Strongback materials for cedar strip construction

33' LOA, Just one 20mm spruce..

 

You might like to consider this - if you are building strip plan with upswept ends, you usually have pointy formers that are supposed to be removed after the planks are glassed.

Then you have to clean out the really tiny ends and try and lay fibreglass inside the hollow pointy ends. Thats really irksome

If I ever build in strip plank again, I am going to build the first 2 feet of either end as a hollow fibreglass mould (two sides, joined together). Then I will lay the strip planks over the mould, and epoxy them directly to it, so the mould becomes the inner fibreglass that you would normally have had to insert and layup very awkwardly.

It would also have the advantage of ensuring a very fair shape to apply the planks to, much easier than fiddling with individual plank ends. It might seem more trouble at first sight, but I am convinced it would end up being much easier.

 

That's an interesting idea, I'll have to think about that. Of course, I'll probably have more questions once I do some thinking

 

You could have a small bulkhead near the stem and fill the gap with foam.

 

Yeah, that's what I was thinking too.

Does it make a difference that I'm going to build this like an open topped canoe and then put a separate deck on later? I admit I'm a little fuzzy on the order of operations for this construction method. My understanding is that I'll build the hull on the strongback, glass the outside of it, and then remove it from the strongback to do the inside. The deck is going to be made from a different material (plywood or doorskin) and will go on as the last step, meaning that i should be able to reach the inside fairly easily.

Am I missing something here?

EDIT: OK, looking at the model again, the ends are very pointy and so the stems will probably be solid wood for at least a little ways back. How much of the interior fiberglassing is for strength and how much is for waterproofing?

 

My take on the 'end building' process, is that it has to achieve 'strong enough to be slammed up against a rock' with 'wont hold water a long time after immersion, and start to rot' because of poor layup results.

Even with the two halves mounted on top of each other, getting back into the ends to add the final layer of waterproofing and inner strength is a real pain.

Check out

http://www.youtube.com/watch?v=d5zOSFwrcFo

building the Guillemot kayak. They do a nearly solid end point, Watch the end bit where he does the impossible job of the inside taping at either end. This is what we need to reduce as much as possible.

The Guillemot method really is building strips on moulds, on a big scale. They build their kayaks like violins, even using carbon fibre inner linings.

If you can, watch the entire series on YouTube - one of the best introductions to Pointy Strip Plank Kayak Building I have seen.
You must see
http://www.youtube.com/watch?v=JGPgEPXuDrc&feature=related

also

 

I drew a picture of the concept. Hope it makes sense

 

Speaking of decks, I’m a bit hazy about exactly what kind of boat we’re talking about here. I have read both canoe and kayak with mention of very large canoes but not much reference to decks until now.

I haven’t done a full kayak-style deck with small cockpit but I understand the deck is built over the same molds as the hull but not attached at that time, so it can be lifted off to be glassed inside. Keeping the hull and deck the same shape so they fit together later requires lots of spreader battens.



For a sailing-canoe type deck with side decks and a large cockpit the usual construction method is similar to a decked sailboat, with side deck knees, coaming, all the usual stuff but no seats along the sides. More robust than a kayak deck as you can expect to spend a lot of time hiking out.



If you just want (relatively) small decks in the classical paddling canoe style, my simple method is to complete the inwale with a rabbet for the deck to lay flush, and add the outwale after trimming the deck edges flush. That makes it easy to get neat joints where it shows. A small deck is not an elaborate thing but does wonders for the stiffness of the hull. At its most minimal, it's a breasthook of course . . .

 

Definitely a kayak. The advantage of the cedar strip method I see is in durability when doing things like hitting rocks, logs, bottom, being drug up on the beach, etc. I don't trust fiberglass and am dubious on SOF for this. On the other hand, the deck shouldn't take as much abuse as the hull and so doesn't need to be built up from 1/4" thick wood.

That's my understanding of the process as well. I still need to watch R Watson's videos above, but from what I've read you make two pieces (hull and deck) off the same strongback, glass them inside and out separately, and then join them together. My plan is to make the hull, fit permanent bulkheads in to create separate compartments, and then put a deck made of lighter material over it.

 

Just bear in mind, its the fibreglass that gives almost all the strength to the little bits of wood. The timber is virtually just an attractive core material, and if you use woodworking glue to join the planks together for ease of construction, the fibreglass and epsoxy layup becomes even more critical.

When I built my 16ft canadian canoe ( no top deck to speak of) , and had only glassed the outside of the hull, the whole boat would twist and bend when lifted. As soon as the innner FG was installed, it became super strong, and survived being blown of its 1 metre stand, 30 metres along the ground, by a strong gust of wind.

Yes, thats the go. You may want to consider making the deck quite strong. I used to row racing shells that has little bits of 'canvas' at each end for lightness, and they were always getting splits and holes from someone dropping somthing on them, or tripping over them on the ground.

As you plan to use the craft in surf, the decks also form a major structural member, to prevent the hull from sagging and bending longitudinally, especially if you hit something head on. The rigidity of the whole hull depends on a strong 'top half' to the 'clamshell' assembly.

Dont forget, even with bulkheads, if the centre cockpit fills up with water, you have half a tonne of weight to push the hull into sand or rock - so structural strength may be stretched to its limits.

 

In case you're unaware of it, the Kayak Forum is a great resource for related discussions and pictures as in this htread http://www.kayakforum.com/cgi-bin/B...read/id/205285/sbj/strip-end-blocks-question/

 

Don't jump into hasty conclusions. The tensile strength of wood is generally btw 5000-15000N/sqcm, and give (to quote Brent) a huge longitudinal strength. In longitudinal strip planking gf gives the tranverse strength and as a whole this kind of structure doesn't have it's full stiffnes before all the components are ready.
BR Teddy

 

Thats in compression
( http://www.engineeringtoolbox.com/young-modulus-d_417.html )
which is what a core material has to do.

Also, glued timber loses between 40- 50% of its strength on the joined surface at best. Unless you meticulously epoxy every little bit of the 4mm thickness, ( which most strip kayak builders dont - they use quick woodworking glue or Cyano Acryl ... whatistname), all you have without the fibreglass is a bunch of loosely attached 4mm skinny strips that couldnt stand someone as heavy as me standing on it, let alone contribute any longtitudinal strength. Like I said before, until both inner and out glass is applied, the hull wobbles like spaghetti.

Once the encapsulation is done, that all changes, and you have a stiff moncoque shell, but you would have got the same effect if you had used a high compression foam ( like a surfboard does - they used to be wood stripped too)

You could use high density foam or balsa on its end grain and get the same hull strength result ( many do).

By the time the wood has to take any strain other than compression, the fibreglass has been destroyed.

 

With the same reasoning gf by itself doesn't hold anything (without resin).. so please stop that nonsense...

 

I dont know what you mean by that 'nonsense' - if you have a problem with something I said, feel free to explain it.

If you are upset about my downgrading the role wooden strips play in the average strip plank Kayak, try coating the hull with just two layers of epoxy ( no glass fibre) after gluing the planks together with woodwork glue.

Then, if you stand on the hull and jump up and down, it will fracture and buckle.

Or - if somone large like me steps heavily in the cockpit when the Kayak ends are held off the floor by say, chairs, the hull will split and fracture.

The wood is largely just pretty filler, unless you carefully epoxy all the strips together, ( which no one does), and even then the edges of the planks will come apart (the edges of the wood wil break away from the epoxy) under a decent load.

There is only so much 4mm of lightweight timber can do with no other re-inforcing.