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Old 15 September 2004, 11:13   #41
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25kg of Lead = 25kg of water displaced.

I imagine a 30litre volume might be starting point so its just a matter of calculating the cubed root of the volume divided by about 1.3 times pi.. hmm..this'll be about 40cm diameter buoy size but that'll submerge it almost.

So we could go for 1.5 to 2times the volume giving us a buoy about 50cm. So not that bad, but you could use a couple of smaller ones or do what we do sometimes last person up ties on a lifting bag that wont quite lift when full. This reduces the effort required to haul it up a lot.
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Old 15 September 2004, 11:20   #42
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Quote:
Originally Posted by Simon B
or do what we do sometimes last person up ties on a lifting bag that wont quite lift when full. This reduces the effort required to haul it up a lot.
It does and I have a video of this one with a swiftly made lift bag in about 25 mtrs depth by the undersigned, deployed by the undersigned and also recovered by the undersigned. The other diver was filming.
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Old 15 September 2004, 11:49   #43
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More Ramblings

In terms of the (general) buoy size my own experience is that you need *much* more buoyancy than the simple weight of water idea suggests. Maybe it's because we're using thicker rope than your average washing line, but the drag of the any current present can easily cause the buoy to 'go under'. Infact we've even lost one completely this way before now - we couldn't wait out for the next slack for it to come back up.

I just got the largest barrel I could get from a farmer friend and that seems to do the trick. Must be at least 50L.

I think it's this extra tensioning effect, in combination with any swell, which has resulted in the weight bouncing along the bottom - probably exacerbated by us often getting there well before any slack water to be on the 'safe' side. Certainly not a diver effect the times it's affected us.

One idea I'll maybe have a crack at someday involves the pivoting-toothed-cleat type of things I think used for locking off ropes on yachts. Attached to a large buoy and the line towed through as already discussed. Which would be perfect if I had ready made ropes for each depth we dive - I currently rely on shorter lengths joined with shackles etc.

For now we'll be using the lift bag approach but for e.g. 70m dives this can take up a silly %age of your available bottom time.
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Old 15 September 2004, 12:02   #44
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Quote:
Originally Posted by Simon B
25kg of Lead = 25kg of water displaced.

I imagine a 30litre volume might be starting point so its just a matter of calculating the cubed root of the volume divided by about 1.3 times pi.. hmm..this'll be about 40cm diameter buoy size but that'll submerge it almost.

So we could go for 1.5 to 2times the volume giving us a buoy about 50cm. So not that bad, but you could use a couple of smaller ones or do what we do sometimes last person up ties on a lifting bag that wont quite lift when full. This reduces the effort required to haul it up a lot.
Great but this dose not take in to account the force that the pull of the rib will be putting on the buoy!
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Old 15 September 2004, 12:10   #45
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Neil,

I'll try and test out the proportion of rope and drag to guage how far it all gets pulled down to gauge the amount of extra buoancy required for the calc so far we have only used the the tow lift for small shots.

Also being tested is a tripping arrangement whereby the shot wont slide back down and is held in place under the buoy. Nifty and simple.

Re the shot skipping along I have seen this with no one on it and in shallow water 30m it's movement appeared to be a function of the swell tugging at it. It did look just like a diver was doing it so my mate and I watched for a while.
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Old 15 September 2004, 16:25   #46
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I ran through all these various options over the years until I came up with an easy working method.
Large weights ( I used to use 56lb'ers) are useless, they bounce off the wreck and along the bottom and cause more problems than they are worth.
Small grapnels and lightweight ones are OK if they catch but they can "fly" over the wreck and don't drop fast enough on deep ones.
Tying a lifting bag is also a problem, wastes some of the dive time (little enough on deep dives) and is forgotten, filled too soon or similiar most of the time. (ie doesn't work well)
I use a 30kg grapnel made from a bit of solid bar with 3-4m of chain on it. Shot rope is old climbing rope (11m). Top bouy is a 50l one, anything smaller gets dragged under by divers and it must be able to support the whole shot weight just in case. This works cos it drops fast enough to hook quickly. We normally hang the rib on it and never had a problem dragging even in fair currents and winds.
Recovery is by boat, climbing shops sell a one piece ascender which is clipped to the bouy after tying to the boat, drive away till the slack is taken up away from the direction the boat was hanging then a bit of welly ans once off drive away till the bouy is at the shot. Pull into the boat (the gadget stops the shot moving back down again) and stow. No effort whatsoever.
We use this on wrecks from 20-65m and it works perfectly unlike most other methods.
You can't dive Scapa without a licence from Orkney council, the hardboats arrange this for you. Not sure if they would be keen giving them to RIBs.
Corryvreckan is on my "to do" list but can only be done with good weather and the right tide which is only every few months.
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Old 15 September 2004, 16:29   #47
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Rupert clever idea. We do something similar in Greece when we dive the wreckss but they are not that deep 35 mtrs max around Cape Sounion and an old II WW war ship at 45 mtrs.

Something else 65mtrs is deep (very deep). I suppose you use gases.
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Old 16 September 2004, 10:48   #48
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Calling Rupert Bear

Very helpful response - thanks - especially like the climbing ascender idea - just hoping you might share more detail on the grapnel. Presumably it has welded on fixed hook parts? Have you sized these to bend if you have to pull out? Thanks.
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Old 16 September 2004, 16:16   #49
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The grapnel has four welded on arms which bend to release it, welded to about two feet of solid rod approx 2" diameter and a welded loop at the top for the chain. Weighs approx 30kg including the chain.
If you need details I can post a pic.
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Old 16 September 2004, 19:09   #50
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RB,

I dont quite understand the way you do it? does the "acender" get pulled down the shot rope which is lying at a shallower angle in the water away from the bouy lifting the shot as it goes against the resistance of the large buoy being dragged?
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