DIY double-bungees - for A.I. or T.I....

One of the things I've had to do as part of the process of adding TI amas to my AI
is to add double-bungees to the amas. With the standard single-bungees there was
far too much 'bounce', 'looseness', call it what you will.

I probably overdid the planning/calculations, but in the end it all worked pretty
well and came in much cheaper than forking out 4 x $20 for the Hobie product.

One thing to keep in mind is that not all shock-cord has the same amount of stretch.
I made a couple of sets from one piece of shock-cord and another, which must've been
from a different batch. One had a unstretched to fully-stretched ratio of 1:1.6 and
the other was 1:1.9. This stuffed up my calculations as the second batch results in
bungees that were far too tight.

Planning makes perfect? Not sure, but it was entertaining...


The original TI single bungees were 1/4" shock-cord, which is all but impossible
to find here in Oz. The closest size is 6mm, so that's what I went with - Aussie
made multi-core shock-cord. To make up the 4 double-bungee assemblies took 2 metres
of the stuff, plus I also used a meter more for a couple of 'prototypes.

For the cord I used for my final set, (1.6x stretch) the unstretched length between
the shockcord ends (when all knots were tied) was 19cm for the lower and 23cm for
the upper section.

I also had some pull-tabs made up using some spare bits of 25mm nylon webbing I had
lying around. These make it far easier to pull the cords over the eyelet posts on the
akas.

The parts required...


The original plan involved pulling a loop of cord through the shockcord end, adjusting
the lengths of the upper and lower sections, then using a couple of heavy-duty cable
ties to secure the cord.

Pulling the cord through...


Securing with cable-ties...


Trimmed and fitted into the shockcord end...


While this worked well I wasn't 100% happy with the durability of the cable-ties and the
potential for a failure while out on the water. So I came up with a simpler and hopefully
more secure option: tie a simple overhand knot in the cord. It was slightly more fiddly
as once I'd tied the knot I had to force one end back though the shockcord end, but with
a bit of stretching and swearing I managed it. There's no way the knot will pull through.

A simpler solution (top)...


Once the knotted end was in place, I threaded a pull-tab over each of the cords and then
pushed then both through the other shockcord end - a flat-bladed screwdriver worked well
for that task.

Almost complete...


After adjusting the upper and lower lengths of the cord I tied an 'unknown' knot to
secure the two ends. Getting the knot to pull tight in the just the right spot was a
bit of an art. Once the knot was pulled tight, I trimmed the ends and applied a bit of
heat to melt the tag-ends. Finally a few drops of super-glue around the knot as a bit
of extra insurance.

The mystery knot, which seemed the best and most compact way to tie the ends...


Then I reinstalled them onto the TI amas...


Pulled tight onto the eyelet-posts...


I tested the amount of 'give' in the cords and compared it to that of a 'factory' 2016 TI
and it was almost identical. With maximum force exerted on the ama and the bungees at maximum
tension, there was a gap of about 12mm between the top of the ama and the bottom of the aka
collar. I actually wanted a bit less stretch than this, ideally the aka collar wouldn't come
past the top of the ama, but I think that would be putting too much strain on the cord and
the shockcord ends.

Just the right amount of give...


The all-up cost for the 4 double bungee setups was about $10, which is a fair bit cheaper
than the factory items and (hopefully) just as reliable!

Wow Mingle, the definitive double bungee post!
Nice work.

Yea you can do it that way, and it will work just fine.
I did mine slightly differently, instead of double redundancy mine have 5 levels of redundancy.

They took about ten minutes to make 3-4 yrs ago and are still going strong, (total cost for all 4 was $.60 cents). The Hobie spectra 300 lbs test rudder line costs $.30 cents/ft (you only need 1-2 ft total), available at any Hobie dealer or ACK.

I just used the existing 1/4" bungy cords that came with my boat, cut them in half, then re-fed the line back into the hole Now I can use those little mushroom head buttons for other things (those little buttons are very handy, I hang my rescue ladder, tramps, sail control pulleys, docking bumpers, and the kitchen sink on those crazy mushroom buttons. My front mushroom buttons have 5 things hanging off of them, and the rear mushroom buttons have three things hanging on them.

We are divers and snorkelers and these bungy's easily withstand 300 lbs of direct weight, I can have two divers sitting on the AMA's putting their masks and fins on. We also sit on our AMA's to put our scuba tanks on, all geared up both my wife and I weigh around 300 lbs ea.

Total cost for everything $.60 cents, and 20 minutes of my time. You just take a knife, cut the bungy in half. Unscrew the buttons with a pair of needle nose pliers. Shove the loose end back into the hole in the button, tie a knot, then pull both knots back into the bottom of the button. For added redundancy Tie a knot in small piece of spectra wrapped around one of the bungy's, then pull it thru the hole along with the two bungy's.
Actually the mod didn't cost me anything at all, because I keep a wad of the grey Hobie rudder line in my repair kit (everyone should), I just used some of that. Even if all 4 bungys break while on the water, the spectra string prevents the AMA from falling more than an inch, that single piece of spectra can withstand more that 300 lbs of force (why I call it five times redundancy, (4 individual bungys, plus one spectra equals 5). Once all completed and tested, one drop of super glue on each knot prevents the knots slipping or moving (who doesn't have super glue in their garage).

Just practical ideas that's all.



This pic was taken in May 2010, the bungy's in the above pic are the exact same bungy's, I didn't buy new ones (no need).


FE

EDIT:
The bungy's in the above pics have over 4000 hard (mostly offshore) sailing miles on them, and over 200,000 road miles (sitting on the roof of our vehicle, or pulled behind on our trailer).
Our rig:

Thanks for the comments guys...

I was going to add the spectra-cord 'safety' line as well, but thought it overkill (I may add them in the future - once I've done a few on-water trials).

In my opinion (nothing but gut-feeling) the most likely point of failure would be the screw-in shock-cord ends or possibly the eyelet posts. Adding
extra lines wouldn't help in the failure of either of these.

I'm not sure of the rated loads the screw-in fittings can handle, but they don't seem the most secure items in the 'chain'. On my A.I. amas the
screw-in fittings were very tight and I'd have to use pliers to unscrew them all the way. With the T.I. amas, the screw-in fittings only had to be
tweaked loose with the pliers, after which I could easily unscrew then by hand.

Anyone want to try and yank out on of the screw-in shock-cord ends and see how much force it actually takes?

Mike.

I haven't done any measurements, but I reckon those "mushroom" buttons would take a shedload to fail, as the force is lateral. On the other hand, the large screw-in fittings are such large scale that despite them only screwing in to "wheelie-bin material, they too would take a huge force to actually fracture. I don't know that you can draw any conclusions about the ease or otherwise of unscrewing the fittings either, as failure would depend on the amount of threaded area which would not necessarily change if one is easier to unscrew than the other.

As for the need for the safety straps, I personally rate them as far more important that double bungees, and fusioneng's exampl of two full-size divers complete with scuba gear as a pretty convincing proof of their strength. Your call of course.

PS. I am full-size, which meant I needed a TI

_________________
Tony Stott
2012 Tandem Island "SIC EM" with Hobie spinnaker

is there a method or special tool to remove the screw in shock cord holders?

I just use needle nose pliers.

Hey,

Just got my TI. Stoked to find this as one of the first listings as this needs replacement on my vessel. Thanks for the instruction.

Yes, there is a tool to remove the screw-in shock cord holders. It's much easier than using needle nose pliers, etc. Can be purchased at Hobie.com - part # 71115101. Well worth it.

bill