Are trim and buoyancy fundamentally related?

[MichaelMc]

Here is a video I have permission to use which I think demonstrates that trim and buoyancy are not related. There are a few things wrong with this diving - but you can't really say he isn't neutrally buoyant. You can see that his knees come up when he goes to frog kick, meaning that he is in trim for that part of the dive. The frogkick is a bit of a poor effort but that's not the point here.

That is a great video for this discussion. I might ament 'that part of the dive' to 'that instant'. I think his move prior to kicking shows how buoyancy, trim, depth, and propulsion are tightly related. If he did not do that little dive the body angle down move before the first kick, where would he go? I claim he would go up, as that is how his thrust will push him, and his wing will expand changing his buoyancy. If he were in horizontal trim already, that would not be the case, and he would not need that little dip the body angle down move. Without that move he can not kick without changing depth, if neutral.

 

[HantsDiver]

That is a great video for this discussion. I might ament 'that part of the dive' to 'that instant'. I think his move prior to kicking shows how buoyancy, trim, depth, and propulsion are tightly related. If he did not do that little dive the body angle down move before the first kick, where would he go? I claim he would go up, as that is how his thrust will push him, and his wing will expand changing his buoyancy. If he were in horizontal trim already, that would not be the case, and he would not need that little dip the body angle down move. Without that move he can not kick without changing depth, if neutral.

I agree. He clearly knows what he is doing and is making it work for him. I know the bloke in question and he's not a bad diver. Not up to my own standards I'd say but not awful either.

But the idea was (at least how I've taken it from comments on here) was that he'd need to be in trim to be able to stay neutral. And that divers who dive in a seahorse position are negative. I think this is more an example of him being lazy than poor buoyancy control personally. Which kind of proves you can dive like that and be neutral.

 

[MichaelMc]

I agree. He clearly knows what he is doing and is making it work for him. I know the bloke in question and he's not a bad diver. Not up to my own standards I'd say but not awful either.

But the idea was (at least how I've taken it from comments on here) was that he'd need to be in trim to be able to stay neutral. And that divers who dive in a seahorse position are negative. I think this is more an example of him being lazy than poor buoyancy control personally. Which kind of proves you can dive like that and be neutral.

You agree but are missing the big point.

He absolutely can float neutral in the water in whatever trim he wants. Float, not move.

He is making that 45 degree trim work for him by changing it back closer to horizontal each time he kicks. Showing the tight relationship. If he wants to move, without changing depth and hence buoyancy, he has to do his kick such that its force pushes away from him horizontally. Mostly that means being horizontal at the moment of kick. Setting aside odd positions like sitting upright in the water and doing a bent waist flutter kick to move yourself backwards.

You can dive like that, as long as you are happy changing back to horizontal for each kick. Or just continually kick and control everything by thrust. (Edit: But if you are neutral, you would have to do that continual kick horizontal or your depth will change..., it's all vector physics.)

 

[HantsDiver]

You agree but are missing the big point.

He absolutely can float neutral in the water in whatever trim he wants. Float, not move.

He is making that 45 degree trim work for him by changing it back closer to horizontal each time he kicks. Showing the tight relationship. If he wants to move, without changing depth and hence buoyancy, he has to do his kick such that its force pushes away from him horizontally. Mostly that means being horizontal at the moment of kick. Setting aside odd positions like sitting upright in the water and doing a bent waist flutter kick to move yourself backwards.

You can dive like that, as long as you are happy changing back to horizontal for each kick. Or just continually kick and control everything by thrust. (Edit: But if you are neutral, you would have to do that continual kick horizontal or your depth will change..., it's all vector physics.)

Again i actual agree with everything you are saying.

My point was more along the lines of if someone came on here and sais they were diving with someone who had trim like that people would automatically say they were negatively buoyant. Which I think we can all agree is not the case here. People assume that those who dive at 45 degree angle trim are bad divers. And I don't think that is the case.

I have come in for a lot of criticism rightly or wrongly for saying that trim is a party trick. However, would he be a significantly happier diver if he was parallel? Would he use significantly less gas? Would you look at him and think that is someone who needs to take a ppb course?

I happen to know the bloke in the video. And I also know that if he chose he would be able to be perfectly horizontal. In a wreck he would be. In open water he is lazy and can't be bothered. I don't think that's a bad thing really. It's not making him unsafe in an open water environment.

The thread was about getting people away from thinking that trim is fundamentally linked to buoyancy. People who didn't dive could interpret from reading some stuff on here that he was a poor diver who had no control over his buoyancy. Now there are many things I would critisise him for in that video - the frogkick being the major one. But not his buoyancy personally.

 

[RayfromTX]

Unfollowing this thread now.

 

[MichaelMc]

It is obvious that he has control of his buoyancy, because he stays there when he stops. He's just doing more work to move than he needs to, because he did not setup for static horizontal trim for this dive. He didn't want to, that is fine. I think he would be happier and use less air on that dive if his static trim were horizontal, he would not need all those extra moves, he decided not to set it up that way, fine. Particularly as he is not in horrid trim and is not near a silty bottom nor kicking the crap out of the reef. He clearly knows he needs to get horizontal before kicking if he does not want to go up. No one is saying that diver is unsafe.

Him being fine or safe is separate from the interrelation between buoyancy, trim, depth, and propulsion. A relation he demonstrates. I think we are concerned about those who are trying to learn diving and trying to understand how various things interconnect to make their diving easy or hard, more than those who do not dive.

 

[Ayisha]

...the idea was (at least how I've taken it from comments on here) was that he'd need to be in trim to be able to stay neutral. And that divers who dive in a seahorse position are negative.

The overwhelming majority of posters in this thread said that both buoyancy and trim are desirable, not dependent on each other.

You can't tell that he is neutrally buoyant at all. He is sculling his arms, he never completely stops and holds position. The videographer is all over the place. Every time the diver almost stops kicking, his legs drop. Each of his kicks creates a rocking motion up and down, so he may have too much ballast below his centre of gravity. He would be well served by stopping all motion and let his body fall any way it may and see if he is foot heavy or if he is overcompensating. If he is foot heavy, then he could move some of the ballast higher up his body. This could be achieved by moving the tank up higher or moving trim weights behind his shoulders or on a tank band, wearing lighter fins, tank composition, controlling the bubble in a drysuit, as well as body position.

"Divers who dive in a seahorse position" are not necessarily "negative". Their ballast is not in an optimal place, as noted above. Those who dive diagonally are often overweighted as well, especially newer divers, but certainly not always.

That's pretty basic Fundies 101. Shocked that you can't see that if you have actually passed Fundies.

 

[Bogtrotter]

Generally speaking there are clearly circumstances where an inverted position would come in handy. My comment was actually in reference to the earlier photo which seemed to show a sustained backwards mid-water inversion of about 45 degrees.