Simple Spline ducks
by Derek Waters

Making Spline ducks.

So you've built a boat or two, and your creative juices are flowing. Time to draw your own design. Time for splines and ducks.

After a bit of home boat-building, drafting splines are usually in plentiful supply - there are bound to be one or two candidates amongst all those scrapwood strips lying near the saw. What's needed are modestly priced spline-weights (or ducks) to keep the battens in position on the drawing surface.
These ones are not as elegant as the commercially made product, but they work every bit as well, and the price is right. Most of what's needed is probably out there in the 'boatshop'.


These quantities assume you are making four 'ducks' - although I was taught you can define any curve with only three points the spline doesn't always agree and that fourth duck often proves handy.

20 lb. or so of lead wheel-balancing weights, collected from local tire-fitters
12" length of one eighth inch diameter brass wire from model-shop. Alternatively, cuphooks from the hardware store.
Scrap wood as follows:

Dim.X Dim.Y Length/Thickness Material
1.5 in. 3.5 in. 18 in. '2 x 4'
1.5 in. 1.5 in. 9 in. '2 x 2'
9 in. 9 in. 1/2 in. or thicker G1S ply
16 in. 1 in. 1/4 in. G1S ply

Small quantity of epoxy resin and filler
Waterglass (sodium silicate). (Not essential. Available from marine stores, or online from
Budget Castings supply or Skylighter)
silicone tape or clear silicone bathroom sealant type 'goop'
leftover paint

Making the mold:

Rip one of the rounded-over edges off the piece of 2 by 4. Take care to ensure that the resulting face makes a 90 degrees angle with the flat of the '2 by'.

Now cut out four 'bites' in from the squared edge as shown. The dimensions shown will give a 'duck' weighing about four pounds.

A sabresaw or a bandsaw would be the quickest way, but a coping saw will work. If none of these tools are available just mark in from the corners and drill a large radius hole, saw down with a handsaw then clear out the waste with a chisel.

You should end up with something like this.

Your 2 by 4 'comb' is one and a half inches thick. On the inside face of each of the bites, mark a line one inch from the large face. This will be your 'pour to' level. Cut the 'comb' in half, giving you two capital 'E' shapes.

Make sure your scrap of 2 by 2 has square edges by carefully ripping off any round-over. The width of this piece isn't very important as long as it remains wide enough to have a screw driven into it without splitting. Mark a centreline on the scrap ply and attach the 2 by 2 down that line. Drive screws or nails through the base and into the wood, countersinking or punching as necessary.

Now attach the two 'E' shapes as shown. Make sure they fit snugly against the centre strip. The lead will be poured into the four resulting cavities to a depth of one inch. Remember to position the 'E's so that the 'pour level' line you marked earlier is in the right place, one inch up from the plywood bottom of the mold.

If you managed to find some waterglass, paint it onto the inside surfaces of the mold. The waterglass helps protect the wood of the mold against the heat. Set the mold aside until the waterglass has completely dried. Dryness is very important - at pouring time any hint of moisture will be turned instantly to steam, which will fire gobbets of molten lead everywhere. Be absolutely sure the mold is dry before proceeding.

A few further words on safety:

The fumes from molten lead are a health hazard. Do not breathe or sniff them. Melt the lead outdoors, over a BBQ or camp stove. A cartridge mask is a good idea. For a one-off process like this, a combination of simply keeping upwind and holding your breath should suffice.

Lead is toxic - use a scrap pan to do the melting, and make sure that it cannot be inadvertently used for food preparation by someone else in the future. Buy a pan for a dollar at a charity store then if in doubt, crush it afterwards.

Lead is (of course) heavy - make sure that scrap pan has a solidly attached handle. The old pan I use has a scrap wood handle I fitted to replace a shaky original. You do not want to be standing holding a handle while a pan of molten metal falls on your feet.

Don't be pouring lead wearing shorts and a T-shirt - Long sleeves, gloves and boots are the order of the day. Wear safety glasses or goggles. If you don't have a cartridge mask, at least wear a disposable mask to guard against splashes.

Pouring the ducks:

Set up your mold on a sturdy box or old table. Use a spirit level to ensure that the mold is not sitting tilted. Find an old coffee can or similar metal scrap receptacle to dump dross into. Get into your safety gear.

Fill your melting pan with tire weights. Usually, tire weights are pretty dirty, but there isn't much point cleaning them as all the dirt will either burn off or float on top of the lead. I use a dollar-store sieve-spoon to skim off the dross and steel clips.

It will take a while for the first weight to melt, but once there is a pool of molten metal in the pan the other weights will slump and melt fairly quickly. Additional weights can be added to the molten pool, being careful to avoid splashing.

Once all the metal in the pan is molten and all the dross has been skimmed off, carefully pour the first 'duck'. Pour it all in one go all the way up to the line you marked earlier, swiftly but carefully. Once removed from the heat source the lead will want to set up, especially where it is thinned out on the side of the pan. When you return the pan to the heat the lead will re-liquefy and more lead can be added ready for the next 'duck'. Repeat the melt, skim, pour cycle until all your 'ducks' are filled up to the same one inch pour level. Turn off the heat and leave the mold alone for a while. The lead will set up very quickly, but will retain heat for a long time.

After an hour or two, remove the screws holding the mold together and lift out your 'ducks'. Some shrinkage is to be expected. Any hollow on the exposed 'pour' face can be filled with thickened epoxy. Any odd bumps or irregularities can be shaved off with normal woodworking tools.


While you are waiting for the lead to cool, cut four pieces of quarter inch ply each four inches long by one inch wide. These will be the bases for the ducks. Cut, sand or file one end of each piece into a blunt chisel shape. This 'pointed end gives a single point contact if you are using the ducks pushed against the side of the batten.

Cut four pieces of the brass rod, each two and three quarters of an inch long. File a chisel point onto one end of each section of rod. Measure back from the point five eighths of an inch and using pliers or a vice bend the chisel point over to ninety degrees as shown. Run the hacksaw blade across the long end of the rod to give some tooth for the adhesive. These will be the hooks which press down on top of the batten to hold it 'mid-line'. Cup hooks or large bronze nails with the heads cut off can be substituted here.

Decide what size of batten you will be pinning down with the brass hook-end of your ducks. I've found three eighths of an inch by one eighth of an inch to work well. Using the hooks as a guide, measure and mark for drilling a hole on one end of each lead casting. If you have never drilled lead before proceed with caution. Drill slowly, withdraw the drill frequently and clear the flutes. Drill bits can be re-ground to an angle suitable for cutting lead, but for a one-off job the effort is probably not justified. Lead has an aggravating tendency to seize around the drill and can snap drill bits in a moment. Ethylene glycol (automotive antifreeze) works quite well as a drill lubricant. If you use any lube on the drill make sure to clear all traces out of the hole before gluing the hooks in place. Should you be unlucky enough to break a drill while making the holes for the hooks the simplest solution is to hammer the broken drill flush with the surface and have a go at the other end of the casting.

Once you have drilled all the holes, using epoxy adhesive glue the hooks in place. Next, glue the lead blocks onto the ply bases. In addition to the glue I used bronze nails to pin the bases to the castings but the adhesive alone will prove more than adequate. Once the adhesive has set up it will be time to finish the ducks. Any pits or cavities in the castings can be filled with thickened epoxy before giving the duck a coat of paint. Some kind of painted finish is a good idea, as otherwise the lead will get on your hands. This may not be a huge health hazard, but will certainly smudge off onto your drawings. As the pictures show, I was satisfied with 'workboat' finish - a quick coat of spray paint has held up fine. All that remains is to prevent the ducks from sliding around on the table. I happened to have some self-adhesive silicone rubber high friction tape lying around from another project, but silicone bathroom caulking makes a readily obtained substitute. Just smear a thin layer over the underside of the bases and put them aside until the silicone dries. The resulting rubbery surface will neither slide nor mark your drawings.

Derek Waters