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boats

The Stasha ‘Tweed’ Nesting Dinghy uses Flax

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The Woodenwidget Stasha ‘Tweed’ special edition lightweight nesting dinghy.

The Stasha lightweight nesting dinghy from Woodenwidget has been around now for a few years and I used the prototype for three years as my yacht’s tender. In many ways it’s the perfect dinghy for the Pacific Seacraft Dana, after all it was designed to fit on one! It is easy to stow and launch but more than that it is a fabulous little boat. It is a joy to row and it sails superbly too. When the prototype was showing signs of wear I thought about just putting a new skin on but in the end decided to do something a bit different. And so was born the Stasha ‘Tweed’

The biggest difference between a standard Stasha and the ‘Tweed’ is the fabric. The standard Stasha uses a thin and lightweight heat shrink Dacron covering which is then coated. The ‘Tweed’ uses a Flax fabric woven in the UK. It is specially woven so that it ‘drapes’ well and can conform to a curved surface such as a boat hull. It is normally used with a bio resin that cures with sunlight but I did not try this method deciding to use epoxy resin instead. Not because I think it is better or anything but simply because I had been given a load leftover from another job. If it wasn’t used it would soon be of no use so by using it I avoided buying something and stopped it being wasted.

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Close up of the cloth at the bows. It wasn’t even necessary to overlap the cloth leading to a very tidy look. The fine trim was added to cover the join between the cloth and the panel. This wasn’t entirely necessary as the join was fairly neat but it does lend a pleasant finished look and may even protect the bow panel from damage.

I will be discussing using this material later in this post but if you’re interested here is the link to the site where I bought the Flax fabric. It is the Hi-No twist fabric at £22-50 a metre. The roll is 1.38 m wide so for the Stasha it meant laying it across each section to get enough width to cover in one piece.

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Close up of the raw Flax fabric before epoxy.

One of the problems about using epoxy is that it is brittle and without modification would lead to a fabric covering that could be punctured too easily. In order to make the epoxy a little flexible you can add Benzyl Alcohol. About 2-3% is plenty. It’s a simple and cheap way to do it.

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The test bed for the fabric. This is the finish after the first coat of epoxy. As you can see the finish is very rough so needs sanding and further coats of epoxy.

The fabric is 400 grams a square metre which makes it twice as heavy as the Dacron before the epoxy is even added so using epoxied Flax is not a light option. However the standard Stasha proved to be so easy to stow and use on the boat that it wouldn’t really be a problem if it was a bit heavier. Especially considering that the boat is in two halves so it’s already easy to handle.

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This is how the fabric looks on the inside after epoxy. The finish is much smoother on the inside but far from flat and smooth.

The other difference between the original Stasha and the ‘Tweed’ is that it is made entirely of teak, not ash. Again, this was wood left over from another job and so it would be a shame not to use it. I didn’t know if it would be possible to bend the ribs using teak as it is a much stiffer wood than ash and not known for its flexibility or bending qualities but it wouldn’t hurt to try. If the worst came to the worst, I could always have ash ribs and teak stringers. That would have looked ok.

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Bending the teak ribs in required planning, patience and lots of spare wood. The extra stiffness of teak makes this a very hard job. Ultimately successful however. Wood is an amazing material.

As it happens making the ribs bend was certainly possible but required a lot of patience and spare pieces of wood as the breakage level was high. In a couple of areas I was not able to make ribs in one piece so had to scarf two pieces together. This was no problem but did add time to the build. The orientation of the grain was crucial as well, the slightest run off and the ribs would split. I soaked them before hand for a number of days and kept them wet while I was teasing them into place on the strong back with the hot air gun. It took a very long time to do the ribs and the teak was so much stiffer that the ribs would have a tendency to force the stringers away from the strong back. Ultimately I succeeded but it was not easy.

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One of the wooden rings cut out using two blades in the hole cutting saw. This was then cut into 4 and used to trim the plywood end grain.

Another difference is that every piece of plywood on the ‘Tweed’ has been capped with solid wood so that there is no end grain showing. This is a surprising amount of work, especially where the two sections join as the cutaway has rounded corners. The quickest way I have found to do this is to use two blades in a hole saw and cut out a ring of wood. This is then carefully split into 4 with the grain orientated and then it is glued to the plywood. One of the things that takes so long with adding trim to plywood is the care that you need when trimming it down level.

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Here the plywood end grain trim is being glued on. They will be planed down flush later.

The best approach here is to simply glue on a piece of trim that is slightly wider than the plywood. When it has set, use a hot air gun on the lowest setting, warm up the excess epoxy so it becomes soft and scrape it off using a sharp scraper. This is a very gentle operation. The epoxy will come off very easily with a little heat and a little patience. Once the epoxy is removed it is time to plane down the trim flush with the plywood. Remember that most plywoods have a top veneer of a half a mm or less. You cannot afford to cut into it at all. So a very sharp block plane is needed and also lots of care. Gradually plane down the trim until it is flush. Then use a block and some 180 sandpaper to clean it up.

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Gluing on the inner trim. This is something the standard Stasha in its never ending quest for weight loss never had. It adds a little weight but gives a very sleek look on the finished dinghy.

The last great difference is the addition of inner trim pieces to cover up where the stringers are glued into the end panels. It adds a ‘finished’ look to the boat and also strengthens the glue join. These take a long time to make and fit too as they need to be very neatly made. The quickest way I found was to make a cardboard template for each piece.

The seat used to scratch the varnish on the Stasha so now there are pieces of wood that will remain unvarnished instead. The seat is now made from slats of unvarnished teak rather than a single piece of plywood.

If you were thinking of building a Stasha ‘Tweed’ then I must warn you it’s a lot of work and it demands more skill than for a standard version which is very easy and fast to build. It will weigh about 50% more. With floors and seat it weighs about 17 kilos which is about 5 more than the standard boat but as I said, because it’s in two pieces not one section weighs more than ten kilos.

Asides from weighing a lot more it takes about three times longer to build and uses about five litres of epoxy and two litres of varnish!

The end result is a lightweight but virtually indestructible hard nesting dinghy. It is also extremely nice looking.

The finished fabric is almost two mm thick  so a rebate needs to be cut out of the exterior edges of the panels so that the fabric fits flush in the end.

Also, because of the extra thickness of the fabric it is necessary to remove a couple of mm from the sides of the joining panel on the rear section or it will be too wide to nest without scraping the varnish.

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The varnished front section with panels rebated a little to allow for the thickness of the fabric. The standard Stasha uses much thinner Dacron that doesn’t need to be rebated.

Here’s how the fabric is fitted: The fabric is not wide enough on the roll to fit from one gunwale to the other but it is wide enough to cover from front to back. I bought 4 metres of fabric and didn’t have much left over.

One of the great advantages of this system is that you do not have to rush or panic. You will epoxy only when you are happy with the fitted fabric. It is worth taking your time to get the fitting of the fabric correct because when it is epoxied and varnished it is translucent and any kinks or jumps in the weft or weave of the fabric will be very noticeable.

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The fabric draped over the framework, tensioned and stapled in place.

The fabric needs to be stretched tight before epoxying or it will sag with the weight of the resin. The problem is, how to get the fabric tight when it is such a loose weave. If you pull on one part, it pulls the weave out of line. I used staples to hold the fabric tight. It took a long time to pull it all tight while still keeping the weave straight along the keel. It took a lot of putting staples in and then pulling them out again as I tightened first one side, then the other all the time keeping tension fore and aft as well. This is quite tricky to do as you need as much tension as you can get but without pulling the weave apart or distorting or pulling the weave out of line.

However there is no rush so you can take as long as you like to get the fabric laying right. I found that the fabric lay well over the entire front section if I pulled the aft corners aft first. Then by the time I got to the front, the fabric was able to cover the whole shape in one piece without kinks or pleats. I was particularly impressed that I managed to get the fabric to follow the bow section without having to cut any fabric out.

To make working on the dinghy easier, I varnished the entire framework (except the outer surfaces) with three coats before fitting the cloth. Varnishing the framework is a complete pain in the arse, as is sanding between coats. The plan was to do three coats on the framework and then two entire coats inside making 5 coats for the woodwork and 2 coats for the fabric.

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This is what the front section of the dinghy looks like after its first coat of epoxy. The colour is good but the finish is very rough, far too rough for a boat. It will need sanding smooth and more epoxy then two coats of varnish before it is finished.

The epoxy is brushed on liberally. It is amazing how much epoxy the thick fabric will soak up. I did a test piece first to judge how to apply the epoxy. In the event it was very forgiving and I had no drops of epoxy come through the cloth despite a heavy application. The ideal is to have enough epoxy to wet out the fabric but not so much you are adding weight for no reason. The trick to getting the folded corners to stick down is to apply epoxy to the wood underneath it first, then dab the fabric down with more epoxy.

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The finish on the outside after three coats of sanded epoxy.

Once the epoxy is set it needs to be sanded smooth. If you have applied enough epoxy you will find that even with some quite violent sanding (I used 60 grit on a random orbital) you won’t go through to the cloth. In a couple of places the cloth was visible but that was OK as it needs another couple of coats, each one sanded down smooth. Then the whole surface was sanded down with 120, then 180 and finally 240 grit. Then it received a further two coats of varnish. The varnish is needed as otherwise the epoxy has no UV protection.

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Almost finished varnishing. Four coats on the wood and one on the cloth. It will receive one more coat all over. Note the trim around the stringers where they fit into the front panel. A small touch but makes a large difference to the looks. The epoxy used to glue in the ribs had teak dust blended into it to darken it making it very hard to see it.

The end result is a thick yet slightly flexible and extremely tough skin for the boat. It looks fantastic with the light coming through it. I am happy that I took so long laying the cloth so that the weave was even.

Conclusion:

Not for the faint hearted. Sanding epoxy is a thankless task and varnishing to that level is also soul destroying. If you have 120 plus hours spare then all you need is a set of Stasha plans from Woodenwidget.com and this article and you too can have a splendid looking lightweight nesting dinghy like the Stasha ‘Tweed’.

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View from the inside with the light coming through.

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Categories
boats

How to Bend Wood

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Over 30 strips of 2.5mm in place on the jig. This is just a dry run to check that every thing is ok.

There are many ways to bend wood but it will depend on the job that you want to do as to which method you will use. For example if you only want a slight bend, maybe simple brute force will be sufficient but it depends on the size of the wood. No amount of brute force will bend a foot square section of oak for example!

Thinner wood bends easier than thicker wood and some woods bend easier than others. The greater the curve, the thinner the wood needs to be. These are all basic rules that cannot be broken but if you know what is possible then wood can be shaped into almost any form. Even time can bend wood as can heat and moisture. Wood really is an amazing material.

As wood bends, the inner section compresses and the outer section expands. Sometimes it is possible to stop the wood breaking as it is bent by placing a steel band around it as it is bent into shape, in much the same way that a plumber inserts a steel spring into a copper pipe in order to bend it without it kinking.

Steam, or in fact any kind of heat can be used to bend wood but it helps if the wood is green, or in other words, freshly cut so it contains more moisture and is naturally more flexible. But steaming is only good for relatively thin sections of wood. So what does one do if you want to bend a larger section?

Well, it depends on how much you want to bend the wood. It also depends on the orientation of the grain and as I mentioned earlier on the type of wood. Often brute force and time can bend quite substantial pieces of wood but these days people are in a hurry and can’t wait months for gravity to do its thing.

Perhaps the most effective way to shape wood is to laminate it. This is done by gluing thin strips together using a jig to create the shape required. The thickness of the wood required will depend mainly on the radius of the curve required. The greater the curve, the thinner the strips will need to be.

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Making the three sections of the jig from layers of 20mm plywood. This jig was made in three pieces because there are many knees to make and they are all slightly different. By keeping the curved section the same, only the other pieces need to be modified each time.

The down side of laminating wood is the fact that you have to make a jig and use glue. You can use almost any kind of glue but for the best and strongest results it’s best to use epoxy. The trouble with epoxy is that it is messy and expensive. That said, once the jig is made it’s easy enough to bang out any number of glued laminations.

Recently I was called in to reinforce a 20 metre wooden sailing boat. The deck had lifted due to the compression forces of the mast and rig and in order to put more strength into the boat new and larger hanging knees were required. So that the knees did not intrude into the boat too much they needed a fairly sharp radius but as they were 75 mm square that meant that they would need to be made from very thin strips indeed. In fact sometimes laminations can be made with wood as thin as one millimetre!

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Just some of the strips of mahogany cut to make some laminated knees.

In this case it was decided that 2.5mm would be about right. The ideal is to use the thickest laminate possible as it requires less work and less glue and arguably looks better as there are less glue lines visible in the finished product. 2.5mm wood bends very easily even into a very tight curve but once you pile a load together it becomes much harder. It’s not so much the physical limits that you can bend the wood too but more the force needed to force a stack of laminations into the jig.

To create a 75mm thick knee, over 30 strips of wood were required and because there is quite a lot of force involved, the jig needs to be pretty strong. The jig was made from 4 pieces of 20mm plywood stacked together to create a support for the stack. The wood is actually cut about 8mm thicker than required as it will need planing down on both sides afterwards. When the wood is in the jig, it is slightly proud of the jig. This enables it to be bashed or clamped down into place.

The jig was made in three pieces and then they were bolted through the work table with 10mm bolts to ensure that nothing can move. Where the wood bends around the most, an inner block was also made to ensure that the clamping pressure on the curve was even.

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The inner block is crucial to a quality lamination. This ensures that there are no visible glue lines in the finished product. The other areas are easier to clamp.

When the jig is made it’s time to do a dry run. This is to ensure that you will have no problems when you come to actually glue it up. What you do not want is to discover a problem half way through gluing. Epoxy is unforgiving in this respect. If it goes wrong, the wood will be glued like that forever and since epoxy, wood and your time is expensive, it’s worth taking a little while to do a dry run just in case. If the dry run is successful, remove the clamps and the strips and prepare some epoxy.

The stack is laid down on its side and the first strip is coated in glue. I always mix in a little thickening agent to ensure that the epoxy has a little flexibility and also to ensure that the epoxy goes on thick enough. Making sure you use the right amount of glue is a matter of experience but it’s certainly better to use too much glue rather than too little! Once the first strip is coated with glue (using a roller) the next is laid on top and then that is coated and so on until you get to the end.

Then the stack is placed into the jig which has been lined with plastic to avoid the assembly from gluing to it and thus making it impossible to remove once the glue has cured! The stack is clamped in place starting at the centre of the curve. As the first clamp is tightened a little, it’s worth bashing the stack down onto the work surface. Now work outwards in both directions clamping and bashing as you go.

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The laminations, glued up and clamped in place. Leave to cure for 24 hours before removing.

If you have used sufficient glue and enough even pressure from the clamps you should end up with a solid piece of shaped wood that doesn’t even look like it was made from many pieces. Once it has been in the jig for 24 hours it can be removed. It is possible that the finished piece will ‘spring’ that means that it may open slightly as it tries to go back to its original shape. The more laminates you use, the less chance there is of the laminates springing.

Once out of the jig, an electric plane is the ideal tool for removing the glue that has hopefully squished out from between the strips. A belt sander also works well though sand with progressively finer grades to ensure that no scratches are visible in the finished product.

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The knee, planed down and sanded. A bit of spirits on a rag show how the knee will look once varnished.

If you want to get really carried away you can assemble the stack for gluing so that the colour of the wood is uniform. Often wood, even from the same tree can be different depending on the direction or orientation of the grain so you can disguise the fact that a beam is made of many pieces by carefully matching each strip to the one above it. In reality it’s not really worth it unless you are trying to create something really spectacular. In time, most woods with either darken or lighten with exposure to sunlight which will make the colour more even.

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A close up of the curved section of the knee. No glue joins are visible but it is possible to see a slight difference in colour of some of the laminations. In time these will fade slightly until you’ll have to look closely to see if the beam is made from many pieces.

You can of course make a feature out of using different woods in a lamination. Often boat tillers are made like this. It’s a bit cheesy but it does show what’s possible. Laminating wood is a great deal of fun and also very satisfying. It is also quite amazing how a stack of thin strips of wood that were collectively very floppy suddenly become unbelievably strong and resilient once glued.

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Laminated tiller on a Dana 24. This one is made from mahogany and ash.