The definitive article on spring fin strap construction by Dave Dalton is here.
The ideas below are some variations using the same basic construction.
I have repeated a brief description of my first version so that everything
is in one place, and then described my second version.
VERSION 1 - USING EXISTING STRAP ATTACHMENT HARDWARE
(1) hook the springs directly onto the existing attachment hardware on the fins, and bent the hook end of the spring back a little inside the spring so that it would not come off
(2) take about 2 to 3 inches of the 1/2 inch inside diameter tubing (which was used anyway for the spring heel covers), and cut it in half lengthwise
(3) push it through the attachment hardware, bend it round, and push the two ends together, so that they join around the spring
(4) slip a half inch length of thin bicycle inner tube over the two ends to hold them together (actually a one inch length folded double on my first version - see picture). I had already placed the inner tube over the springs before step 1.
I did a couple of dives like this and they were fine. However
if I was going to continue using them I would glue the inner tube to the
1/2 inch tubing to ensure that all stays together. I experimented
with using "Seal Cement" (neoprene cement) and roughening up both surfaces
first with a hacksaw blade prior to cleaning and gluing. I also used
a band of heat shrink tubing over the whole lot to keep it together, with
the net result that the glued join appears extremely strong (on one test
pull, the pliers I was using to grip the tubing went through the tubing
before the inner tube/heat shrink came off the tubing). I tried several
different types of
tube and glue - Aquaseal is easier to apply than Seal Cement but doesn't seem to bond quite as well. The tubing I used was 1/2 inch internal diameter and 1/4 inch wall thickness, either "Hypalon Rubber" or "Santoprene", McMaster-Carr numbers 51195K33 and 51225K43. Both of these seem to glue well - the joint was the strongest with the Santoprene, but (i) this might have been because the Santoprene seems to have a higher friction surface - the adhesion of the glue itself looked better on the Hypalon and
(ii) I notice the Santoprene is a thermoplastic - which might give problems if the fins are left in the sun, so on reflection I think that I would go for the Hypalon Rubber.
SECOND VERSION - END CAP AND D-RING METHOD
The second style I tried was based on the use of a rubber end cap rather than the split tubing, and the D-ring attachment method from Dave Dalton's instructions. The D-ring's I used came from Beacon (search on the term D-ring).
Also in a slightly different form:
What I did was:
(1) Obtain some rubber end caps. McMaster-Carr supplies both vinyl and rubber end caps with 1/2 inch internal diameter (9753K48 and 6448K53); I got some of each, and there really doesn't appear to be much in it, but the rubber ones are harder to drill neatly so I used the vinyl ones. I don't know which material will last longer.
(2) Place the rubber end caps over the ends of each spring and cross-drill them:
(putting them on the spring is an easy way of holding them for drilling).
(3) Remove the end caps, slightly bend the spring ends in so that they should not accidentally become detached from the fixing hardware, and fit the cave line:
I used a Bowline Knot for the cave line. It's an excellent knot that creates an end loop that won't slip but should have some protection to ensure that the knot doesn't loosen up over time if the loose end flops around. There are a number of ways which this could be done; including using a half hitch, a rubber band, some electrical tape, or by tying a stopper knot (like a granny knot) in the end. I used some heat-shrink tubing over my knots. Greg used a Bunt Line Hitch which is a slipknot that snugs up tightly and doesn't creep or loosen up even when subjected to repeated loading/unloading. This hitch is also great for tying off cave line to bolt snaps for many other diving applications. It's a low profile hitch which requires very little line and has been used by seamen in wet environments for centuries.
Greg also had a neat method using compressed air to blow the cave line through the spring, whereas I looked more like a monkey with a stick ...
(4) Replace the end caps and insert a cut 1 inch D-ring through the cross-holes, checking that the D-ring also engaged with the cave line (by stretching the spring):
(note that the version on the right in this picture has an over length piece of inner-tube over the whole strap - more on this below).
(5) Remove the existing attachment hardware from the fin, and drill out the holes in the mounting lug a little
(6) Cut some one-inch lengths (just under, actually) of hollow steel tube and insert them into the enlarged holes using channel lock pliers:
I used 4.5 mm internal diameter/5.0 mm external diameter/0.25mm wall thickness stainless steel thin wall tubing from McMaster-Carr (50415K214).
(7) Fit the D-rings to the fins and close them up with channel lock pliers - see:
Here's how Greg Porter's turned out. Greg used Duane Liptak's suggestion for using 10/4 electrical cable sheathing for the spring covers:
INNER-TUBE COVER VARIATION
The Extreme Exposure straps have a length of tubular webbing over the spring. I assume that this is to reduce the possibility of line becoming trapped between turns of the spring when they are stretched. For my straps I placed an over-length piece of inner tube over the whole strap prior to step (4) above. It is not fastened to the end caps, but just "floats" over them:
and the Extreme Exposure straps:
The covering could also be used for the first version as well.
I do not know whether or not the covering is a good idea or a bad idea.
However it will be easy to cut the inner tubes off if they are no good,
but it would be more difficult to "retrofit" them later (because the D-rings
would have to be opened up again).
The motivation behind these methods was not to address any problems with the functioning of the Dave Dalton or EE straps, but to come up with something which can easily be constructed with hand tools only.
I have a slight preference for the D-ring fastening method over the use of the original hardware, because the original hardware has some (admittedly very small) "sticking out bits", and it seems good to avoid these if possible. I was initially concerned about the theoretical possibility that the cut D-ring could pull apart, but these fears were immediately dispelled when I tried to remove one of the D-rings with a pair of grips and found it very tough.
The hollow tube idea comes from Dave Dalton's instructions, which he
reports as being suggested to him by Brett Dodson. The idea behind
it here is to reduce the likelihood of damage to the fin lugs from the
ends of the cut D-ring, either when they are inserted or as they rotate
during usage. It should also further strengthen the D-ring attachment
(it increases the angle
which the cut ends would have to bend through to get free). I drilled the mounting holes out a little because I was concerned that the lugs might split after a while with an oversize piece of tubing inserted through the holes.
My earlier posting mentioned that the EE extra large straps are about
12.5 inches long, compared with the 11 inch McMaster-Carr
(#3932K24) springs usually recommended . Dave Dalton pointed out
that this is the longest spring which McMaster-Carr
supply in the recommended wire diameter (0.062 inch) (they do supply longer
"continuous length extension springs" in the adjacent wire
diameters). I adopted the rather pedestrian approach of stretching the center couple of inches of my springs by an inch, to give a 12 inch length (I have size 11 feet). Clearly this is not ideal, but my HOPE is that there should not be a problem (otherwise the ends of springs would also have been failing where these have been bent, when people shorten springs). At least the center part is hidden by the rubber tubing.
I have done two dives with the first version of straps, and no dives with the second version, so this posting should be read as a description of what I have done rather than as a recommendation for others to follow. I hope to be be cave diving intensively for the next few months, so I will report any problems or issues which arise.
PS Many thanks to Greg Porter for his input and for posting the pictures
on his website (he did a great job of cleaning them up and making them
visible); and also to Dave Dalton for some comments on the D-ring attachment
method, and of course for his original instructions.
Web page questions: