Strongback materials for cedar strip construction

Don't know why we have an argument going here. We all know wood is, for its weight, very strong along the grain, and we all know its strength across the grain is around 10% of that. Thats the main reason we have plywood after all.

Cross-grain strength must be provided for a strip-built hull. For my own stripper I do that with ribs, going back to 100 year-old methods but with glue instead of glass.

The problem with added strength is, the only way to test for it is to break the boat, since strength is resistance to breakage after all. However stiffness is testable and demonstrable without breaking.

Wobblyness is a reflection of stiffness not strength, and the greater stiffness that appears after the glass is added derives from the greater modulus of glass compared with wood. When the hull is twisted, its the glass thats providing most of the resistance, not the wood. The wood still has all its longitudinal strength and stiffness and some of the boats resistance to bending stresses comes from the wood, but because the glass is so much stiffer than the wood it takes up the forces that result quicker. Break the glass and the wood is still there to resist bending loads, but the loss of the glass will show up by a failure elsewhere.

Each boat construction technique is a system, with the parts interacting in a complex but understandable manner to produce the end result.

It is a regrettable (IMHO) fact that a glassed boat is no longer truly the wood boat it once was, from a structural point of view it's now merely the wood core used to make a fiberglass boat. There's a bit less F'G than there would be in a solid F/G boat, the wood core allows that by acting as a spacer between the glass layers, but the FG is doing 90% of the work and the wood is just along for the ride. Might as well use balsa instead of cedar - although it's inconvenient and expensive - or a foamed plastic core for all the good the wood does. But at least it's pretty.

I avoid the use of glass on a number of grounds, personal & environmental among others, but's good stuff. So's wood, but once glassed it's mostly a spacer between 2 working glass layers. If you actually want a wood boat, you have to go to a little more trouble to get the strength in all the drections you need.

 

Isn't the wood also adding impact resistance though? The wood can survive a lot of hits without deforming structurally while the glass cannot?

 

I hate to add fuel to the fire but there is no comparison of the impact resistance of foam and wood. The foam is stiff but even when covered with cloth and epoxy on both side it will not take the impact as well as wood. Even different species of wood take impact differently ,balsa does not do well. cedar is widely used , I built one marathon boat out of white pine and cedar couldn't compare with the stiffness of the boat with the same layup
or the ability to absorb impact. Wood is a more important part than is being said here.
Ron

 

True. I was forgetting about impact resistance, but wood has a minor part to play once glassed. I just detest glass unless it is wrapped around a beer.

 

There is no fire, there is only friendly discussion

No, not necessarily.

Marine grade foam comes in a range of weights, take Diab H80 foam, if you were to say drop a hammer on it from 1 metre, it wouldnt dent ( I just tried it). I dropped the same hammer on a piece of Western Red Cedar, and there was a slight dent (must be a soft strip).

The big difference is, if I kept dropping the hammer on the foam, it would eventually dissolve into dust, whereas the wood would still have fibres intact (though bent), and it would take a lot longer to pulverise . There are a number of studies quoted in these forums that indicate wood is much better for long term, constant impact resistance. Also, timber has much better shear strength ( snappability ). Foam has only strength in compression, and in a constant immersion scenario, much less water absorption and rot potential.

Wood is a bit cheaper (depending on species), better at self fairing, and much more attractive.

 

- I deleted the gloating post on the glory of wood that I had penned as an act of supreme self-control, and I hereby dedicate my selfless sacrifice to a return to the thread topic.

 

Bless you my son.

Wood is good!

Boats is .... good fun

 

Concur!

 

I'm finally coming up for air after moving and while everything I own is still in boxes, at least all those boxes are in one place now.

I discovered that there is an infinite supply of free particle board from cheap desks and cabinets at the re-store. Solid wood would be nice, but unless I can find a free or nearly so source for it (old pallet boards, perhaps) I'll probably go with the particle board.

How is alder as a building material? The only reason I ask is that I have four and a half acres of it here. I also went to the exotic woods place and looked at hardwoods for the keel area. Locust or Hickory can be had for around $2/bf and those should protect me on my various rock finding trips.

Can't wait to start building this damn thing.

 

For strip planking, particle board will do fine. Just run some packing case tape around the edges to prevent gluing strips to the moulds.

Make sure you BOLT ( not screw or nail ) the moulds to the strong back, it will be much easier to remove them later when you lift the hull.

If you can get enough particle board, you can use it as the strong back too.

Any light timber that will cut cleanly into strips, and wont warp during construction, will do. As Alder is used for flooring, it should be strong enough, if you can get light and dry enough planks.

This great site http://www.woodweb.com/cgi-bin/calculators/calc.pl
calculates the relative weights of timber, and say that Alder is similar in weight to Western Rd Cedar - which is fine.

Hickory is double the weight, and really too heavy.

Its your fibreglass that will offer the most protection against rocks. If you think you will be punishing the craft much, use some Dynel or equivalent on the damage prone bits - but it adds lots of weight because it is resin hungry.

 

I'm looking to use hickory or something like it just around the keel and ends for impact and abrasion resistance. Would a strip of Dynel or kevlar tape over the keel help much?

 

apparently I gotta spread some rep around before handing you one for that beer remark lmao

cheers
B

 

One or two strips in the centre, and/or an end strip of Hickory wouldnt do any harm, I was just worried you were going to do the whole bottom of the hull with hickory. Dynel tape would be a good thing along the keel and ends, and would save peeling off fibreglass in the event of a 'big bang'.

The Guilliemont Kayaks have a carbon fibre inner shell, so they must be guarding against huge loads - bigger than I would ever want to encounter.

But if you are going out in the surf, having the hull fill up with water and get rolled onto the sand and/or rocks - counts as a 'major event' in my opinion.

 

I agree with the use of pine boards. Though they have to be assembled, the work is not difficult and you can staple and fasten to pine easily. Not only screw but also glue (liquid nails in a caulking gun is handy and cheap).
Invest in a small spokeshave to perfectly trim the molds when you are using a batten to fair the setup. Once perfectly fair, you can lay masking tape over the edges to make goo removal easier.

 

Agreed Alan, though I use plastic packaging tape instead of crape masking tape. I've found this type of masking tape can stick to things with epoxy on it, but the plastic packaging tape never fails to release. I prefer the clear tape so I can see what's going on.