Why is overweighting a bad thing?

[Cudabait]

Hey, Pug, You really got 'em going. However, Don Burke is right on with this one. To be simplistic-- you gotta do your right weighting in the water. The best place to do this is at 5m, or 15 ft. Done with what would normally be you final tank pressure at the safety stop. 400-500 psi. A lot of lesser experienced divers find themselves positive at safety stop and must work to stay down. This is sometimes caused by the less accurate weight calculatiion method conducted at the surface, It's good, but not as good as doing it like Don Burke suggested.

 

[jonnythan]

Custer once bubbled...


The exact opposite.

That's english for "The exact opposite".

He said- "That makes your buoyancy control trickier, especially at shallow depths and increases your workload. "

Especially in shallow depths, with a few extra pounds, I'm more stable.

I find it hard to believe that you'd think a Los Angeles class sub is more stable than a Ohio class sub.

But it could be me.

Repeat after me:

I am not a submarine.
I am not a submarine.
I am not a submarine.

 

[Don Burke]

Custer once bubbled...


The exact opposite.

That's english for "The exact opposite".

He said- "That makes your buoyancy control trickier, especially at shallow depths and increases your workload. "

Especially in shallow depths, with a few extra pounds, I'm more stable.

I find it hard to believe that you'd think a Los Angeles class sub is more stable than a Ohio class sub.

But it could be me.

That is possibly the worst analogy you could have chosen.

You aren't talking about changing your class of submarine, which involves changing the hard displacement. Hanging more weight on a diver is more like bringing a thousands of pounds of weight into internal tanks and compensating by putting a bubble in a main ballast tank. So in addition to the little bit of hull flex, you have a bubble expanding as you get shallow and shrinking as you go deeper.

If you are restricted to slow speeds (like say a knot or so), your planesmen and trim pump will get a real workout with no upside at all.

Been there, done that, got the T-shirt and the coffee cup(and the grey hairs).

You're kidding yourself if you think that bubble makes you more stable.

Don Burke
ETCS(SS) USN (ret)
former Diving Officer of the Watch for Dry Deck Shelter operations on USS James K. Polk SSN 645

 

[Cudabait]

Cudos to you, Don Burke. Well done!! I've just got a 100 ton license, but I can sink a boat as well as the next man!!

 

[Custer]

Don Burke once bubbled...


That is possibly the worst analogy you could have chosen.

You aren't talking about changing your class of submarine, which involves changing the hard displacement. Hanging more weight on a diver is more like bringing a thousands of pounds of weight into internal tanks and compensating by putting a bubble in a main ballast tank. So in addition to the little bit of hull flex, you have a bubble expanding as you get shallow and shrinking as you go deeper.

If you are restricted to slow speeds (like say a knot or so), your planesmen and trim pump will get a real workout with no upside at all.

Been there, done that, got the T-shirt and the coffee cup(and the grey hairs).



You're kidding yourself if you think that bubble makes you more stable.

Don Burke
ETCS(SS) USN (ret)
former Diving Officer of the Watch for Dry Deck Shelter operations on USS James K. Polk SSN 645

I'm glad you got a blast from the past. The bottom line is, more weight equals more inertia in a given circumstance, and we specifically talked about shallow depths. Assuming you have an adequate BC, put on 15 extra pounds (correctly attributed) and tell me if you're more or less stable.

 

[paulintexas]

i had a nice reply all done, and apparantely you can't use too many

quotes

or it erases the message (grrr)

Anyway, thanks for the responses, I'm here to learn

Now, perhaps a better way to ask is this:

"I know PERFECT weight is ideal, for all the reasons given, but what is PREFERRED, 3lbs too light, or 5lbs too heavy?"

As a new diver, I opt for the 5lbs too heavy at this point until I can really understand my dive gear (which is still being assembled piece-by-piece and has a lot of rented gear (getting less rented gear as I go!)).

Opinions on that?

 

[pipedope]

For a new recreational diver - correct weight to slightly light.

For a commercial diver - correct to wayyyyyy heavy. Sometimes you need to stay put in a current or fighting a big dredge or jet nozzle.

 

[Don Burke]

Custer once bubbled...


I'm glad you got a blast from the past. The bottom line is, more weight equals more inertia in a given circumstance, and we specifically talked about shallow depths. Assuming you have an adequate BC, put on 15 extra pounds (correctly attributed) and tell me if you're more or less stable.

Less stable. ...and the shallower you are, the worse it is.

Don't forget that most of the volume change comes in the top 33 feet. If we found a hole deep enough to squash that bubble to zero (impossible, but humor me), half of the volume change would be in the first 33 feet. In the real world, it always works out that more than half of the volume change is in the top 33 feet.

If you get your BC inflated with a bubble to displace the required 15 pounds of water at 15 feet, if a wave takes you up to 12 feet, the bubble displaces 15.98 pounds, making you light and taking you the wrong way.

If a wave pushes you down to 18 feet, the bubble will displace 14.08 pounds, making you heavy and taking you the wrong way.

You said "correctly attributed". I think you meant "correctly distributed". Plus or minus a little belt slip, the weight will stay where you put it. The bubble won't. If you put your head down, the bubble runs toward your butt, trying to force you head down. If you bring your head up, the bubble runs toward your head, trying to force you upright.

...and that extra 15 pounds of mass, once moving, makes it harder to correct yourself because it is moving the wrong way. If you live by inertia, you might die by inertia.

If you were talking about bringing a neutrally buoyant camera housing that weighed 15 pounds, we would be having a different discussion. That would give you some extra mass, but no changing buoyancy, which is the real problem with overweighting.

 

[Uncle Pug]

Cudabait once bubbled...
you gotta do your right weighting in the water. The best place to do this is at 5m, or 15 ft. Done with what would normally be you final tank pressure at the safety stop. 400-500 psi.

A primary goal in proper weighting is to be able to control your ascent during the last most critical phase of the dive: from safety stop to the surface.

Now a lot folks consider their dive over after the safety stop and just blow to the surface. This is unwise. A very slow ascent is much preferred. There are times I will take several minutes to do the last five feet.

Therefore... IMO you should be weighted so as to allow this kind of final control in your ascent. I don't think you can accomplish this with weighting yourself neutral at 15' with 500psi in the tank.

You will gain bouyancy in the last 15' not only from drysuit gas expansion but also from the material itself (unless trilam), especially if diving wet and more so the thicker the neoprene.

Proceedure: in very shallow water (~5') with only a couple hundred psi in your tank, no gas in your BC or wing, no extra gas in your drysuit and carrying weight in small pouches begin dropping weight 1#~2#s at a time. Find the weight where a full inhalation will cause you to rise and a full exhalation will cause you to sink.

This is your correct (minimal) weight. If you are a n00b you might want to carry an extra 2#s until you master your buoyancy control.

Ultimately you should be able to control your final ascent from 5' just using breathing at a 5fpm rate.
~~~~~~~
Now... for another reason why diving overweighted is bad (besides being tough on your drysuit as you crawl over the corral or roll amongst the urchins.)

A too heavy weight belt with offsetting gas in your wing will put you in the typical rototiller position: head up body inclined at a 45 degree angle.

This will direct the force of your finning down destroying the viz... but worse yet it will cause you to plan upward and this must be countered by more weight on the belt.

When you stop finning and the upward planning effect ceases you will begin to sink. This is why rototillers are constantly finning.

Not only will correct weighting allow your to control your ascent but it will also allow you to remain motionless while maintaining depth.

 

[Don Burke]

paulintexas once bubbled...


"I know PERFECT weight is ideal, for all the reasons given, but what is PREFERRED, 3lbs too light, or 5lbs too heavy?"

As a new diver, I opt for the 5lbs too heavy at this point until I can really understand my dive gear (which is still being assembled piece-by-piece and has a lot of rented gear (getting less rented gear as I go!)).

Opinions on that?

That's a trick question. My answer would be about two pounds heavy for a new diver, but by the time you figure out what you should really wear, you don't need the extra weight.