Trim versus buoyancy

[MikeS]

It has been an interesting day on the board with farting regulators and opinionated BCs. So I’m sitting here wishing it were time to go home and start packing for the trip to the quarry tomorrow and thinking about trim. I’ve come up with the following:

Buoyancy and trim are not directly related. However, some things such as adding air to a BC affect both. It is possible to have good buoyancy control with terrible trim and vise versa (although not likely). The more important of the two is buoyancy as a runaway ascent can injure or kill while making an ascent with a “feet up” orientation may look funny but is not inherently dangerous. With perfect trim you should be able to effortlessly float along in any orientation and be able to change orientation with a minimal effort.

The problem is that trim is dynamic. The reason being is that localized changes in buoyancy effect trim as well but differently. For example on descent, as the increasing pressure reduces the buoyancy of your wetsuit and air is added to the BC. Overall buoyancy remains neutral but localized changes in buoyancy will affect trim. (i.e. your feet are now less buoyant).

It would seem that the key to maintaining good trim is to first establish good trim for a given situation (i.e. at 15 fsw) and then to configure you equipment so as to minimize the effect on trim caused by buoyancy control.

So once you’re in the water, what buoyancy changes affect trim? I could only think of two, if there are more please help me out. One, as you change depth and add or subtract gas to your BC to compensate for changes in buoyancy due to compression. Two, as you use gas the buoyancy of your tank increases and you release air from your BC to compensate.

Mike

 

[landlocked]

How about switching your primary light from the right hand to the left! It is negatively bouyant. Adding air to your dry suit. Picking up an anchor, etc. Thats about it for me.

 

[sheck33]

MikeS once bubbled...


Buoyancy and trim are not directly related. However, some things such as adding air to a BC affect both. It is possible to have good buoyancy control with terrible trim and vise versa (although not likely). The more important of the two is buoyancy as a runaway ascent can injure or kill while making an ascent with a “feet up” orientation may look funny but is not inherently dangerous. With perfect trim you should be able to effortlessly float along in any orientation and be able to change orientation with a minimal effort.

It would seem that the key to maintaining good trim is to first establish good trim for a given situation (i.e. at 15 fsw) and then to configure you equipment so as to minimize the effect on trim caused by buoyancy control.

So once you’re in the water, what buoyancy changes affect trim? I could only think of two, if there are more please help me out. One, as you change depth and add or subtract gas to your BC to compensate for changes in buoyancy due to compression. Two, as you use gas the buoyancy of your tank increases and you release air from your BC to compensate.

Mike

I am not sure i completely agree there. My weightplacement and overall gear configuration allow me to swim with a horizontal 'trim', so trim is your body position in the water. Once i have my trim right by shifting weights/gear/arching your back or whatever else it takes, adding air to my BC to remain neutral or adding some air to prevent a painful squeeze does not affect my trim at all. Of course assuming you are weighed correctly to begin with, if you are too light at the end of the dive it will be very hard to have good trim or good bouyancy. As my tank gets more empty it gets more bouyant but i am weighed right so my trim is not affected by it. I agree that from a safety point of view good bouyancy control is more important than one's position in the water perse.

 

[landlocked]

Um... Never mind.

 

[MikeFerrara]

If your balast and buoyancy have the same center your trim will be the most consistant. It can still change some such as with a wetsuit that has a single lower layer but a double upper. As presure increases both ends change by the same percentage but that changes trim. With a dry suit you do away with this problem. You will see the same thing when wearing stages. Again if balast is placed correctly you minimize this but it doesn't go away.

 

[MikeS]

Perhaps I’m wrong here but I think that “trim” and “orientation” are two separate issues. Good trim means that your center of gravity is well places allowing you to achieve whatever orientation you desire effortlessly. Arching your back to achieve a horizontal orientation is an example of overcoming poor trim to achieve a desired orientation.

IMO the only thing that fin pivots are good for is as a demonstration that you breathing effects buoyancy. A task that allows you to touch the bottom does not teach good buoyancy control or good trim. I think that a much better exercise is to pick up a small object off the bottom without coming into contact with the bottom.

Mike

 

[MikeS]

MikeFerrara once bubbled...
If your balast and buoyancy have the same center your trim will be the most consistant.

That makes sense, what can I do to achieve that? I have a back inflation, weight integrated, BC. The things that I can thing of that will affect the center of gravity are:

Left to Right -- shift weights left or right

Head to foot -- position of tank in BC strap

Back to front-- shifting weights between back (non-dumpable pockets) to front (dumpable) pockets

It would also follow that the adjustments should be optimized at the average pressure (depth) and with a half full tank.

Mike

 

[MikeFerrara]

That is one of the advantages of the bp, it has weight and it puts it in the right spot. Then...instead of adding all the additional weight you might need on your waist (belt or integrated pockets) you can add it to the plate. You can put molded weights in the chanel of the plate (p-weight) or between the plate and doubles (v-weight). You only need enough ditchable weight to enable you to swim the rig up in the event of a bladder failure. In recreational diving where you should only be slightly negative with a full tank and neutral with a near empty tank you don't need very much ditchable weight. As an alternative to a p-weight you can add weight to the tank (tank weights or keel weights). In cold water you may want to switch to a steel tank. The important thing is to get the center of buoyancy as close as posible to the center of gravity. Then you minimize the aposing forces. The NACD cave manual "The Art of Safe Cave Diving" has a good chapter on this. In the tropics diving in just a swim suit the problem is reduced because you don't have much compression going on and never add much air to the bc. That is why some warm water divers with a jacket think they are doing ok. Take the same diver and put them in a 1/4 inch two piece suit with 20 pound of lead and they loose all control. By the time they decend a ways they are 20+ pounds over weighted with the weight on their waist and a bunch of air pulling the upper body up.

Even without a heavy wet suit and trim problems you still never be as streamline in any bc as you can be with a bp/wing.

I dive mostly cold water. I don't see more than one or two of a hundred with descent trim. I don't hardly ever see a diver with good trim and good finning technique. I love it when I see an instructor with a jacket bc, alternate stuffed in his pocket , counsol dangling on a 12 inch clip, hoses all twice as long as they need to be (except the primary which is too short) looping way out like a cowboys rope and plowing through the water head up using a forcefull flutter kick with split fins and blowing out the bottom for as far as the eye can see. Then I know why divers have to come to a chat board to learn this stuff. Then they only half believe us because their instructor is trying to sell them the latest in designer magna zippered bc's and telling them that they never heard of a bp/iwng.

When you see a diver perfectly horizantal, knees bent and herdly moving their fins (a relaxed modified frog) yet moving faster than you and..hardly any bubbles being exhaled you will know what I mean.

 

[MikeFerrara]

MikeS,
I rambled alot but I'm not sure I answered your question. If switching to a bp isn't an option you can still use tank weights to get some of the weight off the waist or a steel tank. I haven't made any p-weights yet so I put an ankle weight on the neck of my tank. It only weighs 1 1/2 pounds but that high up it does alot to shift the center of gravity. When adding weights to the tank I try to keep them as close to the bladder as possible (usually). Of course you can raise and lower the tank in the bands also. It's not that hard but most divers don't bother or even know that they are supposed to.

 

[MikeFerrara]

Swimming horizantal doesn't mean trim is good. Staying horizantal when not swimming means trim is good