Trim

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Oh I see what your saying, the butt of the AL80 is more bouyant than the neck.
 
jeckyll:
Brandy: This probably isn't a DIR specific question and I'm pretty sure that the DIR folks aren't the only ones who can trim out, though some might disagree :wink:
Sorry jeckyl ,I diddnt think you canadians were so sensitive.:no
 
No problem asking a question like this in the DIR forum. You'll just get DIR answers, is all.
 
TSandM:
No problem asking a question like this in the DIR forum. You'll just get DIR answers, is all.
Thats why I asked it here. From watching dir guys in the caves here in florida ,they have trim to a science.Including factoring in tanks,neg or pos,and weight placement. Thanks guys and girls
 
TSandM:
get yourself horizontal and really flat, with your knees bent, and try to hover and see what happens. Low feet can be low knees, or legs extended too far, or too much weight low on the body. Also, extending your arms out in front of you weights the front of your body, too.

I'm with you. Bend knees = move CG forward = finding the sweet spot after some practice. His feet can't weigh that much!
 
If you bend your knees, you might have to frog kick :wink:
 
BRANDY:
Oh I see what your saying, the butt of the AL80 is more bouyant than the neck.
Not exactly - AL80s exhibit a characteristic called a "righting arm", which is a moment of inertia that results from the center of buoyancy not being the same as the center of mass/gravity. This is a phenomenon that is well-known in boating - it is the effect that causes boats to want to return to vertical (or "right themselves") when they are rocked by waves.

The center of mass on an AL80 shifts forward as the gas is consumed, which causes the valve of the tank to want to drop (which causes the tank to want to rotate into a bottom-up position). Steel tanks are less affected by this, because their center of mass is further from the valve.

Several people have suggested moving the BC further down on the tank to compensate for this. That's actually not that great a suggestion, because it doesn't really overcome the righting effect. All it does is mask it by changing the location of the tank's centers relative to your own body's centers. Moreover, it is "self-limiting" in the sense that it only gives you so much play, because you can only move the BC so far. It also does not work on all cylinders or cylinder configurations.

Take doubles for example - most doubles tend to be head-heavy, which tends to force the diver into a head-down position (an effect that is particularly pronounced on steel doubles). Logic dictates that to overcome this, you move the BC up on the tanks, by repositioning the bands. The only problem there is that in most cases, you can move the bands as high as they will physically go, and you will still be head-heavy!

It's clear that another, more general solution is going to be required to solve this problem. With doubles, the way we compensate for this is to more fully extend our legs. With Jetfins on (which are negatively buoyant), this increases the moment arm to the diver's rear, which brings him or her back into a more level trim position.

So it's mostly about weighting and body mechanics/positioning, not where the tanks are in the bands. The DIR approach to this problem is for the diver to adjust his or her body to the tanks, not the other way around. This will allow you to properly trim out almost any set of tanks, independent of their buoyancy, what type of exposure suit you are wearing, etc. It's really surprising just how much control you have over this type of thing, just by changing the position of your head or legs.

The key first step however, is to make sure you are properly weighted. My guess is that 10 lbs is too much for you now, and that excess weight is forcing your hips and lower body down. Do the weight check and try making some adjustments with your legs - if you can't get that to work, then start thinking about where or how to redistribute the weight.

This stuff is not that hard to do, but it does take a little bit of practice and some good feedback from someone who knows what to look for. Once you get it dialed in though, it will make you feel much more stable and comfortable in the water.
 
DIR-Atlanta:
Not exactly - AL80s exhibit a characteristic called a "righting arm", which is a moment of inertia that results from the center of buoyancy not being the same as the center of mass/gravity. This is a phenomenon that is well-known in boating - it is the effect that causes boats to want to return to vertical (or "right themselves") when they are rocked by waves.

The center of mass on an AL80 shifts forward as the gas is consumed, which causes the valve of the tank to want to drop (which causes the tank to want to rotate into a bottom-up position). Steel tanks are less affected by this, because their center of mass is further from the valve.

Several people have suggested moving the BC further down on the tank to compensate for this. That's actually not that great a suggestion, because it doesn't really overcome the righting effect. All it does is mask it by changing the location of the tank's centers relative to your own body's centers. Moreover, it is "self-limiting" in the sense that it only gives you so much play, because you can only move the BC so far. It also does not work on all cylinders or cylinder configurations.

Take doubles for example - most doubles tend to be head-heavy, which tends to force the diver into a head-down position (an effect that is particularly pronounced on steel doubles). Logic dictates that to overcome this, you move the BC up on the tanks, by repositioning the bands. The only problem there is that in most cases, you can move the bands as high as they will physically go, and you will still be head-heavy!

It's clear that another, more general solution is going to be required to solve this problem. With doubles, the way we compensate for this is to more fully extend our legs. With Jetfins on (which are negatively buoyant), this increases the moment arm to the diver's rear, which brings him or her back into a more level trim position.

So it's mostly about weighting and body mechanics/positioning, not where the tanks are in the bands. The DIR approach to this problem is for the diver to adjust his or her body to the tanks, not the other way around. This will allow you to properly trim out almost any set of tanks, independent of their buoyancy, what type of exposure suit you are wearing, etc. It's really surprising just how much control you have over this type of thing, just by changing the position of your head or legs.

The key first step however, is to make sure you are properly weighted. My guess is that 10 lbs is too much for you now, and that excess weight is forcing your hips and lower body down. Do the weight check and try making some adjustments with your legs - if you can't get that to work, then start thinking about where or how to redistribute the weight.

This stuff is not that hard to do, but it does take a little bit of practice and some good feedback from someone who knows what to look for. Once you get it dialed in though, it will make you feel much more stable and comfortable in the water.
My thoughts exactly.I do weight checks,but not since the extra 10lbs of muscle.Its not that bad but as I advance in diving im beginning to realize how important it is in the springs where silt and overhead enviorments become an issue.I dont even notice in salt water but when things get a little tight I want my feet up more. Thanks for the great respose.
 
DIR-Atlanta is making some excellent points
 
Yes, tank position is mostly dictated by where the tank has to be in order to allow you to reach and manipulate your valves, so you don't have infinite flexibility in repositioning the tank. Once the tank(s) have been situated properly for function, then weight and body position are used to correct the trim.

There are some combinations of human body and tanks that never trim out comfortably, but as far as I know, that's only an issue with doubles.
 
https://www.shearwater.com/products/swift/
http://cavediveflorida.com/Rum_House.htm

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