BASIC NOTIONS

Depth

The most important piece of information you need to consider when sailing is depth. If you run aground, then knowing that you are 250 metres from the layline or that you should be attaining a VMG of 5.3kts is pretty irrelevant, so a good depth sounder should be one of your first considerations on a sail boat.

Depth sounders are very reliable and rarely give false readings, however, given their importance to the safety of the boat and crew, it is sensible to give them a quick ”sanity check” every now and then. Important things to take note of are:

• Is the depth reading changing?
• Does the depth reading look accurate?

It is important to note that in water deeper han 100m a standard depth sensor will usually stop finding the bottom. Most people simply change their instrument display to a different page long before this point. Depth sounders have a secondary navigational application, they can be used to warn of inaccurate positioning from a GPS unit. If your chartplotter spotsounding shows that you should be in 30m of water, but your depth sensor says you are in 10m then you should be on high alert – either you have a 20m tidal range, the chart is inaccurate or the GPS position is inaccurate, any of which should be cause for concern!

Modern GPS units can be incredibly accurate, but they aren’t infallible – as some sailors have found to their cost. The “disclaimer” warnings that all devices show in the modern age are there for a reason and it is not just to cover liability of the manufacturer. Over-reliance on navigational aids is not good practice and can land you in serious difficulties – your eyes and common sense are the most important navigational tools you will ever have.

Heading

So you know how deep the water is – now you need to know where you are heading. Whether we are using Heading as a reference for the wind direction, or for dead reckoning our way across a channel, bay or merely in an area of low visibility, it is a crucial tool. At its very simplest if you sail South in the morning, then get shrouded in fog, you know that home is to the North and can navigate with this basic information

Boat Speed (Log)

If you know your heading, the boat’s speed and a time reference, you can use dead reckoning to calculate your position quite accurately. This is the method, along with celestial navigation, by which early ocean travellers found their routes.

The Speed Log is now generally referred to as Boat Speed, with the word Log now being used for miles travelled (like odometers in cars). Accurate boat speed is, and has always been, key to accurate navigation and performance measurement.

True Wind

True Wind is the wind experienced by a person or object when stationary relative to the surface of the water.

Apparent Wind

Apparent Wind is the wind experienced by a person or object whilst in motion, if you are stood still on a windless day, you will feel no breeze. If however you then get on a bike and ride, you will feel wind rushing past you – this is the effect of Apparent Wind. The Apparent Wind is not only different in speed to the True Wind, the angle will also be different.

B&G electronics onboard a Beneteau Oceanis 41 in South Dartmouth, Massachusetts.

And this is our tip of the week, but don’t miss the next chapter where we will deepen into the “Wind” topic. See you soon!

⇒ Sailing: Wind