Saturday, 21 May 2016

Fitting the deck-beams part 2

Marking and cutting the tenons
What is described below is what works for me. There are alternatives such as the elegant one described by Chris Cunningham in his book on Building the Greenland Kayak.
 I take the essential lines and measurements with a thin batten wedged against the top of the the mortises on each gunwale and towards the narrowest width between them. As you can see the batten is not so wide as the mortise, and placed in this position can establish the point (where the pencil points) from which all of the necessary lines and markings can be taken. The lines drawn across each side of the batten also give the angle of the gunwales at this point, necessary for determining the angles of the tenons. I rub the lines out afterwards so that I do not get confused when doing the next beam.

These lines are measured with a bevel gauge as shown below, then the lines on the other side checked, and if different, the mean is taken between them. Shown below are most of the other tools required for this job the ruler and another bevel gauge fixed at  the gunwale angle of 77 degrees (13 from vertical). I had forgotten to add the marking gauge set at the width of the mortise (1/4").
This angle is then drawn twice - a gunwale thickness apart at one end of the deck beam. The edge of the tongue at the narrowest edge of the beam is determined by the line drawn at right angles from the drawn bevel to the corner of the tenon. You will notice that the tenon now starts about an eighth of an inch in from the edge. This width is scribed along the edge of the beam to the other end, and as shown in the next photo

 the inner width of the beam marked off from the batten (just held for the photo like that!).
Then the width of the mortise is marked from the top of the beam with the marking gauge (not shown) so that we can cut the thickness of the tenon, and  the angle of the shoulder is now added to lines taken off the batten using the bevel gauge at 13 degrees.

 The thickness of the tenon is then cut (if you use a band saw as shown below you will need to finish off with a handsaw because of the bevel resulting from the shape of the kayak).
This cut can be seen below and the handsaw will be used to cut the edge of the shoulder.
Then the tenon is trimmed. It doesn't need to be trimmed lengthwise; these can be trimmed from the assembled frame.

If you look carefully you can see the tenon edges protruding.

Finally after checking again that the frame is true from a stretched line down the centre by measuring half widths etc, the ends are lashed and trunnels (wooden pins) inserted as shown below.

At the stern a block is also inserted due to the wide angle at which the gunwales converge.

The trunnels are cut from a die. Normally the wood-stock is held firmly with the other hand as it is being hit through the die to stop it bending and breaking. I use oak for strength. (you can alternatively buy hardwood dowel)

Finally with this kayak, which is being built in tandem with updating the computer design, the sheer of the finished structure is measured using a laser level, and then the design in the computer will be updated to reflect the actual sheer measured.

Two raised deck beams still need to be added and can wait until a later stage in construction. Pinning the tenons will be shown in the next post.

Friday, 20 May 2016

Fitting the deckbeams part 1

Deck Beam position
If you are working from plans or are working from a book/CD, the deck beam positions will either be given or else you are shown how to determine where they should go. It comes down to fitting the lower body into the structure at the correct point with relation to the centre of buoyancy of the craft and marking off the beams either side of that.
Specifically there will need to be one that supports the rear of the cockpit, and one not far behind to take the weight of the paddler sitting on the rear deck. In front there will be a beam that also acts as a thigh grip if a small cockpit is fitted; it helps to know the paddler's dimensions to determine the position. In front of that there will be more raised beams and then one which will act as a footrest or to take the front of a foot-peg fitting. Other beams are fitted towards each end of the boat as needed.
On this craft which originated in the "Freeship" computer program I added the positions according to the principles described above around the calculated centre of buoyancy using the "intersections" command.
Securing the deck beams
Two ways are commonly used to secure the deck beams to the gunwales; they can be held in place with dowels or they can slot right through the gunwales in a mortise (the slot) with a tenon (which goes through the slot) to form the joint of that name. Using dowels is easier and quicker but generally reckoned to result in a slightly weaker structure. The latter method is also much more satisfying and is described below.
Cutting the mortises

Beneath the cutting tool is a wedge of 13 degrees which is the angle that the gunwales are planned to set, off of the vertical. This results in quite a gentle sheer. 
The photograph is not quite accurate as I was holding the camera with the other hand which would normally be pressing down hard on the top edge of the gunwale to prevent it riding up and jamming the cutter when you let it rise after the cut.

The picture of cut mortises shows the vertical markings of their position, but not the horizontal which were marked with a mortise gauge. My quarter inch slots are 1 1/8" across and about 1cm below the gunwale top. The width of the slots is dependent on that of the deck beam stock. In the next post I will describe how the tenons are cut.

Wednesday, 11 May 2016

First gunwale assembly

Picture shows first assembly in formers, the edges of which are at 13 degrees from the vertical. When set at this angle they give the boat its classic sheer.

The front of the gunwales have been trimmed at an angle to take the stem piece later. At the stern a block will be fitted to take the stern piece as the gunwales come together there at a wider angle. After having a good look at the set up I decided to take just half an inch off the beam so that it will now be 23" - it had seemed too beamy. Along with checking the sheer I will feed the dimensions of the wood back into the the Freeship programme as the wood takes its own shape.

Stable gunwales

After I trimmed the cedar gunwales to size one of them started to warp. When I checked the moisture content of the wood, one side of the gunwale that warped had a much higher moisture content than the other,  i.e. this was the last to be sawn and still green on the side most into the log. Didn't have this problem when I used this timber before, and it might not be this but a particular run of grain. So it was back to plan A which was the douglas fir gunwales that I had initially intended to use. The stable cedar gunwale will still be used for stem and stern pieces as well for making up the frames.

The picture shows them just having been sawn. Then they were trimmed and planed to size, and the tops cut and planed at 13 degrees so that they are flat when the gunwales set at 77 degrees in their mortices. This will be clearer when first assembled.

Tuesday, 3 May 2016

A new Skin on Frame Kayak

Please find my first post on Blogspot. Facebook although OK for photographs was not very good at displaying larger amounts of text. Then I tried the Word Press website with links to the facebook group but that had a rather inaccessible address, hence the first post here about a kayak I have just started to build. This may be the last of skin on frame as I have wished to move on to epoxy ply or glass-fibre for some time.
I was unable to make a copy of Brian Schultz's F1 kayak as requested by Tom, however as Brian's design was based on the Mariner Coaster of the 1990's (copyrightretained) ;  I was able to go back to that as part of the source material from which to make a design that would work in British waters rather than the North West American coast.
Here we have smaller waves and more wind, so my design whilst retaining many characteristics would have a lower profile. Below are a few of the stages starting with the F1 in the top left corner and the plan from which the sections are lofted at the bottom right. This will need adjustment for gunwale curve corrections in the early stages of putting the frame together.  14 feet long, 23.5'' beam, I have given it the name "Lowen" - Cornish for fun.
Since then I have started preparing the wood-stock including the gunwales from a piece of  Cornish Cedar (shown below) from the Crockadon Estate near St Mellion.

before I can do anything else I have to finish renovating the Polynesian outrigger hull that is cluttering up the workshop so that I can put it on ebay and be rid of it.