Plywood interior?

Hello, beginner here.

I have been asked by a company that designs a boat to draw some of the interior components. Basically, the plywood cupboards, tables, benches and so on.

I'm a mech eng with 20 years of steel and plastic machinery design under my belt, but ships and wood is new to me. So I would be very grateful if any of you veterans would share some opinions and experiences.

Please tell me if I am wrong in these assumptions:

1 - I should avoid glue.
The reason for this is that the boat builders are aiming for a rather expensive, high-end product with a design life cycle of 50 years (and a pronounced eco-friendly profile). Glued joints will inevitably fail within that time span. No?

2 - I should avoid pre-tensioned screw joints.
The reason is that compressed wood sets over time and the screws lose their tension.

3 - I should assemble the plywood pieces with gemetrically locking joints, such as the "IKEA-type" cam/locking screws, or at least barrel nuts and screws.

Such as these:
http://www.home-dzine.co.za/diy/diy-camscrews.htm

While still relying on threads, they are deferred remote from the locking spot and thus (combined with wooden dowel pins) do not bend and shear the threaded zone as much.

Would you agree that this assessment is sound?

Or am I talking crap and glue is great?

 

I was building houses 50 years ago using nails, screws and bolts ( mostly nails ) Many 100 year old houses are still very livable and all original stuff. The cabinets I put in were glued together and small, metal brads were nailed in or shot in. Those houses are just fine 50 years later. Todays adhesives such as epoxy are much better and just as long lasting and will out preform yester years glues. Remember we glue supersonic and space planes together and have for many years even decades. Even our bolts, screws, and nails are much better today and you can get them made out of Titinium, of even dense spent Uranium rods. Modern adhesives and fastiners tailored to your need and materials ARE THE ONLY WAy TO GO. To get fancy use hardwood dowels to make your connections. Looks great........be sure to epoxy them in. Stan PS some of our hardwoods such as black locust, white oak. and black walnut are stronger than screws and bolts. Here is a link to hardwoods http://www.connectedlines.com/wood/wood13.htm Check out their weights and density. My total answer to your questions and asumptions would be A BIG NO..............................

 

Hello rasorinc and thank you for answering!

I have high respect for adhesives, I just have higher respect for steel.

I would assume that a boat is exposed to dynamic forces that a house never really sees. Not only will the boat crash up and down on hard sea, the hull itself will move ever so slightly from the stresses.

Interior components such as cupboards will then both experience G-forces from the motion, as well as follow the (very small) movements of their fixpoints at the hull - and rub together. Causing screw joints to lose pre-tension due to setting in the mating surfaces and clamped material. And possibly glue to crack after a couple of decades?

I'm used to vibrating steel machinery, I may well be mistaken about the nature of boats and plywood. I suppose the damping properties are better than for metals and that the innate elasticity may alleviate many of my feared problems.

Let me put it this way: if you were to build a mobile trailer home for off-road driving across the jungles of the world for the rest of your life, would you glue?

 

The recipies for todays super adhesives (epoxy type) can be designed to glue metals together that have different rates of expansion to heat and cold. The Boats method of build and the materials should be able to support cabinets in all kids of weather and expansion, and torshion. You can make cabinets as rigid as a square hunk of metal, but that is overkill. Wood cabinets give warmth to a home or boat. You can mount them
(hidden) with 2" bolts if you want. I would use an epoxy for a trailer over rough roads but there might be needs for bolts and screws and dowels.

 

Mostly agree with what's said. With epoxied joints, as dynamically loaded assemblies, the panels usually give up, before the adhesive, mostly because of the better modulus of elongation properties, in marine epoxy formulations on wood and plywood (wood products). Of course, it is possible to undersize a faying surface, but this is a design flaw, not material based. The monocoque structures, common with epoxied plywood panels are exceptional capable in this regard, being able to transmit loads fully across panels in all direction, greatly reducing point loading and stress risers.

If mechanically fastening joints, these should be well fitted, across the full faying surface, which should be sufficient enough to prevent undue compressive set. This said, dynamic structure loads have a nasty way of defeating these types of joints, so load transmission and sharing is the usual engineering approach. Simply put, instead of using a big hunk of something screwed to another big hunk of something, it's usually best to employ a more "stick built" technique, with several smaller load bearing elements all sharing and distributing the loads. In the end, though more effort in assembly, it's generally lighter and more durable.

Cam lock and other "over center" attachment devices can work on lightly loaded elements, but I haven't much faith in highly or dynamically loaded structures, mostly because of the intentional point loading these types of fasteners employ, often at a very small contact patch on the cam. These work great on an IKEA cabinet that will spend it's life resisting gravity, not so much in gyration.

 

You may want to look at acrylic adhesives like methacrylate for bonding the furniture to the hull.
Pretty easy to use and you will destroy the cabinet work before the adhesive lets go.
Not sure how "green" though. In the marine environment some compromises are worth it, durability is one of them.

 

I am an engineer that designs wood structures regularly as part of my work, though not cabinetry. My pinon on those barrel nuts and screws is they are a bad idea, they are not very strong and they tend to concentrate the loaded joints, including putting wood cross grain tension (a very bad idea). I think those are used to ease and speed assembly rather than to create a strong joint.

Any metal fastener that will be used in a damp environment, particularly salt water, will eventually corrode away not matter how well covered or coated. If you use metal fasteners them must be of marine grade stainless or silicone bronze (not regular decorative copper or bronze).

Polyurethane and epoxy adhesives are considered water proof, durable and permanant. The only issue is if you intend to make the assembly removeable for maintance or eventual replacement, it has to be cut out. There is no practical way to open up the joints other than cutting the joint. So these kinds of adhesives can be used but only for parts or sub-assemblies that are not expected to ever have to be removed. One should never consider btw, that cupboards, cabinets or closets in a boat will never be removed. it is often necessary to strip out the whole interior to do heavy maintenance and refurbishing of the hull at regular intervals. Trust me on this, the owner and his maintenance crew will love you forever if you design the installations so it can be removed and reinstalled without damage.

Rabbeted or pocketed joints are the most common way to assemble furnishings, some consideration should be made for the shrink and swell of the wood. No matter how well sealed and finished, wood will expand in damp climates, and than shrink when warm and dry. Rather than fight it, just try and design to accommodate it. It will tend to shrink and swell about 3-4 percent of width across the grain, and about 1 percent along the length of the grain. Design accordingly.

Depending on the clients desires and the design of boat overall, you can leave fasteners heads exposed as part of the style. Exposed Silicone bronze screws and hardware (angles and brackets) can enhance the style and appearance if a traditional look is desirable, or you can hide them under wood plugs or have them all accessible from inside the cabinets. But the bests design in my opinion would be ones that all interlock together, and only use the fasteners to hold them in place, but not to take the primary loads, all fasteners should be arranged to have them loaded in shear, never in tension if possible (light tensions loads are usually okay), and NEVER in tension or withdrawal when fasteners go into end grain. All fasteners should be arranged in cross grain shear, and the direction of loading on the grain itself should be considered since across the grain capacity is about one fourth than in the same direction of the grain. Cross grain compression tends to be lower by one third than in the direction of the grain, so design accordingly.

good luck.

 

Another consideration with furniture is they often need to be considered as part of the structure. It's quite common for athwart "partitions" to be bonded, while the longitudinally oriented "faces" are simply hung, making them removable in many situations. Simply, some coordination with the structure designer(s) will be necessary.

 

Hello

VERY good points, all of you! ^_^

Then I think will use rabbets and glue for the bits that can be assembled in one piece, and install them using place-locking devices that does not pierce material.

That way there is tolerance for a little bit of motion (swelling, hull stress, etc), yet it will not cause stress concentrations.

I did not think about the swelling. Or rather, I did not expect it to be so significant. I was planning on adding gaps in certain places to prevent creaking and squeaking, but it seems I need bigger gaps.

Thank you very much for your time and expertise, this was exactly what I was hoping for!

 

Funny that, I still see 50 + year old glued dinghies and small offshore boats (sail and motor) being used regularly. Some of the other waterproof adhesives are OK too and have been well proven. I am very much with Petros and PAR on this. Another way of 'hiding' necessary fasteners which eliminates plugs is to use an upper/outer laminate timber on the surface. It might be a little more work but you did say very high quality!.

If you are familiar with sheet steel, it is useful to mentally think of the 'box' section size you would use and envision that in timber, but the wood as solid rather than hollow. Just a v quick rule of thumb trick for sizing, but you have the benefit of lamination with timber so can achieve more complex curves and forms. These are better on a boat (generally) as they can prevent injury in a sea way and are more pleasant to handle. Hence a lot has been copied in FRP as it also has the ability to allow curvature and it helps the material structurally.

Personal taste is to try and get a mechanical aspect to a glue joint, at minimum increase the shear surfaces if it is under load. Even dovetails and dowels are fine and not hard to do with the jigs around these days. just depends on the exact nature of the joinery.

As Petros says, Stainless 316 (A4) preferably, 304 (A2) at pinch or Silicon Bronze are the best materials for fasteners that are widely available. Titanium bolts are used in aerospace and can be sourced with some greater difficulty.

 

For faces (rails and stiles), don't allow gaps and don't add any place-locking surface-mounted devices or gaps will appear on the face and they will more readily absorb moisture and dirt.
Instead, assemble with thin plywood splines and epoxy. This applies to more narrow (up to 3") pieces where expansion won't be an issue. Wider pieces that join the side of the adjoining piece can be matched plywood.
Most importantly, use quarter-sawn wood if at all possible as expansion and shrinkage are only half that of plain-sawn.
Panels should be plywood if possible.
This is an extremely easy way to build long-lasting cabinet parts.
Most swelling damage to boat furniture occurs from water (usu. rain water) getting into the boat due to owner inattention anyway. In fact, using the right wood, orienting the wood properly, and proper construction methods as well as sealing the wood well, all serve to create a long-lasting interior.
Wood likes to "take up" and let go of moisture slowly and the sealing job helps in this respect. Sealing should therefore be equal on all surfaces inside and out, everywhere. Never underdo the unseen surface. Then should the wood encounter a very wet situation, at least expansion will do the least damage.
I wish you luck with the project. If you are able to, I suggest you build a joint or two and submerge them in water to test the method. You will soon discard the method you mention above. PM me if you need more detailed advice.

 

Nothing personal, more of a management thing, but how does your boss expect to get a 50 year service life and sell to wealthy knowledgeable buyers when he assigns a 20 year machine and plastics engineer to build wood furniture for a boat. Who is doing the structural calculations: a 20 year electrical engineer? Who in their right mind would buy an expensive yacht designed by people working so far out of their training and experience. Sounds like a recipe for failure, I would not suggest taking out any longterm loans based upon this job......

Also I am surprised, given your Swedish location, that you have not contacted the government wood products agency charged with promoting their usage. I suspect that there are hundreds of scientific papers that address the exact questions that you pose here. At the very least, you should be familiar with the requirements of the appropriate certification standards that apply to maritime construction such as ISO, Veritas, Lloyd's, ABYC, etc. When you provide documentation for your build techniques to the insurance companies will you cite the advice of strangers on the internet as a technical reference?

Sorry, but this whole thing sounds hokey to me, if you are what you claim, someone is taking advantage of you IMHO. I cannot imagine putting something as important to a projects success as the interior design in the hands of a marine novice who needs to ask such basic technical questions in a place like this. I also find it hard to believe that an engineer with 20 years experience in any field would do any kind of serious research in a forum like this......

 

You should visit the shop which will be building the interior components to learn their capabilities and preferences. Also learn as much from them about their experiences and recommendations. If no shop has been selected yet then find at least one and preferably several shops who may be doing the construction and visit them.

Designing in wood may require "un-learning" some of what you know about design based on your experience with steel and plastic.

 

So, you'll be attaching things to the panels, so that you can eventually attach the panels?

 

Thank you SukiSolo and alan white for your recommendations!

DCockey - I realize that there are some things I have to unlearn. For instance, working with soft materials such as wood as load-bearing structures.

PAR: yes, that was my though.

Now, Westfield 11 - I understand your sentiments, but you have to realize that Sweden is a small 9-million people country. There is no such thing as just lifting the phone and contracting some specialist, because if any exist then they are usually already choked with work.

Or retired. The current generation has been too preoccupied with leisure that knowledge and skill is fast dying out. I would love to adopt a trainee to pour my own skills into, but I have yet to find anyone that shows a bit of interest. Typical developed nation problem right now, I'd say.

Still, the Swedish industrial success is rather impressve one you realize how precious few engineers created it (mainly one or two generations ago). One key to this is a corporate culture of very flat organization and extensive generalization. It's a small set of people rotating around creating stuff and wielding considerable freedom of initiative, as compared to many other industrial nations with a strict hierarchy that locks people in niches.

While that is fine in itself - spending your life doing one thing tends to make you very good at it - there are quite a lot of niches where you run into the ceiling rather quick. There is only so much you can learn about building steel towers before diminishing returns kicks in. A lifetime of tower design won't make you that much better than someone who did it for a few years.

I'm a generalist mechanical design engineer, and I have been involved in some fantstic projects, simply because I was the only one within the 200 km radius that was both available and had sufficient skills.

And just to clarify: I am not 20 year old, I have done mechanical design for 20 years. Metals, plastics, medical, nuclear, structural, chemical, structural analysis, mathematical analysis. If it is made of atoms, structural mechanics apply. The one thing I am bad at is making things aesthetically pleasing, but I know a couple of industrial designers.

This particular boat-building client needed someone who could transform a designers' fancy surface models into drawings for the woodcutter and assembly crew whithin two weeks. Because govenment institutions are involved and they want to see something at a certain date.

Also, they are building one (1) boat. It is a showcase for new technology, not a serial production. Yet they want to make it reasonably authentic, and keep the prototype indefinitely as it is a nontrivial investment for a family company.

And my part is not actually very demanding. Make the kitchen and bunk areas manufacturable. Using a given material and not uglifying the designers' intent.

Sorry for sounding like a braggart, I just want to put things in perspective.