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Plywood on Ericsons

Christian Williams

E381 - Los Angeles
Senior Moderator
Blogs Author
Questions here, not answers. Delaminated plywood in sometimes found on our boats, notably early models. Was it marine plywood? Is marine plywood necessary for furniture on sailboats? How able is marine plywood to withstand long submersion?

Boats have been made of marine ply for many years, and none of my plywood hulls has ever delaminated. However, the floorboards of the '84 Ericson 381, long wet by fresh water leakage, fell entirely apart. Were they marine-grade ply to begin with?

Sole delaminated A.JPG

Plywood is great stuff. Try finding a flat tree 4x8. Also, they're the stuff of modernity and easy construction. At the old sailing-ship watering port of Perth Amboy, N.J., in the 1960s, was a lovely wooden schooner owned by the Booze family. Her name graced the transom in lovely gold leaf scroll, Maid of Perth. Moored nearby was a jaunty reverse-sheer home-built MORC racer named Maid of Pligh.

Marine plywood is so-called by many manufacturers. The idea is waterproof glue and no voids. But exterior grade also has waterproof glue. And not all marine ply is considered equal. Is it worth the expense and sourcing? Why can't you use exterior ply for boat projects that get wet but then dry off, if you coat with epoxy and varnish? This is why. (I ran out of marine grade, ho ho).

delaminated rudder.JPG

But in fact, some badly delaminated plywood can be restored, veneer and all, if you don't mind going to a lot of trouble. (Link at bottom)

So--are Ericson bulkheads marine plywood? All of them? Should they be? What was the former plywood standard for commercial boatbuilding of sailing yachts, and what is it now?

 
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wynkoop

Member III
I had to replace the two deck plates that give access to my bilge fore and aft. They rotted, so their failure did not allow me to discern if they were marine or not.

I replaced them with some exterior grade ply I had laying around. I have varnished one top/bottom/sides, but not the other. I really need to sand it down and do that. My factory original deck plates were painted on the bottom and varnished on top.

I had a load of exterior plywood submerged in fresh water for several months when Silver Maiden was on the hard a few years ago. Her battery failed and the rain water got over the deck where I was storing the wood. I pumped her dry and once the wood dried out it seemed fine. I have used it in a few projects since, and I expect some of it will go into a rebuild I am doing of an AMF puffer that is to be my new tender.
 

wynkoop

Member III
I should mention I also have two bulkhead rot issues, the one at the forward end of the lazerette port side and the one starboard of the toilet in the head. The lazerette I took care of over the summer with penetrating epoxy and I think I have arrested the rot. I will sister it next summer with a piece of exterior ply that I will impregnate with total boat penetrating epoxy first. I also intend to remove all the WIRING that came stapled to the bulkhead, strip the paint and hit it with penetrating epoxy before I sister it.

The head rot will just be pull the mahogany ply panel out, coat the back side with penetrating epoxy, and cover the areas of actual rot on both sides with it. I may then try to find a trim piece to fasten over the damaged 1/2 inch at the bottom.
 

Loren Beach

O34 - Portland, OR
Senior Moderator
Blogs Author
Note that when water leaks into plywood, it follows the layer of wood, and ignores the insoluble glue. This happens when a plywood core in a deck gets saturated over the years, by a fastener penetration thru all the layers.

As for 'marine' ply, I was told -decades ago- that the glue was the same for 'exterior' and 'marine' but the later had no voids in the layers of plies.
Sometime in the last couple decades the mills started using all waterproof glues, and I have no idea if the plywood that we buy has more than one 'grade' of waterproof glue used to make it.

The gist of it is that the fibers in waterproof plywood will hold together better when damp or even wet, but if they rot, the panel will be destroyed anyhow, or at least separate into veneer layers.
 
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racushman

O34 - Los Angeles
Loren's point about the rot following the layer of the wood and not the glue is key in my experience.

My only dismasting experience to date (knock on wood) was in my beloved O'Day 222 trailer sailer on a nice summer afternoon sail along the Chicago waterfront. The chain plate was bolted thru the deck to a perfect looking teak marine plywood bulkhead. Only unbeknownst to me, under the beautiful teak veneer was mush from 30 years of water weeping in. One nice puff and "pop" the rig went over the side.

The replacement bulkheads I built all got treated with epoxy on the edges. I don't think marine grade mattered much. Exterior is fine for most of my purposes.
 

goldenstate

Sustaining Member
Blogs Author
Christian - I suspect you already know more about the bulkhead construction of Ericsons than most of us ever will...

I think for modern construction, the choices are either balsa or foam cores.


One advantage of foam, as I understand it, is that closed cell foam will not permit the intrusion of water into the core, rendering the rot problem moot. I'm midway through some research on this - others surely know more. PVC foam core (Divinycell is one trade name) is strong and light and resists water intrusion. I think it's the yellow stuff in this video:


Again, as far as I understand it, using fiber-glassed plywood isn't really optimal if the strength is supposed to be derived from the surrounding fiberglass and not the core material.

Kind of (but not really) related: here's a cool video of a CNC lathe forming the male plug for a new form for a J/boat:

 

steven

Sustaining Member
For shrouds anchored to bulkheads. Note that the load is shear.
That is, load is perpendicular to the plywood edge not perpendicular to the plywood face.
Hard to find data, but I believe plywood is much less strong in shear than in compression or flex.

--Steve
 

goldenstate

Sustaining Member
Blogs Author
For shrouds anchored to bulkheads. Note that the load is shear.
That is, load is perpendicular to the plywood edge not perpendicular to the plywood face.
Hard to find data, but I believe plywood is much less strong in shear than in compression or flex.

--Steve
Unless I misunderstand what you are saying this disagrees with what I know about the material. Plywood is very good at resisting shear forces. This is why the walls of buildings are reinforced with plywood. It resists twisting shear forces. Plywood flexes a great deal, but it does not fail when secured in a plane.

This is how houses where I live are retrofitted to withstand earthquakes.

 

Christian Williams

E381 - Los Angeles
Senior Moderator
Blogs Author
My house has been retrofitted twice during construction projects, all that earthquake stuff installed, so I found that video pretty interesting.

Our stays are held up by chainplates in shear on plywood--or anyway, they are on some models. It is instructive to see what the massive stainless chainplates on the 381 are attached to --essentially, the furniture. I think that's why some of my furniture is glued and screwed for strength.

IMG_0583-001.JPG

I asked a surveyor how the stays can be given such a casual change in direction before meeting the chainplates. They go through the deck and can make as much as a 30-degree turn before being secured. Doesn't this put undue force on the deck?

He smiled and said something like, "well, you have to build a boat somehow." He said the building technique had stood the test of time.

The 32-3, unlike the 381, terminates the stays in aluminum deadmen in a TAFG designed to receive them. But the stay still changes direction coming through the deck.
 
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Rufus McCool

Junior Member
The video on shear walls was very interesting. It made me think of the unique construction techniques for various failure resistance, depending on your area of the country. In my case, hurricanes are the villains, rather than earthquakes. Proper construction process is critical if survival is expected. This becomes evident after a storm when one house is standing nearly unscathed right next to a completely destroyed home.

The correlation of shear walls (in the video) to sailboat bulkheads points out the benefits of tabbing. However, it seems most modern boat building techniques have moved away from traditional chain plate attachment, and use fiberglass hull bonding as Christian mentions. It gives you a real sense of security to study the upper shroud attachments via a TAFG system.
 

goldenstate

Sustaining Member
Blogs Author
I have never been inside an E-38 but based on Christian's picture (and Rufus' observation) it would seem the chainplate system combines all of the perils -I mean- benefits of a California shear wall with Florida hurricane strapping :) Good thing they are overbuilt.
 

dofthesea

Member I
Try to get your plywood from a real Sheet goods store. A place where contractors or Cabinet Makers go. They know everything you need to know about real marine play. Just because its marine ply doesnt mean you dont seal it really well on all 6 sides. When I use marine Ply I typically put 4 coats of varnish on it.
 

Second Star

Member III
Marine plywood is made with waterproof glue, hardwood laminates and without voids within the layers. There are different grades of marine plywood and different types of outer laminates The wood can rot so it must be made waterproof with paint or resin but the glue won't fail when wet. Hardwood laminates don't wick water as quickly as softwood. Voids in regular ply will hold water and quickly stimulate rot. IMHO don't worry about using it for interior cabinetry as I don't plan to submerge the cabin very often! Places like the bilge or engine mounts or the floor of an anchor locker really call for marine ply. Ditto dofthesea.
 

steven

Sustaining Member
see http://matweb.com

above source lists shear strength (resistance to load on the edge grain) of sheathing grade plywood as 800-1000psi; and compressive and tensile strengths in the 5000 psi range.

breaking strength (various sources) of 304 stainless 1x19 wire:
3/16 is given around 4,700 lbs;
1/4" is given around 8,200 lbs.
 

goldenstate

Sustaining Member
Blogs Author
I wouldn't dispute those number, but they are measuring different things. The tensile strength of the cable is higher than the shear strength of the plywood. Maybe we are saying the same thing in that respect. All else equal, the plywood may well be the weak point in the system, but looking at the tensile strength of the cable and the shear strength of the plywood is apples and oranges, no?

Being able to distribute shroud load over many adequately-spaced penetrations in the plywood chainplate attachment points also helps to explain why a cable might fail before a plywood bulkhead.

I think the question is more, "What's the best material to use for a bulkhead to which chainplate load is applied?" Plywood is certainly much stronger than solid wood in this regard. I don't know about G10 or other synthetics.

I'm not an engineer by training, so happy to be corrected. I just know plywood is the 'best idea so far' for keeping multi-ton houses standing during earthquakes.
 

steven

Sustaining Member
Tom,

the concern is that the bolt inside hole is held to a large extent by the bolt barrel pressing edge-wise against the laminate layers; not as normally encountered where a bolt is held by a backing plate pushing on the plywood surface. Plywood is strong when resisting surface pressure, but the laminate edges are relatively weak and can potentially be crushed allowing the bolt to pull out.

Not sure if G10 is better for this application.
The listed material properties suggest that it might be.

--Steve
 

Hagar2sail

Member III
Blogs Author
see http://matweb.com

above source lists shear strength (resistance to load on the edge grain) of sheathing grade plywood as 800-1000psi; and compressive and tensile strengths in the 5000 psi range.

breaking strength (various sources) of 304 stainless 1x19 wire:
3/16 is given around 4,700 lbs;
1/4" is given around 8,200 lbs.
Matweb is a fantastic site, but you do have to watch where they get their data from. Plywood strengths are notoriously hard to get, so lets assume their values are correct. Although those numbers might be the "strength" of the material, you also have to understand how that data was generated. I would hazard the compression load was calculated by crushing the plywood through thickness (perpendicular to the surface) in a press. Though impressive, this is not how plywood is typically loaded or used, it is the equivalent of standing on it while it is already on the ground. Similarly the tension test is mostly testing the glue in the material. The shear test could be done a number of ways, but probably was an offset test. This does load it in a way similar to how it is used in real life. I think a more important data point is the American Wood Council lists the shear capacity of red oak parallel to grain as 220psi in number 1 and select structural grades. This shows the advantage of plywood over using traditional solid lumber.
 

Hagar2sail

Member III
Blogs Author
Tom,

the concern is that the bolt inside hole is held to a large extent by the bolt barrel pressing edge-wise against the laminate layers; not as normally encountered where a bolt is held by a backing plate pushing on the plywood surface. Plywood is strong when resisting surface pressure, but the laminate edges are relatively weak and can potentially be crushed allowing the bolt to pull out.

Not sure if G10 is better for this application.
The listed material properties suggest that it might be.

--Steve
To your point, having the backing plate makes sure the plywood can not "dish" in way of the bolt, making a much stronger connection, I think this is what you were saying?
 

steven

Sustaining Member
Wow ! on the backing plates.

Especially compared to the factory 1.25" washers on the E35-2.
 
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