Monday, 24 October 2016

Skinning the kayak - pulling out the concaves

Pulling out the Concaves
The last blog ended with the fabric - pinned at the keel, developing concaves between the chines due to the amount of its longitudal tension. It has been my experience that the considerable force required to flatten the fabric also tensions it around the boat.
As just grabbing the fabric and pulling can distort the weave of the fabric, I prefer to wrap a batten around the piece I am about to tension before pulling it tight. In this fashion with the boat upside down I tighten from the keel downwards one chine at a time. Thus I end up with a row of pins on the first chine stringer holding the fabric in place.

It is important to insert the pin all the way to the flat top of the drawing or upholstery pin so as to prevent the centre of the pin from pulling a hole in the fabric. If the pin does not go in easily first time it is probably best to re-insert it until you find the decreased resistance between the hard winter wood of the growth rings. I find that staples can chop through the individual fibres. Although that is not a problem with a natural fabric such as canvas, it is with polyester which tends to slip over itself.
Once the fabric is secured to the chine stringer, then the pins can be removed from the keel and the process repeated until the fabric is secured to the gunwale.

I use two hands when not taking photos, one pushing down on the gunwale and the other pulling the batten and fabric up and over until the pins are in place.
Above shows the redundant chine stringer pins being removed.
Pinned to the gunwale. Most of the creases are due to slack between the pins which should be addressed when the tension comes from the seam. The boat is now ready for sewing the seams - next post.

Saturday, 22 October 2016

Fitting the fabric

Cutting the fabric to shape
 As a full size piece of cloth is unmanageable, the first stage is to determine the approximate dimensions of the cloth to be bent to the boat. This is done by measuring the circumference of the craft at points from the bow to the stern. It is important that the measuring tape from which these points are taken is pulled along the keel and round the curvature of bow and stern to where they meet the deck. You see, the cloth will take this path. If you measure points in a straight line from bow to stern the piece of cloth will be too short.
To the circumference at each point is added four inches. That is for the one inch overlap needed for the rolled seam and two inches spare so as not to have to waste a piece of cloth through having made a mistake in the measurements etc. That measurement is halved and the points plotted either side of a straight line lightly drawn on the fabric. The outline of the piece is drawn through the plotted points.
The shape of the cloth will be something like that shown below:

The irony is that I had forgotten to add the extra two inches of fudge factor to this piece and it did not give me enough overlap so I had to cut another piece. It was not wasted however - I gave it to someone who was making a shorter and narrower boat!
On the floor is the notebook in which I had plotted the dimensions and the pair of Kevlar shears I used to cut the fabric, after which I sealed the edges with a heat gun. Although this was quicker and fume free, I was not satisfied with the quality of the edge, so the next piece was cut with my old method.

This involves the use of an old chisel filed sharp on one corner, heated to red heat with a blow torch. I find that I can cut up to one metre at a time very quickly. You need something underneath the fabric to take the burn mark - to which it will adhere where it has been cut, but peels off easily. It is really important to wear a good gas mask when you do this and make sure the room is well ventilated - best do it in the open. The fumes that come off burned polyester fabric are nasty - they really go for my lungs.
Fitting the Fabric
Once the fabric is draped over the boat and one is happy with the orientation and that it will fit etc I turn the boat upside down and stitch the stern seam.

I stitch with just a little tension in the fabric, not much - so that it is not slack, and start the seam by making a crease on the corner which gets bigger and bigger as you go round the corner. By the time the picture above was taken I had trimmed and sealed the excess fabric. I made quite a close stitch using two opposing needles and 1.5mm braided wax polyester twine that I buy from my local Chandlers (Macsalvors of Pool). I was careful to make sure I did not pull the cloth. Once the first run of stitches was done, I folded the cloth over on itself and stitched the seam again, and then once more to make a secure seam that would withstand the tensioning of the fabric without pulling. The clamps would probably remain in place after I had taken the seam round to the deck, as I have yet to do in the photo.
I used to tension the fabric the way they say in all of the books I have read: on your back with the foot against one of the bow deck beams pulling with all your might - and if no one was there to help, wondering how on earth you were going to secure the fabric at the bow.
Instead I cut the fabric to about one foot wide at the bow, with an excess of about one foot in length, and this piece of fabric about one foot square I wrap round a spare piece of wood or batten, clockwise; and then keep turning, tensioning against the point of the bow as is sort of (I would have used two hands and had the clamp nearby, ready) shown below.

As you can see I hold the fabric in place with a spring wrench and it remains there after I have done a seam round to the bow, until I am sure that I will not lose any tension by it's removal. The fabric is protected by the masking tape wrapped round the jaws.
The tension enables the fabric to fit round the bow without creasing, and also puts a great deal of concave into the fabric as it drapes round the chines, as is shown in the final photo.

It also shows how I have pinned the fabric securely to the keel with upholstery pins so that it will not pull holes as I conduct the next part of skinning which is to progressively pull the concaves out of the cloth - next post.

Monday, 17 October 2016

one way of skinning a kayak part1

Part 1
There are many ways to skin a kayak. This is what has worked for me so far with 440gsm (13oz) polyester fabric. I am sure it can be improved.
Much of what I do has come from Robert Morris'  "Building skin on Frame boats" for his is the method which seems to have fitted my choice of fabric best of all.
About ten years ago I purchased about 250 metres of ex loom cosmetic second polyester fabric, 2.25 metres across from the recycling manager of Heathcoates in Tiverton. Prior to this I had been able to try out a number of his samples and this was the one which seemed best suited to my needs.
Most people seem to have gone for 8 oz polyester; this seemed a little too light for me - although there are advantages which I will discuss later. I have done a lot of rock dodging in boats skinned with the 13 oz and I have not holed it yet. The nearest I come to it was when inspecting the interior of Hayle sluice at low tide and managed to become impaled on a bolt sticking up from the bottom. Although the fabric ended up with a big dimple which would not come out - and weeped: it did not hole.
As is often the case, one of the fabric's biggest advantages is also a major disadvantage, and that is the looseness of the weave which enables the fabric to drape well; but this also allows the fabric to pull easily.
Thus in situations in which significant stress will be put on one point eg sewing, or the temporary use of nails to set up the sewing in of the cockpit, or perhaps the commonly used way of tensioning the fabric round the boat prior to sewing through the use of a drawstring, the fabric will need reinforcing at those points or it will pull and a hole will appear. I must add that this is not through any of the strands being broken, but through sliding over each other. I am unable to break a single strand with my hands.
 Ultimately Nylon is probably stronger due to it's ability to stretch, however this is outweighed by it's tendency to sag on long immersion and it's inability to stick to anything much, including most sealants.
 The consequence of the tendency of my fabric to pulling stitches is longer working time, primarily due to the use of a rolled seam down the middle of the boat. This takes much longer than the whip stitch that Brian Schultz uses on his nylon covered boats. Where points need reinforcing I rub in Bonda moisture curing polyurethane varnish. This reinforces without making fabric too stiff. Perhaps I could make a quicker seam with fabric prepared this way?
Another advantage of polyester is it's capacity to shrink when heated to near melting point. 13 oz samples I have prepared have shrunk approx 2% under the hottest iron (which does not melt it). The 8oz sample I shrank this way contracted much more but I have forgotten how much. I have found 2% to be enough to remove all but the worst wrinkles (when the seams should really be redone anyway).
Next post: skinning the boat!