Misconceptions and Fallacies

[rjack321]

Bevis, meet butthead

It could be one of those Brown Trout from the Butt Dump they're talking about.

Gary D.

 

[JeffG]

Is this a trick?

http://www.cavediggers.com/Main_Page/Articles/Wetsuits/wetsuits.html

No...its just wrong.

 

[*Floater*]

My favorites:

1.) Ankle weights are a crutch for people who don't know how to achieve proper trim.

Ankle weights are used for lots of things, some tasks having nothing to do with trim but comfort of lead placement as one example. Also apparently more commonly used in the Northeast than other regions.

2.) You should have just enough lead to sink at the start of your dive by venting all air and exhaling.

You need to be able to sink straight down with your tank at 500 psi or less. If you can just barely sink with a full tank, you are underweighted.

3.) Excess lead or gear results in a measureable increase in air consumption.

Short of some extreme change in weight, there will be no measurable difference in air consumed or effort while diving. No measureable change in air consumption taking a deco bottle, two reels, ankle weights, etc. Adding a few more blasts of air in your wing or BC will make you neutral if you are overweighted. Dive #1 with X lbs and dive #2 with X+10 will not yield measureable results. Other factors affect air consumed much more notably including stress, tasks (hunting vs. cruising), current, etc.

4.) The amount of lead you carry is a measure of skill. "X diver only uses 10lbs of lead - wow he's good!"

You need to know the whole story. A neoprene drysuit diver with thick thermals and an aluminum tank will require more lead than a trilam diver using steel tanks for example.

To be continued...

--Matt

I don't disagree with the points per se, except maybe point 3 (depending on what you consider measurable), but I will say that point 2 sometimes works out in practice for wetsuit divers because the wetsuit is more buoyant at the start of the dive than at the end if it's been dry prior to jumping in. I know I've had to swim to get down on the initial ascent, even though I found out at the end of that dive that I was a pound or two overweighted. Also you can be a little positive on the surface with near empty tanks since the real goal (for me at least) is to be weighted so that you can be neutral on your last safety stop at around 10' with empty wing and near empty tanks, and at that depth your wetsuit has already lost maybe 20-25% of it's buoyancy due to compression of the neoprene. So if that 20-25% represents 5# for a given wetsuit, then I would be 5# positive at surface at the end of the dive with empty tanks and empty wing and still be correctly weighted.

Also the explanation of point 4 points to a potential fallacy, namely that neoprene drysuits take more lead to sink that trilam suits. This is of course true given the same undergarments, but in practice trilam divers tend to need thicker clothing underneath so often it's a wash, or could go either way.

 

[Charlie99]

This is one with which I must disagree with you. Buoyancy and mass are two different things. You can be neutrally buoyant with a 2000 pound anchor attached to you, but try moving that thing. Takes quite a bit of effort.

How about the more typical case of being overweight by 5 pounds or so. Some DMs or instructors do that regularly so they can pass weight to underweighted divers if needed. Several times I've picked up lead from the bottom during a dive and adding 5 or even 8 pounds didn't make a noticeable difference in air consumption. Yes, I had to add a bit more air to the BCD, but it doesn't materially change the shape. The drag is higher, but not significnant.

Mike F. has a nice analysis above, where he discusses the impact of TRIM. Post #33.

 

[vondo]

Everyone always stresses sreamlining and reducing bulk to be more efficient, and the BC is probably the biggest bit of kit we wear. Making it bigger by adding air to offset overweighting has got to increase drag, consequently increasing air consumption.

Plus, even though you are weightless when properly bouyant, you are still not massless and you are having to push more mass through the water. This may be a small effect, but it's not zero.

 

[GLENFWB]

No...its just wrong.

Ok, I will be the one to ask....So, why does a wetsuit work if it isn't the layer of trapped water??

 

[vondo]

Ok, I will be the one to ask....So, why does a wetsuit work if it isn't the layer of trapped water??

It's the neoprene that keeps you and the water from losing heat to the surrounding water. If you could build perfect seals that kept out any water so that the neoprene was right next to your skin with no intervening water, you would be just as warm (warmer actually).

Heat flows very easily from you into that thin layer of water. It doesn't flow from that water into the neoprene. If it was just a boundary between your skin and the neoprene, same thing.

 

[JeffG]

Ok, I will be the one to ask....So, why does a wetsuit work if it isn't the layer of trapped water??

The gas trapped in the neoprene.

 

[The Kraken]

How about the more typical case of being overweight by 5 pounds or so. Some DMs or instructors do that regularly so they can pass weight to underweighted divers if needed. Several times I've picked up lead from the bottom during a dive and adding 5 or even 8 pounds didn't make a noticeable difference in air consumption. Yes, I had to add a bit more air to the BCD, but it doesn't materially change the shape. The drag is higher, but not significnant.

Mike F. has a nice analysis above, where he discusses the impact of TRIM. Post #33.

Ya know, one of my problems is that I'm involved in QA/QC. All I deal with is a finite analysis, statistics, yes or no and so forth and so on. To wit, if one is carrying even one ounce of weight more than absolutely required, then it requires more energy, yadda, yadda, yadda . . .
. . . splitting hairs, I know . . .

I'll be sitting over there -----> listening . . .

the K-i Square Analysis

 

[JeffG]

It's the neoprene that keeps you and the water from losing heat to the surrounding water. If you could build perfect seals that kept out any water so that the neoprene was right next to your skin with no intervening water, you would be just as warm (warmer actually).

Heat flows very easily from you into that thin layer of water. It doesn't flow from that water into the neoprene. If it was just a boundary between your skin and the neoprene, same thing.

Almost. Its the bubbles in the neoprene that does the real work. Thats why when you go to depth in a wetsuit, not only does it lose it buoyancy, it also loses it thermal properties.