O2ptimaCM and Trim

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addohm

Contributor
Messages
94
Reaction score
21
Location
Quincy, MA
# of dives
1000 - 2499
Has anyone tried LOWERING the unit to improve "heavy leg" trim? I know it seems counter intuitive but Im curious if moving the counter lungs closer to your hips changes the center of buoyancy downwards or not.
 
On a chest mount rebreather the priority, IMHO, should be work of breathing. Moving the unit down will likely make it more difficult to breath. This could be akin to cutting off your nose to spite your face haha. If a small adjustment downward doesn't negatively affect this and improves buoyancy go for it! Personally, if I could ride my unit higher to get even better immunity to position with my WOB I'd do it.

What's better I think is to think of your rebreather as fixed, delete it from your system in your mind and address your leg issue otherwise. This can be accomplished by 1. moving the effective negative buoyancy center of your weight system away from your feet, 2. using a drysuit and learning/improving your ability to move the bubble. In this situation you'd be adding just a bit of air to your legs for example. With a wetsuit tho... *shrug*

What could also be causing your trim issue is not heavy feet but over-inflated counterlungs creating a leg-down/head-up pitch. When your legs are low you can think of it as the location of your center of buoyancy, not the weight of your legs that's the issue, where your body attitude has to rotate to get your weight and lift centers closer together. Counteracting this with a little weight over the center of your counterlungs could work, but trying for closer to minimum loop is better (if this is actually the issue). I'm sure you know this but, this is accomplished by taking a full breath in and A. if you don't hit the bottom of your counterlungs let some out and adjust with another buoyancy space to get neutral or B. you hit the limit and add just a bit and adjust. Note also that being in a modified frog kick configuration moves your feet closer to your center of buoyancy (and easier to move by changing the angle of your knee).

In an ideal world your center of mass and center of buoyancy are in the same location whether looking from the top, side, or front so there's no torque and. your body can be in whatever attitude you like. In reality this is extraordinarily difficult to accomplish and we settle for prone/near prone.

If none of this works something more serious is going on, hit up your instructor. With time and experience you'll find yourself unconsciously adjusting the mix of buoyancy in your suit/wing/counterlungs constantly to suit whatever attitude you need, but it takes time to get there.
 
Heavy legs?
I dive it with a tail weight and jet fins to get motionless.
 
I’m not saying this is you but lots of people that think they are leg heavy are actually head heavy and they are just finding the balance point by going head up and dropping the legs. You may want to start with exploring that first.
 
I’m not saying this is you but lots of people that think they are leg heavy are actually head heavy and they are just finding the balance point by going head up and dropping the legs. You may want to start with exploring that first.
Hi Ben :) If I get neutral and don't move, my legs fall. With the same wing on OC sidemount, my trim and buoyancy are perfect. Id really rather not buy a second wing just to use with this rebreather, otherwise I wouldn't have gotten this particular model in the first place. Any additional suggestions are more than welcome. When I did my checkout dives, I actually did do it in my dry suit and I do remember specifically moving some air into my boots just to counter this problem. But now, there's no chance for me to be wearing my dry suit. Its just too damn warm down here :)
 
Heavy legs?
I dive it with a tail weight and jet fins to get motionless.
At the moment I dive it with no additional weight at all. I switched to neutral fins to see if that would help. It just slowed how quickly my legs drop when motionless. I used to have this problem with my xdeep stealth tec 2.0 when I first got it and ended up moving the wing downward on my body to shift the center of buoyancy. That worked perfectly. Before shifting the wing downward, I tried adding weight closer to my head but that really didn't work at all.
 
On a chest mount rebreather the priority, IMHO, should be work of breathing. Moving the unit down will likely make it more difficult to breath. This could be akin to cutting off your nose to spite your face haha. If a small adjustment downward doesn't negatively affect this and improves buoyancy go for it! Personally, if I could ride my unit higher to get even better immunity to position with my WOB I'd do it.

What's better I think is to think of your rebreather as fixed, delete it from your system in your mind and address your leg issue otherwise. This can be accomplished by 1. moving the effective negative buoyancy center of your weight system away from your feet, 2. using a drysuit and learning/improving your ability to move the bubble. In this situation you'd be adding just a bit of air to your legs for example. With a wetsuit tho... *shrug*

What could also be causing your trim issue is not heavy feet but over-inflated counterlungs creating a leg-down/head-up pitch. When your legs are low you can think of it as the location of your center of buoyancy, not the weight of your legs that's the issue, where your body attitude has to rotate to get your weight and lift centers closer together. Counteracting this with a little weight over the center of your counterlungs could work, but trying for closer to minimum loop is better (if this is actually the issue). I'm sure you know this but, this is accomplished by taking a full breath in and A. if you don't hit the bottom of your counterlungs let some out and adjust with another buoyancy space to get neutral or B. you hit the limit and add just a bit and adjust. Note also that being in a modified frog kick configuration moves your feet closer to your center of buoyancy (and easier to move by changing the angle of your knee).

In an ideal world your center of mass and center of buoyancy are in the same location whether looking from the top, side, or front so there's no torque and. your body can be in whatever attitude you like. In reality this is extraordinarily difficult to accomplish and we settle for prone/near prone.

If none of this works something more serious is going on, hit up your instructor. With time and experience you'll find yourself unconsciously adjusting the mix of buoyancy in your suit/wing/counterlungs constantly to suit whatever attitude you need, but it takes time to get there.
Thanks for your detailed response. I actually changed my ADV\MAV out with an MAV only valve to counter this. I thought the same thing. Too much air in the loop causing my torso to be very buoyant. At least, that was the idea. Yesterday I did a 2.5 hr dive and shifting the unit downward definitely did not help, but having an MAV only definitely helped keep the loop volume at minimum. I don't like having to press another button, especially with my already busy hands leading cave dives, but I managed. Im going to continue on with the MAV anyway. Using a drysuit down here is really not an option for me.
 

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