Who gets to say how a configuration should look or how it should be used? Is someone claiming to be the font of sidemount DIR? If OW divers want to configure in a certain way for their environment what basis does a cave diver have to criticize?
In this case, the laws of gravity and measurement gets that say-so. The issues are:
1)
Cylinder and diver stability. Horizontal trim is
demanded in sidemount by the necessity to retain stability of the cylinders. Cylinders hang at the divers' sides, and are not clamped on their back. Movement from horizontal position causes cylinder movement and more drastically effects the divers' stability. It differs to back-mounted diving for that reason - and this necessity has been reflected in the training standards of every agency that provides sidemount training. For instance, sidemount is the only
recreational PADI course that specifically guides instructor towards developing horizontal trim.
2)
Equipment Configuration - Hose Routing - Sidemount cylinders are secured in a
different location to where backmount cylinders are secured. Go figure... common sense eh? Top-mounted LPIs are absolutely logical for backmounted diving - they provide a direct, efficient routing of hoses from regulator to LPI attachment, with no need for the LPI hose to be bended or re-directed in any way. In contrast, sidemount cylinders aren't secured on the diver's back - they are under the armpits. That means an LPI hose is routed from the opposite direction. That requires a different approach for LPI hose location - if the same results of direct, efficient LPI hose routing are to be achieved.
btw... for clarification, I am not a cave diver, but I am a sidemount instructor..
On cylinder stability and the need for trim.....
PADI Sidemount Instructor Manual/Course Notes:
4. How do you adjust for proper trim in sidemount? What are your options for refining your trim during a dive?
E. Proper trim in sidemount.
1. You need optimum trim and streamlining to move cleanly and efficiently, which saves energy and gas. Streamlining reduces damage to the environment because youre not dragging gear across sensitive aquatic life.
2. On your first sidemount dives, your instructor will have you establish neutral buoyancy.
a. Relax and hover. See what your natural attitude is in the water (horizontal, feet high, low, etc.).
b. Your buddies and the instructor will help you be sure your cylinders are relatively in line with your body (you cant always see them on yourself).
3. Because sidemount is designed for proper trim and streamlining, you may find you need little or no adjustment after you kit up. As you use gas, however, you may find it changes. Your trim may become more head-down.
4. Regardless, as necessary, to adjust for proper trim and streamlining, arrange your weights so you can hover horizontally with little or no effort, and so you can easily change positions during the dive. Arrange your cylinders so theyre inline with your body.
5. Changing cylinder attachment points and sliding weights up or down on the cylinders (if you have weights on them) can help.
6. It may take some practice initially, but usually becomes intuitive quickly.
Section 3 - Open Water Dives
....
10. Hovering
a. Demonstrate the skill.
b. Students establish neutral buoyancy and hover for at least one minute.
c. Encourage hovering in a horizontal, swimming position.
....
11. Ascent and safety stop
a. Students ascend in buddy teams and make a three minute safety stop at 5 metres/15 feet.
b. Encourage maintaining a relatively horizontal position and keeping stop depth at chest level.
Then, we see...
Sidemount diving is not a new concept, but its application has spread from cave diving to open water recreational and technical diving. Sidemount diving presents divers with adifferent approach to equipment configuration, and a new set of skills to master.
Remember that the philosophy of this course is to provide an introduction of the benefits and proper configuration of the use of sidemount equipment for recreational diving, with
an emphasis on safety.
Sidemount is different to backmount. That's pretty fundamental. Pushing divers towards a belief of retaining back-mount configuration aspects detracts from the overall benefit of sidemount. It's nothing short of a lazy approach. It's different...and requires a different approach. That's why training is given....
Specifically on configuration...
4. How do you configure your right and left side regulators for use when sidemount
diving?
D. Regulators and regulator configuration for sidemount.
1. In sidemount, the regulators mount on the cylinders so that everything is streamlined without loops or protrusions that cause drag and snags. You rig the regulators and cylinders so that when worn, the first stages are inward, toward your body where theyre protected yet accessible
Maybe someone can explain how a top-mounted LPI achieves the concept of "without loops or protrusions' better than a bottom-mounted one? Bearing in mind that the LPI hose routes
upwards from the regulator under the left armpit (rather than
downwards from a back-mounted cylinder).
Top-mounted LPIs require a 180 degree bend in the LPI hose. That is, IMHO, contrary to the philosophy taught on the course. That
isn't cave or open-water specific - it is a general principle that applies to sidemount diving in all aspects, as a motor-skill and configuration issue.
---------- Post added February 9th, 2013 at 07:42 AM ----------
How easily does the 'cave style' model vent as you ascend vertically?
Piece of cake from horizontal. Bad from vertical.
It's as 'different' as learning to use the lower dump-valve on a BP&W or jacket BCD, rather than relying upon the LPI hose. Actually...it's easier... because most (nearly all) sidemount BCDs utilize a simple air-pocket design for the bladder - which optimizes the ability to dump from a horizontal position, from a lower mounted dump location.
From my perspective as a sidemount instructor: If a diver
learns to dive (inc ascent/descent) in horizontal trim, they graduate training able to utilize any model of sidemount BCD. If they are permitted to use a 'open-water only' sidemount BCD...and further permitted to lapse from proper trim whenever they feel like it... then their training has only prepared them to use those styles of 'open-water' sidemount BCD. They will be deficit in skill and technique to use the majority of sidemount BCDs on the market.
For those that don't (yet) dive sidemount: Sidemount is very stable in a horizontal position. When moving into a vertical position, there is more inherent instability in the cylinders - because those cylinders will move around in relation to the diver. At the end of the dive, if the cylinders are 'floaty', this can cause significant instability and cylinder 'rise'. From my experience as an instructor, this places a consequent demand on the diver, who will struggle to control their rig and feel much more 'uncomfortable'.