Too much lift?

[cool_hardware52]

There's tons of great feedback in here, thank you. Based on some of the conversation I've been trying to match up lift needs and the buoyancy of my kit.

Follow-up question - what is the average buoyancy of a full 3mm wetsuit and the corresponding loss of buoyancy at 15ft? FYI, I'm a pretty standard body size - 5'10" 190lbs, typically wearing a large wetsuit.

Wetsuits vary widely, the cut of the suit, the neoprene used, how many times it's been crushed at depth etc. The reality is a 3mm suit isn't going to be very buoyant to start with typically 3-5 lbs, and therefore it cannot change dramatically from the surface, i.e. 1 ata to 15 ft ~1.5 ata. Even if your 3mm suit lost 1/2 of it's surface buoyancy at 15 ft that's only about 2 lbs., anf it
s likely less than that. 2 lbs or less is not enough to get very excited about. 1-2 lbs is easily compensated with breathing. If you get your ballasting right withing 2 lbs or less you get a gold star.

7mm suits or 7mm 2 piece suits that can approach 30 lbs of initial buoyancy are another matter all together.

Tobin

 

[cool_hardware52]

There's tons of great feedback in here, thank you. Based on some of the conversation I've been trying to match up lift needs and the buoyancy of my kit.

Follow-up question - what is the average buoyancy of a full 3mm wetsuit and the corresponding loss of buoyancy at 15ft? FYI, I'm a pretty standard body size - 5'10" 190lbs, typically wearing a large wetsuit.

For divers using 3mm suits and buoyant al 80's:

An empty 80 is + 4 lbs and your suit is 3-5 positive. Lets use 4 lbs for the suit.

4lbs of tank and 4 lbs of suit implies diver that is close to neutral in a swimsuit will need about 8 lbs of total ballast to stay under water at the end of the dive. If you are 4'8" and 300 lbs you aren't neutral, but most of us are pretty close. 2-3 times a year I encounter divers convinced they are 10 lbs positive. I ask them to get in the water in their swim suit with a 10 lbs weight and see just how long they can tread water. Nobody yet has reported they had no problem doing so.

This often a good fit for a Stainless Steel Back plate. Most SS plates are about 5 lbs (a DSS medium is just a tad under 5 lbs) and the typical harness is about another 1 lbs. In reality it's a bit less too because much of the mass of the harness is the webbing and it's only very slightly negative. Let's say a medium SS plate and harness is more like 4 ~ 5 in the water.

Your reg is negative. For short hand I usually use 2 lbs. Depending on model it's probably more like 1 ~ 1.5 in the water.

So now we have a SS plate, harness and reg that combined provide about 5 to maybe 7 lbs of ballast in the water.

That is still a good match for a diver in a 3mm with a buoyant al 80. Many report that their plate, harness and reg is all the ballast they use while diving with 3mm suits and buoyant tanks.

The typical 3mm suit is likely about +3 at 15 ft. The typical al 80 will be about -3 at ~500 psi, that means the diver needs ~6 lbs of ballast and the plate, harness and reg provides it.

What if the diver needed another 1 lbs? Use your lungs.

What if the diver is 1-2 lbs over weighted? Rejoice they are doing better than 95% of recreational divers. Nearly everybody can swim up 2 lbs....

What if they are 10 lbs under weighted? Something is wrong and the basic assumptions need to be re examined.

Tobin

 

[REVAN]

There's tons of great feedback in here, thank you. Based on some of the conversation I've been trying to match up lift needs and the buoyancy of my kit.

Follow-up question - what is the average buoyancy of a full 3mm wetsuit and the corresponding loss of buoyancy at 15ft? FYI, I'm a pretty standard body size - 5'10" 190lbs, typically wearing a large wetsuit.

It depends heavily on your size, and the type of suit. I have a 3mm suit that is the same material type, cut and size as my 6mm suit (2 piece freedive style with farmer john). For reference, I'm a 6 foot 170 pound person.

In theory, it will have half the buoyancy of my 6mm, and should come in at 7.5 pounds. In reality, it is a little less than that now because it gets 10 times as much use as my thick suit and has been past 200 feet several times now. It's now closer to 6.5 pounds at the surface. At 15 feet, it will be about 4.3 pounds buoyant.

My freedive suits are made from Yamamoto 45 neoprene. That has more gas bubbles and less neoprene than a typical cheap scuba wetsuit (usually made from a 65 or 55 material), so it will have more buoyancy than is typical for the thickness and size of the suit.

 

[admikar]

A grossly over weighted diver swims out using his huge BC, descends, panics, kicks to the surface, doesn't ditch ballast, doesn't add gas to his BC, fatigues, and is found dead on the bottom with their weight belt on and gas in their cylinder.

This entire sequence started with the gross over weighting, which the huge BC's enable.

I understand what are you trying to say, but.......
I am a little confused...since diver adds no gas to BC, how come huge BC is a problem here? Diver will get drowned with a tinny BC with no gas in it all the same?

I made a tow system in my 20 meter pool. An operator pulls the tow line with a fish scale at a fixed force, like 20 pounds. This tow line pulls the diver through the water at some achieved speed. Another operator times the towed diver between fixed markers to calculate the achieved average speed. Note that this measurement must be done after the towed diver's speed settles to a relatively constant value. With average speed and force you can calculate a drag coefficient. This measurement gets taken multiple times and averaged to knock out the variables and noise in the measurements. It gets pretty time consuming.

Real question here is, what is average speed used to calculate your 40% increase? I am not saying your results are wrong, but if I want to prove a point I can choose a speed that would be 10% or 100% increase. At last, I got the picture, when I dive I don't race.

Very much agree with the last 2 sentences. I had always heard the claim that a BP/W had less drag than a stab jacket, but I never saw any numbers to back that up. To me, it seems like whether the bubble surrounds your body, or is on your back, it wouldn't make a big difference for drag. If anything, I could see a stab jacket giving someone a seal-like profile (as opposed to I guess a Mickey Mouse head profile), and being beneficial in terms of drag, compared to a BP/W. But Im no scientist or engineer, so I'd like to see smarter folks than I come up with the actual numbers.

And Im not trying to defend stab jackets -- they are not my preference. However, I'd just like to see them discussed based on hard facts, not biased views.

For me it is not about drag, it is about fit. I tried stab jackets, I tried back inflate, and I tried BP&W. Last option took most of fidgeting, but I get the best fit. So, it suits me the best.

 

[cool_hardware52]

I understand what are you trying to say, but.......
I am a little confused...since diver adds no gas to BC, how come huge BC is a problem here? Diver will get drowned with a tinny BC with no gas in it all the same?

I've said it repeatedly, but why not one more time. Huge BC's allow divers to diver grossly over weighted. All is good until that day that things go wrong. The day the BC malfunctions (rare) or the day the diver panics (far more common) and then the cascade starts.

My definition of minimum ballast is just enough to allow the diver to maintain a shallow safety stop with near empty tank(s) If a diver is weighted this way, and is using a reasonably sized BC they simply cannot be grossly over weighted. Slightly maybe, but not by 10-16-20 lbs. If they try to weight themselves with a gross excess of ballast they will discover it on the first dive, not that day when the poo hits the fan.

I've never claimed that huge BC *require* a diver be over weighted, or that huge BC's force a diver to be over weighted. However huge BC's to allow poor practices. This isn't a difficult concept.

Tobin

 

[The Chairman]

Oversized BC permit gross over weighting. It's really that simple.

Ignorance permits gross overweighting. You go ahead sell more of your gear to solve the problem and I'll educate divers instead. It really is that simple. BCs don't overweight divers. Divers overweight themselves. Putting them in a BC that won't work for all their diving is not a good idea.

 

[cool_hardware52]

I You go ahead sell more of your gear to solve the problem and I'll educate divers instead.

Ah back to assign motives. Completely uncalled for. And patently false.

I'd challenge you to find a single diver I've helped select a BC that I didn't educate.

Tobin

 

[The Chairman]

So here is my real world thoughts on "drag" when it comes to recreational diving. The entire idea is highly overrated and makes for good conversation on SB.

Probably the sanest thing posted in this thread.

Drag is important if you're in a race. Compared to the total profile of the diver, wing drag is almost inconsequential.

Ah back to assign motives.

No motives were assigned. No need to make up crap to defend your position. No need to act like you've been maligned. You're throwing gear at this problem and I'm throwing knowledge. It's just that simple. You're approaching this as a manufacturer. I'm approaching this as an educator. Again, it's just that simple.

 

[decompression]

Ignorance permits gross overweighting. You go ahead sell more of your gear to solve the problem and I'll educate divers instead. It really is that simple. BCs don't overweight divers. Divers overweight themselves. Putting them in a BC that won't work for all their diving is not a good idea.

Gotta say Pete, I understand your point, but far too many fresh off the IDC bus use the convienience of overweighting as there is little consequence during OW and they don't really question what the senior dive instructor says because " they've been doing it that way since Christ was a lance Corporal ". Further, this is enhanced by Manufacturers making a limited amount of BCDs that don't have 55# from small to large.

I think things would be different if they had to weight properly from day one.

 

[The Chairman]

I think things would be different if they had to weight properly from day one.

You can give into the ignorance of others. I'm not going to. I'm not even going to substitute even more ignorance to try to make up for it all.

Never throw gear at a problem when education is really what's needed.That's especially true when the pendulum swings to the point where people die because their BC is too small. I'm sorry that position seems to piss a rather few people off. It shouldn't. They have their own agendas. Mine is education and knowledge.