Scooter tutor

I am thinking of buying an underwater scooter. Can you give me any information on these devices?
Henry Fieldwick

Diver Propulsion Vehicles (DPVs), sometimes called scooters, are a useful tool to cover a very wide area under water. I have used them many times to explore wrecks and itÕs amazing how far one can travel compared with swimming divers. It also makes a great difference to your gas consumption rate. Because you are not exercising, your breathing rate reduces dramatically.
There are two types of DPV. One is a smaller unit onto which the diver holds while being pulled along. The other is larger and looks a little like a torpedo - the diver sits on it. The main difference between versions of each type is in depth rating. Some of the smaller recreational units have depth restrictions of 30m, but the more robust technical DPVs work in the 100m range.
All come with various options, lights, different electric motor sizes (which affects the speed), sizes of batteries and controls. There is even a two-man DPV. You must decide what you want to do with it or where you want to take it. The DPV is a great diving tool, but whichever you choose, please take it into shallow water first while you learn how to use it properly.

Caption: Above: the smaller of the two types of underwater scooter, or DPV. The diver holds onto the unit while being pulled along.

Pros and cons of Hogarth
In your reply about the 2m hose (Where To Put My Hose?, October 1999), you say you donÕt use the Hogarthian method because your second regulator is stowed around your neck on a bungee system and the long hose would sit over it, making access difficult. The way I see it, the long hose is the regulator you donate to your buddy should he need it. In that event, using the Hogarthian method, all you have to do is dip your head, donate your buddy the long hose, and put your bungeed second regulator in your mouth. Where is the problem?
Adri Haine

I have no problem with the Hogarthian system and dive with many people who use it. I am also pleased to see you advocate passing off the primary regulator to a buddy in an emergency, something for which I have been criticised in the past.
My point is that my secondary regulator sits very close under my chin, and any hose crossing it would obstruct me in getting to it, although this is only a slight problem.
I donÕt plan to have too many emergencies (I hope) and stowing the hose alongside the cylinder keeps it tidy, avoids anything passing across the front of my chest (which I like to keep clear) and, at the same time, is very easy to pull out if I have passed my regulator to a buddy.
I am a firm believer that the kit and configuration must suit the individual diver. Give the stowing system a try.

Bands apart
I am setting up a twinset with 12 litre cylinders. I have stainless-steel twinning bands but I am not sure how far apart they should be set on the cylinders. Can you advise me?
Neil Frank

The distance between the studs on the top cylinder band and the bottom one should be 27.5cm. This is a standard setting and is the distance between the holes on a backplate that fits onto the cylinder bands.
The holes in the wing also have the same measurement, which complies with manufacturersÕ equipment. This universal size came about by trial and error.
The distance of 27.5cm ensures a solid fixing for a backplate and, at the same time, allows various heights of cylinder to be used. For example, twin 12 litre cylinders are generally taller than twin 10 litre units.

Gas when you need it most
What methods do you recommend for deploying spare decompression gas for divers in case of an emergency or equipment failure?
Claire Smith-McAlister

It is very important that each diver plans the gas required for every phase of a dive, including decompression. It is not unknown for a diver to lose a decompression gas for some reason - usually equipment failure. I use a system by which all divers return to a decompression trapeze, where redundant decompression tanks are mounted with two second stages on each cylinder. The gas is not for the diversÕ planned decompression, but can be used in an emergency.
We also carry spare decompression gas on the boat attached to a 15m line and large buoy. If the diver could not make it back to the shotline and decompression trapeze, he would deploy a delayed surface marker buoy. If this diver required extra decompression gas, the spare gas on the buoy could be lowered over the side to him.
There are various systems that can be used, but those I have described work well. It is always worthwhile planning redundancy. You might never use it, but itÕs a comforting feeling knowing that itÕs there!

Penetrating question
I am interested in exploring inside wrecks, but I realise that there are potential dangers. Are there any special techniques to navigate inside them safely?
T Prosser

The most common method used to enter and exit a wreck is line-laying. Most divers think this is just reeling in and out of the wreck, but various procedures have to be observed when belaying a line inside a wreck.
These include tying off the line at various points, keeping it under slight tension (not having loose line floating around to snag divers) and using only good-quality thick line (not the standard SMB variety), which is less likely to break. It is also important to use a system when tying line off that indicates the direction of lay, entry or exit from the wreck.
Other methods to assist wreck penetration include lights placed along the route, depending on conditions.

Front, back or sides?
I have started to carry out longer decompression dives and require more gas. Which is the best way to carry my deco cylinders?
Nick Thomas

The answer to your question is - whatever you feel comfortable with. There are a number of options available, including cylinders mounted on your back, side-slung or front-mounted.
The back-mounted system offers the advantage of keeping your chest-area free from clutter, but adds more bulk and weight to your back. This is not a problem when you are in water, but can be at other times.
Side-mounted deco cylinders are the most popular and are easy to get to while on the dive. It depends on the size of the cylinders, but they can get a little cumbersome while you are swimming and do cause drag under water.
Front-mounting makes access very simple, though it does cause clutter on the diverÕs chest (the triangle of access), making it more difficult to get at back-up equipment.
I would suggest that you try out all three options in shallow water and decide which suits you best.

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Jack Ingle is the BSAC's Technical Diving Adviser. He is a BSAC National Instructor, an IANTD Technical Instructor, a TDI Tri-mix instructor and co-author of NSAC nitrox courses.

Keep your queries coming Submit them, marked "Technical Questions": by letter, addressed to Diver; by fax on 0208 943 4312; by e-mail to editorial@divermag.co.uk; or on Divernet's Technical Q&A page. This last can also bring rapid responses from other readers (though these should of course be treated with caution). We regret that questions cannot be answered on the telephone or, generally speaking, replied to individually.

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Appeared in DIVER - January 2000