DON'T
go all
ROUND
the
HOUSES
Given how vital shotlines are, especially in Britain, why are they so often next to useless? John Bantin shows how to take the direct route to and from a dive site
There is one piece of diving equipment that is not sold in dive shops, nor is it ever advertised in the page of Diver, yet it is almost essential on nearly every dive we make in home waters.
A shotline provides a marker for the dive site and indicates the most direct route between site and surface. In water that is moving, and that includes almost all of our coastal seas, a correctly placed shotline is vital to the safety and success of any dive on a stationary site like a wreck.
So what constitutes such an item? In its crudest form, it's a length of line with a heavy weight at one end and a float on the other. And that's what most divers use - a shotline in its crudest form.
How many times have you descended a shotline so over-long it's running almost horizontally through the water? It doesn't mark the most direct route to the site.
How often have you ascended a shotline to find the float crushed by water pressure a long way from the surface, because the shot fell off the wreck or reef? That doesn't tell the cox'n of the boat where you'll be surfacing.
A good shotline should be around the same length as the depth of water in which you will be diving. It should stay where it's dropped, and the float should never disappear beneath the surface.
We come across bright ideas in books, like over-length lines passed through pulleys under floats and the excess taken up by counter-balancing weights. That sort of thing is wonderful when used in the bath, less so in our seas.
Why? Because the current will simply pull the excess line through the pulley and add it to the line deployed between the float and the main weight. You end up with the familiar horizontal shotline that means a long haul between the surface and the bottom.
Yet there is a way to get the line as vertical as possible, that will not cause the float to submerge, will stay where it's dropped, and is easy to deploy.
Most club boats will put divers into a variety of depths, from 10 to 50m. To make it easier to start off with about the right length of line, it's a good idea to have a series of lengths (say of 10m each) with an eye and a karibiner permanently spliced into either end.
Two eyes make it easier to handle the rope but require shackles to join the lengths together, and these can be tricky to assemble in a rolling, crowded RIB.
The line should be of a thickness that is both visible and comfortable to hold onto. Splash out for terylene and enjoy the difference between that and rough polypropylene.
Instead of using a fixed weight, consider the advantages of a suitable length of heavy galvanised steel chain.
You will probably need at least 5m in British waters, with a 2.5cm link. The chain provides the weight and acts as a shock-absorber as the line and float is lifted by passing waves.
It will also take into account the changing height of the water as the tide rises or falls.
If you have a length of line of, say, 5m permanently attached to the chain, this will give flexibility in assembling a length appropriate to the depth of water.
It's no good asking your fellow-divers not to pull themselves down the line. The best of them will swim down using their controlled negative buoyancy and running the line through their hands as a guide, but others are bound to haul themselves down and thereby pull the shot from its position. The weight of the chain might be enough to allow for one diver doing this, but not for several.
To ensure that the shotline remains in one place, a folding grapnel anchor of the type commonly used by small boats in areas where a sand anchor will not hold is ideal. Folded, it acts as a simple weight. Unfolded, it will catch.
The last diver up (using a system of simple duck-tape tags so that the last one knows his role) can fold the anchor and it will still stay in the same place long enough for the diver to ascend and do any stops. The boat will stay with the buoy even if the weight does move off the site.
Under water, divers can hook the end of their bottom winder lines to the chain and head off on the dive, secure in the knowledge that they will, like Theseus in Greek mythology, find their way back out of the underwater labyrinth and safely to the surface.
The buoy at the surface must be adequate to support the line but not much of the chain. If the heavy end of the shotline falls off the wreck (or reef), the float will be pulled under and the cox'n will lose it.
A second, bigger buoy is therefore added to the first buoy using a couple of metres of line, so that it will support the whole paraphernalia should the first one be pulled under. The cox'n will still see what has become a lazy-shot supported by the second bigger buoy, and the disappearance of the first buoy will alert him to what's happened.
Two buoys also give the added advantage of indicating instantly the direction of any current, and any change in that direction, which is essential information for those in the boat. They can approach up-current of it and know in which direction to look for divers who might deploy independent late-deployment surface marker buoys.
When it comes to deploying a shotline for a number of divers who plan to do long decompression stops, it is necessary to upgrade the weight on the bottom and the size of the float accordingly. A number of divers cannot comfortably all hold on to a line at the same point, and even if they spur off by means of a Jon line, this can lead to an awful lot of underwater knitting.
A Jon line is useful for decompressing in the shallows when there is a sizeable surge. It's a length of line that can be spring-clipped to the shotline so that the diver can control his depth, even when the shotline buoy is dancing under the influence of the waves. It too works like a shock-absorber.
Planned technical dives will use a trapeze. This is a rigid bar supported on the line at decompression-stop depth, supported by the primary buoy at one end and an additional line and buoy at the other. The buoys will be adequate to support a number of divers who might make themselves comfortably negatively buoyant, plus a cylinder of deco gas with its regulator on the end of a long hose for those who might need it.
The forces at work in a current when a number of divers are holding on to the trapeze are quite great, and the weight at the bottom should account for that.
It will need careful deployment in the water if it is not to become a cat's cradle, and a winch might be needed to get it back on board the boat. Recently surfaced divers should never get involved in hauling up the weight, as this activity could precipitate a bend.
Appeared in DIVER - May 2000
