Master Neutral Buoyancy: The Importance of Horizontal Trim (Simple Vector Physics)

[Eric Sedletzky]

I like this topic.
But I must wonder, what were the concerns and topics of discussion back before there were BC's, bubble management, and the resulting trim issues to worry about?

 

[The Chairman]

I like this topic.
But I must wonder, what were the concerns and topics of discussion back before there were BC's, bubble management, and the resulting trim issues to worry about?

None that I remember. When I bought my first pair of Jets from the illustrious Hal Watts, "Mr Scuba", at his Colonial Dr Dive Shop in 1969, the only selling point I remember was: "You can kick the crap out of the reef and not hurt these Jet Fins". Hal denied ever saying that, but with a smile and a wink, sheepishly confessed that it sounded like something he might have said. Once. Many years ago. Before he knew any better.

 

[archer1960]

But at least you know what weighting you need for 6'. Using 15' establishes a starting point and sets the minimum weight combo needed for any dive. After that it's just a pound here or there to adjust for 10' or 6' or whatever. The problem is not many people have a clue whatsoever about what weight they need to achieve anything, they just figure they have a BC to cover for it. This is a typical instructing problem, many times instructors themselves have no clue about what weights are needed, all they know is they don't want their students to cork so they weight the hell out of them to keep them planted on the bottom. However this bad info never gets rectified until the student either takes a PPB class or learns it on their own somewhere somehow.

Yup. The actual depth you use for checking your weights is largely irrelevant. It's the fact that you DO check and adjust it to be neutral with no added air in your BC under some specific set of conditions that matters. Once you have that, the rest is pretty easy.

 

[The Chairman]

In January? Are you serious? LOL yes I do.

Wait... are YOU serious? Are they too cold for your widdle footsies? Will your tushie freeze? The ones down here are heated if that's the case. Yah, it don't make sense to me either.

 

[scubadada]

HP100s are about 2" shorter than an AL80.

Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan

Cylinder	Service Pressure (psi)	Diameter (inches)	Height* (inches)	Weight Empty (pounds)	Buoyancy Full : Empty (pounds)
F7 Steel HP80	3442	7.25	20.8	28.3	- 8.1 : - 1.7
F7 Steel HP100	3442	7.25	25.3	34.0	- 8.4 : - 0.6
F7 Steel HP120	3442	7.25	29.1	39.2	- 8.8 : + 0.7
F8 Steel HP117	3442	8.00	24.2	37.6	- 9.1 : + 0.2
F8 Steel HP133	3442	8.00	26.9	42.4	- 9.1 : + 1.5
Comparison Only - Aluminum 80	3000	7.25	26.1	31.9	- 1.8 : + 3.2

From the DGX website, the current Faber steel tanks

 

[boulderjohn]

I like this topic.
But I must wonder, what were the concerns and topics of discussion back before there were BC's, bubble management, and the resulting trim issues to worry about?

None that I remember. When I bought my first pair of Jets from the illustrious Hal Watts, "Mr Scuba", at his Colonial Dr Dive Shop in 1969, the only selling point I remember was: "You can kick the crap out of the reef and not hurt these Jet Fins".

What I am about to say will run counter to what a lot of people like to think.

In the late 1990s, some diving pioneers including Al Tillman (NAUI instructor #1 and original director of the Los Angeles County System) put together a history of NAUI, the first dive agency in the modern sense. That history included a detailed description of the 1960 session in Houston in which dive instructors from across the country gathered to lay the foundation for NAUI instructional practices. In reflecting on that historic gathering, that history included the belief that the average OW class graduates in the late 1990s was starting off their diving careers with better skills than the average instructor had in Houston back when NAUI was founded.

Look at old videos of divers "back in the day," including the people diving with Jacques Cousteau. Divers back in the infancy of scuba did not have the god-like skills many people seem to think they did.

 

[gcarter]

Cylinder	Service Pressure (psi)	Diameter (inches)	Height* (inches)	Weight Empty (pounds)	Buoyancy Full : Empty (pounds)
F7 Steel HP80	3442	7.25	20.8	28.3	- 8.1 : - 1.7
F7 Steel HP100	3442	7.25	25.3	34.0	- 8.4 : - 0.6
F7 Steel HP120	3442	7.25	29.1	39.2	- 8.8 : + 0.7
F8 Steel HP117	3442	8.00	24.2	37.6	- 9.1 : + 0.2
F8 Steel HP133	3442	8.00	26.9	42.4	- 9.1 : + 1.5
Comparison Only - Aluminum 80	3000	7.25	26.1	31.9	- 1.8 : + 3.2

From the DGX website, the current Faber steel tanks

I have 2 of the HP117 tanks, and for me they trim out amazingly well, feel much more natural on my back than my Luxfor 80's or my PST 100's.

The tank used can make a huge difference.

 

[Eric Sedletzky]

What I am about to say will run counter to what a lot of people like to think.

In the late 1990s, some diving pioneers including Al Tillman (NAUI instructor #1 and original director of the Los Angeles County System) put together a history of NAUI, the first dive agency in the modern sense. That history included a detailed description of the 1960 session in Houston in which dive instructors from across the country gathered to lay the foundation for NAUI instructional practices. In reflecting on that historic gathering, that history included the belief that the average OW class graduates in the late 1990s was starting off their diving careers with better skills than the average instructor had in Houston back when NAUI was founded.

Look at old videos of divers "back in the day," including the people diving with Jacques Cousteau. Divers back in the infancy of scuba did not have the god-like skills many people seem to think they did.

"Divers are better now than they were then".
That depends on how you look at it. Divers then didn't know what they didn't have, divers now don't know what they have.
Divers back then wouldn't have known about BC's, spg's, computers, single hose regs/octo's, and a number of other things. They had to learn to dive with what they had. Training was a lot more rigorous too, more swimming, more theory on air usage, SAC rates etc. because you had to know how long that air was going to last without any instruments. Then there was harassment, and all sorts of other pool skills. It was also NAUI you were talking about. Yeah, I know a guy teaching a NAUI course in a university and it hasn't changed much since 1980, except for the introduction of modern gear, so yeah his class and divers are probably better than then, but that's his class. I don't think there are too many modern classes in run-of-the-mill recreational scuba schools like his full semester class.
But yeah, divers got heavy then and bounced off the bottom when they went deep, used their hands a lot, etc., but at least they had enough knowledge about weighting to be able to dive and pull it off. If you were to strip off all the modern gear from a student now and throw them in the deep end the way they dove back then most of them couldn't do it.
Aside of the lack of a buoyancy control device and the "hovering" that divers can do now, divers then left open water much more well prepared for a real world independent dive on their own then divers now...if that's what you mean by skills.

 

[stuartv]

Cylinder	Service Pressure (psi)	Diameter (inches)	Height* (inches)	Weight Empty (pounds)	Buoyancy Full : Empty (pounds)
F7 Steel HP80	3442	7.25	20.8	28.3	- 8.1 : - 1.7
F7 Steel HP100	3442	7.25	25.3	34.0	- 8.4 : - 0.6
F7 Steel HP120	3442	7.25	29.1	39.2	- 8.8 : + 0.7
F8 Steel HP117	3442	8.00	24.2	37.6	- 9.1 : + 0.2
F8 Steel HP133	3442	8.00	26.9	42.4	- 9.1 : + 1.5
Comparison Only - Aluminum 80	3000	7.25	26.1	31.9	- 1.8 : + 3.2

From the DGX website, the current Faber steel tanks

And older Faber 100s, PST E7 100s (3442psi) and older PST 100s (3500psi), are all right at 24".

So, I guess it depends on which HP100s you're talking about.

 

[BHB ScubaTroll]

After a few PMs and a phone call or two, it appears that I've really hit a nerve here for many, many divers. In that respect, it's been decided that I need video to competently convey the next few concepts. In those videos I will show you how to do just this pretty effortlessly.

I think you should put a bunch of us lard arsed divers in the water demonstrating trim and buoyancy... and one slim person who learns trim and buoyancy

---------- Post added December 24th, 2015 at 08:43 PM ----------

Hello NetDoc and BoulderJohn,

Where do you like to position the tank on ones back?

Thanks in advance,

markm

That will be different for everyone. I am a "rather large" (read as fat) guy, but my legs are still very athletic. This automatically makes them sink in both fresh and salt water... Given this issue, my tank is placed very high on my wing to "tilt" or "compensate" forward to help my trim in the water