Chine log joint design factors, guidelines, and testing

I haven't been here for quite some time, but I have a project and I'm sure yall have knowledge that can help. I've built a few small ply boats, mostly with chine-longs but a couple with taped seams. I have a professional background in engineered wood products, making plywood, LVL, and wood I-joists.

I'm building a plywood-box utility trailer with an eye towards production. I'd like to use chine logs with with glue-only joints, somewhat like the web-flange joint used on wood I-joists. I'm trying to gather as much practical design information as I can before I make test samples.

For I-joist web-flange joints, the primary design consideration is longitudinal shear strength. The joint also needs to hold itself together while the glue cures, and resist the web spearing through the flange when loaded in compression.

I'm more concerned with tangential bending forces (changing the angle of the joint) and shear. The type of forces you see in an open skiff with minimal framing and bulkheads.

Can you help point me in the right direction? Would this be somewhat analogous to using bot internal and external chine logs? What size chine logs are used for 1/2" ply?

Thanks!

 

A boats bottom usually has some rocker, so the chine can be smaller. No such thing as standard size chine log for 12mm ply. You would be better off working from your trailer load and compare that to a floor beam. Is it just an edging joint and not structural to the floor load.?

 

How much of an issue are road shock and vibration?

 

Huge.

 

I'm not a NA, or experienced with structural design, so this is outside my wheelhouse. I understand how rocker makes the structure stiffer but not how that reduces stress on that seam.

It's primarily an edging joint but will provide stiffness to the ply floor between cross supports.

 

From a production point of view I would finger joint or dovetail the plywood edges (cnc cut), this increases the glue area enough to have the plywood itself fail. The (necessarily external) battens can then be calculated as either a stiffener or as structural frames as needed. The additional bonus is that the battens will cover the plywood endgrain and increase the longevity.
No taping, that's to much work, if additional strength is needed (corner joints) use riveted metal brackets.

 

Is the trailer built all of wood or is it a plywood box over a steel frame?

 

Ply box on a steel frame.

It's an off road trailer with big tires. Fenders are required, so I figured I'd add side boxes that fulfill the fender requirement while adding storage capacity. Plan is brackets supporting the bottom of the box, and I'm thinking that the sides will be stiff enough without brackets or framing if the joint circled in red is good enough. My guess is that a 2x2 (1.5"x1.5") with a couple dados and epoxy would work just fine, but I wanted to consult with those more experienced first.

brown is the main box. Purple side boxes. Black is the tires.

 

The usual setup is with the wheels approximately in the middle. What is the reason they are in the end? It would be much easier to make a standard box that is full width with fenders on the inside. The corners may have dividers if you want, but otherwise the standard setup is that the space between fenders is about 49". I don't understand why make it so complicated. If you are trying to avoid a steel frame, the first thing to take care of is the weight rating you expect to give the trailer. That will drive the design. Without a weigth rating nothing else makes sense.

 

The wheels are approximately in the middle fore/aft, the sketch is an end view. A full width low box would have much less effective ground clearance, which is why most off road trailers have only the box between the wheels and external fenders. I did quick search for examples:




The other high-clearance option is a deckover/high-boy, which is full-width but the floor is over the wheels, so the lower space between the wheels isn't enclosed.

 

I think if you state the reason to have a plywood box without a steel frame it would be easier to help. Otherwise, you could simply weld angle iron and bolt the plywood panels to the frame. There are plenty of trailers like that. Your examples of trailer are all steel, which makes sense for off road and rough use.

 

The little trailer above starts at $11,500, which I think is absurd. I'm looking to produce something at a much lower price point.
Many people who use trailers will tell you that steel decking sucks, and wood is much better. I suspect that most commercial-use trailers in the U.S. use wood decking, but I don't have the data. Wood is a fine material and I believe it's appropriate here.

The reason for a wood joint here is to save time and money, and I believe it can work. It also has the benefit of capping the ply ends, which are venerable. Any thoughts or guidelines on what the geometry should be?

 

I love my plywood boats, have three.
I would buy a trailer with Ply sides and decking but it would have to have metal base & perimeter framing and cross beams under the deck.
I'm not an engineer but have spent my career in transportation and have seen just about every mode of vehicle failure.
Utility trailers live a tough life of abuse and neglect. Heavy crap thrown in them with no regard to weight or sharp edges. Stored year round under eaves with no gutters, borrowed by your prize brother-in law...
A completely different level of care than a CLC ply travel trailer.