peter radclyffe
N/A
being dimensionally stable, in large sizes & long lasting, plywood has a lot of uses, there is a dutch company that builds all in ply, the interior of the boat first for access, then the hull, good idea
Marine Plywood
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peter radclyffe
N/A
no boat that age is as sound as the day it was built
Tranona
Well-Known Member
If you are in the least bit interested, then read this in the context of the thread, which is about the durability of marine plywood boats. The hull of my boat is Cascover sheathed and I am making the point that there is no rot, delamination or any other signs of failure of the structure. pretty good evidence of the effectiveness of the technique that the wood is as sound as the day it was built.
As I pointed out this comment does not apply to the whole boat structure - just the sheathed hull.
mollyoxford
Well-Known Member
Good tip - will check it out.
See, as I understand resorcinol IS waterproof but epoxy not really... there are some actual water absorbtion numbers for epoxy over wood/plywood here
http://www.woodenboat.com/forum/archive/index.php/t-7241.html
which ties in with what I have read elsewhere. The speculation that keeping the inside surface unpainted may allow the absorbed water to evaporate out is interesting.
For my experimental ultra-cheap self-designed little tender I am using medium-crappy 6mm WPB ply, pine, copper 'hardboard' nails and "PinkGrip" D4 waterproof glue. With probably many coats of B&Q's cheapo white gloss and some leftover granuled-up Blakes deckpaint. (else I'd just sprinkle chunky sugar on the 2nd last coat on the floor then rinse it off when the paint has set. Does a great non-slip surface, that does)
Tranona
Well-Known Member
Seanick
Well-Known Member
Even the best ply does become brittle with age. Try breaking up a few ply boats and you will see what I mean.
Just keep of the rocks!
Just keep of the rocks!
Tranona
Well-Known Member
Do you have any scientific evidence to support that statement? I can think of no logical reason why ply should become brittle with age. It is after all only a composite of wood and glue. I can accept delamination from glue failure, rot from damp and fatigue failure from stress, but not failure just from age.
EastCoastChris
Well-Known Member
Whilst pondering some repairs I need to make to my plywood hull I came across a comment that whilst plywood may rot it is also infinitely repairable.
My understanding is that both sides (inside and out) of a panel should be treated in the same way to ensure that any movement of moisture is balanced and water is not trapped in the ply. It seems far more probable that water would penetrate from the untreated interior and sit trapped between the veneers than it would be to evaporate out having come through the epoxy from outside.
There is an awful lot of advice around regarding plywood boats, I have had bad advice from people with the best of intentions. Your safest bet is to read as much and as widely as you can and garner as many opinions as you can, eventually you start to see the way forward.
Chris
My understanding is that both sides (inside and out) of a panel should be treated in the same way to ensure that any movement of moisture is balanced and water is not trapped in the ply. It seems far more probable that water would penetrate from the untreated interior and sit trapped between the veneers than it would be to evaporate out having come through the epoxy from outside.
There is an awful lot of advice around regarding plywood boats, I have had bad advice from people with the best of intentions. Your safest bet is to read as much and as widely as you can and garner as many opinions as you can, eventually you start to see the way forward.
Chris
Tranona
Well-Known Member
In my experience, rot in ply comes through end grain penetration or fresh water sitting in pools inside, Well known spots are around the cockpit and main bulkhead areas of Eventides and Golden Hinds where multiple joints and water pathways such as locker lids make it almost impossible to keep fresh water away from the inner cores of ply. Other weak points are panel butt joins and deck edges, particularly where rubbing strakes are screwed on with a bedding that fails. Do away with these by epoxy sealing or for the cockpit by using a waterproof cover and rot disappears as an issue. Through deck fastenings can also be a problem again solved by epoxy lining the holes and using a non setting sealer such as Life Calk.
Pretty sure I could build a durable boat out of ply now based on my experience - it would not be too different in construction from an RM although maybe different in style!
Pretty sure I could build a durable boat out of ply now based on my experience - it would not be too different in construction from an RM although maybe different in style!
Seanick
Well-Known Member
"Do you have any scientific evidence to support that statement? I can think of no logical reason why ply should become brittle with age. It is after all only a composite of wood and glue. I can accept delamination from glue failure, rot from damp and fatigue failure from stress, but not failure just from age."
No scientific proof-I am not a scientist!
I have been building and repairing boats profesionally since 1987. I know what a fresh peice of ply feels like, I have broken up bits I have cut out of hulls, and completley broken up the odd 17 'to 26' boats, yes, even disposed of an Eventide. Even the best ply becomes brittle, any experienced boatbuilder will tell you that. Even the older style 1088 Gaboon througout ply, lovely and dark with good cores becomes brittle.Thats not to say the boat is unseaworthy as it becomes older, but the effects of say, hitting a bouy when under way may be more dramatic to the older boat. How many Gallants and Debutants have you seen in boatyards with one bilge plate missing, including a section of hull. The original design utilised a longditudidal stringer, and allowed the ply to flex slightly. After 40 odd years th ply has had enough.
Another example is when reclaimed timber, such as pitch pine, is used when planking up a 'new' boat. Compared to freshly seasoned timber it is harder to bend and breakages are common.
I hope this is scientific enough for you, and I await your lengthy reply-thpough I am off to bed!
I won't mention varnish
No scientific proof-I am not a scientist!
I have been building and repairing boats profesionally since 1987. I know what a fresh peice of ply feels like, I have broken up bits I have cut out of hulls, and completley broken up the odd 17 'to 26' boats, yes, even disposed of an Eventide. Even the best ply becomes brittle, any experienced boatbuilder will tell you that. Even the older style 1088 Gaboon througout ply, lovely and dark with good cores becomes brittle.Thats not to say the boat is unseaworthy as it becomes older, but the effects of say, hitting a bouy when under way may be more dramatic to the older boat. How many Gallants and Debutants have you seen in boatyards with one bilge plate missing, including a section of hull. The original design utilised a longditudidal stringer, and allowed the ply to flex slightly. After 40 odd years th ply has had enough.
Another example is when reclaimed timber, such as pitch pine, is used when planking up a 'new' boat. Compared to freshly seasoned timber it is harder to bend and breakages are common.
I hope this is scientific enough for you, and I await your lengthy reply-thpough I am off to bed!
I won't mention varnish
Tranona
Well-Known Member
No, won't be lengthy! What you are describing is stress failure - not age although of course the two are related. I have pieces of ply from my original deck laid in 1963 that are as good as any new pieces of ply - probably better. No signs or rot or delamination, nor failure of veneers. However only slightly curved and never under any stress. Only reason I have them is because I extended the coachroof about 17 years ago which meant cutting out part of the original foredeck. Good quality 1/2 inch ply offcuts are always valuable - some has been recycled as backing plates for fittings on my Bavaria!
Seanick
Well-Known Member
No, I don't mean stress faliure specifically. You really need to lay into a plywood hull with a sledge hammer to realise how brittle they become. It is unfortunate that in my job I have had to dispose of various boats for the usual reasons. From that fairly destructive act I gained a good insight into parts and cross-sections of boats that you don't usually get to see.
Take a piece of old ply, say 18" by 5", and then a comparable fresh peice, and break them over your knee. The ply may look fine, but put a bit in a vice and whack it with a hammer, it will probably break clean off. As I said, this does not make your boat unseaworthy, but just not quite as pliable as when it was built.
Reclaimed timber, such as pitch pine from warehouses has not been under any real stress, but when converted to planking stock it is brittle when compared to similar species and similar jobs. Even larch, which is very compliant becomes much harder to bend ofter a few years on the rack, even when the m/contents are similar. The last few Sunbeams built in Falmouth and also in Bosham used reclaimed timber. The boatbuilder in Bosham-twice my experience-had a right job getting the planks round, and broke a few. You would never have that problem with fresh timber.
As I said, I'm off to bed now, as I have to bend some particulary nasty sepelle round the back of a Hillyard first thing tomorrow.
Take a piece of old ply, say 18" by 5", and then a comparable fresh peice, and break them over your knee. The ply may look fine, but put a bit in a vice and whack it with a hammer, it will probably break clean off. As I said, this does not make your boat unseaworthy, but just not quite as pliable as when it was built.
Reclaimed timber, such as pitch pine from warehouses has not been under any real stress, but when converted to planking stock it is brittle when compared to similar species and similar jobs. Even larch, which is very compliant becomes much harder to bend ofter a few years on the rack, even when the m/contents are similar. The last few Sunbeams built in Falmouth and also in Bosham used reclaimed timber. The boatbuilder in Bosham-twice my experience-had a right job getting the planks round, and broke a few. You would never have that problem with fresh timber.
As I said, I'm off to bed now, as I have to bend some particulary nasty sepelle round the back of a Hillyard first thing tomorrow.
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maxi77
Well-Known Member
Do you have a better address than "marinplyonline" which if googled only brings up your post.
Thanks
oldsaltoz
Well-Known Member
Do not use polyester cloth (Mat) called CSM,Chopped Strand Mat. It 3 times the voids the epoxy cloth has, this means you will need a whole lot more epoxy resin, and finish up with a weaker brittle structure. Apart from costing a lot more.
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