Delta
by William R. Watt

Delta is a 12x2 foot double ender made from one sheet of plywood using the reversed triangle (hence the name Delta) technique first seen on David Beede's "Summer Breeze", winner of the 2001 Duckworks design contest.

To build one of these boats the 1 foot side panels are cut off, then laid on top of, and temporarily clamped or nailed to, the 2 foot wide bottom panel, while 6 inch by 2 foot triangles are cut off the corners. The triangles are measured in from the ends of the sheet instead of out from the middle because the plywood will not likely be exactly 8 feet long. While the panels are still held together, and as per David's instructions, the "bumps" are rounded off. I rounded off 6 inches by marking the edges 3 inches on each side of the bump, springing a small metal batten through the marks, and sanding off the excess. The radius of the arc is just over 18 inches, a number used in saw kerf bending of the chine battens and bottom skids as described later.

I calculated the relationship between the draft (d) in feet and the immersed volume (V) in cubic feet to be V = 8d + 3.46 d^2 - 2.31 d^3 where d^2, d^3 are d squared and cubed. The freshwater displacement in pounds is 62.5 times V. The wetted surface (WS) in square feet is 8 + 24d. The waterline length (LWL) is 4 feet plus 8 inches per inch of draft. Displacement speed in miles per hour is 1.54 times the square root of LWL. The following table shows results for selected drafts. Its interesting how all the numbers except speed are linear with draft.

 draft volume disp. WS LWL speed (inches) (cu ft) (lb) (sq ft) (ft) (mph) 3" 2.2 136 14 4.66 3.3 4" 3.0 185 16 6.66 3.9 5" 3.8 235 18 7.33 4.2 6" 4.6 286 20 8.00 4.4

Rearranging the pieces shows the sides and bottom of the boat. The plywood used was virola underlayment. This is new in our area. I wanted to try it out for another project due to start in a few days. Virola is from Brazil. It is not rot resistant. The inner ply feels hard but the two thin outer plys are soft, porous sapwood. There were some sheets with no edge voids. The face plys on some sheets were both smooth and free from defects apart from a couple filled splits near the ends.

When I got a selected sheet home and checked with a lamp there were no interior voids. The soft face plys had fuzzy areas. Where the plywood was covered with resin the fuzz could be ignored. Where the plywood was painted the fuzz could be sanded down after the first coat dried. A sheet of this virola underlayment weighs only 8.5 pounds, half the weight of lauan underlayment or 5.22 mm okume marine plywood.

I did some tests on this plywood. A coat of automobile engine antifreeze was brushed on to retard rot, and idea inspired by David Carnell. One end of the plywood was treated and the other left untreated for comparison. Polyester resin was also tested on the plywood, reinforced with polyester or with fibreglass. The cheap polyester fibre is sold as drapery header tape at fabric stores. Barend Migchelsen mentioned using it with polyurethane construction mastic (PL Premium) to tape seams. It does not unravel and create a mess like fibreglass. Unfortunately I was rushed an did not wait for the antifreeze treatment to dry thoroughly before taping the butt joins. I was able to pull the tape off some of the cured joints.

Another problem was the polyester cloth is stiff and would not conform unless the surfaces to be butted were very level across the seam. After curing there were air pockets under the cloth. I assume this is because polyester resin is not as viscous as polyurethane mastic. I rebutted one face with a triangle of doorskin plywood bedded in polyurethane because it will cure on a moist surface. That saved time and kept the project moving along. Later, when the wood was dry the other side of the same butt was taped with fibreglass without problem.

As it happens all butts on this boat are above the waterline so their condition is not crucial. A third of the boat's bottom was coated with polyester resin. Because the wood is porous one coat was spread without any catalyst and allowed to soak in. Then another coat mixed with catalyst was put on so the resin would cure right into the wood. It made for a very slow cure, about 18 hours. (Don't try this with epoxy. Epoxy has to be well mixed with its catalyst in order to cure.) For comparison another third of the bottom was treated with two coats of linseed oil and the last third was left untreated. Consequently the hull is quite a chequerboard of surface treatments.

The entire outside of the hull was given three coats of some exterior oil house paint I wanted to use up. The inside of the hull got two coats of linseed oil. In the course of building this boat I discovered the way to identify an amateur boatbuilder. He's the one with the sawdust and paint on his pajamas.

After the plywood was cut up the nice spring weather turned bad forcing construction indoors where there was no room to swing a camera. I doubt anyone would be interested in photos of the designer/builder on his knees under the hull sawing knots and nail holes out of recycled lumber. People should look instead at David Beede's Summer Breeze photos and at those of the Duckworks editorial board.

This photo shows a chine batten bent in two directions using saw cuts. The dark lines are the glue impregnated saw cuts. A temporary mould and a 12 inch carpenter's square appear in the photo. So does a knot hole in the chine batten on the extreme left. The damp wood from the antifreeze treatment forced a change of plan from trying polyester taped seams to chine battens glued with PL Premium and fastened with wood screws. After ripping 3/4 inch by 3/4 inch battens out of red cedar 2x4's it was discovered they would not bend to the shape of the hull.

Discarding the options of setting up steam bending equipment in the bedroom, or ripping thinner chine battens and running out to buy shorter screws, I recalled something about wood bending with saw kerfs from Fred Bingham's "Boat Joinery". I decided to give it a try. You clamp a piece of scrap wood the same dimension as the wood to be bent and a bit longer than the radius of the bend to a flat surface, saw almost through the scrap, raise the far end until the saw kerf closes, and at a distance out from the kerf equal to the radius of the bend, measure the height of the scrap above the flat surface. That height is the spacing between the saw kerfs in the piece to be bent. Imagine trying to saw 6 closely spaced kerfs in 12 foot battens in two places 4 feet apart without breaking the battens.

Fortunately chine battens do not have to bend in a continuous curve on boats with straight sections. They can be put on as shorter pieces with the cuts falling on the straight sections. The chine battens were cut into shorter pieces to make cutting the kerfs easier and to relocate the knots, nail holes, and rot takeouts in the recycled lumber. Alternating the direction of the kerfs enabled the battens to bend in two directions. I don't know if that's been tried before. It took a while to get a feel for how deep to saw the kerfs but by the last of the four bends I had it down pretty good. Only two of the four broke. I used them all anyway with no problems to date.

The ends of the boat were wonderfully sharp before the gunwales went on. I wondered if I would be arrested for paddling a dangerous weapon. The seam in the photo is one of the good polyester butts. I do not have any data on how strong this tape is. The ends were prettied up with chisel, saw, and sander. I am still undecided about putting on small decks, wrapping the ends in fibreglass, leaving them as is, or what.

Some builders will have noticed the boat has Bolger bows. That is to say the plumb sides come together to meet the rising flat bottom all at the same angle (a gradient of 1-in-4 on this boat). Phil Bolger, a popular designer of plywood boats for amateur builders, claims this provides good performance through the water. The Bolger ends were unintentional. I didn't notice the Bolgerness until the finish was going on.

I haven't seen one of these photos on Duckworks of a boat coming out a second story window so thought I'd include one. I prefer the bottom skids to a keel. A thin plywood bottom needs something to support it over the two foot span. If you don't want to sit on an inside keelson then the stiffening has to go on the outside. If its a keel, when the boat is ragged ashore it drags on the keel and one chine which tends to chew up the chines. When a boat with skids is dragged ashore it drags on the skids so the chines don't get chewed up. I guess that's why they're called skids.

I figured 1/2 inch thick skids would bend to the shape of the hull because the gunwales had been made out of two thicknesses of 1/2 inch thick by 3/4 inch red cedar and these had bent to the shape of the hull. The 1.5 inch wide skids were ripped off an 8 foot spruce 2x4. They would not bend to the shape of the hull. I used the saw kerf technique again. This time it went much better. Only one broke and then only on the final glue up.

Alas, the skids took the unfinished weight of the hull from a lovely light 22 pounds up to a disappointing 28 pounds. That's what you get for rushing ahead without careful thought. Smaller, lighter, shorter skids would have been fine. The skids were glued to the hull with polyurethane and held in place with screws through from inside until the glue cured. Then the screws were removed and the holes filled with drops of polyester resin from the end of a tooth pick. I did not have screws on hand short enough to leave in. The final weight of the boat is 29 pounds. It could have been built to 25 pounds or maybe less.

The edges of the plywood and all glue lines exposed to water were protected with two coats of polyester resin applied with a putty knife and a tooth pick. No fooling around with brushes or sponges on a stick. When the job is done just wipe the resin off the spreader with a rag and go make a pot of tea. I wear kitchen latex gloves and toss them out when the buildup of hardened goop starts flaking off and falling on the work.

A very small car gets a stylish spring hat. I think it looks regency period.

After some practice the Delta boat, with its flat entrance and exit, paddles straight. Using a canoe paddle I heel the boat over until the knuckles of my lower paddling hand dip in the water, and push the upper paddling hand out so the blade is a bit under the side of the boat. When heeled over the boat has a longer waterline and goes faster and straighter. To glide in a straight line the tip of the paddle is left in the water.

The double bladed paddle in the photo was made from a paint roller extension handle and a discarded aluminum snow shovel. (I learned recently a local junk yard sells discarded aluminum traffic signs for \$5.) Attaching the blades off centre puts them deeper in the water while not lifting the opposite arm as high. The boat goes straight with this paddle if the blades enter the water forward, about where the boat itself enters the water. The action is like a swimmer pulling himself along.

In smooth water I've tried rocking the boat a bit on each stroke, getting the chines to dig in. Directional stability is not as good with double blades as with a single blade. A skeg or kayak rudder would help. I'm looking forward to rigging the Delta boat for sail, the one from the one sheet Loonie which appeared in Duckworks in 2001.

The Delta boat nuzzles up to a dragon boat, looking like mother and calf. In the background is a cantilevered swing bridge at the entrance to a lock on the Rideau Canal. The dragon boat was donated by Nortel Networks back in the days when everyone imagined the company was making money. It is paddled by a tribe of wild Amazons who build up momentum but have difficulty deciding which way to turn. I keep a wary eye out for them when paddling in the vicinity.

When not on the water the Delta boat rests on custom made wobbly saw horses at the bottom of my garden. The saw horses go inside the hull for good support. It takes 8 legs to make a pair of sawhorses as I discovered after carefully cutting out 4 and trying to make this pair of sawhorses with them. Its a nice boat. This six foot adult can recline comfortably in it. I built it for an inner grandchild to keep us out of trouble. He seems to like it. I certainly do.

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