Building a stressed ply

Development Rule International Canoe Hull

Phil Stevenson August 2006




This design is provided for amateur construction within the Development Rules of the International (Sailing) Canoe Federation. It can be used free of charge with the provider accepting no liability for the success or failure of the construction, or any consequent success or failure of the subsequent boat.

If however someone wishes to use the design to produce a boat for resale or commercial purposes I request they contact me to negotiate some form of royalty.

While I have tried to be as comprehensive as possible I know I will not have covered enough to suit everyone. I have assumed at least some reasonable amateur boat building skills or experience. Please do not inquire about templates or CAD plots as none exist. This is a cut and fit exercise not a one design jig saw puzzle.

This Design:

This canoe is designed to the draft 2006 Development Rule of the International canoe Association. It is narrow, light and in this case has a Una rig, all variations to the traditional Nethercott canoe. This hull is very narrow at the mast and so an un-stayed mast has been used to do away with low staying angles. There has already been some interest in building a sloop rigged version with stays requiring a wider deck line in way of the mast. Anything would be possible for those who want to experiment.

The prototype has proven initially very fast in light conditions in comparison with the current Aust IC Champion Nethercott, but will prove more challenging when the wind gets up.

The design outlined has a stepped deck to accommodate Australian Skate type rails for seat retention, but it could simply be modified to suit a sliding seat carriage as is normal in most ICs.

All photos included are of the prototype AUS 21 named 21st Century Hollow Log. There are more photos on


This method of hull construction allows very cheap construction of hulls at near comparable weight to modern carbon/foam hulls. The target hull weight for a development rule canoe is about 30kg and this is easily achievable. The method is based on some success I have had in building moth hulls in the last 10 years and some 12ft skiffs in the 1970s. The system owes its origin to John Buzzaglio who made the first Unicorn A Class in UK about 1965, and which method was copied to home built cats of many classes in the 1970s and 1980s.  

To achieve the required strength the hull shape is restricted to curved sections. The 3mm ply skins are jointed by minimal carbon epoxy seams, and small quantities of timber. To maintain this strength the skin is supported internally carbon skins and with ply and 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:

Ø      Make the timber and ply joints fit well,

Ø      Use carbon where strength is important.

Ø      Use minimum resin with the carbon, and

Ø      Use foam where strength is not that important.

Ø      and because timber soaks up water use a good wood sealant inside and outside.

Since this is written in Australia some of my terms may be inconsistent with what you might use in other countries. Eg:

  • The sliding seat of established canoeists is known as a plank to the Aust small boat classes which use them.
  • Vang might mean kicker in UK.
  • Gunwale is Inwale or shear stinger.
  • Frames, Bulkheads, Formers are all interchangeable terms.
  • DAR timber is dressed all round so 25 x25 DAR is finished 20 x20

All units are in mm unless otherwise noted.

Material List for the hull:

Five sheets of 3mm exterior grade okeome / gaboon ply 1220 x 2440.

One sheet of 50mm L grade (11kg/m3) polystyrene foam 1200 x 2400.

Three 5m lengths of 25 x25 DAR western red cedar.

6 sq m of 200gsm carbon cloth.

2.5 sq m of 150g approx Kevlar

3 sq m of 300gsm Carbon Uni

3m of peel ply

6 litre epoxy with hardener, bag of fibre filler, bag of light powder filler.

3 litre of Everdure or other wood sealer.

1 sq m of 75 gsm CSM Glass

2m of 25mm x 2mm aluminium tube.

Small pieces of trim timber.

Lets get Going:

Cut the Plywood:

Lay out the ply to form two pairs of sheets into two big sheets 4820 x 1220. Overlap the edges 20mm to accommodate the splices need later. Staple them together so you can work with the big sheets. This is not long enough for the canoe but it is more economical to source the small bits later.

Draw a pencil line on one sheet parallel to the long side and inset 390. This is the centre of the bottom panel. Mark off the stations from the bow, then the widths at each station square off the centre line. Then with a long batten like the 20x20 timber for the gunwales, supported by nails if needed, draw the outline of the bottom panel. Note that the lines for the centre cut out for the keel at the bow is two curved lines ending tangentially as a single fine cut at station 2000.


Ply Cut out shape:













all these widths are measured from centre line



measured from datum at edge of ply sheet

from bow




from bow



top of fore deck
























600 step 























340 step to 250


































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. Drive small nails into the plotted points and use one of the 20 x 20 timbers to plot fair curves.

The two sides are cut from the second sheet. The measurements are at the same stations but or offset from each ply edge.

Once marked out select a part near the end of the second sheet to cut out the small pieces needed to finish the stern of the three panels to the full length.

Cut all the ply with a 10mm allowance outside the lines so you can fair everything up after the sheets are glued together.

It is easier to splice these all together at this stage rather than earlier, as the splices are shorter. 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.  Sight down the centre line of the bottom and straight edges of the sides to ensure the assembly is straight.

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.

When cured check all the dimensions again and correct any errors due to the splices not being 100% straight. Then cut to the new lines and plane the edges to smooth fair curves. 

Gunwales and spreading:

Glue the gunwales inside the top edge of the side panels. These are 4 pieces of WRC (western red cedar) 25mm x 25mm DAR. One pair goes from the stern to 150mm forward from the step at station 3000. The other pair go from the step to near the bow at the shear level (ie straight from top of ply at 3000 to the bow, leave the foredeck loose).   If there is any short fall, it can be 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. If you intend to have a clear finish staple through scraps of ply to prevent surface damage.

Find a piece of scrap timber about 70x150x20 and fill the gap where the two pieces of gunwale overlap at station 3500.

When cured you have to start joining the wobbly sheets long the chine and keel. Use lots of tape on the outside and some pieces of copper wire between pairs of 2mm holes drilled 15mm back from the edge. You may need these as close as 200mm c-c. Push the ends through from the inside and tighten them by twisting the ends together on the outside. Then push the inside loop down as flush with the ply as possible.

Use masking tape across the outside of the ply to close up the gaps and to seal against epoxy leakage, 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, and that the two side pieces are level at the bow, not one forward of the other.

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:




 2000 from bow

3500 from bow








The seams are taped inside and out with carbon tapes. You can buy carbon in tapes but it is a normally a heavy grade. Alternatively cut carbon cloth (about 200gsm) into 50mm – 60mm wide strips. If the cloth is not a square weave, cut it with the major fibres across the strips. Use one layers inside and one outside.

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. Fill the seam between the ply with a fillet of epoxy and fibre filler, then the carbon tape and epoxy, and cover with peel ply to squeeze out the excess resin, and push the carbon down hard against the ply. Leave the last 450mm at the back of the chines and the front of the keel until later.

When the inside is fully cured turn the hull over, remove the masking tape, cut off the copper wires and clean up the joint. Leave the struts in and the hull spread until the outside carbon is cured.  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 sealant. Also while you are waiting you can make the fin and rudder cases, see instructions later on.

Pull into Shape:

Remove all the spreader sticks and start to pull the sides together. Tape the bow first.  The target gunwale widths are:




At 3500






Nail a timber across at 33500 to hold design beam. The bow is best held together by clamping the ply between two straight pieces of timber.

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 that the bow and stern post are parallel and square to the deck.

Now is the time to check the overall length is not over 5200, and the chine rule measurement. Make a template with two 100mm spaces offset from a 750 wide stick. Make sure the boat measures in before going on. If needed adjust the beam to achieve the rule chine width and deadrise.

When you are happy, carbon seam the inside of the bow and stern. Add a slither of timber to help hold the carbon in the bow and stern and keep everything straight The outside can wait till later when you are finishing things off.

Now join the foredeck down the centre. Pull the sides in so they touch and tape in place. Check that the seam is straight and down the centre line. Make sure that the vertical line from the keel to the back of the foredeck seam is vertical and parallel to the bow and stern. You may need to trim one or both edges with the plane to achieve this.

When satisfied tape up the outside, invert the hull, and do the inside carbon. You need to apply the carbon and resin with a brush on a long stick. Do not worry about spillage just spread it about as you need to seal the inside of the ply too.

The shape of the bow will appear unusual to some. The chines and waterline are fair curves but at deck level the plan view is pinched in near the mast due to the closing of the foredeck. This provides some degree of wave piercing ability with little sacrifice of volume.

The hull should now be fairly rigid and safe from twisting.

Bulkheads and foam

Inside the boat are three 3mm ply bulkheads,

at 1350 just in front of the mast,

at 2700 at the back of the centreboard case and front of the plank rails, and

at 3650 at the back of the rails.

There are also two transverse horizontal 50 thick foam wedges in the bow at about shear level and about mid height between shear and keel, and

There are 6 foam bulkheads; each 50mm thick polystyrene with a folded 100mm wide strip of carbon unis around the perimeter:

            Two equally spaced between the mast bulkhead and the case bulkhead,





            Two equally spaced between the case bulkhead and the rear rail bulkhead and





            Two between the aft bulkhead and the rudder case.





Also the entire inside of the hull is skinned with 200gsm carbon laid on the diagonal to the centre line, from the mast to within 1200 of the stern. This goes in before any bulkheads.

Once the carbon is in it is a good time to give the inside of the hull a coat or two of your favourite epoxy sealant.

The deck from mast back to the back of the case has the same carbon on the underside, and the aft deck from the centreboard case back to the front of the rudder box has a layer of 150gsm Kevlar.

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 bulkhead 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 or foam. Use this tool to mark out all of the ply and foam bulkheads and frames.


Fitting out the interior:

Measure back from the bow 1500 for the end of the foredeck, and 3000 for the back of the centreboard case.

Fin and rudder cases:

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 50mm wider than your fin and as deep as the hull measurement plus a bit, from the off-cut.  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 thick 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  strips of timber to fill the gaps between the ply sheets just in front and back 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.

Once one of these is in and cured mark the centre-line in the bottom of the hull. 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 at the thickest section, so the case is spread just 1mm wider.

I prefer not to cut the slot in the hull yet as there is a chance the shape may distort as the stress is relieved. So I make a small 20 thick piece of timber to fill the bottom of the case and then locate this on the centre line with a couple of nails pushed through drilled holes. Once it is in the right spot, slip the case over the block and the bottom of the case is in the right spot, (back of case is against the middle bulkhead. Align the case so it is vertical and central and glue and carbon it into the bottom of the boat. Use some strips of uni carbon across the bottom at front and back of the case and generous sq weave carbon bandages everywhere.

Repeat process for rudder box except it is easier because it is rectangular to suit the rudder cassette. Mine is located between 600 and 400 from the back. It is 50wide and 300 long.

Mast Bulkhead:

Mark the inside of the hull at 1350 from the bow for the main bulkhead. Use the profile gauge to cut a ply frame to suit the hull shape.

Before you glue this in you need to have given the inside of the bow area a second coat of epoxy sealant, and fit the two foam wedges.

The bulkhead is 3mm ply with a single layer of200gsm carbon and is held in place with a 50wide strip of uni carbon forming a fillet and making a ring frame plus a 50 wide strip of 200 gsm square weave to hold it in.

The mast step tube is a 75 x 1.5 carbon tube. It is located at 1450 to the centre measured from the bow. There is a gap between the tube and the bulkhead so that the vang can be tied around the tube. Tou also need to bind on a SS saddle at the front just above the cockpit so the vang does not ride up.

There is a wooden plug in the bottom with a head less10 dia bolt forming a pivot pin. Buy a 75mm hex head bolt with at about 20mm of unthreaded shank. Cut off the head and round the top. Thread on a nut and washer and screw it into a 9mm hole drilled centre in the wooden plug. Glue this in the bottom of the mast tube. The top bearing is a nylon special to fit the mast dia. You need a lathe for that.

The tube sits on a 80x20x180 timber pad which sits on a transverse layer of carbon unis. It nestles into a socket in the back of the foredeck, which has been strengthened with a layer of 20 thick timber strips and transverse carbon unis all the way from the bulkhead to the aft edge, and down to 70 below the gunwales.

Photo above shows tube in place with carbon and fillet around base, but before carbon around aft edge of fore deck. Note also the deck doubler to allow splicing of the remaining deck.

You also need to install a section of deck at this stage with a hole for the tube. Get all three lined up with suitable rake, and glue in the tube. When cured add extra carbon unis to the underside of the mini deck, around the base of the tube with small timber triangles to form a fillet around the base, and around the back of the tube and aft edge of the foredeck.

Be generous with lots of small carbon. This is the highest load area of the boat so make it strong.

Other bulkheads:

These are similar except the top is dished to make the cockpit. They have timber edges for the decks. The middle bulkhead has an 80mm step between the deck between mast and fin case and the aft dance floor deck, so has two timber deck supports. All these are nominal 20x20 finished made curved by laminating two pieces 10x20

I used about 130mm of dish in the middle BH and pulled a string line from that t o the stern to figure out the dish in the aft one. The centre line must be straight or you will not bend the ply.

To these bulkheads I glued four 75x20 timber posts to support the seat rails. The rails are 400 off the centreline of the boat and 90mm above the gunwales. (all to CL of the 25x 2 alloy tubes.)

I moulded some chopped strand glass around the alloy tubes to make some glass tubes to make sockets for the tubes. No carbon and alloy mix to prevent electrolytic corrosion.

(Hint: for moulding tubes spiral wrap the alloy mandrel in 50 wide plastic tape, two layers, first one sticky side out, second layer sticky side in, then it will release from both the mandrel and the work. When the glass is on and wet out, wrap another layer of tape sticky side out to compact the laminate and squeeze out excess resin.)

The glass tube is cut into 65 long pieces and installed on the posts. Initially just glue but then after trimming the posts add spectra rope cores and carbon. I used the rope threaded through the bulkheads below deck to get a good grip between the tubes and the hull.

Add some extra carbon to make sure that the post loads are distributed to the hull sides. Note the extra timber around the posts at deck level to support the deck ply too.

Top of Cases:

The top of the cases needs support and you also need to have something to glue the deck to around the cases to get a decent seal. Make up some rails and cross bracing. I used a half bulkhead at the front of both cases and some timber to the gunwales.

You also need a triangle of solid stuff for control lines and cleats. Aft corner of the mid deck each side of the fin case works well for me.

Foam Bulkheads:

These are a good fit to the hull shape at the sections mentioned but with the corners rounded off. The foam is edged with a folded wrap of 100 wide carbon unis to form a ring frame. The foam and carbon is glued in with a mixture of epoxy and light filling powder, but only just before the decks go on so you get a wet joint to the deck.


The first exercise is to ensure that all the gunwales, bulkheads and cases are prepared correctly to take the ply. Use some battens to ensure  that it touches everything evenly and  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 back end.  You need to fit it well around the rail posts

Reinforce the underside of the mid deck with 200 gsm carbon and the aft deck / dance floor back to the rudder case with lightweight Kevlar cloth. This makes the 3mm ply very tough so you do not put your knee through it. It is also light.

Put the deck on while this is still soft or leave it no more than 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.

I put a kick tail of 30 x 30 triangle timber down the centre line of the aft deck to help prevent sliding off the boat. I also used a EPA rubber deck coating for comfort and non slip.


The top of the fore deck, bow, stern 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.

Cut the slots for the fin and rudder, trim as needed.

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 sealant. 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.


Things to go with Your Hull:


My mast consists of 4 sections:

Base: 2.7m of 63 id x 2.1 wt carbon with an internal sleave from 400 to 100 from base. It has some extra wet lay up carbon at deck level for wear at the bearing.

Next: 1m of 58 ID x 1.8wt tube

Next: 1m of 51 ID x 1.8 wt tube

Top 1.5 approx is a tapered mid piece of windsurfer mast, starting at 50 OD tapering to about 30 OD

The overlaps are about 150 to 200 and some extra carbon was applied before assembly to get neat fits. I have glued two of the overlaps and left the other one loose so the mast comes into two pieces.  Check the total length from keel to mast top complies with the class rules.

The gooseneck is  a U shaped wrap around the mast which extends out the back as two tangs. The boom is bolted between the tangs.  So there are no holes in the mast and I sealed all the ends so they all float.


This is like a big moth or sailboard sail.  Luff and leach about 5.25m, foot about 2.7m, about 300mm luff curve but match it to your mast bend. Roach to make area: 10sq m + 0.6 IC shape fudge factor + 5.25m x 75mm luff pocket allowance = nearly 11 sq m.

Seat / Plank:

I found out two 50mm x 1.6wt tubes are not strong enough. Mk 2 is two 50x50 x 1.8wt sq alloy with 20mm timber inside plus some 40 x 8 timber glued to the bottom. These are separated by 400 wide 50 thick styrene foam and held together with light glass cloth.(do not mix carbon and alloy). I would like to make a lighter version again with timber foam and carbon for strength, but that wil come later.

I needed to build a rudimentary carriage to keep the plank square to the boat. This comprises two tubes of CSM glass which run on the rails and a cross of 50 x 3 ply carbon reinforced, below and clear of the plank. There are carbon verticals in each corner to link the cross with the tubes and stop the plank skewing. Simple and weighs only 600g.

Rudder and fin:

Almost standard IC fare but the rudder is maybe a bit bigger (190 x 600 below hull, 35 thick) to cope with mono rig tacking. The fin is also slightly smaller (1200 x 170 overall, 25 thick) which does not seem to matter as the hull has more lateral area than normal. These were both remnants from moth hydrofoil experiments, so were not really designed just adopted. The tiller is the top off a sail board mast. The shaft is a home made carbon tube about 2.5 thick and 25 dia. There is a thinner wall similar tube in the foam box and some nylon bearings top and bottom. The gooseneck is two carbon tubes which fitted the shaft and tiller simply carbonned together at the right angle. The tiller is removable for storage.


The mainsheet copies standard moth practice of a simple bridle between the front ends of the rails. The bridle is high enough so the mainsheet blocks come just block-to-block, this allows you to sheet to the centre without vertical loads. I use 4:1 with the ratchet block the last one on the boom.

The Vang is 24:1 with cleats on each side of the middle deck. Cunningham is 6:1 but will change to 12:1 as you need to be able to pre bend the mast to take out the luff curve in light winds.

The out haul is tied off.


Mine is made from 50mm alloy tubes welded into a 3.5m x 1.1m A frame with vericals on each corner. There are glass and polyester cradles for eth bow, max width station and under the mast, all carpet covered. It has plastic wheelbarrow wheels and inflatable tyres.

Cost in $ and time:

The boat was built from commercially available materials with no special deals. The sail and spars tubes were professionally made. It cost just on $A6000 including about $300 for the trolley components. For comparison this is about the 75% the price of a new Laser or about 35% the cost of a professionally built hydofoil moth. I am a part time anmateur builder and it took me about 3 months, but people say I build quickly. I pretty well gave up television and social life for the duration. I have also built a lot of boats over the last 40 years and a have suitable tools and a convenient space at home. So consider your skills, time and commitments before getting in too deep. If you can not do it yourself, I expect you would pay a pro builder maybe three times this cost to cover the time needed, and then other materials would probably be better value.

Follow Up

I would greatly appreciate people advising me of errors or faults in this document. Anything that makes it easier for people to use, to make more boats will be usefull.


Any questions call me on

Australia +61 (0)2 9816 1028 H

Australia +61 (0)2 9350 4697 W



Phil Stevenson.

August 2006