Archive for November, 2006

Streamlined Equipment

Published at November 17, 2006 in Articles. Closed

Nobody ever saw a fish swim on an angle in the water, or an aeroplane fly at a 45 degree angle. The reason is obvious – flying or swimming like this makes us use more energy to move from one place to another.

As diver, we want to enjoy our time in the underwater world, so it makes sense to make our movement as easy as possible, so that we don’t get worn out or tired before time. Remember that the harder we work, the more we breathe as well, so a good streamlined diver is going to be able to spend more time diving than one that is always fighting against their body position and equipment. If we have a long swim to do it becomes even more important to be flat and efficient, so that we can achieve our goals.

The equipment we choose, and how we arrange it, has a big effect on our streamlining in the water. There are basically two ways of improving a divers streamlining. The first is to maintain a flat, horizontal, position in the water. The second is to think about how the equipment is placed on the body, and how it will add to our drag. We’re going to concentrate on the second aspect, equipment placement.

A horizontal divers head and shoulders will be breaking the water first, as they swim forward. Consequently, we want to place as much of the extra equipment that we can effectively in the area of water that is in the “shadow” of the head and shoulders. If we can “hide” the equipment there, then it won’t add much to the drag. Obviously, a cylinder can’t really go under your arm, so that has to go on the back of the diver. Let’s look at the rest of the equipment.

Streamlined diver

Buoyancy compensator
A backplate and wing type of BC has very little extra volume; the wing itself sits snugly next to the cylinder. There is no extra “clutter” in the chest area, and the harness straps are smooth to the body.

Lights
The canister type lights have a small light head, held in the hand. The battery packs can be quite bulky, so we place this on the waistband of the BC. It’s pushed back against the backplate, and it consequently sits under the arm, where it isn’t in the flow of water, and consequently doesn’t add to the drag.

The backup lights are clipped to the harness, and snug up under the arm as well, so you don’t even notice they’re there. Even though they’re small, if they dangle around off the body, they will contribute to the overall drag.

Hoses
Hose routing can add a surprising amount of drag to a diver. If you don’t believe it, fill a bathtub, and with a loop of low pressure hose in your hand, feel the amount of drag produced when you pull it through the water. So, what we do is to route the hoses such that they contribute as little as possible to the drag.

This starts by choosing a regulator that allows the hoses to be routed downwards, parallel and close to the cylinder. Next, we try to keep the hoses as short as possible (without compromising their utility). The high pressure hose needs to be around 56-61cm long, depending on the first stages and whether you’re using single or twin cylinders. The back-up regulator is frequently too long on many divers equipment, and it tends to extend over the right shoulder. It should come over the right shoulder, close to the neck while allowing full movement of the head when in use. A 56-61cm length hose, depending on the first stage and cylinder configuration, is usually about right.

Pockets
All of the rest of the equipment we use on a dive needs to be carried, so we need a place to store it all. An instructor of mine once asked me if I had any trousers without pockets in them. I thought I was being clever when I said that I did, so he then asked me if I ever wore them. To which the answer was, of course, no. Trousers without pockets are pretty much useless – you have nowhere to keep your stuff. OK, I hear the women now saying that that they have trousers without pockets, but you have handbags. And handbags don’t like getting wet!

So, our diving suit, be it dry or wet, needs to have pockets too keep or things in. These are best placed on the sides of the thigh. They need to be high enough so that we can get to the bottom of them without bending double. They go on the side to keep out of the flow and not act like water scoops when we swim, or drag if we’re close to the bottom.

I tend to keep all sorts of things in pockets – DSMBs, spools, wetnotes, line arrows, shears, spare double enders and anything else I might think I need on a dive. It doesn’t matter how full they are, it doesn’t add too the drag. Compare that with clipping everything to the harness – it would be very messy. Put some loops of bungee cord in the pockets, and clip everything to the loops. That way, if you can’t find something, you can empty the whole pocket, and select what you want, without risking losing anything.

Streamlined equipment configuration

Summary
Basically, try and keep all of your equipment as tidy as possible. Keep hoses short, and running down the length of the body as much as possible. Try to keep the chest area free. Try not to let equipment dangle down where it will catch on things. Use your pockets, and practice pulling things in and out of them. Remember that if something has to be carried, try and place it where it won’t add to the overall drag.

Balanced equipment - what does it mean?

Published at November 15, 2006 in Articles. Closed

I frequently get asked how much weight a diver will need for diving a particular equipment configuration. It seems like an easy question, but not many people are aware of how to select equipment, and then how to choose the correct weighting for that configuration. Most people will be familiar with the concept of carrying enough weight to hold a short stop at 6m, with very little gas in their cylinders, and with an empty buoyancy compensator. This allows a diver to hold a decompression stop in the event of having run out of gas. It essentially identifies the minimum amount of weight needed to conduct the dive, and is tuned to the end of the dive. This is an important aspect of the weighting process.

grace-90ft_0157.jpgHowever, there is an often-overlooked aspect of weighting, and that is weighting at the start of the dive. At this point, we are at our heaviest, as we are carrying gas in our cylinders, and we are dependent on the buoyancy compensator to achieve neutral buoyancy. Should this now fail (elbow coming away from the bladder, or the inflator unit malfunctioning) we run the risk of becoming negatively buoyant. The first thing that we should do is to ensure that our BC has sufficient lift to maintain neutral buoyancy at the start of the dive. It really doesn’t need to be much bigger than that, other than to maintain decent floatation if you hang around on the surface in a swell after a dive.

It should be fairly easy to see that failures at the start of a dive can be just as concerning as running out of gas at the end of the dive. We need to be very careful in our equipment choices to avoid placing ourselves in a dangerous situation. For example, very heavy cylinders such as a set of 300Bar doubles, full of gas, would cause us a lot of trouble should we experience a wing failure. A lighter set of doubles, such as aluminium 80’s would be much more buoyant and possibly a better choice.

rhea-turtle_0086.jpgAnother consideration is the use of wetsuits. These require a large amount of weight to sink them at the surface, but once at depth, they compress, and loose their buoyancy. Consequently, the weight we needed at the surface is now effectively “dead weight” at depth. Again, the failure of a wing may leave us no option but to ditch some of this weight. This may then make the ascent less controlled than we would otherwise like.

The combination of heavy steel cylinders and wetsuits start to become a very unattractive option.

Up until this point, the discussion has been rather subjective. I’d like now to try and be a bit more scientific about the whole thing. We can start to do some rough calculations to see how different equipment configurations would affect our buoyancy in the water at both the start and end of the dive. They way we do this is to categorise each piece of our equipment (and gas supply) as either positively buoyant, or negatively buoyant.

Negatively buoyant Positively buoyant
Steel cylinders Wet / dry suit
Backplate Wing gas
Breathing gas
Weightbelt

We need to make sure that at all points in the dive the negatively buoyant components are balanced by the positive ones. This concept is sometimes referred to as a “balanced equipment configuration”. So the next step is to start applying some real numbers to the table above to see what happens. We’ll stick with a wetsuit example for the time being.

Negative equipment Buoyancy Positive equipment Buoyancy
Steel 12L cylinder 2kg 5mm wetsuit 12kg
Backplate 3kg Wing gas 15kg - see working
Breathing gas 3kg
Weightbelt 7kg - see working

We now need to calculate how much weight we should carry. Remember that this is done with no gas in the wing, and an empty cylinder. The Total negative buoyancy comes to 2+3=5kg, and the total positive buoyancy is 12kg.

So we need to carry 12-5=7kg on the weightbelt.

At the start of the dive, we need to offset all of the negative buoyancy (15kg). 12kg of this comes from the suit, and the remaining 3kg from the wing. This will increase at depth, to a maximum of 15kg (assuming wetsuit is compressed to nothing). So 15kg lift wing is all that we need.

OK, now we move on to looking at what happens if the wing fails at the start of the dive. On the negative side, we have: 2+3+3+7= 15kg, and 12kg of positive buoyancy.

That makes us 3kg negative, which should be possible to handle, dropping the weightbelt if necessary. Now, if this occurs at depth (say 30m), when the suit is compressed to 1/4 of its thickness, we can re-do the calculations.

Negative equipment Buoyancy Positive equipment Buoyancy
Steel 12L cylinder 2kg 5mm wetsuit 3kg
Backplate 3kg Wing gas 0kg
Breathing gas 3kg
Weightbelt 7kg

Suddenly we find ourselves 15kg, negative, and only 3kg positive, a discrepancy of 12kg. This is going to be very difficult to manage at depth, and highlights the need to choose equipment with care. We can ditch the weightbelt to make us 6kg negative, but this would still be a strenuous swimming ascent. A solution could be to use an aluminium cylinder, which has a positive buoyancy of around 1kg.

This would make the negative buoyancy 3+3 = 6kg (weightbelt dropped), and the positive 1+3 = 4kg, leaving us with 2kg negative buoyancy. While not being pleasant, this should be manageable by a competent diver.

This example highlights the dangers of using thick wetuits with steel cylinders, and the working through shows you how to go about “balancing” the configuration. Try playing with the numbers and see what you get. For example, try working out what would happen if using a set of double 12L cylinders instead of a single.

You should find that it becomes increasingly difficult to manage a full wing failure as the amount of gas you carry increases. For this reason, dives utilising a drysuit for these kinds of dives is recommended. Firstly, as the dives need more gas, you are likely to be spending longer in the water, so thermal considerations begin to dictate drysuits. Secondly, a good tri-laminate drysuit will not change its buoyancy characteristics with depth, so the problems associated with wetsuit compression are eliminated. Finally, the drysuit can act as a secondary source of buoyancy in the event that you do have a wing failure at depth, and this will allow you to manage your ascent.

Lets just run that through the planning table again:

Negative equipment Buoyancy Positive equipment Buoyancy
Steel 12L cylinder 2kg Drysuit & undersuit 8kg
Backplate 3kg Wing gas 3kg
Breathing gas 3kg
Weightbelt 3kg

Look at the difference in the wing gas requirements - because the suit doesn’t compress at depth, the large 15kg required for the heavy suit is reduced to 3kg. You’d probably still elect to use a larger wing, purely for surface buoyancy though.

So, from this, you should be able to see the importance of correct equipment selection. Don’t go changing bits of your equipment, particularly ones that are substantially negative or positively buoyant without first considering the effect of the balance of your overall equipment configuration. Likewise, when purchasing equipment, make sure that it will work well with the equipment you currently have, and that you are not going to compromise the overall performance of your gear.

If you’re unsure about how any of this works in a real-world example, then feel free to get in touch, and I’ll try and work through the problem with you.

How to tie the perfect boltsnap

Published at November 11, 2006 in Articles. Closed

I often find myself showing people how to tie boltsnaps to backup lights and hoses, so I thought I’d try and write a description of how I do this. There are a number of ways of doing it, but this is the one I use – it works just fine.

This is the thing were trying to produce. It needs to be nice and tight so that it doesn’t slide on the hose, and help the boltsnap “stand up” to make it easier to clip onto the harness d-rings. The same knot is used on backup lights, but it doesn’t have to be quite as tight.

So, you will need the following things:
1. 60cm of cave line
2. A boltsnap (!)
3. The hose or light
4. A pair of cutters or a sharp blade
5. A lighter

Start by putting three turns through the boltsnap and the hose. Then pull the loops tight.
Next we want to start the knot. We’re going to put a turn around the loops which will apply tension to them, and lock the boltsnap in place. The first step is to cross the two loose ends. Pass the end in the right hand over the end in the left so that it looks like this.

Next, we’re going to tie a knot on the other side to lock the knot. Flip the boltsnap over from the right hand side in the image above to the right hand side. Pull the two loose ends of line over to the right hand side.

The next step is to tie a reef knot to secure the knot. Start by pulling the loose ends to tension the turns. It takes a bit of practice to get this set right, but essentially you want the turns to be pinching together behind the eye of the boltsnap, but not touching.

Tie the first half of the reef knot and it should look like this.You can make fine adjustments to the tension at this stage.

The next step is to tie the other half of the reef knot. Remember that for a reef knot, it ties “right over left, then left over right” otherwise it’s a granny knot and won’t be stable.When the knot is pulled tight, the reef knot should look square and symmetrical.

To finish, melt the knot so that it can’t come undone. Be careful not to melt through the line. Melt the ends of the line, and mushroom the ends with the end of the lighter to flatten them out.
The final thing should look like this.
Job done.