Waiting For More Plywood

I'm still waiting to hear about the next batch of 5mm plywood; the mill suggested February 4th as a possibility but I'm not holding my breath. In the meantime I set about making the outer part of the stem.

This outer stem is 75mm wide and 80mm thick at the maximum point but it tapers towards the sheer. I decided to laminate it out of 10 x 8mm thick strips but these would be difficult to bend and would probably break. So I built a steam box and read about steam bending timber. It sounds straightforward and making the steam box was. Using it was easy too, just put the wood in and cook it at 100C for an hour per inch of thickness. I expected that the wood would bend fairly easily while it was hot but...... Here's what I found out:

• The timber cools and stiffens up very quickly so you have to bend it and clamp it in a matter of seconds,
• The outside of the bend must be supported by a flexible strap to help prevent the fibres breaking away where the grain runs out.
• A strong fixed former is needed to withstand the force involved. It has to be fixed so it doesn't move about when you heave on it.
• It is difficult to do single handed, holding the bend in place and putting clamps in place needs 3 (or more) hands.
• Each laminate would have to be bent individually because 2 or more would need more force to bend than one person (ie me) could apply. This would be tedious and would take several days.

Here's a photo of some of my test pieces; there were quite a few more!

I think my problems were: the test pieces were all too dry because it gets very hot in the shed (up to 40C), they were not carefully selected (the grain should be straight with no run outs where it has to bend), they might have been too thick (about 10mm), maybe they were not a good choice of timber.

Having spent about 3 days learning about steam bending I decided to Build up the outer skeg from solid pieces glued together in 4 layers. This is the same method that I used for the inside part of the stem. It is quick and easy. Here is the finished result which only took a couple of days to make. There are 4 layers of 19mm thick timber each made up of 3 pieces. The joints in each layer are staggered so that each joint lies against 3 layers without a joint. The gap between the hull and the stem at the upper left is the hooked scarf joint where the stem joins the keel. There is more work to do after the hull is finished and before the stem is glued in place.

I started building up the skeg but before I could do that I needed to know the size of the hole for the stern tube and to decide on that I needed to know the diameter of the propeller shaft and that depends on the propeller, etc, etc. This brought me to a standstill because there are too many variables. I muddled around for a while trying to work out what was needed. Eventually I managed to contact and meet a marine engineer (that took several days because it's a) holiday time and b) he was then working elsewhere on a boat) who cleared up all these issues in a few minutes. He said: use an off the shelf fibre glass tube because it is cheaper than buying the materials to make your own and is probably better quality, use an off the shelf "cutless" or "cutlass" bearing which is made to fit the tube, use a 1" diameter propeller shaft because the vast majority of propellers in the sizes I am interested in have a 1" taper bore (a smaller shaft would be strong enough but ground round 316 stainless steel smaller than 1" has to be made to order and is therefore very expensive).

So the outside diameter of the tube is 1.75" but he suggests making the hole a bit bigger to allow some wiggle room to get the tube and propeller shaft correctly aligned. Once the tube is lined up the space around it in the hole is filled up with epoxy "bog".

Here is the start of my built up skeg. There are 2 pieces at the moment, above and below the hole for the tube, The skeg is almost 100mm thick where the tube goes through and 75mm thick above and below that. I made it in 2 halves so that I could rout out a channel about 12mm square exactly along the middle of the join. I also routed out 2 grooves towards the edges of the join for splines to be inserted to locate the joint. Then I clamped the 2 halves together and ran a 16mm diameter drill through the hole.

You can see that the skeg has to be built up more to reach the bottom of the keel that will run the length of the hull. Once that is done I will glued the joint around the hole. Believe it or not but I ran out of timber the day before the Australia Day long weekend!

When the hull it complete the skeg and keel will be fitted in place and the 16mm hole used as a pilot hole to drill the rest of the way through the hull. Then a block has to be fitted inside the keel where the stern tube comes through and this will be glued in place when its location is known. The 16mm hole will be extended through this block. Finally the pilot hole will be opened up in stages using long drills and a boring bar until it is big enough (almost 2") for the stern tube. I have a selection of drills up to 1" and will make up an extension to work through what will be a hole almost a metre long. From 1" up I will make up a boring bar with an adjustable cutter because drills over 1" are expensive and of limited future use.

Second Layer Complete

Put the last of the strips in the second (middle) layer this morning. Traditionally this 3 layer cold molding has the diagonal strips in the first 2 layers and the outside (3rd) layer is longitudinal. This is because it looks better in a clear finished boat. I'm no longer sure why I chose to put the middle layer longitudinally but I am glad that I did because it is much easier to get a tidy pattern with the diagonal strips. Also, they are shorter and easier to handle.

The first side of longitudinal strips was a learning exercise; it doesn't look very tidy! The second side is better but still not very good. The appearance is irrelevant of course because it will be covered by the next layer and I know that this will look OK. Even there the appearance is only important to me because it, it turn, will be covered by one or two layers of fibre glass cloth and many layers of paint!

The thing about longitudinal strips is that the shape to be covered is maybe twice as wide in the middle as it is at the ends so the strips need to be tapered for it to look pretty. This leads to a lot of waste material and is perhaps why the recipes for calculating the material needed add a 25% allowance for wastage. Here is a picture of most of the "waste" material after my 2 layers are complete.

As you can see there are not many usable pieces there. The third diagonal layer will have a larger wastage because I will only use complete strips without any butt joints.

Here's a photo of the stern where you can see the different patterns as the strips run out. The starboard (left hand because it is upside down) is tidier but...... There are strips tapering out along the keel (at both ends) and along the "great circle" but the strips along the sheer are neater and don't have as much twist in them.

For all the boats I have built I have used plastic drink glasses to mix epoxy because they are cheap, maybe 10c each depending where you shop, and I use them once and toss the in the bin. I have used 100s of them, maybe even a few thousand, and never though twice about it. With this cold moulding I was using them at a much greater rate and made a discovery. If you let the epoxy go off overnight it is easy to remove and the cup can be recycled. Let the epoxy harden, squeeze the cup and put out the epoxy leaving a clean cup. Here they are with epoxy in them.

And here they are ready to go round again. I am getting about 5 or 6 uses before the plastic splits and they have to be binned. Just think of the money I am saving by building this boat!

Next steps are to scrape off the excess epoxy and sand the whole hull. I started the scraping this afternoon but my hot air gun gave out a big flash and became a cold air gun. Hmmm, I wonder if it is still under warranty?

I also need more plywood and can't get this until the mill goes back to work and makes some more 5mm ply. They are still on an extended Christmas holiday so there is going to be an extended delay.

Second Layer Started

I'm putting the second layer of strips on. My intention was to put these on vertically because I thought the shorter strips would be easier to handle than the longer horizontal ones, I now know that I was right, the long strips are more difficult in all sorts of small ways. What I found out when I tried the first vertical strip is that they don't want to bend around the sharper curves on the bilges so they have to go on horizontally.

I started with the first 100mm strip just above the sharp curve of the bilge and worked towards the keel from  it. As the strips approached the keel I saw that the 100mm wide strips would not fit nicely in the hollow section closer to the keel so had to switch to 50mm wide strips. This left me with a 75mm wide strip along the keel.

I found that the narrower strips are quicker to apply; they reduce the amount of spiling and fitting because they will bend sideways to some small extent and they need less glue so the mixing and applying doesn't take as long.

Here is a photo of the bow section.It takes two and a bit 2.4m long strips to go from end to end so I'm using up the offcuts from the first layer. Simple butt joints between the strips will be lost when the third layer of strips goes on. The challenge in putting these strips on is to make sure there is thickened epoxy filling any spaces between the layers. The only way I can see of doing this is to put plenty on so it squeezes out when the strip is nailed in place. then the excess can be scraped off and, if I can work quickly enough, used on the next plank before it goes off. Where the narrower strips come to the stem the twist makes for a gap 10mm to 12mm between the strips. I decided to fill these tapering gaps with offcuts of ply rather than spiling and fitting the strips. This is quicker and less wasteful than shaping the strips. The ply "buttons and black plastic are holding one of these filler pieces in place.

Here is the stern end. I knew there were hollow sections in the first layer and the coloured patches are epoxy bog filling them up. The hollows are there because there were not enough ribbands to support the strips in this area where the strips have a lot of twist. There were not enough ribbands because I had decided  during the lofting to notch the ribbands into the frames. This was a mistake, I should have drawn the frames smaller to allow the ribbands to be fixed to them without the notches. Next time I will not fall into this trap!

More of the offcuts from the first layer are visible in this photo. I have a lot of these and will use them up in this second layer of strips.

Another view of the stern shows the first of the 50mm strips coming onto the sharper curve of the bilge. There will still be ridges where the strips join but they will not be as pronounced as with the 100mm strips and will sand away. Also visible are some of the ridges in the first layer that have been sanded away showing the glue between the plies and some of the middle layer. These were amongst the first strips I put on and as my spiling and fitting technique improves there were less of these ridges to sand away.