Picking up a sensor?

[Jaco Versfeld]

Good day,


We have an acoustic sensor (recorder) with a "lower cost" acoustic release system that we are busy testing to deploy and retrieve. We currently test in relative shallow waters - 11 meters to 17 meters, in case of a failure - so that we can still dive out the sensor. We use a Flamenca 25 sail boat (https://sailboatdata.com/sailboat/flamenca-25/) to do our deployments.



The system comprises two parts: a weight and then the sensor and acoustic release itself. The sensor is basically 75mm (approx 3 inch) PVC pipe. The total length of the sensor and release system is about 1.5 meters. We fitted a foam collar, so that the sensor will indeed surface after we issued the release command.


The weight is a 15 kg gym weight, and we screw a thinner pvc pipe (63 mm) with length 0.5 meters to it. We use about 35 meters of a thin dynema rope (2 mm) to retrieve the weight. The weight and sensor is tied together with the dynema rope.


What would be the best way to retrieve the sensor, once resurfaced? In light winds we managed to do two successful retrievals, and did it from the cockpit. However, when the wind picked up, with our third retrieval - we struggled quite a bit.


I assume the best would be to approach the retrieval similar to picking up a mooring buoy. Thus, under engine, come from right downwind, and approach the surfaced sensor. Use a boat hook to catch the sensor.


I wonder if we should we attach a line from a cleat on the bow, through fairlead. With the crew member at the mast stays, use boat hook to catch the sensor, attach the line to the sensor, and then leave it in the water. As the boat drifts downwind, we can then pull in the sensor and anchor from the bow, almost like an anchor?


The thin dynema is not the most comfortable rope to work with, but we intent to deploy at deeper depths - 50m - 120m - and space for thicker rope becomes a problem.


Any thoughts on this?


Thanks,
Jaco

[psk125]

It might help to explain what your sensor is sensing, to help people understand what you are trying to do.
You use a "thin dyneema rope" to retrieve the weight. Why not attach a similar light line - or even lighter, like monofilament - to the sensor? If you are planning to use it in very deep water where this would not be practical (a mile of line would get difficult to handle) perhaps you need a loop of line at the top of the sensor that can be easily snagged with a boathook. If you are using it in the open ocean you may also want to add a staff with a flag on it or some sort of locator signal like an AIS MOB setup so you can find it more easily when it surfaces.

[leecea]

Sounds like you have some type of experimental rig that presumably doesn't weigh very much and can be caught with a boat hook.

BTW, everyone will want to know details are what you are doing, even though what you're asking doesn't really depend on whether you are trying to sense water temperature, fish, or sunken alien space craft!

The way you describe picking up a mooring ball at midships and letting the boat drift down is reasonable and might be easier than a normal bow pickup. One thing to consider is how fragile this device is, if you misjudge something and it hits against the boat.

Another thing people sometimes do is back up to a ball and pick it up from the stern, but I assume you have an outboard motor (not an inboard engine) so that might not be a good idea.

Here's a link that might have some useful details: https://sailing-blog.nauticed.org/ho...-mooring-ball/

The only other thing that comes to mind is that I see working fish/crab people in my area use large hooks to quickly grab things - easier that the small boat hook. Not sure where they get them but perhaps a pool hook like these could make things easier: https://www.marine-rescue.com/rescue...rds-crook.html

If you are doing this a lot you could also install something like this arm and pulley so the device is kept away from the boat while you pull it up: https://www.fishonsports.com/product/crab-pot-puller/