Buoyancy of fins & the effect on horizontal trim

[SouthSideScubaSteve]

With 30 dives logged, I am still a newbie, but I know enough to understand that my horizontal trim (or lack of) is still not what it needs to be. I also think I have a pretty good understanding of the physics involved (Center of Mass, Center of Buoyancy and levers in general) and I have a theory I want to float by all of you ….. First some basic info:

On a couple of recent dives while on vacation in Hawaii, the guide I dove with sent me several pic’s from the dive, including several of me. They confirmed what I already knew, when I have my legs bent at the knees with my shins perpendicular to the rest of my body (in a frog kick position) my trim is nearly horizontal; when I am swimming using a traditional scissors kick I tend to be positioned roughly 20-30 deg off horizontal… I’ve done weight checks, moved my BC down on my tank as far as possible (without hitting my head on the 1st stage while swimming) and have the max weight I can load in my BC trim pockets (60% of the total) and I just can’t seem to get horizontal when scissor kicking or when I’m at rest in the water. My current theory is that my fins are relatively heavy as fins go and have almost no inherent buoyancy which in turn creates significant changes in my CoB when my legs are extended in a scissor kick position. I am currently using Aqualung Slingshots fins (which I like and also happened to get a great deal on), but as I’ve said, they are heavy and sink like rocks …..

I would like to try a set of fins that are closer to “neutral” buoyancy to see if that helps with my trim but really don’t know what style & brand fin I should try …. Ideally, I could (with the cooperation of my LDS) take various fins into the pool and determine relative buoyancy of each with a mesh bag and handheld scale; but for some reason I can’t see them letting my do my experiment with all the pretty new fins hanging on the hooks in the store. I’m also not trying to start a “split fin” debate; I’m just looking for insight on relative buoyancy of various fins ….

Does anyone out there have any data, or at least an opinion to share??

 

[D_B]

fin buoyancy will definitely affect trim ... mine are darn near neutral , SP Jets are quite the other way .. I've heard good things about Slipstream fins, they look and perform like jets but are lighter in the water

BTW .. you might not need a scale to tell which fins are lighter, just put two different ones in the water and let go

Al80 tank? .. note: it will only be negitivly buoyant for the very beginning of the dive, it will switch to positive pretty early on

 

[Walter]

when I am swimming using a traditional scissors kick

Most folks with only 30 dives have not yet started using a scissor kick. Most are still using a flutter kick. Congratulations on thinking outside the box.

They confirmed what I already knew, when I have my legs bent at the knees with my shins perpendicular to the rest of my body (in a frog kick position) my trim is nearly horizontal; when I am swimming using a traditional scissors kick I tend to be positioned roughly 20-30 deg off horizontal

Looks like one of two things will work for you. Either switch to a frog kick - try it, you'll like it - once you get the hang of it - or simply lean forward.

Unless you are using a chunk of steel (spring straps) on your fins, there's not as much difference between one set of fins and another as you might think. I use SP Jets, a set of fins most people think are heavy. With my boots in the fins, they are neutrally buoyant. If you are in halfway decent shape, it's your legs, not your fins, that is pulling you off horizontal trim. Leaning forward or switching to a frog kick should do the trick. You might want to move some of your lead to ankle weights which you can place around your tank valve.

 

[H2O 70]

The OMS Slipstreams are near neutral in salt water.

 

[Vlane]

This may be a little drastic, but switching from a typical BC to a BP/W can help with trim. A BP/W will put more of the weight on your back rather than hips which is one of the causes of bad trim. I also struggle with being feet heavy and I have yet to be able to solve it, I dive a Zeagle Stiletto by the way. I have not completely solved the issue but I have got it to where I'm not as feet heavy. I haven't gotten to the point to where I want to switch fins yet but it certainly could help if you have heavy fins.

 

[Garrobo]

Up until lately I was pretty much leg-heavy and when stationary in the water I would go immediataly to a vertical position. My fins are just a little negatively buoyant. They will sink slowly. I tried putting some of my lead weight as far up as around my neck which helped some. I went back to a moderate weight-lifting and dieting program, lost the BCD (using a horsecollar now) and 12 pounds and I am now almost horizontal in the water. Body fat was the problem.

 

[SouthSideScubaSteve]

I had been using the terms "scissor" and "flutter" interchangeably, not realizing there was a difference .... Opps!!

I do try to frog kick as much as I can (and keep my legs I position to do so), however I have to make a conscious choice to do so. My natural tendency is to extend my legs & flutter kick

... If you are in halfway decent shape, it's your legs, not your fins, that is pulling you off horizontal trim ...

So you are saying that the buoyancy effects from the fins are negligible compared to the effect on my CoB by straightening my legs?

 

[SouthSideScubaSteve]

Al80 tank? .. note: it will only be negitivly buoyant for the very beginning of the dive, it will switch to positive pretty early on

Yes, I use an AL80 (I got a great deal on a couple when Gander Mtn got out of the scuba business)... Any idea at what pressure an AL80 goes Positive?

It occurs to me that my problem typically gets worse as my dive goes on... I'm now wondering if I have my tank too high up and my tank is lifting my head & shoudlers up as the dive goes on???

 

[SouthSideScubaSteve]

... I went back to a moderate weight-lifting and dieting program, lost the BCD (using a horsecollar now) and 12 pounds and I am now almost horizontal in the water. Body fat was the problem.

I'm sure that could be part of the issue in my case as well.... I'm more than a little soft around the middle :depressed:

 

[Blackwood]

Differences in fin buoyancy aren't all that significant. They can be overcome with leg positioning (i.e. lever arm).