[PSUBS-MAILIST] Pilot Fish unmanned -255m dive report

Sat Jul 2 16:07:27 EDT 2016

oops early send...

Hello Psubbers,

Yesterday the Pilot Fish hull went down to -255m off the coast in the SE of
France.

For a long time I have wondered how to pressure test the Pilot Fish hull.
There was the Aberdeen Hypobaric center option, but way up in Scotland, and
also a test chamber in Cherbourg, France operated by DCNS, closer but much
more expensive and requiring loading the hull vertically, so fully stripped
out, with a special support.
Other option: deep lake. Unfortunately there are not that many options in
France with easy access. At last, a deep spot in the sea. Ever wasted time
on the internet looking for things like where is the deepest point on the
planet or where is the farthest point from land in the ocean? I did some of
that, discovered the existence of a mysterious deep cutting canyon (Gouf de
Cap Breton) near my town in SW France. But it is narrow and swell is big
there.
Then the mediterranean sea with Toulon came as natural choice. the depth
drops off to 1000m less a few miles from the harbour. And a friend of mine
I did my military service with in the area, found the perfect boat
(Coquimar) for the job, not too expensive, but only with perfect weather
though, and careful preparation.
For the last few weeks we were watching weather, and availability of the
boat and all people involved. this week the planets aligned so I prepped
the sub and drove to Toulon on Wednesday to do the job on Thursday.
I had loaded a wooden berth on the back of the rental truck to use on the
boat to put the hull, keeping the trailer on shore. So we leave shore at
5h30 on Thursday. Nice weather but a bit foggy. When exiting the harbour,
we are contacted by radio by the harbour control tower asking us what we
are up to (probably spotted the hull on the deck...). After a short
discussion which we cannot escape, we are told to come back as we do not
have an authorisation to do the job. We did not know about this and soon
discover it normally takes one week to get approval, even when it is the
French navy who asks for their exercises... Really bad news. Weather window
would close. I have to say that on top of the French administrative
complexity, Toulon is where the navy hosts their nuke attack subs, hence
the local authorities are looking closely at all underwater activities.
(and which is why I did not want to use acoustic pingers on the hull for
the test in case of retrieval, just out from the subs hide nest...). And it
does not help to tell the authorities we are testing the hull of a british
flag registered submersible!!
Back to the harbour. To avoid a pricey cancelling of the operation, the
only and near impossible hope is to get approval during the day, and test
the next day, for which the weather forecast improves. Long story cut
short, tens of phone calls later to various offices and people, including
cold calling a retired admiral back from the golf course to grease the
process, I get a grant at 5pm.
So same process the next day. Weather just perfect. See the pics on the
facebook page. We even see dolphins.
at 6h30 we arrive on location.
The boat has a crane just rated over the hull weight. But accounting for
dynamic loads due to movement of the crane boom, and suction effects when
pulling the hull out, we come close to the rated capacity of the crane,
hence the need for a nice weather window with almost no swell. Swell is
also not good as a hull low on water can easily bang the boat and fenders
are too high to help. And the crane boom cannot reach the winch tie point,
so we decided to first crane the sub in the water, having it slightly
floating (-50kg) then move the boat to bring the sub on the other side
where the winch is, and then, clip some lead weights on the sub's hull to
get it to sink for the test.
I filled the sub with sand bags and sodalime buckets to make it just
buoyant. I check all the through hulls, and add a touch of silicone on the
hatch oring. A Scubapro 330m bottom timer is attached on the nose frame.
To minimize the use of a diver, we attached  7m long twin ropes at both
nose and tail of the hull with a pressure resistant trawler float at the
end I got from Emile. This helps orienting the sub, especially when the sub
will be coming back up. On each rope, at around 2m from the tie point on
the hull and 5m from the float, we clipped a lead weight from the boat's
deck and then lowered it in the water. This gave the hull a negative
buoyancy of around -50kg.
We then lowered the hull with the boat winch. We did a station at 20m for a
few minutes to check tension. I had bought 300m of semi static nylon rope
in 3 pieces, which could hold the weight of the sub in case it flooded, but
not by much margin, so I spent time making special knots which would not
lower the overall resistance of the rope by more than 20%. The ropes were
marked every 10 and 50m.
The usual best practive is to test at 125% of the max operation depth, so
in our case we would go at 120m max in the English Channel so needed only
150m.
Since our hull is designed for 250m operational depth, (rather than
carrying lead we d rather have a thicker hull) and all the conditions were
ok, we pushed down to the 250m mark on the rope. We arrived at 250m at 7h10
and waited an hour. Tension was ok, no air bubble. An hour later we started
ascending slowly. At around 9am, we saw the sub by transparency. We could
grab one float and unclip some lead, but the other float was entangled so
we sent the diver to clear the leads and ropes. The sub became buoyant
again. Good news, as a sign it did not fill with water. Then the delicate
crane operation went ok, the weather still perfect, but we just first
checked for boat wake around us.
The depth gauge was recovered. Max depth 262m, but that s configured in
fresh water so it is actually 255m in the mediterranean. The rope stretch
minus knots consumption explains the 5m off the 250m rope mark target.
Opening the hatch we check the hatch closely as the mismatch between
conning tower diameter and acrylic dome diameter induces massive stress on
the dome ring, and hatch flange. We had FEA done for that, but still...
We got scared as I saw water in the sub. But soon tasted, it was fresh...
relief. Actually I had embarked fresh water bladders to add weight to the
hull, and noticed they were leaking due to weight placed on top of them. I
once read on psubs emails about bladders that the question is not about
whether they could leak but when...  the other through hull seem tight.
At 11am we are back to the harbour. and at 9pm the sub is back in Pau after
a long drive. Mission complete.

regards,
Antoine

On Sat, Jul 2, 2016 at 9:01 PM, Antoine Delafargue <
antoine.delafargue at gmail.com> wrote:

> Hello Psubbers,
>
> Yesterday the Pilot Fish hull went down to -255m off the coast
>
> For a long time I have wondered how to pressure test the Pilot Fish hull.
> There was the Aberdeen Hypobaric center option, but way up in Scotland, and
> also a test chamber in Cherbourg, France operated by DCNS, closer but much
> more expensive and requiring loading the hull vertically with a special
> support.
> Other option: deep lake. Unfortunately there are not that many options in
> France with easy access. At last, a deep spot in the sea. Ever wasted time
> on the internet looking for things like where is the deepest point on the
> planet or where is the farthest point from land in the ocean? I did some of
> that
>
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