How do I control my bouyancy and trim?

[perpet1]

MikeFerrara once bubbled...


This isn't correct Pete. If your center of gravity is in the same place as your center of buoyancy there are no apposing forces so assuming the same body position you'll stay balanced even as overall buoyancy changes and your whole body will rise. If those centers are offset then it's as you describe but if that's the case you won't trimmed.

However it is still possible to do a fin pivot but you have to arch your back lika a son-of-a-gun to keep your feet on the bottom (like you would want to. When you do this you're essentially doing something similar to the DIRF COG drill but with your feet touching the bottom.

You know you are right to a point. if you were entirerly correct you would never have to worry about trim as you would always be trimmed. you can create a situation where the two points are the same by using trim. Here I will use a flying analagy, where the center of gravity is almost always different the center of lift and this is adjusted the trim on the plane.

since you can adjust your buoyency on your upper body only you are shifting your center of buoyency BUT your center of gravity stays consistant relativly as long as you are in the same orientation. If you could adjust your center of buoyency over your entire body evenly then your arguement would be correct.

your center of gravity will change with attituce from vertical to horizontal and this is very much independant from your center of buoyency.

I agree when you are perfectly trimmed the two points are the same but add air to your BC your center of buoyency shifts while momentarily your center of gravity remains constant and your upper body will lift and you will be orientated at an angle. I agree your center of gravity will shift as a result but you will already have shifted positions.

lets go diving and you can show me a short ascent (say 10 feet)in the horizontal position just by using your BC. (i.e not forward movement)

EXAMPLE:
I know you or anyone else is not going to take my word for this so try this next time you are in the pool or shallow water (10 - 15 feet): (just remember this will not be a controlled ascent so be careful to breath and not be at the end of a deep dive)

lay flat on the bottom with no movement. Make yourself neutral. Now try to ascend to the surface without moving by only putting air in your BC. Make sure you are perfectly trimmed before you start. Keep you body in line with no arching your back or anytihg like that.

If you can maintain a perfect horizontal attitude all the way to the surface then you will have prooved your point. IPlease post here and tell me how it goes.

Thanks,
Pete

 

[MikeFerrara]

perpet1 once bubbled...


You know you are right to a point. if you were entirerly correct you would never have to worry about trim as you would always be trimmed. you can create a situation where the two points are the same by using trim. Here I will use a flying analagy, where the center of gravity is almost always different the center of lift and this is adjusted the trim on the plane.

since you can adjust your buoyency on your upper body only you are shifting your center of buoyency BUT your center of gravity stays consistant relativly as long as you are in the same orientation. If you could adjust your center of buoyency over your entire body evenly then your arguement would be correct.

your center of gravity will change with attituce from vertical to horizontal and this is very much independant from your center of buoyency.

I agree when you are perfectly trimmed the two points are the same but add air to your BC your center of buoyency shifts while momentarily your center of gravity remains constant and your upper body will lift and you will be orientated at an angle. I agree your center of gravity will shift as a result but you will already have shifted positions.

lets go diving and you can show me a short ascent (say 10 feet)in the horizontal position just by using your BC. (i.e not forward movement)

EXAMPLE:
I know you or anyone else is not going to take my word for this so try this next time you are in the pool or shallow water (10 - 15 feet): (just remember this will not be a controlled ascent so be careful to breath and not be at the end of a deep dive)

lay flat on the bottom with no movement. Make yourself neutral. Now try to ascend to the surface without moving by only putting air in your BC. Make sure you are perfectly trimmed before you start. Keep you body in line with no arching your back or anytihg like that.

If you can maintain a perfect horizontal attitude all the way to the surface then you will have prooved your point. IPlease post here and tell me how it goes.

Thanks,
Pete

I think you're still missing the point. By proper weight placement we get the centers very close. Certainly as you decend a wet suit compresses losing buoyancy and as the dive progresses the tank becomes lighter and there will me minor shifts which are compensated for with body position.

BTW, I always ascend horizontally without foreward movement using only buoyancy control.

The rig that I have the best control in over the widest range of depths and tank pressures is my double 104's and dry suit. Assuming a fairly constant air volume in the suit compression isn't a factor. Now, where are my tanks and wing? They're sandwiched together in the same place. When using a single tank and wet suit I put some weight up on the tank. Why? because that's where the wing and buoyancy is at depth when the suit compresses and I have some air in the wing. The center of gravity and buoyancy are very close together.

 

[GeekDiver]

Wow this is an old running thread.

Since I started this a while back ago (and 40 or so dives later) I thought I would chime in and give an update.

As to some of the questions sorry it’s taken me so long to respond back. I dive wet and fresh water at this point. I have a 3mil wetsuit and 12 lbs weight, at 10 lbs I’m a bit light. I have been renting steel tanks.

I put a quote from what I posted because it was what I think was a big part of what my problem was. I was way overweighted. I now dive with 6 lbs wt. (basicly the same gear except I now have al 80 tanks) I now use alot less air and have better control and a flater profile.

I think it's just something that comes with time and pratice. I can now let my breath out, sink to 5' and hold that depth with no air in my BC and an empty tank.

I think time underwater and being aware but not overly thinking about it is the best solution. I have always liked being underwater and felt secure. I had to stop worring about trim, boyancy and being an airhog which in turn gave me a better SAC, helped to reduce my wt and in the end gave me better bouyancy. Only time, patiance and learning to be truly comfortable in the water could bring about this change for me. Now I have much more control I can really relax and enjoy.

Geek

 

[CBulla]

NICE! Good to see that you've found a good comfort zone and your able to enjoy what you can see around you and not worry about all your underwater profile variables!

GeekDiver once bubbled...
Wow this is an old running thread.

Since I started this a while back ago (and 40 or so dives later) I thought I would chime in and give an update.



I put a quote from what I posted because it was what I think was a big part of what my problem was. I was way overweighted. I now dive with 6 lbs wt. (basicly the same gear except I now have al 80 tanks) I now use alot less air and have better control and a flater profile.

I think it's just something that comes with time and pratice. I can now let my breath out, sink to 5' and hold that depth with no air in my BC and an empty tank.

I think time underwater and being aware but not overly thinking about it is the best solution. I have always liked being underwater and felt secure. I had to stop worring about trim, boyancy and being an airhog which in turn gave me a better SAC, helped to reduce my wt and in the end gave me better bouyancy. Only time, patiance and learning to be truly comfortable in the water could bring about this change for me. Now I have much more control I can really relax and enjoy.

Geek

 

[perpet1]

MikeFerrara once bubbled...


I think you're still missing the point. By proper weight placement we get the centers very close. Certainly as you decend a wet suit compresses losing buoyancy and as the dive progresses the tank becomes lighter and there will me minor shifts which are compensated for with body position.

BTW, I always ascend horizontally without foreward movement using only buoyancy control.

The rig that I have the best control in over the widest range of depths and tank pressures is my double 104's and dry suit. Assuming a fairly constant air volume in the suit compression isn't a factor. Now, where are my tanks and wing? They're sandwiched together in the same place. When using a single tank and wet suit I put some weight up on the tank. Why? because that's where the wing and buoyancy is at depth when the suit compresses and I have some air in the wing. The center of gravity and buoyancy are very close together.

Don't get me wrong it is possible to achive a perfect trim. BUT when you add air to your BC the attitude will change and then you will have to do things to overcome it. All things being equal and you add buoyency on one end and not the other then your attitude will change (especially if you were at perfect trip to start with). I am glad you ascend horizontally as it must make venting your BC and drysuit that much easier during your ascent.

Next poll: how many people ascent horizontally (with no forward motion just flat and up). I am guessing not to many. I have actually never seen it.

 

[MikeFerrara]

perpet1 once bubbled...


Don't get me wrong it is possible to achive a perfect trim. BUT when you add air to your BC the attitude will change and then you will have to do things to overcome it. All things being equal and you add buoyency on one end and not the other then your attitude will change (especially if you were at perfect trip to start with). I am glad you ascend horizontally as it must make venting your BC and drysuit that much easier during your ascent.

Next poll: how many people ascent horizontally (with no forward motion just flat and up). I am guessing not to many. I have actually never seen it.

Shoot over to Gilboa this weekend there will be a bunch of folks doing it.

I don't know how else to explain it to you. You are right if the weight G center and bladder are in different places and a certain amount of air is required to balance it. However if the G center is in the same location as the bc (or close) then there is little or no shift when air is added.

In adition, If using a dry suit, the dry suit will have essentially a constant volume of air. Also for changes in depth with a constant tank pressure the bc will have a constant volume so balance doesn't change with depth. And if the tanks are the primary source of neg buoyancy (like my 104's) and are right on top og the bladder then changes in tank presure will also have little or no effect on balance.

BTW when I ascend horizontally I don't add ait to the bc. Starting from a neutral motionless hover, I just raise my head and shoulders slightly along with a deep breath or two to begin the ascent. As I ascend I'm dumping as needed of course.
Check out the 5th-d videos. There's a link around here someplace.

 

[MikeFerrara]

Pete,

Check out the videos

Just select the DIR Fundimentals class from the left and then select Horizontal ascent from the list of skills to the right. The diver on the left looks a little slicker than the one on the right. Notice the only time there is any leaning foreward is when the rear dump is used.


here

 

[saying]

... on "trim" and didn't really get any results that I liked. So here it is.

Tips on trim?

My legs are still dropping a little. It's not enough to be a hinderance, or even a nuisance. I'm just being perfectionistic about it. I'm trying to get it where I can float completely motionless completely horizontal and being able to get a little negative trim with the bend at the knees.

I've got the tank about as high as I can keep it without cold-cocking myself every time I try to look up.

I've tried weights in back trim pockets, but using even one pounders back there rolls me like a turtle pretty much instantly.

So, what to do? I've been thinking about trying to rig a one or two pounder around my tank neck. Or trying to find a way to strap little trim pouches really high on my BC somehow.

So I differ to the better judgement of ye experienced divers of the interweb. Any ideas?

 

[learn-scuba]

Remember there's more than one way to skin a cat... you can add weight up towards the head, or add buoyancy back towards the feet. What thinkness boots are you diving? Maybe thicker boots will give just enough added buoyancy?

Are you diving weight integrated or a weight belt or both? Make sure your heaviest weights are to your front as that will offet the tendency to turtle. Sounds to me like you have too much wieght behind you and not enough in front. That may also help shift your trim slightly.

If you move more weight to the front, then you should be able to add a little in the back without turtling. One option is to replace four weights of small size (two on each side) with two weights of a larger size (one on each side) which will help offset the tendency to turtle when you add trim weights in back.

Anyway... I'm just shooting in the dark sinceI don't know your current equipment configuration and can't "see" what is happening to you in the water.

 

[saying]

learn-scuba once bubbled...
current equipment configuration

Weight Integrated (Zeagle Ranger).

Wearing 18 lbs in fresh water. A five and a four shot bag on each side (One thing I didn't think about... I've been wearing the fives further back in the little seperated Zeagle pocket bags, I guess I could try them in the front?) The trim pockets I used were the Zeagle ones designed to go on the lower tank straps.

Oh, and 7mm Hyperstretch boots. Do they get thicker? Does Timberland make dive boots?