Pilgrim Diaries #4 - 25 November, '08

Only a month until its Christmas Eve, here of course that’s about the beginning of “real” summertime although its pretty warm now, and the vegetable garden is producing already, the lawns need mowing every week, and the maize crop in the paddock next door is a halfway up my shins.

Having spent a large part of the day working on the house and the garden, I got a sheet of plywood out on Sunday afternoon, took it inside where the near gale force wind would not blow the plans around and started to draw out the transom.

Now before I go any further, there are often discussions on internet boatbuilding groups about plywood quality, grading and standards.  If you have any doubts, it doesn’t hurt to buy one sheet or an offcut or a piece damaged in shipment and test it.

I’ve taken some offcuts here to demonstrate the method and what to expect.  I’ve several sheets of plywood with Agathis Australis (Fijiian Kauri)  face veneers, and inner veneers of what we know as Fiji maple.  The former is a particularly nice looking wood with good painting qualities, fair rot resistance and reasonable strength.  I had found some cheap but useful looking plywood at the local builders mart so made some enquiries of the yard, then their supplier who had imported the product and then through them of the manufacturer.

Sure enough it was marine grade glued, had no voids (spaces in the plywood interior where the veneers did not meet, leaving empty spaces) and was deemed suitable for exterior high hazard use when painted.  It was in fact cosmetic rejects off the marine plywood line which augured well for my intended use.

I bought two sheets, loaded it aboard the trailer and dragged them home.  Now, if they failed my testing, I had a project where I could use two sheets, one with a corner nipped off, for lining in my wifes office to be so this would not be wasted in any case.

Samples for testing

The boiling pot

I took the samples you see here, measured the weight, the thickness and (not really necessary but I have one so why not use it) tested them with a moisture meter  then put them in a pot with a weigh thereon to keep them submerged.  Boiled them vigorously for 30 minutes topping the pot up regularly to stop it from boiling dry.  Put them on the windowsill where the sun would warm and dry them, and 24 hours later repeated the hot water treatment.  Did that three times!

At that stage there was about 0.3mm increase in thickness, a slight swelling which was not unexpected as the moisture content had gone from about 9% to 16%.  On slicing a piece open I found that the inner veneers were only about 12% which says a lot for the integrity of the glue bond.

There were no hints of delamination, a slight bump in one where two inner veneers had overlapped and the surface had slight grain raising.

I’ve put them in the fridge buried in some leftover mashed potato,  and will try the boil test one more time in a couple of days.  I expect this product to be fine, its actually behaving better under these trying conditions than the BS1088 Red Meranti that I have tested in the past and which is the most common plywood used in boatbuilding here.

Checking its square.

putting in the centerline if you measure at both top and bottom you dontneed a square

The centerline and water line are in You can see where the measurements are intersecting

So, with the sheet of plywood laid out on the rug in my lounge, I started work. Now, I use a system based upon the waterline, and the centreline to draw each of the frames in my boats.  The measurements are up and down and out from the intersection of those two lines.

You can pick up from my photos that I have measured in from one end of the sheet far enough to have maybe 25mm ( an inch or so ) to spare from the transom centreline , and up from the plywoods long edge a little to draw in the waterline. Note that in this case the transom begins slightly above the waterline, and with other frames we need to measure up from that edge far enough to allow for the measurement down below the waterline plus a few mm for working space.

Having checked that the sheet itself is square, and here you can see I use a housebuilders roofing square, about US$12 or so when I bought mine, to do that check and in this case its perfect.  It is rare for plywood to be out of square but it can happen, and if you are relying on it being right that can lead to an odd shaped boat so it pays to be a bit careful.

Joining up the dots

Jazz come to supervise After all its wet outside

Rather than use the square, I just measure in from the corner, and then at about the top of the transom measure across again. Then run a ruler, between the points and run the pencil along it. Mines a 1m long (about 3ft 3in, and has metric along one edge and imperial on the other for convenience.  Same for the waterline.  Given a square sheet of plywood these will be perfectly at right angles.

With the vertical and the horizontal baselines established,  I measure up along the centreline to give me the heights, and then repeat those measurements at about the full width of the transom each side and draw a light pencil line across about where I expect the width to be at that point.

I then measure along the waterline from the Centre, and again, put a light pencil line across , in this case I am going to be putting a baseline where the measurements for the top curve of the transom/afterdeck will be so use that for the top end of my lateral measurements.  Measure out and where the lateral and vertical measurement lines intersect, make a dot.  Join the dots and voila! The transom shape appears.

The shape is a good replica of the one on the plans

My improvised drafting weights - Still wet outside

A veneer blade greatly reduces splintering of the plywood

Measure across at 100mm intervals along the top baseline and working from the centre outwards each side and using a square to get your measurements at 90Deg to that baseline measure downwards.  You get a series of marks that show a curve. 

I have a set of carefully cut and planed battens for drawing curves, but a narrow strip of reasonable quality this plywood, or a light strip of even grained wood perhaps 6mm square, or one of the strips of plastic used for joining the panel linings in bathrooms (you can get these from you big box hardware store and they are very cheap) will make a useful substitute. Its worth the investment as you will use this “batten” a lot.

Held with weights, and you can see that as it was pouring with rain I did not bother going across to my drawing office to get the proper drafting duck weights, just raided the larder for ( has a look) tins of peaches which held the batten in place while I ran the pencil around. Had some with ice cream for dessert that night!

ALWAYS try for a fair curve - that is one where the batten has a nice even curve and where you have not had to force it to conform. It should just lie around that curve naturally and a mm or three here and there is not an issue, that fair curve is what you want.

At this stage, I have the transom outline, and have only to mark out the framing and tiller slot, so I drew the seat top curve in the same way that the afterdeck curve had been determined, and then the stringer notches in the frames.  Note that the plywood on the transom is NOT notched, only the wooden framework which will be added later.

Zoom Zoom Zoom

All cut out ready for the framing This is a lot bigger than it looks

Note that I often don’t bother with a compass to draw a radius, that large tin of peaches is about 10mm smaller than specified for the corners of the tiller slot, and it was still raining so that’s the radius I used.  I also decided to put a slight curve on the top edge of the tiller slot to compliment the curve at the top of the transom.

A day later, the rain had stopped and I took the sheet outside, put a veneer blade in the old Makita B4200V jigsaw which must be close to 25 years old now and still a favourite among my power tools, and started cutting.

Here it is, the transom outer skin, framing still to be fitted but there is the shape.

Note that its over 1400mm ( about 4 ft 8in ) wide by 830mm ( 32 ½ in ) high.  Its big for a dinghy, a promise of space and shelter.

I’ll draw the next frame forward during the week and between all the other things I am working on will attack that this weekend.

JohnWelsford,   A boatbuilder again and it feels good to be making sawdust once more.