Summer STEM Challenge: Solar Spray Schooners

It’s a sunny day at the seaside. A high-powered jet ski whizzes across the sea, a jet of water shooting high into the air behind it, pushing it along. WRONG! That jet of water is NOT what makes jet skis go. They need a lot more power than that little spray. (Underneath they have a high-speed prop or a much more powerful underwater water jet.)

That small jet of water – the ‘visibility spout’ or ‘rooster tail’ is mostly just to look cool, although it makes the jet ski more visible, and useful for a small craft. But you can actually power a boat – or jet ski – with a big enough spray of water into the air. Here’s how…

You’ll need a boat hull 40cm or more long, the longer the better, something that can be loaded up with 1,000g without water above its Plimsoll line. You’ll also need a solar fountain – the type with a separate solar panel is best so that you can balance up the boat while putting the water spray near the stern.

Rather than adapting a model boat hull, you could make your own schooner, maybe a catamaran, using plastic downpipes and downpipe ‘shoes’. Some ballast at the bottom of the hull will help avoid a capsize if the boat ends up top-heavy. A keel and/or rudder on the bottom will help your Solar Spray Schooner boat hold a steady course.

The solar panel should be horizontal, while the fountain should face backwards and upwards at 30° or so for high speed, maybe 45° or more for a more spectacular fountain but a slower voyage.

Everything installed? Time to test your Solar Spray Schooner in a pool. If it doesn’t work when switched on, check that the water sensor is getting wet. Many solar-powered fountains have a water sensor – it often looks like a little pad or rod of metal. It switches the pump off when there’s no water.

Try different numbers and sizes of water jets. If the pump is restricted on its outflow by having a very tiny nozzle, the boat will go more slowly, despite having a slightly higher-speed jet. Similarly, widening the nozzle a lot, allowing the pump to ‘max out’ on flow, but at very low pressure and thus low speed, may also restrict the speed reached. With a 5W pump, we found that a single 3.5mm nozzle was better than 2.5mm and 5.5mm nozzles. Alternatively, bunch of smaller jets worked well too.

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

You may notice that the fountain trajectory is not quite the parabola you expect from simple physics of trajectories in a vacuum, but falls short on the far end of the path through the air – that’s all down to air drag, of course:

What about trying out a longer, thinner hull? A longer hull will give a higher ‘hull speed’. Boat power required rises rapidly beyond the hull speed. Why? Because ordinary boats, when they go fast, create a high bow wave by pushing water out of the way and a low stern wave by letting the water back into the hole they just made. They have to climb a ‘hill’ between those waves. If they go slower, then the water from the bow can fill in the hole at the stern – that’s what the waves are doing – and that saves power.

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

What about controlling your Solar Spray Schooner? You could 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 with a Hall Effect device for reading the Earth’s magnetic field to change the position of the rudder servo, depending upon the deviation from the selected course. If the sun keeps shining, your Solar Spray Schooner could sail for miles!

Easier than magnetic control, though, is to use the sun for steering. The angle of the sun to North doesn’t vary too much over an hour or two. You could use a light-dependent resistor to change the pulse length of a 555 oscillator and activate a servo rudder.

Model control servos need a pulse every 20 milliseconds of between 1 and 2 milliseconds, with 1.5 milliseconds giving the central position. With a little microprocessor like a Microbit, which has a light sensor, you could program in the sun’s movement through the sky and get a true compass course.

If you liked this, you will find lots more fun science stuff in Neil Downie’s books, like ‘The Ultimate Book of Saturday Science’ from Princeton University, and for lots of other things (and a free copy of the ‘Exploding Disk Cannons’ book), visit www.saturdayscience.org. In line with this experiment, Neil’s current work includes developing a new ventilator system to support people with breathing difficulties – get more information on this great project at Exovent.org.

There is a back catalogue of STEM-related challenges from the past year to choose from if you are looking for more options. The IET also has a host of resources that adults can use to engage children with the world of STEM.