[HELP] How improve standard tether cable buoyanancy with classic corcks?


#1

hi folks,
now that i usually pull most of the tether cable out when i dive i need to improve his buoyancy.
i have a bag full of 300 corcks wich is more than required.
now i have a question for wich i can t find the answer:

when you use the classic tether cable, how many meters or inches do i need to separate each cork in order to have the perfect bouyancy?
did anybody of you tried?
thxx for your help
best regards


#2

it seems it s 1 every 5 meters:

any feed back of this setup?
thx for your time


#3

Yep

It is working fine I have currently a couple of different tether set ups that I play around with depending upon what I am trying to achieve with the ROV (just a bit of simple soldering to change the set up)

One with 40-50m of OpenROV neutral buoyancy and then the standard twisted pair to the surface (sort of if I am dropping the ROV off a boat and at a fixed location and just looking around that area eg surveying a wreck site)

One as described in the post with Corks every 5 meters so it is neutral all the way (a bit more based if covering a bit more distance or running off the shore)

And I some times I use a hybrid of the pair with 40-50m of OpenROV neutral buoyancy and then the 100m of tether with Corks every 5 meters (more of use for some deeper water wreck survey)

Scott


#4

Your corks will over time soak up water and lose buoyancy, so you’d need to add to them from time to time. Your 3mm drill and a craft-knife to cut a slot along one side, then feed the tether in and hot-glue gun … should do the job.
More complexly - as your corks descend, they will compress. I once sent a colleague’s packet of cigarettes to 1.8km in the Eastern Med, in a polythene bag filled with sugar - they came up uniformly compressed to about 1/3 size. It’s less severe at 40m, but the effect will still be there. At some pressure (depth) they’ll go from being positively to negatively buoyant. With time, that depth will change, so the buoyancy characteristics will change with time too.
OTOH, you’ll probably account for 90% of the buoyancy issue.
The industry produces buoyancy foams good to 10s of km depth, but they’re not simple materials. and I don’t know where to get them. Then again - for a 40m working depth - wouldn’t trawler floats do? Or for that matter, the foam from inside a life-jacket or buoyancy-aid? (Many of these are designed to be released from stowage by 10-20m of water pressure, if the stowage is not manually opened first. So SOLAS jackets should have non-trivial crush resistance too.)


#5

Generally adding lumps of flotation to a ROV tether is bad. The lumps will tend to snag on things and either get the ROV stuck or at least stress the tether at specific points so it flexes and breaks at those points instead of flexing gradually throughout.

You are better off distributing the flotation along the whole tether. Polypropylene rope is cheap, readily available in different sizes, and floats. If you twist or braid the tether with one or two lines of polypropylene rope of the right size you can get the group to float, sink, or be neutral as desired. As a bonus you get more tether strength for when things go really bad and you have to haul the beast back by its tail. Don’t wrap the length of the tether/rope combo with tape. That will give you a stiff soggy mess that will never dry out and in pond or sea water it will stink.


#6

I am inclined to agree with Douglas, above, In that the corks present a snag hazard. However, I have read several posts where people have spent a lot of time and effort trying to thread the cable through a rope core. The rope can also present as a problem in that it can harbour micro-organisms which can subsequently be contaminating other sites.

I don’t see an easy cover-it-all solution at the moment, but noted that Andrew Thaler designed a 3d printable option on “thingiverse” giving removable floats. This enables moving them in case of different water density, ie. fresh or salt water - neutral density, positive flotation, or removal altogether to allow as a “drop line”.

I appreciate that these floats still present as a snag hazard and would invite anyone to come up with a better streamlined design with say a “break away” option, remembering that as a floating tether attached to say 25m of neutral buoyancy tether, there should be less chance of being in proximity of a snag.

In any case, good tether management topside and the pilot being aware of the lay of the tether is essential to good ROV work whatever you use.