Shore dive gas management

[pengwe]

Sometimes we do shore dives where we jump in off the end of the headland, then swim back along the reef. There won't be a lot of spots you can safely exit over the rocks, so we want to make it back to the designated exit point. Generally, the best part of the dive will be off the headland, and as you swim back into the bay, it gets shallower, dirtier, and less interesting.

How can I know how long to linger in the nice, clear, deep portion of the dive, and still leave enough gas to make it back safely?

 

[spectrum]

We use the rule of halves (calculated from your beach pressure) for dives like that in cold water and it gets us back with plenty. On the first half the cylinder gets the chilling hit, your usage will be higher and we return in a more direct fashion. After all that I still come back with too much.

Maybe it's hack gas planning but in cold water and on sites where you do not have a current concern on the way back turning sooner is a waste for us.

Pete

 

[H2Andy]

another way would be to find how how fast you travel up the headland to get to the exit point. say you come up with a 60 feet per minute rate.

now you need to know your SAC, and you will be able to calculate your cubic feet used per 60 feet.

for example, if your SAC is .6, then you know you will use .6 cubic feet of gas per 60 feet swam.

say the distance to be travelled is 2,000 feet, then you will need 20 cubic feet on the surface or (2000/60) x .6

at 40 feet, you will need 1.32 cubic feet per 60 feet you swim, or 44 cubic feet.

so you know that to make your swim back, you will need a minimum of 44 cubic feet.

in a AL 80, 44 cubic feet is about 1500 psi, so to turn back without any emergency reserve, you would turn back when you have 1500 psi left

since you can surface at any time and swim on the surface (i assume), you probably don't need much of an emergency reserve, say, 500 psi

so you end up with 2000 psi needed for return trip and emergency

which gives you 1000 psi to play with

 

[TSandM]

Basically, what you've got is a situation where you would PREFER to return to your starting point, but if you can't, you CAN surface. This is the situation where using halves is appropriate.

 

[H2Andy]

hmmm... do i understand the scenario to be that he splashes in, swims around in a nice spot, and then heads to the exit point?

i'm not sure halves would work in that scenario, since he doesn't know how much gas he needs to get him to the exit point

what if he uses 1500 psi but he needs 2000 psi to get back out?

i guess what you are saying is that since he can swim on the surface, he can surface once he is low on gas and swim up top the rest of the way

 

[pengwe]

I understand how to use halves to go out and back, but that's not what we do. We go start at Point A and go to Point B.

It's not crucial, because we can surface. I've had to do that a couple of times, when we had problems on the way back. I've also arrived back at the exit with plenty of air, which always seems like a shame, because we could have spent longer on the dive. I'm just wondering if there is a better way than guessing wildly.

Andy, are you saying I need to measure how much gas I use to swim a certain distance? Any hints on how to measure distance underwater? I find that really difficult.

 

[spectrum]

I think the problem for all of here is visualizing the OPs dive. Like TS&M described I have many sites where coming up early is just a minor inconvenience. Buddies who did not heed their gauges have had to do it, no big deal, they catch on.

Also the OP describes enjoying the "deep" part of the dive. Around here We rarely go past 40' on a shore dive so consumption and so forth are fairly forgiving. If he is doing significant depth (>70'?) there is a point where he at least wants to make sure he is back in the shallows.

Pete

 

[H2Andy]

Andy, are you saying I need to measure how much gas I use to swim a certain distance? Any hints on how to measure distance underwater? I find that really difficult.

ok, well... you got a point (in caves it's easy cause we know the distance between various points)

you can swim from a pre-set point to the exit then. that way you will know how much gas you need to swim to the exit from that point

add about 500 psi, and the rest is yorus to burn

 

[MauiScubaSteve]

Are you saying I need to measure how much gas I use to swim a certain distance? Any hints on how to measure distance underwater? I find that really difficult.

To have a full handle on underwater navigation you really need to be able to estimate distance traveled. Today my divers covered 100' in ~75sec (big newbie went faster than me?), he averaged 21 kick cycles, svelter buddie 28. On our very large triangle I counted 97 kick cycles to the first turn (24/100') with them at 92 & 132. My calculations are ~400', svelter one gets 470', bn got 440'. Kona crab on the second leg gave them a chance for a breather, all of us ready to make second turn within 15' of each other.

Third leg kc's let you know when to look around. Both of them spotted the starting point, one well before reaching it, the other when he counted 92 and looked to his right 10' (40' vis). Probably not quite 400' on a side, but with a 24' ave depth we used ~500psi, 640psi (both 80cf tanks) and 850psi (100cf) respectively.

With training you could learn how to continue getting better.

 

[S. starfish]

Hmmm... lot's of complex planning going on here...

There's a local dive around here that I like to do which goes form point A to point B, ending up in a bay. You could try and calculate how much gas you're going to use depending on kick cycles, or how fast you're moving and all that, but once you stop to look at a fish or there's a different current than normal I think that's pretty much out the window.
Best thing to do is keep it simple. The only bit of real gas planning you should have to do is to ensure that you have enough to get you and your buddy to the surface in case someone is ooa. After that the best thing to do is just get familiar with the site so that you get an idea of where you are exactly and how much you're going to need to get to shore. If you get low on air (there's only your reserve left) then just surface and swim in, no biggie unless there are stupid surface currents.