Trim weights

[jeffsterinsf]

Uncle Pug once bubbled...

...
Well... it has to do with the way you hold your body. The u/w world is dynamic and just trimming yourself with static weights will never really solve the ever changing problem of being trimmed out. But if you learn how to trim yourself using body *english* then you will be able to handle the environmental imbalances.
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Ok, I'm a bit puzzled by this one. Perhaps someone can elighten me on it. As I recall my physics, there are several possible forces in action here. Since I agree that the less energy expended wile diving to achieve a goal is the "best," we can eliminate forces generated by moving the hands, arms, fins, etc. Equally ignorible should be any contortions intended to change the position of the body within the constraints of conservation of angular momentum.

So that leaves two:

1) A downward force, effectively acting on the body's center of gravity (CG), and

2) An upward force, effectively acting on the body's center of bouyancy (CB).

So long as these two forces act in such a way as there is no moment generated (horizontal separation, i.e., CG in front of CB), the body should not rotate, but only rise or sink. If these forces are equal in magnitude, the body should not have a tendency to rise or sink. But as soon as they are not acting in a straight line, the body will tend to rotate. There are always two positions where there is no rotational moment, with the CG directly above or below the CB. Any other position will induce a moment that will need to be countered to prevent rotation (unless the CG is coincident with the CB). Other than those two points (probably close to upright or close to head straight down), if I can get my CG and CB to align at the same point, no matter what my position, head down, up, on my side, Buddah-oidal, I should not tend to rotate!

Now, when I swim, I know my feet sink. That tells me that my CB is closer to my head than my CG. If I can add weight or bouyancy in such a way as to bring the two together, I won't have a tendency to rotate in any direction. Since weight is a necessary evil (be it in lead or other scuba gear), and it is hard with even the fanciest bouyancy control device to pinpoint bouyancy to a specific location on my body, it makes sense to me to adjust my CG using "trim weights" (or moving my gear, as UP suggests) so that it aligns with my CB for a "nominal" amount of air in my tank and my BC device. By doing this, I should then only have the "environmental imbalances" (currents, eddies, etc.) to deal with.

Now that said, where the weights are placed is important. Remember that the CG is a point in three dimensions. If you place weights in a "bad" place, you can balance yourself in one direction, but make things worse in another. For example, a weight on the back of your tank might make you "balanced" while prone, but when you are upright, tend to tip you back. The "ideal" place would be on the line connecting your current CG and CB. Unfortunately, this usually runs through your body. I'm not much for surgical implants of lead into my sternum, which is about the perfect place. One option would be weights back and front -- so that the CG of the added weight is about the right place. I have not tried this in the water, but think about it when I select trim weight locations.

Then again, since your tank's CG and CB are about a foot behind your own line, and generally change about 5-6 pounds (depending on capacity), you'll need to pick the most appropriate air level to set your trim. Your BC device also sits behind you and changes during a dive. Physics doesn't help me picking that one, diving requirements do -- and I'm sure there is more than one thread covering when and why to have magically neutral bouyancy.


Now, if someone can get me a good airline ticket to some nice warm water, I'd be glad to perform some tests, video it all, and report back to the group

 

[MikeS]

Uncle Pug once bubbled...

You don't have to be respectful to disagree with me Mike

I added the respectfully because I felt like Luke telling Yoda he was using the Force wrong.


Mike

 

[seabass]

Uncle Pug once bubbled...


Now if you can't or won't *waste* a few tanks of tire filler to play with and practice finding the solution to poor trim then Uncle Pug cannot help you.

So what you are saying is that there does not exist a body type for which given the following equipment

Al 80
Al backplate
SS STA
No ditchable weight
neutrally bouyant

would not allow the person to be horizontally trimmed? The fact is that if my quadriceps, hamstrings and calves were slightly more muscular (2lbs more equivalent non bouyant mass) it might not be possible to have horizontal trim with that gear configuration regardless of arching my back and pulling my legs in. I am very close to that point, but shifting the tank does work for me.

 

[seabass]

I would also add that I am horizontally trimmed with my legs crossed and my arms in. I am not head heavy, I am foot heavy.

 

[divernva]

Sorry it's been almost a decade since my last physics course, and don't have my engineering books with me. But bear with me for a minute...

Moving the CG is the right track, but you don't necessarily need to position your weights to faciliate the movement of the CG. Leg extension essentially allows you to move the CG to counteracts the CB force you mention.

Think of the classic see-saw with a heavy weight on the left, and a lighter weight on the right. If the fulcrum is fixed 1/3 from the left, is it possible to balance the plank by varying the location of the lighter weight on the plank?

Same sort of concept can be associated with a divers body. Picture this: Head (heavy weight) to the left, fins (lighter weight) to the right and axis of rotation somewhere near the chest / abs. Legs extended, fins farthest point from fulcrum, the diver will most likely rotate clockwise. Legs curled, fins closer to the fulcrum, the diver will most likely rotate counter-clockwise. The diver should be able to find a leg extension range which provides little to no rotation.

Arm position (stationary as Uncle Pug mentioned) can help reduce or enhance rotation as well.

Moving the tank will work, but the delta in tank movement is minimal for safety reasons. If the tank is too low, the valve won't be reachable. If the tank is too high, the back of the head will hit the 1st stage. If you can reach the valves and avoid the 1st stage, you're in good shape.

Give the legs and arms method a shot. It just might work.
Ken

 

[seabass]

I understand the center of gravity/see saw analogy. What I have been trying to stress is that for certain body types such as muscular legs, it may not be possible. As I have indicated in my posts pulling my legs in isn't enough of a shift. Most of the weight in my legs is in my hamstrings and quads rather than my calves. My tank position is ok, although it does look a little odd, with my tank shifted, I can use the bend in my legs to shift trim. However I still believe it is possible for someone to have my equipment configuration that wouldn't allow them to shift the tank to achieve horizontal trim.

Because I am neutrally balanced, the solution would seem to me to remove the STA (which I only use for weight) and add additional weights. Adding them at the waist (on a weightbelt) would only exacerbate the problem. Therefore weights towards the head would be required. Those as I understand them would be trim weights (v weights for example in the channel of the BP) or additional trim weights such as a chest weight belt. In this case why would the trim weights be training wheels?

 

[divernva]

Are you arching your back with a slightly shoulder down trim? I think you mentioned that you were. But this would bring the farthest point of your thighs closer to the pivot point... as opposed to a completely flat horizontal trim. So instead of the stomach being the most bottom point, it may be your chest.


I also agree that trim weights are a sort of band-aid to the problem. But again at some point you have to consider comfort. I vote for trying a few more times...

Good luck!
Ken

 

[seabass]

divernva once bubbled...
Are you arching your back with a slightly shoulder down trim? I think you mentioned that you were. But this would bring the farthest point of your thighs closer to the pivot point... as opposed to a completely flat horizontal trim. So instead of the stomach being the most bottom point, it may be your chest.


I also agree that trim weights are a sort of band-aid to the problem. But again at some point you have to consider comfort. I vote for trying a few more times...

Good luck!
Ken

Sometimes it feels like people aren't reading my posts..

I am not using trim weights, tank shifting is enough to do it for me. I have reasonably good, although not perfect trim. When my tank is full I tend to roll onto my back when I am completely motionless, although horizontally front to back I am trimmed out ok. To keep from rolling onto my back I have to lean my body back and forth, like balancing on a beam.

My point is that I am borderline, which means that unless I am the six sigma freak of nature, it is likely that there are plenty of people with more muscular legs than me with a thin upper body (sprinters). For those people no amount of back arching or leg pulling is going to work. My point is that a few people on the message board deride trim weights as being a band-aid or training wheels, when for some people it may be the only way to get perfect trim.

I am going to start doing upper body weight lifting to see if I can get 2-5 pounds of lean upper body mass. That may help (is that a band aid? )

 

[divernva]

Is that what we're supposed to do here? Sorry I remember seeing that tid-bit... just slipped my mind.

I used to bike and play volleyball, so I know that particular body structure well. Didn't dive back then, so really don't have a solid solution for those folks. Upper body development sounds like it will help.

Good luck!
Ken