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Building a stressed ply Skiff Moth Hull – Phil Stevenson June 2000

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Building a stressed ply Skiff Moth hull. Phil Stevenson June 2000 Introduction: This method of hull construction allows very cheap construction of Moth hulls at comparable weight to modern carbon/foam hulls. To achieve the required strength the hull shape is restricted to curved sections. The 2mm ply skins are jointed by minimal carbon epoxy seams, and small quantities of timber. To maintain this strength the skin is supported internally with polystyrene foam frames. Remember carbon and wood weigh about the same, but resin and glue are heavy, and foam is light. Also ply and foam are cheap, carbon and resin are expensive. So: ��Make the timber and ply joints fit well, ��Use carbon where strength is important. ��Use minimum resin with the carbon, and ��Use foam where strength is not that important. Two Models: As at June 2000 two versions have been constructed: 1. The middle aged version ‘ANACHRONISM’ and 2. The youth version The first has a waterline width of about 500, while the later is about 300. Both have V shaped Pin Tail sterns and flair-less rolled in fore decks. The middle aged version is much more stable and is suitable for newcomers to skiff moths or the senior sailors who’s agility is fading. The youth version is comparable with modern pin tail designs. Both hulls were built to about 12kg. The completed boats, with aluminium wings and boom, timber foils and a carbon mast weigh 36kg. Material List: Three sheets of 2mm exterior grade hoop pine ply 1200 x 2400. One sheet of 75mm L grade polystyrene foam 1200 x 2400. Two 3m lengths of 50 x25 DAR western red cedar. 1.5m of 50 x 50 DAR pacific maple. 18m of 50mm carbon tape or 1m x 1m of 170gsm carbon cloth. 3 litre epoxy with hardener, bag of fibre filler, bag of light powder filler. Small pieces of trim timber. Lets get Going: Cut the Plywood: Buy three sheets of 2mm x 1200mm x 2400 hoop pine exterior plywood. Put one aside for the deck and smaller parts. Take another and cut it into two 1200 squares. Splice these two to the end of the last sheet to form a big ‘T’, 3600 high, 2400 wide at the top (bow end) and 1200 wide at the bottom (stern end).


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1/2 Sheet

1/2 Sheet

Full Sheet

The splices need to be about 20mm wide. Feather the edges with a hand plane and glue them with epoxy, staple down to a board covered in plastic to keep the joint flat.

HINT: Epoxy resin as sold is for laminating with glass or carbon and for sealing wood. To make a glue mix it with fibre filler till it spreads like butter. For filling or low strength glue (ie for foam) mix resin with the light weight filling powder. Draw a pencil line down the centre and mark out this shape: 1. The Middle Aged version:

1/2 Sheet

1/2 Sheet

Full Sheet2nd CUT


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2. The Youth Version:

1/2 Sheet

1/2 Sheet

Full Sheet 2nd CUT

It is very important that the lines are plotted accurately, symmetrically about the centre line and that all the curves are fair. The keel and chine cuts must end in a single saw cut, fairing in smooth curves to the shape shown. Cut out the ply and plane the edges smooth. Gunwales and spreading: Glue the gunwales inside the top edge. These are two pieces of WRC (western red cedar) 25mm x 50mm DAR (Dressed All Round so it ends up 20 x 45) and about 2.8m to 3m long. The short fall is at the front. They do not need to go to the bow, only about half way between the mast and the bow. They can be tapered down to half the width from the mast forward. Use staples and epoxy glue. When dry put a rope around the middle and start to bend the hull into shape. Use masking tape across the outside of the ply to close up the gaps and form the chines and keel seams. Start at the thin end of each seam and work your way to the ends. Make sure the joint is pulled in tight, and keep checking that the line is fair and true. When taping is complete set the hull upright on a trestle at each end and adjust the width with ties, sticks and tape so that the widths are: Boat Width at bow Width at mast Width at front of chines With at stern Middle Age 175mm 750mm 925mm 600mm Youth 350mm 1000mm 1100mm 350mm Seams: The seams are taped inside and out with carbon tapes. You can buy carbon in tapes but it is a heavy grade and one layer is enough. Alternatively cut carbon cloth (about 150-200gsm) into


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50mm –60mm wide strips. If the cloth is not a square weave, cut it with the major fibres across the strips. Use two layers. Set the hull up so that it is supported well and will not wobble around while you work. Make sure that it is symmetrical and not twisted. Tape the seams on the inside. Use epoxy, and cover with peel ply to squeeze out the excess resin, and push the carbon down hard against the ply. Leave the front 250mm back from the bow clear. When the inside is fully cured turn the hull over, remove the masking tape and clean up the joint. Round the edges a little so the carbon tapes go around easily. When ready put the carbon on the outside of the chines and keel seams. If you want to finish the hull with varnish, do a neat job here so the black lines have neat edges. Again cover with peel ply to push it down tight and minimise the sanding later. Leave the epoxy to fully cure, maybe a few days, so that it will not bend when you start to pull the sides in. You can remove the peel ply in a day and give the entire inside a first coat of resin to seal the ply. Also while you are waiting you can make the fin case, see instructions later on. Pull into Shape: Remove all the spreader sticks and start to pull the sides together. The target gunwale widths are: Boat Width at bow Width at mast Width at front of chines With at stern Middle Age 0 330mm 700mm 640mm Youth 0 260mm 520mm 460mm The bow is best held together by clamping the ply between two straight pieces of timber. Tape together the joint down the centre of the foredeck. It is handy at this stage to put a string line down the centre of the boat at about deck level. Use it to ensure that the boat is symmetrical. Turn the boat upside down about now. Be careful that the hull does not get twisted. Check for twists at every stage. Check the bow angle, and that the foredeck centre join is on the centre of the boat. When you are happy, carbon seam the inside of the bow and foredeck seam. (It is easier to do if it is upside down.) You need to apply the carbon and resin with a brush on a stick. Do not worry about spillage just spread it about as you need to seal the inside of the ply too. While you are at it put in a tapered piece of timber where you want to screw the forestay fitting. Measure the overall length 3355 back from the bow. Mark a line for the transom 20mm in front of this, and cut the gunwales here and remove the bits behind the line. Make the transom from an off cut of ply to fit just in front of this line. Add a piece of 25 x 50 DAR WRC across the inside about 100 up from the base to take the lower rudder gudgeons. The top should be cut down about 130mm for the deck curvature. The top edge needs to be trimmed with some timber to glue the deck to.


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HINT: Make a profile gauge. Take a piece of timber 20 x 20 x 500 long and drill a 2mm hole across it every 20mm. Push a bamboo cooking skewer through each hole, they should be a friction fit. You have a profile gauge. To use: hold the timber outside the hull (like outside the transom position) and push the skewers through until they touch the hull. The shape at the end of the skewers is the shape to cut the ply. Use this tool to mark out all of the ply and foam bulkheads and frames. Check the hull again for twists and symmetry before gluing in the transom. Use glue fillets inside and outside. When it is dry add carbon taping inside the joint. The hull should now be fairly rigid and safe from twisting. Fitting out the interior: Measure back from the bow 975 for the end of the foredeck, and 1700 for the front of the centreboard case. The foredeck can be trimmed back from the 975 point on the centre to about 900 at the gunwale level. This allows for the rake in the stump and the wings. Fin-case: The fin-case is made from ply wrapped around your fin. Measure the width of your fin and the depth of the hull from the keel to the deck centre line (probably about 60mm below the gunwales at this point. Cut two pieces of ply about 40mm wider than your fin and as deep as the hull measurement plus a bit, from the third sheet (leaving one side complete for the cockpit deck). Give one side of these pieces (the inside of the case) two good coats of epoxy. Cut 4 more pieces the same width but only 75mm wide. Wrap the fin in plastic. Wrap the big pieces of ply around the fin, gluing the back edge together and clamping. Pull the front together until the ply contacts most of the fin section. Plane a strip of timber to fill the gap between the ply sheets just in front of the fin. Glue this in with plenty of glue and clamp to dry. When dry add the extra strips of ply to reinforce the top and bottom of the case. I have found this is lighter than a moulded case and they do not leak.

To stop the stressed hull skin changing shape when the fin slot is cut, the area around the case needs to be strengthened before the case is installed. This can be either; . a series of strips of ply glued across the inside of the hull, . a diamond shapes piece of ply glued to the inside of the hull . an area of carbon applied inside the hull, or .a ply bulkhead glued into the hull at the middle of the case. I have not determined which is lighter. Once one of these is in and cured mark the centre-line in the bottom and cut a slot to fit the case. Remove the actual fin from the case to protect it and replace it with a piece of timber, which is just a little thicker than the fin, so the case is spread just a tad wider. Align the case so it is vertical and central and glue and carbon it into the bottom of the boat, protruding slightly through the bottom.


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This is the prototype Middle Aged ‘ANACHRONISM’. Contrary to these instructions it has an extra ply bulkhead aft of the fin case, and the main bulkhead is in one part rather than two. The gunwales are also smaller than specified, which required all those extra bits to cover the lacing holes. Note the ply bulkhead at the fin case and the ply sub deck to ensure a water tight joint between the deck and case. Wing Mounts: Cut four pieces of timber, Maple or something harder than WRC, about 300 long x 40 x 40. Fit them under the gunwales each side, butted up to the front of the transom, and where the front bar goes, from 800 to 1100 back from the bow. Make sure these fit well, no gaps allowed.

Main Bulkhead: Mark the inside of the hull at 950 from the bow for the main bulkhead. Use the profile gauge to cut a ply frame to suit the hull shape. Cut the top away to the deck shape, about 100mm down.

skin

gunwale

Mount block


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Before you glue this in you need to give the inside of the bow area a second coat of epoxy, shape the gunwales and fit the first of the foam bulkheads. Shape the Gunwales: You need to get the angle for the deck just right. If there is any gap the boat will leak through the wing mat lacing holes. You need to get the string line down the centre at the right level, and then bend a narrow strip of ply down into the deck trough to see the angle the gunwale needs to be. Try going working along the boat with this template and a saw, cutting little slots at the right angle. Then chisel off the rough and finish with the plane down to the depth of the saw cuts.

Check all the time with the piece of ply. Vacuum up the mess and give the rest of the hull its second coat of epoxy. Foam Bulkheads: The major foam in the bow is horizontal from just behind the bow to the main bulkhead. This is supported by a couple of smaller upright bulkheads above and below. This is because it is easier to cut and fit that way, and because it keeps the waterline fair.


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The foam is glued in with a mixture of epoxy and light filling powder. Glue the main bulkhead in at the same time with a fillet of epoxy and fibre powder. Top of Fin Case: The top of the case needs support and you also need to have something to glue the deck to around the fin case to get a decent seal. Use the aforementioned string line to mark the straight line from the top of the main bulkhead to the top of the transom, along the side of the fin case. If you chose to fit a ply bulkhead here then it needs to be cut down to the deck shape. If you didn’t then a foam one needs to be made to fill this space. On top of that ply or foam bulkhead there needs to be a ply deck support, bent to the deck profile and extending to 50mm fore and aft of the fin case. The centre need s to be cut away to allow the case to come right through. Glue this in place and when cured put some carbon tape underneath to positively seal the joint.

Remaining Foam Bulkheads: The rest of the cockpit area is fitted with vertical foam bulkheads. These support the hull and the deck. They are spaced at about 300mm c/c. which leaves quite small panels of unsupported ply. Use the profile gauge to cut the bulkheads. Fit with filled resin and shape the top in a fair contour to support the deck. Remember to provide drainage holes down the centre and at the gunwales. It is useful to fit extra pieces between the bulkheads to hole them all in place while the glue dries. See following photo of the Youth Version under construction.


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Deck: The first exercise is to ensure that all the gunwales, bulkheads and fin case are prepared correctly to take the ply. Cut a half meter square of ply and bend it down into the trough and check that it touches everything evenly. Also use a long batten to check that it is fair along the boat too. Do a last check that the boat has no twists or serious asymmetry, and correct any faults. Vacuum out all the debris, and you are ready for the deck. The deck is slightly longer than the sheet of ply so you need to splice a short length to the front end. Make it 180mm longer than the distance from the transom to the main bulkhead. Push the ply down into the correct position and mark the underside, then turn it over and cut it out with a 15mm margin. Now reinforce the underside with light weight Kevlar cloth. This makes the 2mm ply very tough so you do not put your knee through it. It is also light. You need to bend the ply roughly to the deck curvature around a jig or something, before you put the Kevlar on. Leave it only one day so the epoxy is not too hard, before you glue it to the hull. It is very important that it is a good fit all around the gunwales and the fin case. Put plenty of glue around her so that there are no leaks. The photo below shows some extra carbon mixed in the glue around the top of the case for extra security.


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Fore Deck Bulkhead: This is the last piece to close of the last hole. It is set in under the fore deck by about 120 – 150mm forward of the main bulkhead. This is so the stump sits over the main bulkhead and there is enough space for the stump, vang and fittings on top. Mark this small top bulkhead the same way as the others using the profile gauge. If you plan to install an access port cut the hole before installation. Remember to give it two coats of epoxy on the inside. Glue it in with a fillet of glue. From the back of this bulkhead to the back of the fore deck needs to be doubled with another layer of ply on the inside. This just stiffens the thin shell of fore deck and saves it getting easily broken when handling the hull.


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Finishing: The top of the fore deck, bow, and the gunwales can be trimmed with a strip of contrasting timber. Plane the edge flat and hold the strips on with tape. When dry sand them to shape, but be careful not to go through the very thin veneer of the ply. Measure the length overall and cut it off just under 3355mm. There should be 15 to 20mm extending beyond the transom. The stump sits on a piece of 10mm ply. A disk, which fits into the stump, is securely screwed to the 10mm ply to locate the stump. Wait until the wings are assembled to locate this. Similarly you need the wings to cut the sockets in the gunwale, and to attach the hardwood chocks outside the gunwale for the wing hold down bolts. The transom bar adds lots of strength to the back end. It needs to be about 50mm x 20mm and best laminated for strength. It needs to be securely screwed and glued to the gunwales. Drill the holes in the gunwale for the mat lacing. These are at 200 c-c along the gunwale and 20mm in from the edge. Also the holes for the mainsheet bridle, level with the back of the fin case. Check all the holes for leakage. Block one side with a finger and blow in each hole. Any leaking holes will need to be filled with glue and redrilled, or have a plastic tube glued in. Now you are ready to start sanding. Do not start with the carbon areas, the dust will stain everything. Start with the wood, and avoid the carbon until all the wood is smooth, clean and has had two coats of resin. Then sand the carbon areas. After all this mix some epoxy and filler and with a good metal blade fill in all the hollows, holes and dints. Keep sanding until it is as smooth as you desire then varnish or paint. Any questions call me on 02 9816 1028 H 02 9350 4697 W e-mail: [email protected] Phil Stevenson.

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