Bath-tub Steam boats


Bath-tub Steam boats
by Derek Waters

Toy steam tug

Although just over six inches long, this little tug has a working 'live steam' engine. With a hull easily carved from scrap wood, it uses a spirit lamp heating a simple flash boiler. Once the 'boiler' has been filled with water and the fire lit the boat will pause for a minute before puttering off across the bathtub, making the 'putt putt' noise that gives these boats their name.

Putt-Putt or Pop-pop boats have been around for a long time; tin toy versions which have scarcely changed in the last century can still be found in the stores. The undisputed home of Pop-pop boats on the web is the Pop-pop pages. Collected there is information on the theory of (and patents relating to!) pop-pop boats, a list of retail suppliers and more.

Getting started:

(click image to enlarge)
In addition to the wood for the hull, you will need a couple of copper end caps of the type soldered in place to close off 1/2" diameter copper piping. They cost pennies at just about any plumbing or hardware store. Craft shops or model stores sell the 1/8" inside diameter (5/32" OD) copper pipe which carries the steam. The same stores will have the thin brass sheet used for the heat shield 'funnel'. If you have a choice of thicknesses, 5 mil (0.005") works well.

Building these little boats can be done 'by eye' - the rough plans shown here (click for a full size version) are intended as a starting point for your own experimentation. The bold gridding is inches, subdivided into quarters; print the diagram as 6" by 8".

The wood used should be light in weight. Where I live, knotty cedar is sold rough for use as fenceboards. It sands to an attractive finish, and it's easy to find enough clear stock for a project as small as this one. Use whatever you can find locally as long as it is lightweight. Basswood or even balsa would do. Any waterproof wood glue will work.

Making the hull:

Topsides layout Square up a piece of wood an inch and a half thick and long and wide enough to fit your hull. A seven inch length of '2 by 4' will work. From stiff card or thin MDF cut a plan view template. Mark a centreline as shown. Draw round your template and flip it across the centreline to draw a symmetrical plan.

Using a coping saw, bandsaw or jigsaw cut around inside and outside. The example was cut on a bandsaw - note the cut through the bow on centreline. Other saws can have their blades passed through a hole drilled in the waste wood and so will not need this cut. Once the cut is closed by gluing and clamping it becomes almost impossible to see.

topsides cut Once the glue joint at the bow is set, glue the topsides onto a suitably sized piece of three quarter inch thick board (nominal '1 by 4'). While the '1 by 4' still has straight sides, mark the topsides for a sheer line and cut or sand down to shape. When you are happy with the sheerline, cut around the outside of the topsides to remove excess stock. Sand the basic hull to refine its shape. Note that deck house and boiler weight is concentrated towards the bow; looking at the plans you will see that the forefoot of the hull is shown deeper than the run aft, leaving enough wood below the waterline forward to create buoyancy to support this weight.

bare hull Drill two 5/32" holes through the hull, one on either side of the centreline aft, to emerge just below the waterline. The twin pipes from the boiler will be glued through these holes later. Once you have checked that everything fits together as expected it is a good idea to drill a shallow recess in the deck into which the lamp can nestle. Otherwise when the motor is running there will be enough vibration to shake the lamp out from under the boiler.

deckhouse walls deckhouse roof Walls for the deckhouse can be cut from one piece as shown, or assembled from 5/16 thick 'boards'. A roof for the deckhouse looks good with slightly more overhang at the front than at the sides or back. Note that the hole in the roof for the 'funnel' needs to be large enough not to touch the metal heatshield.

funnel templatesThe 'smoke stack' is cut from thin brass or copper sheeting with a tubular top section formed by crimping and soldering. Two and a half by three and a half inches is a good size, but check against your deckhouse to be sure. Make a stiff paper or thin cardboard template to check the fit. Metal sheet as thin as the 5 mil suggested can be cut with ordinary scissors. The completed 'funnel' can be attached to the deckhouse with small nails or tacks.

At this point the deckhouse can be glued down onto the deck and a coat or two of waterproof varnish applied, but do not attach the deckhouse roof yet. It would be in the way while fitting the plumbing.


The simplest 'pop-pop' motor would be a metal tube, closed at one end like a test-tube, and filled with water. The open end of the tube is placed under water and heat is applied to the closed end. As the end of the tube heats up some of the water boils into steam. The expanding steam forces water out of the open end of the tube, and Newton's laws suggest that whatever the motor is attached to will experience a push. There's a fuller explanation of the theory available here.

The plumbing parts The motor shown without its top on the right here is slightly more complicated than the test tube example, but not by a lot. Having two pipes rather than one makes it much easier to prime the system with water, since the second tube allows air to escape as water is poured into the first. Making a 'Boiler' from the end cap for standard 1/2" copper plumbing pipe allows for a top in very thin brass shim stock. Once soldered in place the thin brass can be gently pressed into a dished shape which will click in and out with each pulse of the motor, producing a much more dramatic 'sound effect'. At the left of the picture is an unsoldered lamp as described below.

As with any soldering job good preparation is key to good results. Sand the open end of the caps square for a good fit, and remove any oxidation with fine wire wool before making the joints. Although I've silver soldered some of these little motors, soft solder is more than adequate. If your motor gets hot enough to melt its own solder, it is trying to tell you something. Plumbers paste flux works fine. Plumbers solder works too, although it is easier to solder some of the small joints tidily if you use fine electrical size solder wire. To make the clicker top for the boiler you will need a little scrap of very thin sheet brass - anything much thicker than one mil [0.001"] will be too stiff for the steam pressure to flex. If there's no thin stock available just use some of the leftover material from the funnel, or a small copper coin; your motor won't make the same noise but it will run just as well.

With the boiler soldered together, test fit it into your hull. If the boiler is sitting up in the air gently curve the pipes. The idea is to bring it down to a height where it will just clear the spirit lamp's wick tube by enough to allow a small flame. remember that the ends of the tubes must be consistently below the waterline when the boiler is mounted in the hull.

It is a good idea to rig up a test for your motor at this point before gluing it into place in the hull. The test rig need not be very sophisticated...

The small spirit lamp
The small spirit lamp A source of heat for the boiler is needed. This can be very basic; a bottlecap full of olive oil crimped up to hold a cotton string wick will work. The commercially made tin-toy boats are sold with a bag of tiny little candles. A simple spirit lamp gives more controllability; Too much heat is a bigger problem than too little. Time for another plumbing cap. With its filling tube at one side and wick on top of the lamp, this one needs to be placed under the boiler, placing the weight further forward than is desirable. By placing the wick down an extended side tube the lamp can be moved further back, as shown in the plans.

braided glass fibersWick can be as simple as piece of cotton string but braided glass is much more durable. Craft stores sell glass wick for decorative oil lamps, but it is easy to make your own from strands taken from a scrap of glass cloth left over from boatbuilding. If you do not know how to plait just ask someone with long hair to show you. The glass braid will do its best to unravel before you can install it in the lamp, and knots just come undone. The trick is to dip the ends of the braided section in to molten candlewax.

After a few seconds when the wax solidifies the braid can be cut to length; the wax will burn off once the wick is installed. Almost any combustible liquid will burn in these little lamps. Methyl alcohol (Methanol, Methyl Hydrate, sold as fuel for fondue sets amongst other things) works well and is reasonably safe; just do not get it near your mouth since it is very toxic.

refuelling toolsTwo squeezy bottles or syringes will be useful; one to contain fuel and the other for priming the boiler with water. To start your finished boat first fill the lamp and trim the wick to length. Set the lamp aside unlit and hold the boat so that the open ends of the pipes from the motor are facing upwards. Gently pour water into one pipe until it flows out of the second pipe. Tilt the boat around to be sure there are no air bubbles inside the boiler. Place a finger over the ends of the pipes to prevent the water running out and set the boat in the bath; the ends of the pipes must stay under the water surface.

Place the lamp into position and light it. After a minute or so you may see a few bubbles escape from the ends of the pipes and shortly after that the motor will start its cycle and your boat will be on its way with a 'pop pop pop'.

finished boat

Derek Waters