Dive Planning Question

[1000RR]

New here... recently certified... me and my daughter (13) did our first independent shore dive (Blue Heron Bridge) a week ago (tons of fun). I seem to spend a LOT of time reading a LOT of things here on Scubaboard and have also read Bob's article on dive planning as well. I wanted to find out if I'm thinking about our upcoming (1st for us) boat dive this coming weekend in the right frame of mind in regards to planning.

First, there will be a DM in the water, although this person will be there for the boat, not just us. We'll be diving out of West Palm Beach, FL: and will be doing what they consider their shallower reefs (45'-60'). It is a 2-tank dive and I am pretty sure the dive outfit pretty much made it 2 shallower reefs (as opposed to one deep, one shallow) because of my communications with them and having us as two new divers, one being a Jr. This dive op came recommended for that reason as well (they're good w/new divers and Juniors). Also, there is a good chance our instructor from our OW class may try and join us to to tag along (and provide support in however it might be needed). Very awesome of this fella to go out of his way to do this.

We are both comfortable in the water so that is not of concern... but being new divers, we don't know what we don't know (or have not yet experienced). So this first boat dive I plan to let my daughter know that our focus will be being good buddies (side by side, paying attention, etc.) and paying attention to our ascent rate as well as safety stop. Routinely checking pressure and working on buoyancy are givens we'll be working on. I also want to not rely on our instructor or DM to understand when to turn around and would like to have our own target so in case something happens, we have the context in which to fit our reactions in to. I've only got two data points for each of us for RMV rates. One was during training, one was on our own shore dive when we were exerting ourselves quite a bit early on. Daughter was 0.50-0.70, I was 0.72-0.89 with the higher numbers being when we were exerting a bit more.

I'm renting her AL80's w/Nx and me HP100's w/Nx since she seems to breath like a mouse (and I do not). We have all our own gear aside from tanks. So here are my thoughts. I am going to assume at this point, that it'll be a drift dive and the reef will be under the boat when we begin (ie. there's no navigating out and back). So for our turn around pressure, I was going to use 60' (figuring that should keep us on the conservative side) and figure that we'd use a combined RMV of 1.75 cf/min (I just added our higher RMVs and then added a tad extra on top of that).

- Assent from 60' to 15' = 1.75*(37.5/33+1)*1.5min = 5.61cf
- Safety Stop for 3 minutes = 1.75*(15/33+1)*3min = 7.64cf
- Assent from 15' to surf= 1.75*(7.5/33+1)*0.5min = 1.07cf
- Total = 14.32cf

- For the AL80, 14.32cf translates to ~555psi, add 500psi for minimum ending pressure, my daughter's turn around pressure is 1,055psi.

- For the HP100, 14.32cf translates to ~ 493psi, add 500 psi for minimum ending pressure, my turn around pressure is 993psi, call it 1,000psi.

Do you guys/gals agree with this? Should I have additional considerations? I would mentioned we have AI dive computers too... but those only assume they're going to get me back to the surface, not both of us since they calculate ATR based on my consumption rate (not our combined).

Thanks in advance... new to all this and want to begin our journey with good habits.

 

[RyanT]

Let me congratulate you on dive planning. Early on in your diving career it can seem a little burdensome and complex, but it gets easier and really helps avoid surprises.

One thing I'm not clear on is your "combined" RMV. You should be calculating your gas consumption based on your individual RMV and then plan for the buddy who is the limiting factor in your team's gas supply.

 

[1000RR]

Hmmm, I think I am following you, but not 100% sure. The reason I combined hers and my RMV (and then added a little more to it) was we'd both be using one or the other's tank in an OOA situation so I figured adding ours together would equate to what we would use together on one tank. I think what you said above is another way of saying the same thing, yes? The numbers I have above show each of our RMVs... then I just added them together and put a little "fluff" on top of that to make the 1.75.

Is that ok?

 

[uncfnp]

Hi 1000RR

I will let others address your plans but I will tell you a little bit about drift diving WPB and if you want to share your dive ops I or others may be able to give even better info.

The dives out of WPB/Riviera Beach/Jupiter are all drift dives. The boat will drop you over or near the reef. The guide will pull the surface marker and drift along the reef while the boat follows. As new divers you will drift along with the guide until you hit your target end pressure and will then ascend with your buddy. In this case there is no "turn pressure" but rather a minimum ascend pressure. 750 psi is often used but as new divers you may wish to ascend when one of you hits 1000 psi. Most likely the guide will not go up with you so you need to be comfortable with free ascents and drifting safety stops. There will be no anchor line.

Ideally you will have a SMB, some boats in this area require one and will furnish if you do not have, and know how to inflate it on the surface if you do not surface with the guide and if the boat is not right there. Make sure you look and listen for boat traffic as you ascend.

We love drift diving WPB. You will have a great time. Its goliath grouper season so definitely plan on a GG dive if that is an option.

 

[1000RR]

Thanks uncfnp, great info.

Everything you mentioned seems spot on with what my research and understanding is as well... except I wasn't aware of the "minimum ascend pressure". I understand what you mean though. I guess me wanting to calculate a turn pressure on my own becomes even more important when you were describing what the DM will/might state the ascend pressure is. I want to know what that really means in regards to if I go up based on their direction of waiting till [say] 750psi and something now unplanned occurs. If I go past my own calculated turn pressure, I would now realize that I'm venturing into an area where if something happened I am going to be pushing both of us either coming really close on air OR having to reach the surface w/out a safety stop (which might not be a big deal, but not ideal I guess either).

Based on what you said as an example, if the DM said the ascend pressure is minimum 750, according to my calcs above, that would put us at both ending our dive right around 500psi... which although seems ok, I guess leaves little room for an unplanned event.

Do you usually just go back up based on what the DM suggests or do you typically have an idea what it takes to get to the surface based on your own RMV and do you take into account your buddies too?

GG time is definitely going on down there from what I read. Last year we were in the Keys and took our wave runners out 5 miles to Looe Key and did some snorkeling... had one of those giants right under our wave runner. That thing had to be darn near as long as the wave runner and those are ~11' long.

 

[1000RR]

Hi 1000RR

... if you want to share your dive ops I or others may be able to give even better info...

That picture is awesome! How big was that guy?

We're going out with Pure Vida Divers.

 

[snowdog61]

What uncfnp said and I'll add a bit. Depending on the charter, you may or may not be required to pull a marker buoy. If not, you are required to have an SMB. I will strongly encourage you to be able to launch an SMB from depth as there is a good bit of boat traffic and waiting until you are surfaced could put you at risk. There are some good videos out there.

 

[RyanT]

1000RR, ah ha, I see your reasoning now. I suppose combing your RMVs is one way to do it. Another is to just calculate the minimum amount each of your needs to ascend given your normal RMV, then just plan to ascend at a pressure that will give you enough reserve left over for your buddy's ascent. Fudging your own RMV a little higher into the overall calculations is also a good way to ensure no nasty surprises.

.

 

[1000RR]

What uncfnp said and I'll add a bit. Depending on the charter, you may or may not be required to pull a marker buoy. If not, you are required to have an SMB. I will strongly encourage you to be able to launch an SMB from depth as there is a good bit of boat traffic and waiting until you are surfaced could put you at risk. There are some good videos out there.

We both have our own SMB, mine is a DSMB w/spool, hers is a regular SMB. I have not tried to launch mine from depth yet, but did watch a handful of videos as you suggested. It doesn't look overly complicated but at the same time, like anything, probably a little clumsily at first until practiced.

 

[1000RR]

1000RR, ah ha, I see your reasoning now. I suppose combing your RMVs is one way to do it. Another is to just calculate the minimum amount each of your needs to ascend given your normal RMV, then just plan to ascend at a pressure that will give you enough reserve left over for your buddy's ascent. Fudging your own RMV a little higher into the overall calculations is also a good way to ensure no nasty surprises.

.

Thanks Ryan, so if I calculated I needed 250psi to ascend and my daughter (just for her) needed 250psi to ascend. Would you say to ascend at 750psi, or would you say to begin your ascend at 1000psi (assuming you'd like to have 500 psi when you are back on the boat)? Both would seem to get you back to the surface with enough air (turning at 750 or 1000) if there were an OOA situation, but if we turned at 750, I suspect one would be at ~0psi, the other ~250psi when back on the boat (if something bad happened... OOA).

Am I thinking correctly on that?

Thanks again for the help and insight!