The physics of trim and aerodynamics

[Blackwood]

You read the whole thing? Yeeeesh

I read most of it, and it seemed like more of a discourse about buoyancy than trim (which I think of as position in the water).

 

[MauiScubaSteve]

a trim tropical weather diver might have great trim with his BC set up when diving with swim trunk, and yet have terrible trim with the same BC and 25 lbs of weight with a dry or thick wetsuit (which compresses at depth, forcing the diver to add air to his BC to make up for the lost of buoyancy). So when an instructor tells me on SB that it is better for a new student to be overweighted, I just shudder at the illogical thinking. It is better for he/she to struggle with perfect weighting and learn to manage a small BC bubble, than to struggle with overweighting and the drastic volume change of a large BC bubble.

Am I wrong or have we seen you start this discussion a number of times and a number of ways?

First off, "an instructor on SB" could be anywhere from the Red Sea to Alaska or the Great Barrier Reef to Indiana, so it helps if all the instructors fill out their profiles so all the noobs can look at the profile to judge how valid the instructors perspective is to their own diving.

Second, many instructors will say "overweighted" when they actually mean the significant amount of weight necessary to complete beginning dives compared to after the student figures out the mechanics better. For the vast majority of beginners, "perfect weighting" is not something that is easy to dive with.

To dive with "perfect weight" you must have "perfect breath control", "perfect buoyancy control", "perfect trim", "perfect propulsion", and "perfect reaction to changes in the local conditions" (surge, current, depth,...), and there are very few perfect divers out there, much less perfect students in Open Water classes.

The perfect amount of weight might also change from month to month, or even week to week, depending on the physiology of the individual diver. If you are one of the few who gets it all and achieves perfection in enough categories to dive with perfect weight, I propose the following situation:

You are tourist diving Maui, on a 2-tank Molokini charter. Because of the stupid dive magazines you do the deep wall drift on the backside of Molokini, and then because of turbulent near shore conditions your second dive is the St. Anthony wreck (50'-60' depth). Near the turn point of the second dive, the guide spooks a white tip reef shark out of the artificial reef, and the whole group puts a little leg into it to follow away from the boat a little. On the safety stop your buddies Suunto computer is showing 4 minutes (mandatory), but he has only 300 psi left.

You plug him in to your octo and before the end of the SS you are both well below 500 psi. The trades have picked up and combined with the surf swell that forced your deep'ish second dive, the anchor line is jumping furiously. You are hanging on to each other, and hanging on to the yanking anchor line, and possibly not breathing as calm as any other SS in you life. If both of you were "perfectly weighted" by definition, would you be "perfectly weighted" for the situation?

It is not uncommon for experienced divers to dive with 4 lbs more weight than "perfect", because there are rarely any dives that are actually perfect (by definition).

Going back to students, giving them 2-4 lbs more weight than the amount currently needed for them to dive IS going to look like overweighting in hindsight when the diver has a few dives under his belt, but it is pretty arrogant for a non-instructor with less than 200 dives to make blanket judgments with limited sand to stand on.

 

[ClayJar]

I read most of it, and it seemed like more of a discourse about buoyancy than trim (which I think of as position in the water).

I'm going to guess you stopped about halfway through (around the second interstice). The second half is where the concepts of the first half are applied to trim. (Hehe, next time skim or skip the first half, instead. :biggrin


(The OP *did* ask for a chapter. All I wrote was a short essay. It's not really even an article, although with some large, pretty pictures, perhaps it could be. )

 

[Blackwood]

I'm going to guess you stopped about halfway through (around the second interstice). The second half is where the concepts of the first half are applied to trim. (Hehe, next time skim or skip the first half, instead. :biggrin

haha... will do.

 

[Hank49]

Too bad we don't have a good chapter on this matter. Any aerospace/submarine engineers out there?

My belief is that trim is a combination of static forces and dynamic issues. Center of gravity is changed every time you add air to the BC, and depends on the shape of the BC, and where your weight is suspended.

As close as we can get to this is a heavy steel backplate under a back inflate BC.

.

You just answered your own question. Anyone not diving a SS bp and wing is a lizard.

 

[RiverRat]

Interesting stuff. Especially since I've just started diving doubles and have to "retune" all over again to some degree. I'll have to print it out and read it all. Maybe I'll pickip a tip on something. Other than that I'm just going to keep diving and changing one little thing at a time while experimenting with shifting weight around. Maybe switch back from my AL plate to steel. Tweak the band location, wing location etc.

 

[fisherdvm]

Going back to students, giving them 2-4 lbs more weight than the amount currently needed for them to dive IS going to look like overweighting in hindsight when the diver has a few dives under his belt, but it is pretty arrogant for a non-instructor with less than 200 dives to make blanket judgments with limited sand to stand on.

If you care to read many of my comments, I usually say to take a few extra pounds over weighted to make entry easier. The first time you ever met me, you were not pleasant. Subsequently, it was the same. And now, you are saying I am arrogant? But name calling goes both way. I never said to be an authority on anything. Just simply picking the brain of experience folks who are willing to share without pulling out their "experience card". I guess when scubaboard require that you must have a gazillion dives before you can ask a question, I'll stop asking for opinions. But now you've clarified your comment on being "overweighted". Thank you.

But you are right on the rest of your comments, in fast currents and rivers, one would really want to be overweighted.

 

[Blackwood]

Interesting stuff. Especially since I've just started diving doubles and have to "retune" all over again to some degree. I'll have to print it out and read it all. Maybe I'll pickip a tip on something. Other than that I'm just going to keep diving and changing one little thing at a time while experimenting with shifting weight around. Maybe switch back from my AL plate to steel. Tweak the band location, wing location etc.

I use an aluminum plate with my steel doubles, and a steel backplate with my aluminum doubles.

 

[fisherdvm]

You just answered your own question. Anyone not diving a SS bp and wing is a lizard.

Actually, lizards are quite streamlined. I remember someone diving with a plastic bp and large wings, and they were quite out of trim.

 

[RiverRat]

I use an aluminum plate with my steel doubles, and a steel backplate with my aluminum doubles.

Interesting but makes perfect sense. I'm trying to find the "sweet spot" right now. I'm a bit head high so I'm experimenting with a little lead on the belt or maybe moving weight back up into my plate. I was told in another thread that I may actually be "head heavy" and I'm "compensating" by forcing a head high trim. Not sure just yet. I just got a new can light, so after I get my complete kit squared away, including reel etc. I'll see where I'm at with the AL plate. It may just be a technique issue anyway, not arching my back enough etc. I think the steel plate will put me a little heavy anyway, it's a 9#er. Lots of dynamics going on.