Summer STEM Challenge: Solar Spray Schooners

In our latest STEM challenge for summer 2022, Neil Downie walks us through how to make a boat powered by a solar panel.

It’s a sunny day by the sea. A super-powerful jet ski speeds across the sea, a jet of water spurting high in the air behind it, pushing it. WRONG! This water spray is NOT what makes jet skis work. They need a lot more power than this little spray. (Underneath they have a high-speed propeller or a much more powerful underwater water jet.)

This little water jet – the “visibility beak” or “rooster tail” is mainly meant to look cool, although it does make the jet ski more visible and useful for a small craft. But you can actually propel a boat – or a jet ski – with a big enough jet of water through the air. Here’s how…

You will need a boat hull 40cm or longer, the longer the better, something that can be loaded 1000g without water above its Plimsoll line. You will also need a solar fountain – the type with a separate solar panel is best so you can balance the boat while putting the water jet near the stern.

Image credit: Neil Downie

Rather than adapting a model ship hull, you can make your own schooner, perhaps a catamaran, using plastic downpipes and downspout “shoes”. A little ballast on the bottom of the hull will help prevent capsizing if the boat ends up being too heavy. A keel and/or rudder on the bottom will help your Solar Spray Schooner boat maintain a steady course.

The solar panel should be horizontal, while the fountain should be angled back and up at about 30° for high speed, maybe 45° or more for a more dramatic fountain but slower travel.


Solar spray schooners on a bench

Image credit: Neil Downie

Everything installed? It’s time to test your Solar Spray Schooner in a swimming pool. If it does not work when turned on, check that the water sensor is wet. Many solar-powered fountains have a water sensor – this often looks like a small pad or metal rod. It turns off the pump when there is no water.

Experiment with different numbers and sizes of water jets. If the pump is limited on its flow by having a very small nozzle, the boat will go slower, despite having a slightly faster jet. Similarly, widening the nozzle a lot, allowing the pump to “max flow”, but at very low pressure and therefore low speed, can also limit the speed achieved. With a 5W pump, we found that a single 3.5mm nozzle performed better than the 2.5mm and 5.5mm nozzles. Alternatively, a bunch of smaller jets worked well too.


Solar Spray Schooners in action in a paddling pool

Image credit: Neil Downie

The best angle for the water jet from the boat’s speed point is horizontal, but it’s much more dramatic to direct it upwards at an angle. The thrust at the angle α is given by cos(α). So an angle of, say, 30° upwards gives you 87% of the thrust you get with a horizontal jet.

You may notice that the path of the fountain isn’t quite the parabola you’d expect from simple physics of trajectories in a vacuum, but it falls short of the end of the path in air – this is all due to air drag, of course:


Graph showing the effect of drag on the trajectory of the fountain

Image credit: Neil Downie

How about trying a longer, thinner case? A longer hull will give a higher “hull speed”. The required boat power increases rapidly above the hull speed. Why? Because ordinary boats, when going fast, create a high bow wave by pushing the water out and a low stern wave by letting the water back into the hole they just made. They must climb a “hill” between these waves. If they go slower, the water from the bow can fill the hole at the stern – that’s what the waves do – and that saves energy.

Now the speed of waves in water is proportional to the square root of the wavelength, just like the speed of the hull. At 40 cm, the hull velocity is 0.8 ms-1. To double that, you’ll need a giant 1.6m model!

What about controlling your Solar Spray Schooner? You can add a radio-controlled servo rudder, like a boating pond RC boat. An even more interesting challenge would be a schooner that could follow a compass course.

This, however, would need to be connected to a Hall effect device to read the earth’s magnetic field to change the position of the rudder servo, depending on the deviation from the selected heading. If the sun keeps shining, your Solar Spray Schooner could sail for miles!

However, it is easier than magnetic control to use the sun for direction. The sun’s angle to the north doesn’t vary too much over an hour or two. You can use a light dependent resistor to change the pulse length of a 555 oscillator and activate a servo rudder.

The model control servos need a pulse every 20 milliseconds between 1 and 2 milliseconds, 1.5 milliseconds giving the center position. With a small microprocessor like a Microbit, which has a light sensor, you can program the movement of the sun across the sky and get a true compass heading.

If you liked this, you’ll find lots more fun science stuff in Neil Downie’s books, like Princeton University’s “The Ultimate Book of Saturday Science”, and lots more (and a free copy of the book “Exploding Disk Cannons”), visit www.saturdayscience.org. In line with this experience, Neil’s current work includes the development of a new ventilation system to help people with breathing difficulties – find out more about this great project at Exovent.org.

There’s a catalog of STEM-related challenges from the past year to choose from if you’re looking for more options. The IET also has a wealth of resources that adults can use to engage children in the world of STEM.

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