Good Trim and the Fin Pivot?

[detroit diver]

Uh oh. Somethings wrong here.

I completely agree with Genesis.

Aaaarrrggggghhhh.

This is one of the things that REALLY pizzes me off about dive training today.

I was taught to "fin pivot". Teaching me that was wrong, as it promoted incorrect trim and weighting (too much and in the wrong places!)

Of course its easier for the instructor to just throw lead at you rather than figure out WHERE you need weight and exactly how much. But without proper weighting, in the right place, good trim is almost impossible to achieve without MAJOR fighting all the time.

My g/f and her daughter also were shown this asinine exercise. We're still sorting out THEIR trim and weighting - they're CONVINCED they need more lead than they almost certainly really do, and of course it goes straight to their trim!

We WILL overcome..... Once the water warms up anyway (they're WWWs)

 

[yoda4x4]

Let me add a reasonably new divers opinion to this. While I'll agree that practicing fin pivot doesn't translate directly towards making you a better diver. For me, took it as a two-fold lesson. First, it shows a new diver how to begin to control your bouyancy with just the expansion/contraction of your lungs. And secondly, for me, beginning the exercise was some additional time practicing bouyancy control. How??? We'd start by letting the air out of our BCs. Then, go and find the "sweet" spot. For a new diver, with zero experience, how do you know if you are perfectly neutral? By adjusting the air in your BC with an instructor watching, you could feel what neutral felt like by following your instructors advice of increasing/decreasing the air in your BC.

Mind you, this is all my humble opinion.

David

 

[MikeS]

Uh oh. Somethings wrong here.

I completely agree with Genesis.

Aaaarrrggggghhhh.

That's scary! You should be placed in quarantine immediately; this could be an outbreak of Mad Diver Dieses.

 

[donacheson]

Don - ever dove with a BP/Wing?

I find that when you're dialed in - properly trimmed and weighted, moving your entire body in the water column comes automaticaly. Some are better than others - I have an instructor friend who has been described as having an elevator button in her butt somewhere.

Once you're dialed in, it's quite relaxing, anyway... I would hardly describe it as an "unstable equilibrium." When I want to ascend, I can horizontally ascend, all of me, at once. It's not hard. (or vice versa.)

Yes, I dive regularly with a BP/wing and I can, by controlling my breathing, go up and down easily over a limited distance. My daughter must also have an elevator button in her butt as well, since she can do it over a much greater range of depths and throughout most of the dive - having no need to add or dump air throughout a dive, being able to compensate with one's lungs, is an enviable skill.

The "unstable equilibrium" I referred to was perfect horizontal trim. All other things being equal, it's unstable. The same comment applies to being neutral; all other things being equal, go up a little bit and one goes up further and go down a little bit and goes down further. However, all other things are rarely equal - it's most unusual for a diver to be absolutely still and holding his breath. But it's fun to do sometimes just to see how neutral one can get and how good one's trim really is. It's not even necessary to hold one's breath - just be taking small, fairly frequent breaths and don't move. A good time to do this is while hanging during a safety stop in quiet water. It's difficult to remain horizontal at one depth for very long. I always seem to end up head or feet down and often changing depth.

 

[perpet1]

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

Mike, I agree with you that good trim comes when your center if gravity and your center of beuyency are very close. Now lets assume you have all the conditions necessary to be perfectly horizontal and your trim is tweeked. I will go as far as saying that your center of gravity and your center of bueyency are exactly the same.

NOW add bueyency to one end and not the other by doing something as crazy as putting air in your BC (last I checked that was on the upper part of one's body) or putting more air in your lungs. Without doing anything but hanging out superman style (horizontal) you WILL shift your center of bueyency and as such you will not be able to maintain a horizontal attitude without action.

I say this because physically that is what has to happen. Now you can employ techniques that will shift the center of bueyency back so that it stays roughly consistant but then you are contorting yourself and going out of horizontal (yes arching your back will have this effect but then you are not jsut going up horizontal with no intervention are you?)

I guess my simple mind looks at pjysics in simple terms of fources acting on an object. That said, I am probably Doing It Wrong (DIW).

Pete :rasta:

BUT that all said Mike, I would love to dive with you and check it out some time. If you are ever in New England look me up

 

[detroit diver]

That's scary! You should be placed in quarantine immediately; this could be an outbreak of Mad Diver Dieses.

Saw the doc tonite. He suggests a lobotomy.

 

[Genesis]

If you are PROPERLY weighted and diving a reasonable wet configuration (or ANY dry config!) you should be able to handle significant changes in depth (10'+) with nothing more than breath control.

Now with full doubles and such it gets significantly tougher, because you have more gas on board and as such you have more gas in the wing, and it likes to change lift as you change depth. I find that I have to pay a LOT closer attention to the inflator when diving doubles, especially in shallower water. Below 80-100' I can pretty much dive them as singles in terms of trim/buoyancy shifts, but the percentage change absolute as you rise goes up, and it gets tougher to control without using the wing.

With a single I can go between stops easily without having to touch the dump, and in most of my single configs I can make a full atmosphere of change in depth without having to adjust my wing.

If you're overweighted this is impossible; you're constantly chasing the goal and the gas is moving around and fighting you every step of the way.

The FIRST key to getting trim and buoyancy sorted out is proper weighting. Without that its a LOT harder than it needs to be, as you wind up with way more gas in the wing that you should have.

Instructors pay almost zero attention to PROPER weighting. If you're tense you won't be able to get down properly weighted - you need to be comfortable in the water. Ditto if you have gas trapped in things (exposure suits, BCs, etc.) The solution to this is NOT more lead.

 

[detroit diver]

Perpet,

I know you believe what you say is true, but I can attest that you are just plain wrong in your beliefs here.

When one is trimmed and weighted properly, the center of gravity is approximately the center of the chest. By inflating your lungs, you DO NOT change the CG and therefore your trim does not change. You can EASILY (when practiced) change your position in the water column-straight up and down- by just using your lungs. Actually, it becomes so easy that you don't really even think about it. And you NEVER go head up or feet up unless you wish to.

You maybe cannot do this, and obviously you have never seen anyone do it, but those with the proper technique use it all the time.

One place that it is taught is the DIR Fundamentals class. It's one of the basic requirements of that class. Solid trim, and solid bouyancy control. Of course there are others that dive this way also.

Find some of the DIR divers in your area, and ask them to go for a dive. Then report back to us what you see.

Mike, I agree with you that good trim comes when your center if gravity and your center of beuyency are very close. Now lets assume you have all the conditions necessary to be perfectly horizontal and your trim is tweeked. I will go as far as saying that your center of gravity and your center of bueyency are exactly the same.

NOW add bueyency to one end and not the other by doing something as crazy as putting air in your BC (last I checked that was on the upper part of one's body) or putting more air in your lungs. Without doing anything but hanging out superman style (horizontal) you WILL shift your center of bueyency and as such you will not be able to maintain a horizontal attitude without action.

I say this because physically that is what has to happen. Now you can employ techniques that will shift the center of bueyency back so that it stays roughly consistant but then you are contorting yourself and going out of horizontal (yes arching your back will have this effect but then you are not jsut going up horizontal with no intervention are you?)

I guess my simple mind looks at pjysics in simple terms of fources acting on an object. That said, I am probably Doing It Wrong (DIW).

Pete :rasta:

BUT that all said Mike, I would love to dive with you and check it out some time. If you are ever in New England look me up

 

[d33ps1x]

...would they agree to the dive though.

 

[detroit diver]

...would they agree to the dive though.

I think some of them would. One needs to inquire as to the options available.