What is Trim?

[MoonWrasse]

40kg, that's trim

 

[JeffG]

How would you compensate for roll?

Why would you roll?

 

[dherbman]

Why would you roll?

The same reason you would pitch, imbalance.

 

[Uncle Pug]

Snipped from NASA:

When there is no rotation about the cg the aircraft is said to be trimmed

I would adjust that to read neutrally trimmed.

The reason I mentioned airplane is because an airplane's trim must be considered in the context of forces produced by movement through the air. A diver is moving through a much denser medium and the resultant forces are proportionately greater.

 

[Fastmarc]

I think my bc has pouches on the back on either side of the tank. Maybe I should move some of my weight up to there?
Could the weight there cause a tendency to roll over?

 

[Uncle Pug]

How would you compensate for roll?

That is harder, especially diving with a steel tank like I do. When I roll unto my side like I did swimming along a low relief wall last Saturday I need to keep from spinning on over to my back. I personally use a weightbelt and have the weights position anteriorly so that I am balanced on my side.

 

[JeffG]

The same reason you would pitch, imbalance.

Well, ankle weight on tank valve is pretty much in the middle, so no roll. Trim weight packs usually come in pairs. So one to either side would not cause a roll. If you put only one on with a lot of weight...maybe. I haven't rolled yet with tanks hanging on just one side though

 

[Uncle Pug]

and/or could bring his feet closer to his CG. ie Bend at the knees.

Fin weight is an issue as well. Using rubber (Jet or Turtle) fins you can change your orientation significantly in the water by moving them forward or backward.

Light weight plastic fins don't help much in this regard.

 

[Uncle Pug]

I think my bc has pouches on the back on either side of the tank. Maybe I should move some of my weight up to there?
Could the weight there cause a tendency to roll over?

It could cause you to roll over if you dive a steel tank and keep the weights on your belt toward the back. If however you keep the weights on your belt toward the front then you should be able to balance out.

The goal, IMO, is to balance yourself all the way around so that you can hang statically in any orientation without rotating to another orientation.

 

[nolatom]

Ah!!!! Like the trim on a boat. Got it...
Thanks guys!

Good observation, it's the same principle: "trim" refers to forward and aft orientation/depth, while "list" is side-to-side. On a ship, a little trim aft is a good thing--in diving, you try to be level, or what the old salts would call "even keel".

The airplane dynamics above are not simple, but probably more comparable to diving, since the same meduim (air) covers the whole craft, just as water covers all of you while you dive. A ship's partially in both mediums (media?).

You got way more info than maybe you needed, but try to dive level and you'll like it, and so will your ease of propulsion and air consumption.

And, in diving as in so many things, "the only dumb question is the one you didn't ask"--keep asking, and you keep learning.