Trimix Diving from a RIB

[Neil Radcliffe]

Was just wondering what the collective wisdom would be on doing this.

The scenario would be two tekkies plus a competent cox/experienced ex-diver in a 5.5m 'proven' RIB.

Diving in the 50 to 65m range, twinsets plus 2 stages, spare gas already rigged at 6m on the shotline.

My main current concern is finding a better way of recovering the shot than simply pulling it up. Partially inflated lift bag being one obvious candidate, but I'm not too happy about the idea of it potentially bouncing along the bottom in this state.

Thoughts welcomed on the overall viability on this sort of diving, plus any tips on things to particularly think about.

Thanks, Neil.

[Ben Field]

It'll be an expensive trip but assuming everyones competant why not?

I would reccomend bagging the shot fully to begin with and then drifting deco under DSMB, you will have plenty of time once the bags are up to lower the spare gas/deco station onto the divers positions.

Not sure any sites have long enough slacks for shot line deco from a gas dive? :)

Even when diving off a hardboat I prefer drift deco, far less effort and the skipper gets an accurate and instant count of the number of divers leaving the bottom as we always run with one bag each.

[nick kay]

Have I missed the obvious???

Whats wrong with deploying a trapeze with a spare (say) 36% mix at the 12m or 9m bar?

When you complete the dive, attach a lift bag to the shot weight and partially deploy it, i.e. so that the boat handler gives it a small tug and up it comes )once the trapeze has been released from the shotline...)

Easy recovery of the shot and a decent "target" for the boat handler to the follow (presuming 2 x 25L drums on the trapeze)

[Iain T]

Have done dives very similar to what you outline. We went for the lift bag on the shot (actually an inflatable DSMB) just to take some of the weight off the shot without actually lifting it and then a detachable deco line. The deco line was attached to the main shot at 9m (ie where the lengthy stops begin) and weighted with the emergency deco cylinder. When we got to 9m, we release the deco shot and float happily with the tide whilst the boat cover relieve the boredom by lifting the main shot (now made easier by the lift bag) whilst still able to keep an eye on us.

It works well with a RIB because you only have two divers and dont have to bother with trapezes (which might be impractical in a small RIB). Of course, you have to have some understanding mates for boat cover who may charge you lots of beer for the privilege of watching your bubbles without getting a dive themselves :)

Iain

[Ben Field]

> When we got to 9m, we release the deco shot and float happily > with the tide whilst the boat cover relieve the boredom by
> lifting the main shot (now made easier by the lift bag)
> whilst still able to keep an eye on us.

Fair solution but why the need for such complicated shot arrangements? For 60mtr diving as suggested deco under a DSMB is more than satisfactory as you can carry well in excess of the gas required. Wouldn't consider the waste of BT that is required for returning to shot and deco station unless it was major expidition type dive?

Either way you can drop a station onto divers on bags without using a shot.

> Of course, you have to have some understanding mates for
> boat cover who may charge you lots of beer for the privilege > of watching your bubbles without getting a dive themselves <

HA HA, Agreed, make sure they are not the sleepy sort as well :)

BEN

[Neil Radcliffe]

Thanks to all,

I must admit I sort of like Iain's idea most - mainly because it will be the two divers who are likely to get the job of hauling in the shotline - which is obviously not ideal after a trimix dive.

But the final decision can be made after I find a new weight and know how heavy it is - the last shot-weight still needs a search and recovery operation (left behind folling an interesting boat swamping incident!) Even at 15kg the old one was a bit painful to lift, but on the other hand it was also prone to dancing around the bottom.

Obviously another (strong) helper and things all get a bit easier - but getting more than one is proving difficult. But point taken on not wasting bottom time.

Neil

[Iain T]

Fair solution but why the need for such complicated shot arrangements? For 60mtr diving as suggested deco under a DSMB is more than satisfactory as you can carry well in excess of the gas required. Wouldn't consider the waste of BT that is required for returning to shot and deco station unless it was major expidition type dive?

Either way you can drop a station onto divers on bags without using a shot.

Yes, you could do that. We didn't for a couple of reasons: 1) if you are doing deep stops then you have to deploy the DSMB from a fair depth which is a bit of a hassle 2) We were trialling out a technique that could be used at greater depths, and 3) I'm a bit of a wuss and like the 'security blanket' of a nice meaty shot as a datum to work my way back up.

However you're right, it did seem a bit of a waste when we were coming back over the same ground again to get back to the shot when we don't have massive amounts of bottom time anyway.

Horses for courses and all that - I'm sure we will evolve things with time.

Iain

[Ben Field]

> 1) if you are doing deep stops then you have to deploy the
> DSMB from a fair depth which is a bit of a hassle

Funnily enough I prefer to bag from the bottom on gas dives, little gas is needed to lift the bag (I use AP Valves DSMBi anyway) and skipper gets early record of who's coming up.
Mid-water deployment is what I would call- a hassle :)

Depending on what your diving I guess (and the current) it could be a pain.

> 3) I'm a bit of a wuss and like the 'security blanket' of a > nice meaty shot as a datum to work my way back up.

Not going to disagree there, just don't like the idea of being a flag in quads just to get to my station... you have to carry a dsmb anyway, I like to use it :)

> However you're right, it did seem a bit of a waste when we
> were coming back over the same ground again to get back to
> the shot when we don't have massive amounts of bottom time
> anyway.

But if its for safety then you have to bite that bullet... depending on conditions you might get a free ride (drift) back, just means the ride up will be fun!

BEN

[Ben Field]

> 1) if you are doing deep stops then you have to deploy the
> DSMB from a fair depth which is a bit of a hassle

Funnily enough I prefer to bag from the bottom on gas dives, little gas is needed to lift the bag (I use AP Valves DSMBi anyway) and skipper gets early record of who's coming up.
Mid-water deployment is what I would call- a hassle :)

Depending on what your diving I guess (and the current) it could be a pain.

> 3) I'm a bit of a wuss and like the 'security blanket' of a > nice meaty shot as a datum to work my way back up.

Not going to disagree there, just don't like the idea of being a flag in quads just to get to my station... you have to carry a dsmb anyway, I like to use it :)

> However you're right, it did seem a bit of a waste when we
> were coming back over the same ground again to get back to
> the shot when we don't have massive amounts of bottom time
> anyway.

But if its for safety then you have to bite that bullet... depending on conditions you might get a free ride (drift) back, just means the ride up will be fun!

BEN

[Tim Pentall]

Hi All,

First posting so am hoping not to get panned.

I am a fairly newly qualified Trimix Diver and would not consider a RIB to be the correct platform for this type of diving.

Diveing to these depths with mixed gasses has an element of risk greater than normal sport diving, thats why more planning and backup are required.

The point I am labouring to make is that in the event of an incident a RIB is not the ideal platform in which to administer first aid or conduct a rescue from.

the chances of an incident are greater and we should plan

Was just wondering what the collective wisdom would be on doing this.

The scenario would be two tekkies plus a competent cox/experienced ex-diver in a 5.5m 'proven' RIB.

Diving in the 50 to 65m range, twinsets plus 2 stages, spare gas already rigged at 6m on the shotline.

My main current concern is finding a better way of recovering the shot than simply pulling it up. Partially inflated lift bag being one obvious candidate, but I'm not too happy about the idea of it potentially bouncing along the bottom in this state.

Thoughts welcomed on the overall viability on this sort of diving, plus any tips on things to particularly think about.

Thanks, Neil.

[Philip Smith]

Obviously another (strong) helper and things all get a bit easier - but getting more than one is proving difficult.

Yes, a crewman is advisable anyway for general boat tasks, bringing the divers back in and dealing with emergencies.

Philip Smith

[Ben Field]

> First posting so am hoping not to get panned.

Fear not- this is the oh so friendly BSAC forum :)

> I am a fairly newly qualified Trimix Diver and would not
> consider a RIB to be the correct platform for this type of
> diving.
> Diveing to these depths with mixed gasses has an element of
> risk greater than normal sport diving, thats why more
> planning and backup are required.

The original post was for gas diving in the 50-65mtr range, currently BSAC DL can do a 50mtr dive anyway.... A busy wreck dive to 50mtrs for a DL with reasonably BT and decompression is going to be over an hour possibly? (I had a 75min RT on the Kyarra a week or two back for example and thats juat 35mtrs)

While I'd agree it isn't perhaps ideal, it is doable as depths are only just beyond recreational limits and times comparable.

> The point I am labouring to make is that in the event of an
> incident a RIB is not the ideal platform in which to
> administer first aid or conduct a rescue from.

By the same token it isn't the right platform for a incident after a 10mtr dive either!

For the record one- of our boatees once recovered, administers first aid and then relayed to a Coastguard helicopter an unconsious diver on his own from a 4.5mtr inflatable!!!
(BTW- He wasn't a diver from our club, but the hardboat he was diving off was too big and crowded for him to be recovered too!)

Trimix diving does require more planning but in the event of a serious accident the first aid and evac proceedures differ little from more common diving depths- which should be a sobbering thought.

BEN

[gordon henderson]

Was just wondering what the collective wisdom would be on doing this.

The scenario would be two tekkies plus a competent cox/experienced ex-diver in a 5.5m 'proven' RIB.

Diving in the 50 to 65m range, twinsets plus 2 stages, spare gas already rigged at 6m on the shotline.

My main current concern is finding a better way of recovering the shot than simply pulling it up. Partially inflated lift bag being one obvious candidate, but I'm not too happy about the idea of it potentially bouncing along the bottom in this state.

Who cares? Thats the skippers problem, not the divers... The divers can help by making sure it's not fouled up though.


Thoughts welcomed on the overall viability on this sort of diving, plus any tips on things to particularly think about.

Use a lazy-shot with the bail-out gas attached to it at whatever depth you are happy with. the top-end has a large buoy, the bottom a small weight with a loop with a jib clip and a small loop of shoelace type cord.

So the skipper of the boat locates and shots the wreck as normal.

Once the skipper is happy the shot in on the wreck, motor up to the shot and attach the lazy shot to it with the thin loop.

As the divers descend, they have the pull the loop down the shot line, then when it's uncomfotrable to pull it further, re-fasten it in a prussic type loop, using the jib clip to attach it to the lazy shot with the bail-out cylinder on it.

(Occasionally the divers may opt to jump in with the bottom end of the lazy in their hands, but on smaller boats it should be easy to motor up alongside and connect it from the boat)


Do the dive. At the end of the dive, make sure the shot isn't fouled at the bottom, come up the line and when at 21m, detach the lazy from the main line and deco in free water, using the lazy line as a reference.

The skipper will see this, and then know that he can safely lift the shot. Thats his problem now.... :)

Remember to have a nice easy rest on the surface, and de-kit fully before getting back into the RIB.

This technique works very well, and gives the divers a good reference to use during deco. I've used it on a hardboat as well as RIBs. When there are more than 1 pair of divers, then you can attach a tag to the jump line, and remove it on the way up with the last pair pulling the pin on the jib clip to let everyone drift freely. At that point, some people will deploy a DSMB to avoid crowding the line, but the line still gives them a reference and keeps everyone together which the boat skipper will appreciate.

The next stage up is using a trapeze system rather than just a line which is better suited for more divers, or deeper dives where you might have more than one emergency stage bottle clipped on.

Some people don't like coming back up the shot-line though, prefering to just bag-off on the ascent... In this case, I
prefer to bag up from the bottom, not halfway up. My reel has almost 90m of line on it. Much easier to deploy when hanging onto the wreck rather than being mid-water. (Although mid-water deployment is a valuable skill, but why give yourself the extra stress when you don't need to?) Personally I'd rather get back to the shot-line, especially on deeper dives with more people in the water as it's much easier to manage from the skippers point of view, and theres always safety in numbers!


Enjoy!

Gordon