Marine Plywood

[(37645)]

As some of you may know, I'm currently searching for a boat. Recently I have come across some interesting boats made out of Marine Plywood and I was wondering if any of you have had any experiences with it. Is there any disadvantages? Advantages?

Regards,

Mike

 

[ksutton]

RM yachts claim that with Plywood epoxy, the hulls of RM's are lighter and more rigid than polyester boats. This allows them to achieve not only better performance but also to have a more durable hull. On an RM there is no risk of Osmosis or delamination.

 

[Poignard]

I used to own a YW People's Boat with a plywood skin and it was fine. Strong, watertight and easy to repair.

But watch out for duds where rot has been allowed to develop due to rainwater collecting in undrained corners. Some attempts at sheathing have been poorly done and can result in trapped water causing rot. Have surveyor examine it first

 

[DownWest]

Most opinions are that if done with epoxy and glass (or Dynel) from new OK. RMs are built this way. If done later in life, esp with polyester resin, stay away.
A

 

[oldharry]

A good properly built marine ply boat is every bit as tough as it's GRP counterpart, and considerably easier to repair in the event of accident. Unlike GRP it can be varnished so that you get all the warmth and character of a wooden boat. Many thousands were built in the 1950's and 60's, and were very practical little boats, which many of us older members learned to sail in. Marine ply lends itself to home construction, as a decent solid boat can be built only using basic tools and skills, and people are still building excellent seaworthy boats in this material. A good example is the Eventide designed by Maurice Griffiths in the 60's, and still being built by a few home builders, while many more are being re-built after thirty years or more service, even from a fairly advanced stage of decay. See the Eventide Owners Group website.

But the big shortcoming of the material is that it has a practical life of around 25 years even if carefully maintained. And it is rather too easy to cover up defects in a plywood hull so that a reasonable looking example may actually be very near the end of it's life. An older hull needs careful examination by someone who knows ply. The problem is two fold: the first being that there has always been a lot of 'Marine Ply' on the market that is not up to scratch. In proper Marine Ply all the laminations are of equal thickness, and made of the same hardwood throughout. Many imitation marine plys have core laminates made of inferior wood, and a thin veneer of hardwood on the outside. Also proper marine ply has no voids larger than a certain size where internal laminate sheets are joined. Cheaper plywoods can have quite large voids between the sheets, creating serious weakness.

The result is in a boat the cheaper boards are not only not strong enough, but will rapidly absorb water along end grains and in to the voids, creating ideal conditions for rapid rotting and decay. I have seen cheap 'marine ply' board failing within three or four years while still looking fine outside. I have also seen a board split in two along a core lamination of inferior timber. The trouble is it is difficult on a boat to find out if the plywood used was of acceptable quality, as the end grains and edges of the board will all be well covered up.

Marine Ply life can be extended indefinitely by painting it with epoxy while it is new or sheathing the new construction with an epoxy or cascover sheeting. But this must be done from new: Marine ply from the store should have a very low moisture content - around 5 - 10%. This is needed to allow the epoxy to penetrate properly. Once the boat has been launched the moisture content of the sheeting will rise rapidly, and it is virtually impossible to bring it back to the maximum 15% needed for succeful sheathing.

And ANY, repeat ANY, plywood boat that has been sheathed using Polyester Resin - ordinary glass fibre that is - is already dead. Polyester resin does not stick to timber, actually draws moisture in, and holds it there permanently, creating the perfect conditions for rot. A polyester sheathed boat WILL be rotten and dangerous. Or if not now, will be in 18 months time.

So yes plywood boats are perfectly viable provided they have been properly built, using good quality materials, and have been properly maintained.

 

[Ubergeekian]

RM yachts claim that with Plywood epoxy, the hulls of RM's are lighter and more rigid than polyester boats. This allows them to achieve not only better performance but also to have a more durable hull. On an RM there is no risk of Osmosis or delamination.

No risk of delamination with marine ply? PM me your address and I'll send you a sample or two ...

 

[Ubergeekian]

And ANY, repeat ANY, plywood boat that has been sheathed using Polyester Resin - ordinary glass fibre that is - is already dead. Polyester resin does not stick to timber, actually draws moisture in, and holds it there permanently, creating the perfect conditions for rot. A polyester sheathed boat WILL be rotten and dangerous. Or if not now, will be in 18 months time.

Thanks for an interesting posting. As a matter of interest, what about all those sheathed seams on Mirror dinghies? Aren't they polyester?

 

[ksutton]

No risk of delamination with marine ply? PM me your address and I'll send you a sample or two ...

Not my words, this quote was straight off the RM web site, so please send them your photos not me. I don't dislike the concept, but grp will always be my first choice, for many reasons including maintenance.

Had an RM rafted alongside me this summer in South Brittany, it had just been launched and delivered. The new owner then fell totally in love with my Vancouver 34C.

 

[Cloona]

I sail a 1959 marine ply Rort Tucker Debutante -

If the paint work is kept up its fine - and these boats were sailed transatlantic (altho I am happy occasionally going trans solent on a bright day)

Did punch a hole through it thanks to a lack of attention, a 4 knot tide, no wind and the mooring ring on a very big red bouy (and it was fixed/patched easily)

the whole maintenance thing is overdone - you have to paint any scratches, scrapes or peels in the paints as soon as poss so water doesn't get under the surface - treat the exposed wood with hrydrsol - so might get a brush ouit for ten minutes once a month ....or when its needed ... If you want to enter a perfect yacht paint job competiton you might be kept busy but other wise-

not what i would choose to across the Bay of Biscay but 50 years later Cloona seems fine.

 

[TQA]

You see loads of Wharrams still afloat after years of benign neglect. Ply can't be that bad.

 

[oldharry]

Thanks for an interesting posting. As a matter of interest, what about all those sheathed seams on Mirror dinghies? Aren't they polyester?

Yes - horrible, but cheap! They survive because Mirrors spend 98% of their life ashore on a trailer or in a dinghy park.

There are a few 60's plywood boats about still - some in remarkably good condition. These were built by builders who understood the need to protect end grains, and used top quality glues. If plywood lasted as well as carvel or clinker timber, then where have all the Debs, Mystics, Sillhouettes, Felicitys etc etc gone? And just because a plywood boat is still afloat, it doesnt mean it is sound or even useable! Also quality marine ply of the 60s is way superior to even the very best available today, although the glues tended not to be so good.

Another problem was that the glues commonly used to assemble boats in those days have not stood the test of time, and one of the copmmonest faults is failure of the glue in the joins - in turn allowing water to penetrate the end grains, which is the beginning of the end for ply. Also, rain water on marine ply has much the same effect as battery acid in the longer term.

 

[snowleopard]

I had a dinghy built of marine ply where rain water had been allowed to lie in the bilges. I had rotted all the way through. I currently have a Mirror with extensive rot (a future project). I don't think the original kits used proper marine ply. Surprisingly the seams are still perfect!

 

[oldsaltoz]

Quote oldharry;
A good properly built marine ply boat is every bit as tough as it's GRP counterpart.

Excelent post Harry,

I am sure there are still a few WW1 mine sweepers floating around, all ply construction, made in hurry and not expected to be around too long.

The key to modern ply craft is the application of epoxy, cut and shape each panel and epoxy the ends / edges several times starting with a penetrating epoxy or thinned to ensure end grain is well sealed;follow this by sealing both sides, minimum 2 coats inside and 4 outside. All ply below the water line to have 5 coats on both sides.

Most important of all. All timber MUST be very very DRY before and resin is applied.

Avagoodweekend......

 

[Ubergeekian]

I am sure there are still a few WW1 mine sweepers floating around, all ply construction, made in hurry and not expected to be around too long.

I've just sold my 35 year old plywood glider. Mind you, aircraft ply is superb stuff, and very long lasting, even in the amazingly thin sizes used. Mostly 1.8mm, the glider ...

 

[Tranona]

As the owner of a 46 year old Marine Ply boat I can say that they will last as long as a GRP boat provided a few important conditions are met. First, of course is good quality ply, second is good surface protection, third is tackling any failure quickly and effectively.

old harry has identified all the major issues. In my case most are avoided by having a Cascover sheathed hull which is is sound as the day it was built. The unsheathed deck and coachroof are now mostly epoxy coated at the key exposed points such as deck edges and joins in panels. The non draining cockpit has always been covered with a waterproof cover and the boat is kept afloat all year round with a cover over it in the winter.

Preventative measures and regular maintenance deal with all the problems easily. Difficulty is that very few boats from the 50's and 60's have received such care, so many are past it. But if you can find one similar to mine (which I have owned for 30 years) you have a lifelong companion!

 

[oldharry]

I've just sold my 35 year old plywood glider. Mind you, aircraft ply is superb stuff, and very long lasting, even in the amazingly thin sizes used. Mostly 1.8mm, the glider ...

Interesting - which harbour did you keep it moored in?

 

[mollyoxford]

Very interesting.

Can you still get Cascover? I was under the impression that it wasn't made any more.

Also where is the best place to get decent plywood? Gotta be some stuff better than what the local timberyard have.... It is just about OK for my experiment on designing and building a cheap small tender, but I really wouldn't expect it to last....

 

[DownWest]

Molly, if you choke on Robbin's prices, try marinplyonline. Bought a batch of 9mm sight unseen. Pretty good. Actually worked out cheaper than the price listed.
A

Based by Bristol

Cascover is, I think, a nylon fabric set in recorcinol resin. The modern equivalent would be Dynel in epoxy. There has been discussion elsewhere on using polyester cloth, which is much the same, but loads cheaper. Some dress fabrics were mentioned.
A

 

[oldharry]

Very interesting.

Can you still get Cascover? I was under the impression that it wasn't made any more.

Also where is the best place to get decent plywood? Gotta be some stuff better than what the local timberyard have.... It is just about OK for my experiment on designing and building a cheap small tender, but I really wouldn't expect it to last....

The glue used in cascover was a resorcinol based compound - which is still easily available. The cloth was simply nylon cloth - so presumably it is still possible to do it? However modern epoxies do every bit as well, are more versatile, and probably easier to use. Resorcinol IIRC needed near perfect conditions for it to cure properly and thoroughly.

And you are right, timberyard 'marine ply' very rarely is. May even be marked BS1088, but that doesnt mean a thing, unless it is the official kitemarked stuff - and the Far East have no scruples about how they 'sell' their timber! Good Brynzeel ply is the minimum standard, and if you shop around there is better to be had.

 

[Tranona]

I had the good fortune to meet the guy from the predecessor to Wessex Resins who sheathed my boat at Hartwells in 1963. His description of the process was illuminating and not for the faint hearted! Dr Fitzgerald who commissioned my boat told me he insisted the "factory" did it because it was not always successful. A case of coating the hull with resorcinol, laying on the cloth, rolling in more glue and walking away!. It clearly worked on my boat as there is still total adhesion except where there has been physical damage. One section resheathed by Mitchells in Poole after a collision and local repairs such as on the stem where I use glass cloth and epoxy which bonds well.