Trim and Buoyancy

[Mike Klieb]

While out at Diving Sunday I was trying to work on trim and buoyancy I noticed that as I was swimming I was staying flat in a good diving position but when I stopped and was attempting to just hover my legs would invariably start to sink so I figured id move my tank up on my back to no avail... any suggestions ??? I had 2Lbs one in each weight pocket I could hover it would just end up being hovering in the standing position.

 

[loosenit2]

what fins do you use? Are they negative, neutral or positive? Also what types of boots are you wearing? Both questions go to the same thing, are you weighted on your feet or is it just that you have muscular legs that tend to drag you legs down?

 

[rsingler]

With that little weight requirement, your trim options are limited. You've already tried shifting your tank up in your bcd. That's correct for steel. If steel, you might try getting a shorter, fatter tank in your desired size, if available. If it's aluminum, you might actually try shifting it DOWN on your bcd: since when 3/4 full or less, it's positively buoyant, having most of the tank below the attachment, the tank buoyancy will help lever your legs up.
You can thread a wide Velcro strap through each 2# weight, and attach each to the top of your shoulder straps, rather than your pocket.
Finally, there are some nice fins that are slightly positively buoyant, or at least not negative. That may make more difference than you think, given the long lever arm of your legs. And when you hover, make sure your knees are bent at 90°, again to reduce a heavy lever arm.

Good luck!

 

[Mike Klieb]

"what fins do you use? Are they negative, neutral or positive? Also what types of boots are you wearing? Both questions go to the same thing, are you weighted on your feet or is it just that you have muscular legs that tend to drag you legs down?"


Loose I am diving the ScubaPro Twin Jet Max Fins I was told they are slightly positive and ScubaPro boots. My legs are somewhat muscular but I didn't figure them to be that heavy.

 

[Mike Klieb]

With that little weight requirement, your trim options are limited. You've already tried shifting your tank up in your bcd. That's correct for steel. If steel, you might try getting a shorter, fatter tank in your desired size, if available. If it's aluminum, you might actually try shifting it DOWN on your bcd: since when 3/4 full or less, it's positively buoyant, having most of the tank below the attachment, the tank buoyancy will help lever your legs up.
You can thread a wide Velcro strap through each 2# weight, and attach each to the top of your shoulder straps, rather than your pocket.
Finally, there are some nice fins that are slightly positively buoyant, or at least not negative. That may make more difference than you think, given the long lever arm of your legs. And when you hover, make sure your knees are bent at 90°, again to reduce a heavy lever arm.

Good luck!

The tank is a rental AL80 I will try moving the tank down on my BC and see what happens. If I try t lift my legs to 90 degrees is defiantly makes the rotation happen faster so I am figuring that I would need to move my weight high up on my torso ? to counteract this

 

[rsingler]

You're right about the need to raise weight up higher (or buoyant tank lower). The reason your rotation accelerates when you bend your heavy legs is probably that when your legs are out straight, your fins were acting as retardants. That water resistance disappeared when your knees bent and your fins became parallel to the direction of rotational movement.
You're on the right track.

 

[Mike Klieb]

You're right about the need to raise weight up higher (or buoyant tank lower). The reason your rotation accelerates when you bend your heavy legs is probably that when your legs are out straight, your fins were acting as retardants. That water resistance disappeared when your knees bent and your fins became parallel to the direction of rotational movement.
You're on the right track.

I'll get a chance to test this out next weekend thanks for the input hopefully that will help fix the problem.

 

[loosenit2]

"what fins do you use? Are they negative, neutral or positive? Also what types of boots are you wearing? Both questions go to the same thing, are you weighted on your feet or is it just that you have muscular legs that tend to drag you legs down?"

Loose I am diving the ScubaPro Twin Jet Max Fins I was told they are slightly positive and ScubaPro boots. My legs are somewhat muscular but I didn't figure them to be that heavy.

That particular model the buoyancy varies by color. If they are black they are negatively buoyant and the other colors are slighly positive. If they are negative that may be contributing in a small way to your trim issues. You may just test it by putting your fin in the water and seeing what happens and then doing the same with the boot in the fin.

A couple of other thoughts.

1. Are you using a jacket BCD with integrated weights or a BP/W? If jacket I assume the weights are in ditchable pockets near waist? Do you have trim pockets higher on your back? Moving weight higher (even though you don’t have a lot to play with) may help.

2. Breathing. Do you take large full breaths or do you breath out of the middle or bottom of your lung capacity? How you breath may have an effect on your trim.

3. Leg/Foot position. Do you keep your legs out straight or do you pull them into an L-shape with your fins above you and directly above your knees. How you position your feet can have an effect on weight distribution and effect your trim. If you do a regular flutter kick with your fins straight out you may try out bending your knees and seeing how/if that changes your trim when kicking and resting.

 

[Khrissi]

Definitely found its how I breathe. Weights in my pockets 8kg. 81kg body in 4mm. Concentrated on my breath and it seems to happen k

 

[Mike Klieb]

Definitely found its how I breathe. Weights in my pockets 8kg. 81kg body in 4mm. Concentrated on my breath and it seems to happen k

Yeah breathing has most defiantly helped with my buoyancy and being able to maintain a specific depth while swimming.