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I posted a few days ago describing some experimental ways of making spring
heel straps for Jetfins. Since then I have tried one of the alternatives I
suggested, and Greg Porter has kindly put some pictures on his website. I
believe that Greg has also constructed some straps along the lines of the
first version I described.
BACKGROUND
The definitive article on spring fin strap construction by Dave Dalton is
at:
http://www.deeperstuff.com/
The ideas below are some variations using the same basic contruction. 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
The first version is shown at:
http://www.porter-az.com/scuba/images/simon-spring-strap.jpg
What I did was:
(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 lengthways
(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. It is shown at:
http://www.porter-az.com/scuba/images/sr33.jpg
and in a slightly different form at:
http://www.porter-az.com/scuba/images/sr32.jpg
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 - see:
http://www.porter-az.com/scuba/images/sr28.jpg
(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 - see:
http://www.porter-az.com/scuba/images/sr31.jpg
http://www.porter-az.com/scuba/images/sr27.jpg
I used a bowline knot for the cave line because it is the only know I know.
Greg used a "Bunt Line Hitch" ("a slipknot that snugs up tightly and doesn't
creep or loosen up even when subjected to repeated loading/unloading. It
uses the least amount of line for hitches of this type and has been used by
seamen for centuries".) - see:
http://www.porter-az.com/scuba/images/buntline-hitch.jpg
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) - see:
http://www.porter-az.com/scuba/images/sr30.jpg
(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 - see:
http://www.porter-az.com/scuba/images/sr26.jpg
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:
http://www.porter-az.com/scuba/images/sr33.jpg
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 - see:
http://www.porter-az.com/scuba/images/sr32.jpg
and
http://www.porter-az.com/scuba/images/sr34.jpg
and the Extreme Exposure straps at:
http://www.porter-az.com/scuba/images/sr35.jpg
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).
OTHER COMMENTS
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 possiblility 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 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.
Simon Richards
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.
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