SMELLING THE BOTTOM

ese ays e a e t e e efit f te y t e s a s e t e tt but things have not always been so easy.

The staycation fuelled recent increase in boat ownership has manifested itself in many ways, but one of the less happy ones was demonstrated for all to see when, on a sunny weekend in the Solent, sailors heading out afloat were passing a string of various boats that had been caught on the many banks and shallows that are such a feature of the area.

Although this prompted some laughter and a few jokes, it is worth taking the sight of a boat high and dry to heart, for like sinning and casting the first stone, there are few sailors out there who can put hand on heart and say that they have ‘never smelt the bottom’.

At the heart of the problem lies the fact that the water can be a deceitful medium, with what appears to be, on the surface, an innocent stretch of water which can hide a very different story beneath the waves.

The wonderful harbours of Poole and Chichester are great examples of this issue; large, open expanses of water, in lovely locations that beckon the unwary sailor in, only to leave them stuck (hopefully on a rising tide). It is a problem that is as old as getting afloat in the first boats that were able to venture to new shores, with the first solution being long, thin rods that could be used to probe the water to establish the depth.

Whilst this worked when very close to the shore or in shallow waters, as shipping developed so did the nature of working out the depth of water under the keel, which in turn would give us many terms and expressions that are still in general use today.

Fathoming the sea bed At the heart of the solution was a simple weight, with lead being the obvious material of choice, connected to a long line. This could be dropped over the side and then the length of the line measured, with the action of recovering the line to the extent of the outstretched arms giving us the old English word of fæom, which itself could have roots in the Viking term of favn: both hold the same meaning, of a pair of outstretched arms or roughly six feet.

By Shakespearian times this had been further Anglicized to fathme, yet even once the language had finally settled on the term fathoms, there was still a good deal of uncertainty about what the depth might actually be, as the measurement could range from five to seven feet.

It would be the British Admiralty that would help define the fathom as six feet in depth, almost as a by-product of the work that had been done in the late 1500s by the Oxford based mathematician Robert Hues, who in 1594 calculated that around the Earth’s equator, one degree would equal 60 miles, which meant that each arcminute would equate to one nautical mile, which in turn, happily came out ‘more or less’ at 1,000 fathoms!

Swinging the lead Just relying on the arm span of the sailor ‘casting the lead’ was clearly going to give some strange results, so the line would be marked at fathom intervals by knots, with cloth or leather ties added every second or third knot, so in shallower waters, an experienced leadsman could quickly give an indication of how deep the water was at that point.

Once the boat was underway, the common practice was that the leadsman would stand in the bow and would start by casting the lead forward, by swinging it. In deeper water he would then have to walk aft, paying out the line as he went, trying to keep the line as vertical as possible.

It is this action that gave us the term ‘swinging the lead’, as this described a lazy sailor who appeared to swing his lead but did not release it – giving rise to the term for someone who is not doing what he should be.

With the addition of a plug of tallow (rendered animal fat) placed on the base of the lead, an experienced sounder could see what was stuck to the lead when he pulled it in, allowing him to report what the seabed comprised of, with this giving another indication of the possible location of the ship. However, with the arrival of steam and faster vessels, a better method for measuring depth was needed, with the Royal Navy investing firstly in Edward Massey’s sounding machine (a mechanised sounder) and then Peter Burt’s ‘Buoy and Nipper’ system, but even when a motorised drum was used to wind in the line, the principles were the same, with a weight being lowered on a calibrated line.

A glorious quiet anchorage in Poole Harbour looks wonderful but is very deceptive. You could walk ashore from here and not get your shorts wet (as the saying goes, the water only went half way up the ducks). Image: David Henshall

The latest in CHIRP - multi-frequency sonar can show the shape of the bottom, fish in the water column, create a 3D image and can even ‘look ahead’... and people will still end up sniffing the bottom! Image: Lowrance

a ate t r a t es ea ya ts a a S a r e er t e ar f ra e sse t e tt s a y t es e seas t at a a s este fitt ee s t s ee

LEFT: Modern depth sounders are wonderfully reliable and tell you exactly what you really need to know - how much water is there under your keel. This is more than sufficient for most sailors. Image: Nasa Marine Echoes of the past Thankfully, the new technologies were arriving at speed and with the increasing pace of new developments in electronics, the search was on for better ways of keeping ships safe. One major driver in this had been the loss of the Titanic in 1912 and just a year later Alexander Behm, a German inventor, applied for a patent for his echo sounding device.

The principle was simple, in that a sound wave was transmitted downwards from the hull of the boat, then the time delay when the echo back from the sea bed was detected was measured, with the difference then being used to calculate the depth of the water.

The idea might sound simple, but the technologies needed to send and receive the signal, then to process it accurately, would put ‘depth sounding’ right at the forefront of the technological revolution at sea.

The demands of the First World War would accelerate research in this field and by 1919 the first practical depth sounders were being tested. By the mid-1920s a US Survey ship had been equipped with one of the new ‘Hayes’ (named after Dr Harvey Hayes) echo sounders and within a few years, the true complexity of the nature of the seabed were finally being revealed, which rather than being flat and uniform as had been previously thought, consisted of deep canyons and mountain ranges that were the equal of anything on land.

Fingers on the pulse From the 1960s onwards the leaps forward in the technology would start to rapidly filter down to recreational applications, but at this point the goal was improving on the existing techniques. The military had long known that whilst a single pulse of energy would give an adequate reading of depth, a more sophisticated pulse transmitted could tell a far more detailed story.

This, too, would start to become available to the leisure boating community and today, most depth sounders employ ‘CHIRP’ technology (for Compressed High-Intensity Radiated Pulse), which instead of a short blast of a single frequency, is a longer pulse that covers a wide spectrum of frequencies. The signal processing that underpins CHIRP is incredible, but the outputs, once displayed on a screen, reveal details as small as individual fish in the water column, along with a 3D representation of the sea bed.

Anglers could now see where their prey was beneath the waves (catching it might be another matter!) and even better enhancements to the system allowed sailors a ‘look forward’ capability, giving them a warning for when they might start to ‘run out of water’. However, despite all this fabulous technology, ‘kissing the bottom’ will remain an ever-present part of going afloat for all, from the unschooled beginner to the best of the best. Back in late Victorian times, leading yachtsman and Squadron member, the Earl of Dunraven, kissed the bottom so many times one season that a wag suggested fitting wheels to his keel.

In the end, though, all the technology in the world will not stop you running aground if you ignore the marks laid out for your guidance and the detail that is there on your chart, but my guess is that… by the time you have read this, some of you will have gotten closer to the All images: Andrew Wisemansea bed than is normally advisable!