Diver Daves' Mark-15 Backplate Page

Mark-15 Backplates and Wings

Making or buying backplates and wings for the Mark-15 series has always been the first job any new Mark-15 diver needs to deal with. The issue is that the back of the chassis of the Mark-15 has a definite angle to it, and thus a standard backplate will not fit. In addition, there is the problem of Work of Breathing (WOB), which increases dramatically if the rig is spaced at any distance from the divers back. Lastly, the counterlung needs to displace water into and out of the chassis as each breath is taken, and the water was designed to flow through a series of large holes in the back of the rig. For these reasons, differing success has been had using various systems to add a BC or wings and a backplate.

The BC or wings can be any of the usual selections. One modification that is something to consider is to cut out the fabric material between the bladders of the inflation system, to expose the water venting holes on the back of the chassis. Some people feel that this is important, and others do not. I've dived rigs both ways and I cannot tell the difference. Halcyon made (makes?) a set of wings designed for the Mark-15 that has this "cut-out" feature, but the last time I went to order a set (I'm a low-volume Halcyon Dealer), I was told that I'd need to order 5 of them in one order as they were a special item. So, rather than doing that, I've simply modified existing wings by cutting the fabric out with scissors and then having the edges seamed at a local canvas shop. It works fine, and costs little.

As far as the back-plate is concerned, there are a number of choices. Perhaps the most simple is to buy a Dive Rite "Transpak" and simply bolt it to the chassis. This works fine. A further step is to bond with epoxy resin a small backing plate of some sort into the interior of the rig so that the harness may be removed. The upper attachment area of the Mark-15 lies under the center-section, so if you do not take this extra step you will need to remove your center section in order to remove your harness.

The second method to attach a harness is to use a two-piece backplate. If you want to do this with extreme simplicity, simply cut any commercially available backplate in half with a bandsaw and then remove some of the additional material on both the top and bottom halves and then bolt the remaining pieces to the top and the bottom of the rig. The only drawback to this method is that the 'kink' in the backplate that normally lies between the two cylinders of a set of doubles now spaces the rig away from the divers back. This slightly increases the WOB.

In order to fix that problem, Halcyon made up a set of small plates for the top and bottom of the rig that are slotted to hold straps. These work fine, with only two issues: First, the bottom plate is held hard against the chassis, so that no amount of pulling on the waist straps will tighten the shoulder straps. This is OK if you dive only one suit, but if you change between a drysuit and a wetsuit it's a pain. The second issue is that of availability. I've had a set of these on order for the last 7 months (as of February 2001) and I don't have them yet. Again, this is a small-run item for Halcyon and they just don't seem to be that interested in making them.

The fix that I've decided on to satisfy both of the issues with the Halcyon is to simply design my own and to have them fabricated locally. Following are some photos of the new backplate, which I feel is a real improvement in design for utility, and it's available within a few days of order.

Here is the Halcyon backplate and wings. The wings are the normal style, only cut-out in the middle of the back to allow water to pass to the counterlung. An alternative is to simply cut out the fabric of any other style of wings desired, or to simply ignore the issue. I've done it all three ways and all three work.

Removing the Halcyon backplates, you can see the simple construction. Interestingly, the backplates are pierced with the same square holes that would be used to retain a bolt on a regular set of doubles. These are not used on the Mark-15 installation, and I must wonder why they bothered.

This is, in my view, a flaw in the Halcyon system: The bottom plate is a flat plate, which allows absolutely no adjustment of the shoulder straps once the backplate is tightened to the rig. This, as well as the fact that I was not able to get more of these plates delivered in anything even resembling a normal delivery period, spurred me to have an improved backplate constructed locally.

Here is the locally fabricated version. It owes some features to the Halcyon variety, but has substantial differences. The webbing attachments are welded loops made from bent stainless-steel rod. This will allow adjustment of the shoulder straps while dressing. The plates are made from 300 series stainless, 1/8 inch thick. These are beefy and should last forever.

Installing the backing plate for the top plate requires removal of the center section. Once the area is exposed, I bonded the plate to the fiberglass of the chassis with a glue called "Outdoor Goop". This is a soft, err... GOOP, that is available in any camping store. It sets soft, and the plate may be removed later if desired by simply sliding a putty knife under the plate. The fiberglass will not be damaged doing this. The plate shown is the original Halcyon one, the new one is attached in the same place. The welded-on acorn nuts prevent the end of the screw from abrading the counterlung once the center section is reinstalled.

Once the backing plate is installed, you can then flip the rig over and see where the backplate will attach. The tapped holes are for 1/4-20, and the backplate is simply attached with machine screws. Originally I used stainless steel screws, but I had a screw jam into the threads and I needed to drill out the stub once I broke it off trying to remove it. Another diver also had the same problem. I've solved that issue by now using brass machine screws. Even if they do end up breaking, they are a LOT easier to drill out.

This is the lower backing plate. Once again this is the original. Again, it is bonded in using Goop. The size and placement of this plate is critical, as the next photo will show.

The area where the plate must be installed lies directly under the electronics pod. You can see how critical the placement of the drilled and tapped holes are, as there is only a fraction of an inch of area that is correct. This is a 'Busy' area of the rig, and it needs to be PERFECTLY placed. The screws will protrude through the plate when tightened, and you can see that there is just a few millimeters of space between the holes and the electronics pod. At the divers option, the plate may be mounted slightly higher (to the left in this picture), allowing a rang of perhaps 1 1/2 inches of adjustment to allow fitting to individual torso lengths.

Once the bottom plate is installed, this is the view from the outside. Just two 1/4 inch holes offer a clue as to what lies underneath.

The wings are laid on the rig, and then the backplates are attached with the brass screws. The fabric of the wings has been pierced with a flame-heated nail to provide a hole for the screws.

A similar process is performed on the bottom of the rig. You can see clearly how the straps of the harness will be passed through the welded loops, and also you can see here how the backplate is contoured to clear the structure of the rebreather.

Add your own style of webbing, and you are done! I use a simple setup, with one buckle at my waist. The geometry of the loops allows the webbing to be tightened with ease while strapping-in, yet the webbing 'locks' in place when it is tightened. Even with an unbuckled waist-buckle the shoulders do not loosen. Just for safety, I'd put a 3-bar slider on the left side, where there is no buckle, just to absolutely prevent the webbing from slipping back through the loop when the buckle is unfastened.

Here's a better detail of the loops. They are a big improvement over the original.

And at last, here is the rig all ready to go. I use the Halcyon weight-pockets, as I only carry about 8-10 pounds on my waist. This makes a neat and clean system. To say that I am pleased would be a gross understatement! I am VERY happy with the way that this came out. We have the patterns for these, and if anyone would like a set, just let me know.

Just a preview of another project: This is a 'Butt Mount" for a utility bottle for drysuit inflation and offboard air diluent. The Mark-15 has a very short body, and the rig does not sit down when the diver does. This makes the diver carry the full weight of the rig on his back while dressing. This mount will hold a standard pony bottle under the rig, and will lengthen the rig just enough that the weight will be taken on the lower plate while dressing. This was the first prototype. It is placed between the rig and the bottom backplate and holds the bottle under the rig perfectly. We found it was a bit too heavy, so look below:

This is the second version. It's lighter, and very strong. It will sit under the Mark-15 chassis and take a real pounding while holding the bottle and making the rig stand-up on it's own. This is a nice piece of work, but was still too heavy. That's all part of prototyping, though!

We were not really satisfied with either of the two "Ass Mount" systems shown above, so we've taken off in a new direction completely, using aerospace grade Titanium for a newly designed mount. This is the new mount. It weighs just over one pound and features a large knob for tightening the band to the utility bottle, no tools needed. The angles were cut to allow the rig to sit upright when the diver dresses into his gear, and the center of gravity of the Mark-15 sits directly above the center of the bottom foot. This makes dressing into the too-short Mark-15 chassis a breeze: With the added length the rigs "sits" as the diver does, taking a lot of weight from the divers shoulders and back while getting ready to jump.

This mount is designed to fit under the existing lower backplate, with the wings between the ass-mount and the backplate. Thus it's instantly removable when not desired by simply removing the two machine-screws holding the backplate on, and slipping the assembly off. Here it's shown with the upper and lower backplates. We took the opportunity to weld two D-rings to the lower backplate for attachment of lift-bags and reels, or whatever other stuff we want to carry. We think that the backplates are just about perfect, and will be testing the new ass-mount shortly in the water. Remember that this is just for an inflation and diluent-switch bottle, not for open-circuit bailout. Side-mounts still work best for that application.

If you like these backplates, or would like any other custom stainless steel work done, I highly recommend that you contact Enrique Alverez at Diving Equipment Specialties Corp. His work is famous on the New Jersey Coast, and he is a real craftsman.

Back to Main Mark-15 Page

	Here is the Halcyon backplate and wings. The wings are the normal style, only cut-out in the middle of the back to allow water to pass to the counterlung. An alternative is to simply cut out the fabric of any other style of wings desired, or to simply ignore the issue. I've done it all three ways and all three work.
	Removing the Halcyon backplates, you can see the simple construction. Interestingly, the backplates are pierced with the same square holes that would be used to retain a bolt on a regular set of doubles. These are not used on the Mark-15 installation, and I must wonder why they bothered.
	This is, in my view, a flaw in the Halcyon system: The bottom plate is a flat plate, which allows absolutely no adjustment of the shoulder straps once the backplate is tightened to the rig. This, as well as the fact that I was not able to get more of these plates delivered in anything even resembling a normal delivery period, spurred me to have an improved backplate constructed locally.
	Here is the locally fabricated version. It owes some features to the Halcyon variety, but has substantial differences. The webbing attachments are welded loops made from bent stainless-steel rod. This will allow adjustment of the shoulder straps while dressing. The plates are made from 300 series stainless, 1/8 inch thick. These are beefy and should last forever.
	Installing the backing plate for the top plate requires removal of the center section. Once the area is exposed, I bonded the plate to the fiberglass of the chassis with a glue called "Outdoor Goop". This is a soft, err... GOOP, that is available in any camping store. It sets soft, and the plate may be removed later if desired by simply sliding a putty knife under the plate. The fiberglass will not be damaged doing this. The plate shown is the original Halcyon one, the new one is attached in the same place. The welded-on acorn nuts prevent the end of the screw from abrading the counterlung once the center section is reinstalled.
	Once the backing plate is installed, you can then flip the rig over and see where the backplate will attach. The tapped holes are for 1/4-20, and the backplate is simply attached with machine screws. Originally I used stainless steel screws, but I had a screw jam into the threads and I needed to drill out the stub once I broke it off trying to remove it. Another diver also had the same problem. I've solved that issue by now using brass machine screws. Even if they do end up breaking, they are a LOT easier to drill out.
	This is the lower backing plate. Once again this is the original. Again, it is bonded in using Goop. The size and placement of this plate is critical, as the next photo will show.
	The area where the plate must be installed lies directly under the electronics pod. You can see how critical the placement of the drilled and tapped holes are, as there is only a fraction of an inch of area that is correct. This is a 'Busy' area of the rig, and it needs to be PERFECTLY placed. The screws will protrude through the plate when tightened, and you can see that there is just a few millimeters of space between the holes and the electronics pod. At the divers option, the plate may be mounted slightly higher (to the left in this picture), allowing a rang of perhaps 1 1/2 inches of adjustment to allow fitting to individual torso lengths.
	Once the bottom plate is installed, this is the view from the outside. Just two 1/4 inch holes offer a clue as to what lies underneath.
	The wings are laid on the rig, and then the backplates are attached with the brass screws. The fabric of the wings has been pierced with a flame-heated nail to provide a hole for the screws.
	A similar process is performed on the bottom of the rig. You can see clearly how the straps of the harness will be passed through the welded loops, and also you can see here how the backplate is contoured to clear the structure of the rebreather.
	Add your own style of webbing, and you are done! I use a simple setup, with one buckle at my waist. The geometry of the loops allows the webbing to be tightened with ease while strapping-in, yet the webbing 'locks' in place when it is tightened. Even with an unbuckled waist-buckle the shoulders do not loosen. Just for safety, I'd put a 3-bar slider on the left side, where there is no buckle, just to absolutely prevent the webbing from slipping back through the loop when the buckle is unfastened.
	Here's a better detail of the loops. They are a big improvement over the original.
	And at last, here is the rig all ready to go. I use the Halcyon weight-pockets, as I only carry about 8-10 pounds on my waist. This makes a neat and clean system. To say that I am pleased would be a gross understatement! I am VERY happy with the way that this came out. We have the patterns for these, and if anyone would like a set, just let me know.
	Just a preview of another project: This is a 'Butt Mount" for a utility bottle for drysuit inflation and offboard air diluent. The Mark-15 has a very short body, and the rig does not sit down when the diver does. This makes the diver carry the full weight of the rig on his back while dressing. This mount will hold a standard pony bottle under the rig, and will lengthen the rig just enough that the weight will be taken on the lower plate while dressing. This was the first prototype. It is placed between the rig and the bottom backplate and holds the bottle under the rig perfectly. We found it was a bit too heavy, so look below:
	This is the second version. It's lighter, and very strong. It will sit under the Mark-15 chassis and take a real pounding while holding the bottle and making the rig stand-up on it's own. This is a nice piece of work, but was still too heavy. That's all part of prototyping, though!
	We were not really satisfied with either of the two "Ass Mount" systems shown above, so we've taken off in a new direction completely, using aerospace grade Titanium for a newly designed mount. This is the new mount. It weighs just over one pound and features a large knob for tightening the band to the utility bottle, no tools needed. The angles were cut to allow the rig to sit upright when the diver dresses into his gear, and the center of gravity of the Mark-15 sits directly above the center of the bottom foot. This makes dressing into the too-short Mark-15 chassis a breeze: With the added length the rigs "sits" as the diver does, taking a lot of weight from the divers shoulders and back while getting ready to jump.
	This mount is designed to fit under the existing lower backplate, with the wings between the ass-mount and the backplate. Thus it's instantly removable when not desired by simply removing the two machine-screws holding the backplate on, and slipping the assembly off. Here it's shown with the upper and lower backplates. We took the opportunity to weld two D-rings to the lower backplate for attachment of lift-bags and reels, or whatever other stuff we want to carry. We think that the backplates are just about perfect, and will be testing the new ass-mount shortly in the water. Remember that this is just for an inflation and diluent-switch bottle, not for open-circuit bailout. Side-mounts still work best for that application.