Scarves in winter!

Ok job 2!

The not-necessarily eagle-eyed amongst you will note that I cut 8ft blanks from the wood I bought, when actually the boat is 16ft long.  I'm obviously therefore going to have to join some panels together.  At this stage I have 8 x 8ft blank panels, which will make 4 x 16ft long panels when this job is complete.

So how do we join the panels together? We'd like a joint in the wood that is not really going to show - a kayak has nice curving sides (even in a hard-chine hull like the Chesapeake) and we don't really want a flat spot in the curve half way down the boat where we created a simple butt joint (butt our 2 panels up against each other, then put a backing pad behind it).  The way to do this, keeping a single thickness of wood is to use a Scarf Joint.

Imagine you just put the 2 panels next to each other, and put glue between them - not very strong at all!  Now instead imagine that the point of contact was made larger by making it a diagonal slope - this creates a much larger surface area to glue against.  The recommended slope for such a joint is 8:1 or 12:1 (if you were doing a really structual joint under load like a mast).  That means for a 4mm thick panel, we need 8 x 4 = 32mm wide slope.

The manual and the internet describe a number of ways to create such a joint, such as using routers, belt sanders, etc but I've also read on the internet time and time again on other people's blogs that the best way is actually by hand, and however they try other techniques, they always gravitate back to the one I'm going for.  No point finding this the hard way me thinks, so lets stand on the shoulders of some giants and follow the recommendation: take your wood, and stack a number of panels on top of each other (I did 4 panels at a time).  Step the panels such that each end sticks out 32mm from the one above it.  Take your (sharpened!) bench plane and just plane down the slope - the steps set you at the right angle and provided you have a sharp plane, this is straight forward to do.

Having cut my 11inch wide blanks I had a 6inch slice of wood spare to play with, so I used this to have a practice.  It's worth doing, as you'll get an idea of how evenly you can plane the scarf.  I also think that at the beginning of a project, you sometimes need to have a few plays at things to convince yourself you're safe to get going.  Anyway, you should see how well you're doing because we're using ply, and therefore as you plane, you'll see the layers appear as a horizontal band.  If you've planed evenly, the bands will be straight.  The only thing I'll add is that I found the bottom most panel suffered a bit at the end when I tried this, so make sure you're planing on top of a hard surface.  This job is definitely easier with a sharp plane - I definitely noticed when mine got blunt and the difference when I resharpened it.

Right! Epoxy time!  This is good, but scary stuff!  Again, I used my test piece to have a little play with the epoxy before I started - I plan to do this as I go along with various bits so I can reduce the screw ups!

Epoxy is a 2 part plastic adhesive.  It has a resin, and a hardener which arrive separately.  When mixed, they cause a reaction and start to go hard.  As this reaction takes place, toxic vapour is given off, and the mixture will get warm (it didn't noticeably for me when I tried it, but I suspect will as I use larger quantities later).  The warmth speeds up the curing time, so its best to use shallow trays to spread the mixture out and stop it curing itself.  The other fiddle is you need to keep the resin and hardener stored at room temperature (18-20 degrees) or it starts to go off.  The vapour isn't good for you (and there are dire warnings about sanding semi-cured epoxy and breathing in the dust) - the long and short is take the safety instructions seriously with this puppy, and always wear vinyl gloves when handling it.

The building manual said to lay all 4 planks (well 8 halves at this stage) on top of each other and separate the layers using celophane.  You should then put epoxy between each layer and then clamp it down.  It warned that the diagonal joints of the scarves could slip apart as you clamp them, so you'll need to prevent that with clamps elsewhere on the wood.  Sounds relatively simple yes?

My work area!

Reality, as always, was not so.  Firstly, these are now 16 ft long bendy planks, and you're trying to line them up straight as this will make drawing on them easier if you have a good baseline.  Secondly, and this is a point I should have spotted, its a right PITA to glue the bottom planks when you've got 3 others stacked on top.  I didn't want to do one then the next separately because a) that would take ages and b) I don't have a huge space in the garage to store the planks all over the place gluing so I wanted this to work if I could do it sensibly.  I settled for using an array of chocks to lift the ends up in the air so I could get in to the lower layers to glue them.  I'm sorry I didn't get a picture of this (new to blogging, sorry!) as my head was definitely in epoxy mode!  Anyway, I did a bit of a dry run, and proved I could get it to work, then mixed up the epoxy and got going.  It all went reasonably well - I remembered advice about changing the vinyl gloves often - you don't want to have epoxy blobs everywhere because you couldn't be bothered to change them.

A close up of the joint

The joins on all the panels were pretty good - the dark bit is where the epoxy came out (probably put too much on, but I was worried that this is quite a critical joint and I didn't want to starve it - will get better at guessing quantities hopefully!).  It looks like its just wet, but its actually like a lump of smooth plastic or glass that sticks up a fraction of a millimeter.  Sanded down with my shiny new random-orbit sander, it looks good.  I'm glad I got the tapered join straight here as it will make the kayak look much better if the line is straight and not ragged.  I did have a few places where the celophane wasn't wide enough and the epoxy just caught the layer of wood stacked on top, but fortunately it was only a tiny amount - close call though, I'll have to be more careful.

My next task will be to draw on a baseline (any ideas how to get a 16ft long straight baseline anyone? Maybe I need to buy a chalkline..) then I can mark up the panel shapes from the planks.  More on this next time!