Center of Gravity

[SouthSideScubaSteve]

Is there a good method for determining where your center of gravity is when you are in a horizontal (swimming) position??

I ask because I have been gradually reducing he amount of lead I’m carrying to dive, but as I have been doing so, I am begging to struggle to stay in a horizontal position while swimming…. I dive in the quarries around Chicago with a 7mm full-suit, combined with a 3mm hooded vest. I started the season with 28#, dropped to 24 and then to 20#... On my last dive, I did a weight check with 600# left in the tank and think I can drop another 2#; but I also had great difficulty in staying horizontal while swimming. I am diving with a Black Diamond BC (back inflate) and am careful to position the tank (Luxfer S80) as high as I can without hitting my head on the valve. I have been consistent in loading the max of 10# into the “trim weight” pockets and have been reducing the weight in the ditch-able pockets ….

I had assumed that my COG was right around my ditch-able weights, but based on the problems I had last dive I am wondering if it is actually somewhere around my upper thigh?? This would explain why I am becoming more heads-up as I reduce total weight (finally, an application for my high school physics courses) . I figure if I could easily find my COG, it would be easier to dial in my trim & continue to dive with reduced total lead … otherwise, the only thing I can think to do is go back to 24# and see if that fixes problem

Any suggestions on how to find my COG (other fix the trim issue) would be greatly appreciated!

 

[ppo2_diver]

Most average bodies have a center of gravity around the belly button area. Once you find the COG, you should be able to simply go head up/down by breathing and leg movement.

 

[Diver0001]

If your problem is that your legs are sinking then you either have too much weight (which doesn't sound like your problem), or your weight is too far down on your body.

Some BCD's have trim pockets behind the shoulders. I don't know if the Black Diamond does but see if you can put 2lbs behind your shoulders to push the top of your body down.

Also, attaching a couple of pounds to the cam band that holds the tank on to the BCD might help.

It might also help to actually move the tank DOWN a bit, making the top of the backpack about level with the tank valve. An AL-80 acutally floats butt-up when it's nearly empty so moving it down may actually help to hold the lower part of your body up more.

Failing that, you might try renting a steel 12l tank (what is that? 100cf?). They come in two forms, short and fat and long and skinny. If you want to get your top half down more, then maybe a short-fat 12l/100cf steel tank will do the trick. It's worth playing around with at any rate.

Any combination of small changes can have a big effect too, so don't change everything all at once or you'll have trouble working out what you did that helped.

good luck.

R..

 

[knotical]

You can get an idea of your center of gravity (COG) by balancing yourself on a narrow bench. As ppo2 said, you’ll probably find yours somewhere near your naval. But everyone is different.

You also have a center of buoyancy (COB), and it’s not usually near your naval. Think about the air in your lungs, and you’ll realize your COB is higher. Then there’s the COGs and COBs of your equipment.

COB changes during the course of a dive as gas is depleted from your cylinder and released from your BC to compensate. Also as your exposure suit compresses and expands upon descent and ascent, and as the air in your BC shifts with your position. And even your breathing patterns.

Your COG changes with your position. Do you keep your legs straight or bent? Where are your arms?

The trick is finding and maintaining a compromise amongst all the resultant forces. I know of no reliable way other than what you are doing – experimenting.

 

[Walter]

If you are head up, lean forward and raise your legs. Switch to a knees bent frog kick.

If you are head down, lower your legs.

 

[Steve50]

ankle weight around the tank valve

 

[TSandM]

It may seem completely counterintuitive, but one of the causes of a knees-down diving position is being head-heavy, and it sounds like this might be your case.

Putting the tank high, and all your weight in the trim pockets, and wearing heavy neoprene, means you have quite a bit of lift on your hips and legs, and quite a bit of weight on your shoulders. This will tend to put you in a head-down position, but the temporary fix is to rear up and shorten the lever arm that the weight high on your body has to push you down. (This is a very common issue with people new to diving doubles, which make you head-heavy.)

The way to diagnose the problem is really pretty simple, but it requires some quiet water and a buddy. Get yourself into your normal diving posture, and achieve a horizontal position, even if it requires swimming forward to do it. Get your buddy to check that you ARE horizontal. Then stop moving altogether, and see what happens. If you are feet-heavy, you will slowly tilt that way. If you are head-heavy, your head will go down. This will give you the information you need, to know where to move weight.

An awful lot of trim problems can be fixed with good body posture and adjustments of knee bend and arm position, as well. Depending on the nature of your fins, for example, extending your legs can place weight at the end of a long lever arm, to balance head-heavy gear. Dropping your head will tend to tilt you head-down. Once you have your static weighting close to where it needs to be, you can play with body position and see what has the most constructive effect on your trim.