Deep Water Question

Depends on how the wood was treated...If it was in a vacuum chamber and the air was evacuated as the treatment solution was allowed to completely saturate the wood, I would think it would be fine....The integrity of the wood will depend on the stress involved and the length of service expected...Why not just fiberglass the wood and gelcoat...or use a powder coated steel plate...They do make wood hardener that could be applied to the plywood....Maybe a hardwood plank....

That equates to a pressure of 5272 pounds per square inch or 759,272 pounds per square foot..

I would expect the wood fiber to crush into dust.

https://crev.info/2019/10/wood-buried-at-sea/

https://www.newsweek.com/mh370-shipwrecks-among-deepest-ever-discovered-910287

I can't argue with WJMFIRE's logic but I think it might not work that way. This link https://www.livescience.com/63899-oldest-intact-shipwreck-found.html has a wooden ship that has been 1.2 miles down since 400 BC. It has seen better days but the wood doesn't seem to have been crushed and destroyed. There are other stories out there about wooden ships 2+ miles down but in my quick search I didn't find a story with pictures.

You are correct about those shipwrecks but we are talking about plyed pine, not seasoned dried virgin oak.

I think it would depend on if there are any voids in the wood. Cheap plywood does sometimes have voids. Solids and liquids are hardly compressible under hydrostatic loads, unlike solids with air-filled voids. A prime example of solids with voids is styrofoam cups that have been submerged deep into the ocean and reduced about a quarter of their former size.

Rixter, I agree with you but I think there is a difference between solids like plywood and lead.

https://www.trioforest.com/wp-content/uploads/2013/08/APA-Plywood-Design-Specs.pdf

These guys list the compressive strength in psi topping out at 1100 psi, for wet wood somewhere about 970psi, compression in the plane of the plies, which I think applies in this case. Table 3 Page 16. You might get double that if you found the right piece, but 6 times higher might be a stretch. That being said, I doubt the failure mode is dust, so an aluminum sandwich with no other stresses might survive. It would be helpful to know the reason for this particular design.

See response #19

Thanks. You do have interesting problems(!)

If you have the time, you could immerse the plywood in a tub of water for a several days (weeks?) to raise the moisture content. Then incorporate in your assembly. Water can't crush water. The wooden hulls of ships that remained intact at great depths would have had very high moisture content before sinking. (Note: a couple of years ago I used marine grade plywood in a project. As a test, a immersed a small piece in a pot of water for a week. The sample showed no sign of delamination after that time.)

The difference between marine grade plywood (off the shelf) and any other type of plywood is that it’s denser that regular plywood, and has stronger and water proof glue.

the problem that you might face at that depth is marine grade plywood may be denser, but there is still air pockets in the wood as well as the laminates glue. It may delaminate when the air pockets are forced out.

Marine grade plywood can be made from different types of wood. The product I purchased from a local plywood specialist was manufactured from okoume, an African wood that is classified as a hardwood, but is neither as dense nor as hard as, say, maple. I have worked with standard Baltic birch plywood and can attest that it too is harder and denser than the okoume product. The marine grade plywood is not impregnated with preservative or waterproofing. The glue itself, is of course, waterproof. The plywood is void free, and even if there were the occasional small void in the body or in the glue line, I suggest that under the described pressure there would be no possibility of delamination.

Why the wood? If you are thinking of sandwiching it between aluminum, why not just use aluminum? (5086 being one of the choices for seawater immersion)

What goes down does not necessarily come back up.....when comparing the prices here I would much rather it be plywood than aluminum....

Looks like Netmaker only needs it down there for 20 minutes, so I’m guessing it is expected to come up on a regular basis?

The researchers will install Simrad Sounders which will let them know when the gear hits dead bottom. Given 90 seconds for the Otter doors to form the 38 degree angle of attack on the 4 chain bridle, the 20 minute tow starts then.

Rate of descent will be arbitrary as their test site, between the Western edge of the Mariana Trench and a large sea mount, there are predicted to be ,7 different currents from 2 through 5 knots, going in almost complete Compass Rose directions.

The wood is needed to give some buoyancy so the doors hit bottom standing up. There will be a 3:1 scope of towline 51K of 14mm UHDPE ( Dyneema™) towline.

THIS is what happened a few years back on a similar research expedition.

This expedition used a Misago Sled Beam. On its third deployment, it hit "something" and the 6"x 6"x 3/4" steel beam was torn in two. Both sections and the net came up still attached to the 16mm Dyneema Towline.

This is the reason for trying to use more economical gear to spread the trawl net as the loss ratio can be severe.

They told me they only felt a "slight bump" and then the Load Cell meter dropped 45%, letting them know the gear had lost its integrity.

That looks like a failed weld. Metals should not fail in the weld and definitely should not look llike that if a failure does happen in a weld zone. Notice the base material does not look at all deformed.

...or is that something other than a weld?

If that is a weld, then the structure would have been far more robust with a decent weld.

CORRECT! See Pictures............

See Response #19

The wooden piece is used to open a deep sea trawl net. The customer does not want to spend big money on something that might not come back up ( snags etc in the crevasse he will be sampling for fishes and crustaceans). Hence the idea of an " inexpensive Otter Board ( set of two) to open the trawl net.

The wood will allow the Otter Door to partially stand up and not lay down on the sediment where it can dig in like a flounder and get stuck.

HDPE and UHMW sheets 12" x 24" x 1" are available from Amazon at about $20 - $30 per square foot. I don't know what the data sheet will say but I doubt that the pressure will do anything to the plastic that would be problematic for your application.

A5) The rate of ingress of water into the wood will be slowed, as water will need to go round the aluminium.

The aluminium might require a sacrificial anode to reduce the potential for corrosion, depending on its size, shape and the length of time that it is immersed.

I'd say it depends how slowly it goes down!
If it was to be subject to that pressure instantaneously it would crush... but it is porous!
The only real answer is try it and let us know the result.
Ironically I'd suggest that the more holes it has in, it the better it will survive as it will allow the water to soak in and equalize the pressure more quickly and more evenly.
Bear in mind there is a lot of surface area and not much thickness, so the water can get in there pretty easily.
If I had to put money on it, I'd say it would be fine.
Del

Consider cast plastic sheet for this project. Most plastics are tested to compressions of up to 10,000 psi. I can recommend "corbell plastics/research solutions/plastic-properties" as a good site to view the properties of different plastic materials. The best option would be PTFE but it is far too expensive. HDPE compromises on quality but is cost effective. Corbell sales team are very helpful if you tell them your application and ask for the lowest cost solution. Your problem is that cast plastic comes in set size sheets and to get the 54" length you need to buy the biggest standard sheet. The plastic cost if you were making 2 pairs in 25mm HDPE would be about $250/board. For 5 pairs that would drop to $200/board. More expensive than marine ply but about the same cost as ply clad in aluminum and for a guaranteed solution it may be worth the premium.

FYI

some of the weird stuff from 2.75 miles down in the Pacific.

Yikes! I'm not eatin' any of that....creatures from the deep....

An interesting question, please let us know the results of your test.

It wont be a test. It'll be the real thing. In cases like this I don't get to test anything. It either works like it should the first time or I have a p!$$ed off customer on my hands. ha ha

I've had good luck with MDO plywwod

I left a test piece half buried near a downspout in mud that would freeze for years with little effect.

I tested it because I didn't trust the surface witch reminds me of MDF which disintegrates unless kept in a cool dry place.

what is MDO?

Medium Density Overlay...

there’s also HDO High Density Overlay...