isn't made with waterproof glues. Went home and took a piece of exterior fir ply and a piece of Baltic birch and boiled them both. After a while I checked and sure enough the fir was intact but the birch was separated into its various plys. Definitely not exterior grade waterproof glues - maybe "water resistant" at best. I like working with the birch as it's consistent in quality and cuts well, finishes well, etc. Next choice marine grade fir, Westwind has it in their catalog and web listings but it's out of stock. I'm told that they can't get marine grade domestic fir plywood. I decided to use Hydro-Tek marine grade Phillipine mahogany BS1088. 3/4" is 13 ply and waterproof with no voids whatsoever. Excellent to work with and amongst the best marine ply available (another is Joubert brand Occume (African mahogany) ply which is similarly rated. This was for two athwartship partial bulkheads in the galley and one adjoining fore & aft bulkhead. Probably overkill but I feel better about it. They were glassed to hull with foam between hull and bulkhead and epoxy resin (West) used. I did use some Baltic birch for cabinet faces and such above the bilge area. Even the Hydro-Tek was protected with epoxy on all edges likely to get exposed. For the inside face of the aft bulkhead I also used Hydro-Tek in 3/8 thickness. I guess I just feel better using what I know to be "waterproof" marine ply. The cost is higher but not by that much and the labour is the same either way. If I were replacing main bulkheads or building new I would still use either Hydro-Tek or Joubert. While exterior fir is waterproof, there are voids I would want to avoid. Whoever said that ply is not as good as it once was is right. 30 or so years ago it was hard to tell which side of "good one side" fir was the bad side, now it's hard to tell which side is the good side.
Brian
currently refitting a CS27 in Victoria, B.C.

Brian,

I have read through the thread, and see a lot of plywood material suggestions...so, I do not feel bad about launching a tangent...

When you line the space between hull and bulkhead, a few questions arise:

What is the material used - product name, description?

What thickness is the material?

How is is bonded to the hull: hot glue, bed of thickened epoxy?

Do fillets cover the foam material and plywood bulkhead, or just a radius covering the foam for good tabbing lay-up?

Thanks,

Brian

I think you will find that Hydrotek is a meranti plywood. Meranti is not a true mahogany, only some what closely related. Meranti has a lower strength / weight ratio than okoume, but is more resilient to abrasive damage. Some small boat builders prefer meranti for hulls or at least bottoms and okoume for the rest. For bulkheads either one is more than satisfactory. Meranti is also heavier than okoume by about 20%

I don't think it matters what foam you use, how thick it is, or how you bond it to the hull. All it is doing is helping to hold the bulkhead in place while avoiding hard spots and filling in the void at the hull to help in filleting and tabbing the bulkhead in neatly. By in effect suspending the bulkhead just away from the hull during tabbing you get a near perfect structural joint that carries loads evenly along the entire tab. Any procedures that does this will work.

You may be right, but this is how Westwind Hardwoods lists them. Prices are Cdn dollars.
Brian