Bouyancy using backplate system

[gnulab]

Hi there,

I see video clips of people diving, especially when they're doing the frog kicks, their bodies are in the horizontal position.

The last sea trip, my setup up includes Halcyon Eclipse, 3mm shorty & the Scubapro Jet Fin. Using that setup, I couldn't maintain a horizontal position for long. After few minutes (didn't keep track), my leg will sink, (9 o'clock position), then followed by my upper body rose, and now 12 o'clock position.

I guess I do not need to be 100% horizontal position, and that means the 10-4 o'clock position is fine. But eventually, the above process repeats again.

I've tried on the Scubapro twin jet fin, and I last longer, but again, eventually I'll be in the 12-6 o'clock position!

Is it possible that my legs are negatively buoyant? What should I do to maintain horizontal position? Being in the 10-4 o'clock position isn't acceptable, since when you kick, instead of moving forward, we'll tend to ascend.

Thanks.

Henry

 

[DA Aquamaster]

The options are:

1. Move the tank higher on the plate.

2. Move the wing lower on the plate (mount the wing using the highest set of holes in the wing and the lowest set of holes in the plate.) You may only have one set of holes in the Eclipse without adding another set of holes and gommets, but most plates offer 3 holes in the plate.

3. Use lighter fins such as OMS slipstreams. They are essentially a monoprene verion of the SP jet fin and have almost identical performance in the water but are a lot lighter and less negative in the water.

One or a combination of the above should resolve the problem. If it does not, a 3mm full suit will add some buoyancy in the legs. The last resort would be to add a weight high up on nthe tank (an ankle weight around the valve, a trim weight on an upper cam band, etc.)

 

[SDAnderson]

Is it possible that my legs are negatively buoyant? What should I do to maintain horizontal position? Being in the 10-4 o'clock position isn't acceptable, since when you kick, instead of moving forward, we'll tend to ascend

Yes, it's possible that your legs are negatively buoyant. You're entirely correct that you don't want to swim in a pitch up attitude - 100% prone is where you want to be.

Establishing good trim is a complex process that has to take many factors into account, achieving satisfactory results requires experimentation. Sometimes a lot of experimentation. In the end, you'll end up adjusting the position of your cylinder(s) and (usually) strategically re-positioning small bits of weight. Having a buddy/mentor to help, preferably one who has experience with the process, is almost essential. For what it's worth, the PADI Peak Performance Buoyancy course is highly underrated and can be worth every nickel. Take precautions to avoid wasting your time and money, however: make sure you see the instructor in the water and evaluate his trim before you sign up - if s/he doesn't walk the walk...

• Start by getting in the water with your usual set-up.
• Get relaxed and adjust your weighting so that you aren't carrying any more (or less) than you need.
• Remove four lbs of weight from your rig and, holding it in your hand, drop down to ten feet of water.
• Get as close to horizontal in the water as you can and hold the weight in your hands, moving it around, backwards and forwards and from side to side, until you trim out perfectly. Your buddy should verify your trim - it can be hard to tell by yourself.
• Once you find your trim, hold that position for at least a couple of minutes. If you're balanced, you should be able to hover motionless, no sculling, no kicking, no inflating, no deflating, without losing trim.
• The position of the weights will demonstrate how offset your natural center of gravity actually is from where you want it to be. From here the process becomes one of adjustment - if your center if gravity is too far astern, you need to move weight forward, etc. Move the tank up and down in the bands, reposition some of your weights, adjust body position, etc. Try a number of solutions for each problem. Your buddy can be a big help with this process, too.
• Repeat as necessary until you dial it in. Then be prepared to repeat the process all over again with each new piece of gear you add or subtract.
• Remember that fine control comes from proper body positioning. Use your breathing and hand/arm position as well as back arch to compensate for minor changes in the water.

 

[BabyDuck]

i didn't see where you said how much weight you needed or where it is, but a weight belt will tend to make you pivot like that. if you need 10 lbs, for instance, maybe 8 on a belt and 2 around your tank valve would help. the guys above said basically this - i'm just restating.

your tank valve should be touching your head...not high enough to poke a hole in your skull, but close.

this next is much more a 'fine-tuning' tip. reef hinted at it above.

pay attention to how your body is positioned. are your legs straight out? if so, try bending at the knee to about a 90 degree angle. starting to pitch too far forward? straighten them out a tad.

make sure your back is quite arched. try your hands in different positions - out front like a 'begging puppy', crossed over your chest, along your sides. many folks dive in 'begging puppy' position, which has the added benny of being able to glance and see your gauges (if they're on your wrist, of course. )

this is a great thing to do in a pool if you can get access to one - being able to control yourself in 6-10 feet is reasonable proof you can do it anywhere!

 

[Bob]

Hi there,

I see video clips of people diving, especially when they're doing the frog kicks, their bodies are in the horizontal position.

The last sea trip, my setup up includes Halcyon Eclipse, 3mm shorty & the Scubapro Jet Fin. Using that setup, I couldn't maintain a horizontal position for long. After few minutes (didn't keep track), my leg will sink, (9 o'clock position), then followed by my upper body rose, and now 12 o'clock position.

I guess I do not need to be 100% horizontal position, and that means the 10-4 o'clock position is fine. But eventually, the above process repeats again.

I've tried on the Scubapro twin jet fin, and I last longer, but again, eventually I'll be in the 12-6 o'clock position!

Is it possible that my legs are negatively buoyant? What should I do to maintain horizontal position? Being in the 10-4 o'clock position isn't acceptable, since when you kick, instead of moving forward, we'll tend to ascend.

Thanks.

Henry

Henry,
I dive the same rig you do and I had the exact same problem, leg heavy. I'm tall, 6' 4" and weigh 270. Make sure your plate is adjusted properly. If it's too low, it will tend to drag your hips down. Most people I dive with, dive with their tanks too low, adjust your tank upwards. It'll take a couple of dives to find the optimal position. Finally, I had to ditch my Jets for wetsuit diving. After doing everything above, I was still slighty leg heavy, and my feet would still tend to sink if I remained completely horizontal and motionless. Someone suggested the OMS Slipstreams, so I tried them. The Slips are neutral, and that did the trick. After diving with the Slips a couple of times, I put SP spring straps on them, and that makes them slightly negative, but fortunately not enough to matter. The only problem is, the Slips are only avaible in XL and XXL. Hope you have big feet!
I tend to be more leg heavy in a shorty than say, a 3 mil. For my drysuit, I still use my Jets, they balance me out perfectly. Hope this helps. almtree: Bob

 

[dumpsterDiver]

....[*]Remember that fine control comes from proper body positioning. Use your breathing and hand/arm position as well as back arch to compensate for minor changes in the water.
[/LIST]

I have been diving a long time and sometimes use a BP/W set-up. I also tend to assume a vertical position if I become completely still for a while (say 30 seconds). I understand that you don't want to be plowing through the water at an angle, but getting perfectly trimmed horizontally is not a big issue to me. When swimming, I am horizontal. more or less.

How is it efficient or desirable or practical to use a back arch (which I am sure I would find uncomfortable to maintain) and hand position to maintain a perfect position? My hands are always filled with something; one or more of the following on nearly every dive: speargun, pole spear, lobster net, float line hook, scooter. I really need to have my hands free to do stuff, not trying to fly like superman.

I also collect lobster and fish during a dive and this makes changes in trim in that the fish float somewhat and the lobster bag clipped to one side is definitely heavy (hopefully) and it pulls on one side due to being clipped to the hip. Seems that I would never be able to maintain a perfect trim anyway.

It seems to me that some people over emphasize the need to maintain perfect horizontal trim. I do recognize this is much more important in a cave or wreck, but I don't dive in those environments.

Anyone else get the sense that it is not that critical to be perfect?

 

[hudson]

Since it hasn't been explicitly stated, you can also purchase two XS Scuba weight pockets and mount them on your upper camband. This will let you easily add a few lbs of weight to trim out.

Xs-Scuba Quick-Attach Single Weight Pocket from LeisurePro.com

Note that there are two versions of this - one needs to be in place before you thread the cam bands through, the other is a quick attach (the one I linked to). For this application it doesn't really matter.... the quick attach is more flexible, the other is a little cheaper.

However try the other suggestions first. You also tend to use less total weight as time goes by, which makes these trim weights unnecessary.

 

[redacted]

If you are having this problem at the beginning of a dive while your tank is still full, you may need to make a fairly large weight distribution to correct your trim. If it only occurs later in the dive after your tank has lost 4 lb of gas, some relatively minor shifts should correct it. If you are doing a frog kick, you can even bend your knees a bit more, getting your feet higher and closer to you body to shift your center of gravity enough to have some effect.

 

[String]

The last sea trip, my setup up includes Halcyon Eclipse, 3mm shorty & the Scubapro Jet Fin.

You've got a very light suit and very negative fins. If adjusting the tank or putting weights on the top cam strap doesnt work, try lighter fins.

 

[SparticleBrane]

You've got a very light suit and very negative fins. If adjusting the tank or putting weights on the top cam strap doesnt work, try lighter fins.

...and yet I can maintain perfect trim indefinitely while skin diving, in a bathing suit, with Jets.

Everyone here is so quick to shout that it's a gear problem -- it couldn't possibly be an issue with technique, could it?? :shocked2:

To the OP -- do what BabyDuck suggested -- arch your back slightly (you don't have to do this all the time while diving; it is good practice when you first start out to teach you the correct position). I will add that tightening your butt muscles helps raise your legs into position.