[PSUBS-MAILIST] some further lake tests

Thu May 19 19:50:10 EDT 2016

Thanks so much for the dive report, I really enjoyed reading that. With Snoopy I once had the same exact thing happen to me, with the weight tray dropping onto the trailer as I passed over a speed bump right at the entrance to the lake! It took hours to reload, but what I did forever after was just put a half dozen cable ties where they would prevent the tray from jumping. It worked.

Best,

Alec

> On May 19, 2016, at 5:57 AM, Antoine Delafargue via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
> 
> 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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