Good Trim and the Fin Pivot?

[ClassAction]

Hi,
I'm brand new to diving and trying to get my "trim" under control. I'm about to testdrive my new BP/Wings with tank weights. It occurred to me that the fin pivot drill just confirms that you're not weighted properly. You inhale, and your upper body (only) rises, exhale and you sink. Does this make sense? I've read that if you have proper trim/weighting, your body should be horizontal in the water. Any insight? Thanks.

 

[WreckWriter]

It makes sense but its wrong. Fin pivot comfirms proper neutral buoyancy but its very rudimentary exercise of little value.

 

[rab]

I hadn't really thought about this before (I'm also new to diving), but I agree. There were a few in my OW class that had to have their fins held down. Hmm.. I guess they were already in fairly good trim! It does seem like this "drill" would promote poor trim, but I suppose if all your weight is concentrated on your hips with a weight belt, you're likely to be properly weighted for the fin pivot. :33:

Hi,
I'm brand new to diving and trying to get my "trim" under control. I'm about to testdrive my new BP/Wings with tank weights. It occurred to me that the fin pivot drill just confirms that you're not weighted properly. You inhale, and your upper body (only) rises, exhale and you sink. Does this make sense? I've read that if you have proper trim/weighting, your body should be horizontal in the water. Any insight? Thanks.

 

[ClassAction]

I agree that it shows that you've achieved neutral buoyancy (even if you're over-weighted, you can just add more air to your BC). But doesn't the fact that you're pivotting on the axis of your fins show that your weight isn't distributed properly?

 

[ScubaRon]

In theory you're right. But reality is that as you inhale your lungs expand in multiple directions and exactly compensating for this is not humanly possible. Ergo there is no such thing as "perfectly horizontal".

The fin pivot is really only a technique to demonstrate that you are able to:
[1] use your BC to achieve neutral buoyancy
[2] use your breathing to limit vertical motion to approx. 1 foot
I don't think the instructor cares whether you actually pivot or rise in a horizontal position.
ScubaRon

 

[MikeS]

Hi,
It occurred to me that the fin pivot drill just confirms that you're not weighted properly. Any insight? Thanks.

A fin pivot drill does not address weighting or trim. It’s a simple exercise to demonstrate how lung volume affects buoyancy.

It makes sense but its wrong. Fin pivot comfirms proper neutral buoyancy but its very rudimentary exercise of little value.

IMO it does not address neutral buoyancy as your fins are still on the bottom, implying negative buoyancy. The purpose is to demonstrate how lung volume affects buoyancy. In this context it’s of great value to new divers as an intermediate step to learning how to achieve neutral buoyancy. However, once you figure out how to achieve neutral buoyancy it it’s no longer of much value.

Mike

 

[jeadiver]

I agree that the Fin Pivot drill mostly serves to provide an illustration of the affect of changing lung volume. In other words, when you have the right amount of air in your BC to be neutrally bouyant at whatever depth, you can ascend or descend in the water column by simply breathing in. This shows new divers that they do not need to use the BC inflator to go up and down in the water column (i.e., over reefs or other obstructions), but simply need to take a deper breath.

Note: some people's legs are more bouyant than the rest of their body, making a fin pivot difficult. I've found that if most people will simply arch their backs and stretch out their legs the fins will sit on the bottom for the pivot maneuver.

 

[perpet1]

In theory you're right. But reality is that as you inhale your lungs expand in multiple directions and exactly compensating for this is not humanly possible. Ergo there is no such thing as "perfectly horizontal".

ScubaRon

EXACTLY! There is another thread around here someplace where I had to argue with somone about this exact point http://www.scubaboard.com/t8232.html.
There is no way (without using a drysuit) to ascend perfectly horizontal with either lung volume or even placing air in a BC. When you are perfectly horizontal with no forward motion this means you have established a "perfect trim". Now if you put air in the BC or in your lungs you upset the "perfect trim" and you will tend to go more vertical or at least towards vertical (hence a pivot anlthough I agree the fin pivot is a basic skill).

Now, if you have a drysuit, you can account for this by moving air in the drysuit and maintain the perfect trim and ascend horizontally (with practice).

And I agree, this skill is used only to establish that the lungs can be used to fine tune beuyency (NOT TO CONTROL AN ASCENT).

Pete

 

[MikeFerrara]

EXACTLY! There is another thread around here someplace where I had to argue with somone about this exact point http://www.scubaboard.com/t8232.html.
There is no way (without using a drysuit) to ascend perfectly horizontal with either lung volume or even placing air in a BC. When you are perfectly horizontal with no forward motion this means you have established a "perfect trim". Now if you put air in the BC or in your lungs you upset the "perfect trim" and you will tend to go more vertical or at least towards vertical (hence a pivot anlthough I agree the fin pivot is a basic skill).

Now, if you have a drysuit, you can account for this by moving air in the drysuit and maintain the perfect trim and ascend horizontally (with practice).

And I agree, this skill is used only to establish that the lungs can be used to fine tune beuyency (NOT TO CONTROL AN ASCENT).

Pete

LOL, you keep saying that it can't be done and I keep teling you that we do it on every dive. Check out the 5th D video you can watch them do it to your hearts content.

There my be no such thing as perfect but the buoyancy shift that takes place when you put air in the bc is partially compensated for by where weights are placed (center of buoyancy and center o gravity as close together as possible). The remainder of the compensation is done with body position as in the bend in the knees, arch of the back and raising or lowering of the head and shoulders.


When all are correctly combined it most certainly is possible to ascend perfectly horizontal with no foreward movement by just using the bc and breath control. It's how we do on every ascent.



Fin pivot...
When correctly trimmed the fin pivot takes real effort. One must severly arch the back to keep their fins on the bottom.

The principle is similar to what GUE calls the CG (denter of gravity) drill which consists of hovering in a horizontal position and then using changes in body position (as I mentioned above) to tilt fore and aft. Whith correct horizontal trim the legs can be kept on the bottom using those same techniques. Why do it? I don't know. I know instructors who wear ankle weights so they can demonstrate a fin pivot. LOL I just arch my back til it hurts cuz if I don't my feet come up with the rest of me. LOL

 

[neil]

MikeS said "A fin pivot drill does not address weighting or trim. It’s a simple exercise to demonstrate how lung volume affects buoyancy"

And that's all it is. If you are on the bottom you can't be neutral, can you?
The real problem with this exercise is PADI's insistance that it be performed in open water, so a great many divers think this is some kind of real skill. You spend effort getting trimmed and horizontal in the pool, and then take that big step backwards. No diver should ever have spent more than 5 minutes in their life doing a fin pivot. Mike F., be careful with your back, you aren't getting any younger, you know .
Neil