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.../tbone's statement about tow cord orientation lacks experience. That mindset comes from a couple of well know cave diving instructors who preach 3 & 9 o'clock tow cords, even shaming the students who could not defend their reasoning for vertical tow cords. Most people that use 3/9 do so because that is what they were taught, never questioned it and developed the muscle memory of it over time, which makes anything different feel awkward. If you dissect the physics of the DPV thrust, it's reaction and pivot points, friction of the tow cord through the bolt snap and size of the muscles in your arm used to control the DPV, vertical tow cords make more sense. With a vertical tow cord, small direction changes can be made with the wrist because the tow cord pivots align vertically with the wrist. You cannot easily use your wrist with a horizontal tow cord, because it would require the tow cord to slide through the bolt snap at your waist, which has friction that increases with thrust. Vertical tow cords benefit from the friction at the waist bolt snap, since you want the thrust to pass under you, which requires the top leg of the tow cord to be shorter than the bottom leg and difference is held in place by that friction. Horizontal tow cords require the diver to constantly pull back on the handle to direct the thrust under themself, because the pivots want to self center and direct thrust at the divers waist. Large direction changes require larger arm movements with either cord orientation, but the upper arm muscles are much stronger and able to overcome the bolt snap friction of a vertical tow cord easier than the wrist muscles are able to overcome the friction of the horizontal tow cord with small direction changes. It's a rather complex force vector study, when thrust and drag are not coaxial.../
You got off the rails in the first sentence. I know Tom, and Tom knows what he is saying.
More directly, I and my wife have been cave DPV divers for years, and we have experience on a number of different scooters and we have placed the towcords at both 12 and 6 and 9 and 3 on most of them.
Your second statement that the 3 and 9 approach comes from a couple of well known cave instructors who preach/insist on it is also a bit off.
You also make a large assumption about friction on the line at the scooter and the bolt snap when that's a variable, not a constant.
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In practice, and based on over 15 years of DPV experience with about 10 of that in caves, my advice is that the optimum tow cord position depends on a few different factors:
1) Large heavy scooters can benefit from a horizontal tow cord placement (3 and 9).
That's because the mass of the scooter relative to the torque and power make them more stable in the vertical and less maneuverable in the horizontal. A bolt snap that is free to move on the tow cord and a horizontal tow cord placement work really well for comfort and low muscle input on a long scooter run, while improving the turning ability of the DPV.
There is some validity in your comments about some cave instructors, but they date back to the days when the scooters in question ranged from a 53 pound Oceanic Mako to a 90 pound UV-26. Rolling a large scooter to the right or left is not always sufficient to get it to turn easily and if you have to also pull on it slightly to get it to pitch up into the turn, that's easier to do without having to deal with the friction of a vertical tow cord sliding through the bolt snap. Tom is exactly right on that point.
2) Some scooters can benefit from a 12 and 6 placement
Often with a smaller scooter the reduced mass of the scooter and reduced stability make a 12 and 6 tow cord placement advantageous as maneuverability in the horizontal plane usually isn't an issue. Rolling the scooter to the right or left is usually sufficient to get it to turn easily. A smaller scooter is also light enough that pushing or pulling on the handle to control the pitch in the vertical isn't a big deal - but it will *potentially* be a bit more work on a long run to keep it running straight and level, if the tow cord is free to move through the bolt, snap (see #3).
3) The attach points and methods make a difference
If you run the tow cord through a quick link at the scooter attach point, the cord is free to move through the attach point on the scooter, making it easy to adjust the tow cord length for different conditions. Alternatively using a stainless cable clamp or a polymer attach point with a hole not much larger than the line, will greatly increase friction and take less pressure off the prusik knot used to adjust the tow cord length - but make it slightly harder to adjust the length. The line used can als make a difference. For example 11 strand 750 paracord with its kernmantle cover is a bit stiffer than braided nylon and interacts differently with the bolt snap and free running attach point.
Also, if you just run the line through the bolt snap eye, the scooter can pivot around the bolt snap attach point on the diver with minimal friction. That usually works well on a larger scooter.
On a smaller scooter it will usually improve stability in "cruise" if you use a half hitch to attach the line to the eye of the bolt snap, or just larks head the bolt snap onto the bolt snap eye. This will allow you to more or less ix the position of the bolt snap on the tow cord. Once adjusted to the right position on a vertical tow cord, the scooter will maintain its attitude in the vertical plane with minimal input, which can make longer runs at a more or less constant depth easier on the arm.
Similarly, a fixed bolt snap on a horizontal tow cord can sometimes be advantageous of the scooter is a bit unstable side to side under power.
4) In summary, what works is what works
For example, my wife struggled a lot in her cave DPV course, where precision mattered, until we switched the tow cord to the 3 and 9 position. This was on a 53 pound Mako and it made a huge difference in terms of her ability to control the direction of the scooter and in terms of not rolling to the right due to torque.
Fast forward a few years to when she started using a Piranha P1, she had concerns about the 12 and 6 tow cord placement, but found it worked just fine for her on that scooter. It's smaller, lighter, and much more maneuverable and even with more thrust its very controllable for her with the 12 and 6 tow cord position.
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Consequently, my advise to anyone setting up a DPV is to be flexible, and experiment a bit until you find what works best. Ideally, you'll have an open minded instructor or a knowledgeable mentor who can help shallow out the learning curve.
But be aware that someone who insists on a a 12 and 6 tow cord arrangement is just as counterproductive as someone who insists on a 3 and 9 placement.
There's a time and place for both, as well as a time and place for different bolt snap attachment and line choices.
