Sailing instruments are hard to get right. They’ve got to be accurate, durable and easy to read in all kinds of conditions; from blazing sun to seething tempest. Even on a good day, nothing about measuring a heaving mass of water and gusting wind is straightforward.
All of the components used in a sailing instrument installation need to be compatible with each other, too – so it’s a good thing that all parts from A+T Instruments are compatible with B&G products, and can directly replace them in a refit.
Anyway – while there’s no substitute for good sailing experience and knowledge of the sea, electronic instruments are essential in larger vessels. You might be able to sail a small boat or dinghy with just your senses to guide you, but on a larger craft, sailing instruments extend your senses and intuition, allowing you to “feel” the environment beyond your reach. This
allows sailors to make better decisions at sea and react faster.
In this article, we’ll cover the fundamentals of sailing electronics – like using a sailboat wind sensor and depth sensor – for a barebones, from-scratch sailing instrument installation. We’ll give you the basics of what’s needed and why, going deeper (no pun intended) into some of the more important sensors for sailing.
1. Depth sensor
Above all other sensors, depth is the most important. Running aground is a dangerous mistake, even if it’s just a keel strike, so knowing the depth is vital.
A depth sensor works by emitting pulses of ultrasonic sound, in a range about 10 times higher than the highest-pitched sound humans can detect.
The sound travels through water, and then bounces off the seafloor as an “echo”. The echo is then collected by the depth sensor, which times the difference between the pulse going out and the echo being received. Then, by calculating the speed of sound in seawater (around 1,500 metres per second) against the time it took to hear the echo, the depth sensor provides a depth reading.
They’re very accurate, and give a depth reading up to around 100m or so – more than enough to prevent running aground.
2. Heading sensors: compass and GPS
GPS is standard equipment on even the smallest of seafaring vessels – along with compasses. And the humble compass is a mainstay of maritime instruments for good reason. It’s a good idea to have analogue and digital compasses installed on a sailboat, and to be able to read both.
A digital compass can be fed into a main display for getting quick overviews at a glance, and can provide greater accuracy. But in the event of a power failure and poor visibility, an analogue compass might be the only reference point you have to guide you home.
3. Speed sensor
The speed of a craft on water is deceptive, thanks to fluid dynamics. Even an accurate measure of speed can still be misleading, depending on where you measure from – it’s all relative. But it’s important to get right, because it’s one of the main ingredients to calculate position by dead reckoning (plotting an object based on heading, speed and time).
Speed through water (STW) and speed over ground (SOG) are the methods by which boat speed can be measured. SOG is relatively easy to get with GPS – but STW is variable. For one, currents can be moving with or against the vessel, affecting the measured speed through water. On top of that, the vessel itself creates a flow field as it moves, which can
affect the speed of the water around it.
The effect of the flow field can be mitigated by positioning the sailing speed sensor as close to the centre of the hull as possible, while being as far from any turbulence-inducing surfaces as possible.
There’s a lot to consider. So, why not just use SOG? Well, SOG is helpful for navigation between fixed points – but currents will affect the STW. This will profoundly affect your navigation and the proper calculation of wind if it’s not accurate.
4. Wind sensors
Okay – finally, we get to wind. Every sailor can use their senses to tell where the wind is blowing from, and use it to move, navigate, and calculate routes. But, like anything in fluid dynamics, the wind is a trickster, and to measure it properly takes some cunning…
True wind
The true wind is what a stationary object on the water experiences. The direction and speed are the same for any stationary object in that vicinity – hence the term true wind.
Apparent wind
The apparent wind is harder to describe, but we’ll give it a go. Imagine it’s a windless, summer’s day and you decide to go for a bike ride. On the way, you zip down a big hill – and even though it’s a windless day, you experience the sensation of wind blowing on your face and arms. That’s because your body cutting through the air has created a sensation of wind
rushing past you. The same thing happens when you’re sailing. The true wind and the apparent wind are different, because of the motion and heading of the vessel.
So, in order to get an accurate reading, true wind direction must be calculated from multiple sensor inputs – but factors such as leeway (sideways “slipping” on the water) and heading all impact the course of the boat.
Wind sensors and wind calculations are a world unto themselves, so we’ll be sure to cover
them in more detail soon.
And that about covers it – these are the four main categories of sensors and instruments that will form the backbone of any sailing instrument installation. Of course, now you’ve got all this data, you need somewhere to process it all and display it.
Processors and instrument displays
Sailing instrument processors are highly specialised, super-rugged computers, made to calculate and crunch all the data as quickly and accurately as possible. They’re like the “brain” of the boat, gathering all the information from the sensors, and making sense of it.
The processed info is then sent to a display, for the sailor to read. Now, sailing electronic displays are a wholly different kettle of fish to your garden variety iPad or mobile phone screen; they have to work reliably more than they have to look pretty. We’ll cover sailing instrument displays in detail soon, in another article – but it’s not uncommon to find seemingly basic monochrome LCD displays or analogue dials on even the most advanced ships and superyachts for this reason.