[PSUBS-MAILIST] some further lake tests

Thu May 19 05:57:21 EDT 2016

Hello psubbers,

Last week end we drove to the same lake as the first to do some more test
dives for our little human powered sub. Quite another event rich sequence
worth sharing:

-The preparation session went ok. We just had another tv crew, which is
fine but is slowed us down, as they often want you to repeat some moves
even when they say they just want to film you do your things. Interviews
themselves are really short.

-When driving to the harbour’s launch slope on a bumpy path, we heard a
noise a km before arriving. We found out that our 100kg emergency drop
weight had fallen from the sub. Hopefully it only fell 15cm onto the
central beam of the trailer, from where it could not go anywhere… The
vibrations had moved the release pins, and it also moved apart by a few mm
the lower legs of our nose frame, which normally closely bound each side of
the drop weight and where the release pins go. With a few hands we pushed
the weight back up in place. Lesson learned: use longer pins, retighten the
frame and add a thread bar to keep the legs of lower frame close enough
together. Also, we should have the drop weight rest on 3 support points (2
pins, 1 ledge), not 4 (2pins, 2 ledges) which will never be fully stable.

-We then launched. It went smoothly as we had done it before and knew
exactly where to bring the trailer and its rope extension to have the sub
float.

We then deflated the ballasts to check the trim inside the harbour. This
time it was perfect, having captured last time’s 25kg discrepancy in our
reference excel file and recomputed the lead weight we needed this time
given all the food and stuff we had taken.

-then the boat towed us out, until we reached a 4m water depth to pedal a
bit. This went ok. We were just intrigued by a tiny high frequency
vibration coming from the prop shaft, which was there regardless of
rotating speed, but only when pedaling in the forward direction. It was not
there during the first dives, and it disappeared the next day. We are
wondering whether it came from the water lubricated bearing , or the
pressure seal assembly. The length of the prop shaft of over 2m makes it
probably prone to resonance.

-This time we had better navigation instruments, a compass mounted right
outside the dome in front of us, on the aluminium dome seat, with quite
lower magnetic deviation due to the hull than we feared. Also a diver depth
gauge was there, very useful. We were eager to test our imagenex
sonar/sounder combo, but could not get it to work for that dive. Will be
next time…

-The sub turned out quite stable, keeping straight when we stop pedaling,
but we noticed that having a buoy attached at the front of the sub with a
short rope to remain clear of the propeller gave a bit of instability on
top of extra drag. Depth control worked well with our 80kg tray on rollers
beneath batteries and pilot seat, although we were not aiming for very
large swings of depth. We just need to rework a bit the handle so it is
more convenient to use, especially when the sub is at large angles.

-The oxygen consumption turned out lower than we expected. Michael and
myself are quite lightweights I should say. We were running at 0.75L/mn for
2 people at rest, it actually went down to 0.5L/mn during the night, with
one sleeping. It also shooted up to 2L/mn after I went out for a swim
around the sub in cold water to remove our buoy rope from the prop the next
day. So feeling warm is really good to lower oxy consumption, and a down
jacket, socks, hat and gloves may be worth kgs of sodalime in emergency
mode! When pedaling it ranged between 1.25 to 2L/mn, not as high as
anticipated, but the pedaling resistance and heart rate remained quite
moderate. This brings our ‘theoretical’ life support autonomy to a whopping
16 days in pedaling mode, and 37 days in rest mode.

then we went for a tow to a nice diving spot on the other shore of the lake
where we would spend the night and find deeper water for the next days
dives. The tow was quite choppy, and we occasionally dove a 0.5 to 1 m with
dynamic effects, although since the first dives we moved the towing point
below the nose tip which improved considerably the behavior under tow. At
some point we had to slow down the tow as we noticed that the dome had
slightly moved on its seat, under a combination of : effect of slight
internal overpressure caused by temperature increase due to sun light in
the dome and slight CO2 build up, wave action, occasional UW dips, and
primarily because the force on the dome retainer strap was not high enough.

When we arrived at the other shore, we moored, inflated ballasts and
delicately removed the dome to check the oring and force on retainer strap.
We noticed the oring had a pinch point towards the dome pressure seat, so
we cleaned and re-placed it in a way to keep low pressure tightness. We
also added 1mm spacer between the dome and retainer strap to increase the
strap down force, and ensure a large enough oring compression so the dome
rests on its seat with no gap. the general theory says that a gap should
not be an issue as when you dive, the force closing the gap should be many
times higher than the force pushing the oring in. But here a combination of
waves, overpressure etc may have moved a bit the oring in at some point.

We also removed the prop to see if some debris could have caused the subtle
vibrations, but could not find any thing.

-After that we had dinner and landed the sub to spend the night near the
shore in less than 2m water depth. Deployed our anchor also to stay put. It
worked well. Inside the sub we deployed our bunker bed and tested switching
positions between Michael and I. We slept one after the other, so we could
check on O2 and CO2 levels. Comfort was not bad, because the bunker bed is
really nice, but space is really small inside. We can grab all the controls
and isolation valves in case of emergency, but you can t help but feel like
trapped inside... During the night we used the red lighting as in real
subs, to feel like in the submarine movies, it was really cool!

-Next day, after a beautiful sun rise seen from underwater, we went for a
dive to 10m water depth where we could have space above and below us to
really operate the water ballast and trim weight tray on rollers. Thanks to
our depth gauge, water circuitry and hand pump we could get in trim within
less than 1 kilo, and feel the inertia of piloting moves. It was really
good feeling piloting that thing in three dimensions. After the dive we
surfaced with our ballast and pedaled we got the rope in our large prop,
which led me go out, take a swim to cut it, and shorten the buoy rope as
our dive support boat had not arrived and our dive support was on the
shore. We ll probably add a duct around our prop... Then before I came back
in, some fishermen in another boat came and asked us whether we were fine,
they initially thought we were a boat upside down. It was funny seeing
their hallucinating faces.

  -we then had our own support boat come over and we went on for a stretch
of pedaling at 2m water depth. We could measure our speed at 1.2kt which
falls in the ball park we anticipated. We may gain some without a buoy, and
once our composite shells are polished, and also with a more stable
steering than what we had (the short buoy rope attached to the front of the
sub made it difficult to keep a stable course). On the other hand adding a
duct around our large prop may decrease our speed. But a duct would also
increase a bit the power we can put in at 70rpm, so we might compensate
extra drag effect on speed by extra thrust.

-Over the week end in the sub, we managed to log quite a few hours with our
scrubbers. We reckoned we got over 200LCO2 /kg of sodasorb, which is better
than the typical figure given by the manufacturer (185LCO2/Kg). The
scrubber starts being less efficient indeed at around that amount but we
can still manage <1% in the cabin some time after that. On top of that, we
have probably extra absorption capacity from our nearly worn scrubbers as
we saw it can still run with an exhaust lean in CO2 in parallel or series
to our second fresher scrubber. But it is difficult to measure how much we
can extend the absorption capacity with this technique.

regards,

Antoine
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