Doubles trim epiphany

[MichaelMc]

I'll explain it topside.

Thanks! Very nice. So this tells you where the center of lift is for wings, under roughly doubles, at different angle of trim and different inflation amounts, relative to their 11” mount hole.

Pick an angle, move frame weights to get there. Seems like you would want to zero and trim the frame and test with a back plate in place as the plate may restrict wing inflation shape. Then you add wing, extra weight, and inflate back to zero’d lift point. Move that extra weight head to tail until frame is back to trim. The location tells you where that much lift puts the upward force.

Do you see much difference in where wings put the lift as they inflate? Presumably you typically dive doubles? Do you zero the frame and test the wings with your backplate?

In the bigger picture, you know where your center of trim is, and all the amounts and distances of negative and positive forces and different stages of the dive. Finding the centers of gear like tanks would take hanging in water from a loop of string.

 

[lowviz]

... So this tells you where the center of lift is for wings, under roughly doubles, at different angle of trim and different inflation amounts, relative to their 11” mount hole.

And more.

It also tells you where the force should be applied to place the wing into your chosen angle of trim.

Pick an angle, move frame weights to get there. Seems like you would want to zero and trim the frame and test with a back plate in place as the plate may restrict wing inflation shape. Then you add wing, extra weight, and inflate back to zero’d lift point. Move that extra weight head to tail until frame is back to trim.

An aluminum BP is part of the rig, no webbing. It isn't shown. It's weight disappears when you trim the frame.
It is there to constrict the wing the same way my BP rig does.

Do you see much difference in where wings put the lift as they inflate? Presumably you typically dive doubles?

Those "results" aren't ready for prime time yet. Hardware improvements along the way compromised a lot of old data. No, big problem. The tests are easy to re-run. Sometime this summer...

Do you zero the frame and test the wings with your backplate?

No. The light aluminum BP becomes part of the rig. My heavy SS BP becomes part of my center of balance and net buoyancy.

Finding the centers of gear like tanks would take hanging in water from a loop of string.

Easier way.

Put a length of SS angle on the pool bottom (on webbing so the edges don't dig into the pool) between the uprights of the horizontal bar. The angle has a 1/4" wide flat milled where the sharp peak used to be. Balance your tanks, bands, and regs on that and it is easy to mark the tanks as they will stay in place. The horizontal bar makes a nice visual reference that you got the tanks pretty much square.

Another unanswered question: How does that balance point change with tank fill?

 

[MichaelMc]

Another unanswered question: How does that balance point change with tank fill?

I understand you can get the engineering diagrams of Faber, I think, tanks if you ask them. So the air inside has center of the geometric center of the tank interior. You could likely get a rough approximation with a picture of the tank. Or, measure the trim center at two very separated PSI's, calculate the weight of the air difference, and compute the center of air that would create the change in combined air and tank center that you saw.

And does the center of BC lift correspond to the center of tank air that you are lifting with it? Diving dry you do not have to worry about where the wetsuit loss of lift occurs.

Finding your center is the first crucial step. How accurate did you find you could get that or that it needed to be? Roughly how much shift in it did you see for frog vs flutter?

 

[lowviz]

... So the air inside has center of the geometric center of the tank interior. You could likely get a rough approximation with a picture of the tank.

Yes, probably not a big deal. Problem is that I balance the tanks with the regsets attached so the center of mass of the gas won't align with the balance point. I need to test the swing in balance point.

... And does the center of BC lift correspond to the center of tank air that you are lifting with it?

No, but they are connected in an exact way by the backplate.

... Finding your center is the first crucial step.

Yes, you obviously understand what I'm doing. The slots on the rack that attaches to my BP are on 1 inch centers. Thus, I'm at most a half inch off on my balance point. I don't need more precision than that for my zero. I get into frog position and extend my arms, hands clasped. I next retract my arms. I then seek the most "natural" or comfortable position to place my hands. Legs in what I think is perfect frog position. The GoPro is the after-the-fact judge of that.

Here is where I will speak to "skills". I am absolutely convinced that 80% of "skills", so often referred to, is simply being able to alter leg and arm position to make up for imperfect gear balance. How about we start with correct gear balance and THEN add skills?

... How accurate did you find you could get that or that it needed to be? Roughly how much shift in it did you see for frog vs flutter?

A lot. Heavy fins and ankle weights make a big difference. Remember that you have two things to consider, neutral buoyancy and trim.

Buoyancy is just adding up all the scalar vertical forces (float, sink) and figuring out the best way to deal with the deviation from zero.

Trim is found by making all the moments of rotation cancel. Simple vector sum. Again look at the deviation from zero and adjust placement of weights. Rotational force of anything (stuff you add to "the basic you" like a can light) is its U/W weight times the distance from horizontal trim zero (marked on BP). Keep the signs straight, pick your own convention. I use positive as rotating my head upward, negative for forcing me head-down. Bookkeeping.

You can have perfect buoyancy and be diving a balanced rig (Hogarthian meaning of balanced) and still have horrid trim. That was me for years. The trick is to align all three.

 

[MichaelMc]

No, but they are connected in an exact way by the backplate.

Yes, and a key point is it's unlikely they just align, and thus your trim varies as you use the air, until you compensate with your leg position, so it's best if you set up your ballast placement/trim so that range of leg movement, to account for tank air loss, centers around where your legs are most frog natural.

The slots on the rack that attaches to my BP are on 1 inch centers.

The slots are fairly deep and you hold yourself in one particular slot with your base body lift, correct? So you attach a bar with two 11" centered holes to your plate and harness, no wing. The bar has deep slots on the exposed side that fit around a stable and grounded horizontal bar in the pool. Your buoyancy and the depth of the slots keeps the bar in a particular slot while you judge your trim. You just need to tell which slot you were doing that in. And you subtract out the effect of that bar.

Edit: Your two supports and a bar seems the cleanest setup. One alternative would be a 3' pvc square plus a thin edge added below the top side in the water to fit and pivot in the slots, either a round rod or a thin plate, with weight belts to hold it down. Or, a rod floating between weights on the bottom and some lift jugs. Using a triangle of two weight belts per bar end would keep it more stable, but add lines by your arms and legs.

 

[lowviz]

Yes, and a key point is it's unlikely they just align, and thus your trim varies as you use the air, until you compensate with your leg position, so it's best if you set up your ballast placement/trim so that range of leg movement, to account for tank air loss, centers around where your legs are most frog natural.

Exactly.

The slots are fairly deep and you hold yourself in one particular slot with your base body lift, correct?

Yes. I do this alone so I had to make it easy. You can get away with fewer conveniences if you have another diver helping you.

So you attach a bar with two 11" centered holes to your plate and harness, no wing. The bar has deep slots on the exposed side that fit around a stable and grounded horizontal bar in the pool. Your buoyancy and the depth of the slots keeps the bar in a particular slot while you judge your trim. You just need to tell which slot you were doing that in. And you subtract out the effect of that bar.

Yes. I made a pair of T-bolts to fit in the slots so that the two slots weren't filled with bolt-heads.

Edit: Your two supports and a bar seems the cleanest setup. One alternative would be a 3' pvc square plus a thin edge added below the top side in the water to fit and pivot in the slots, either a round rod or a thin plate, with weight belts to hold it down. Or, a rod floating between weights on the bottom and some lift jugs. Using a triangle of two weight belts per bar end would keep it more stable, but add lines by your arms and legs.

You might be able to pull that off with help from another diver. Trying to make accurate measurements while fighting an unstable support can be maddening. BTDT. That pretty much defined the rig I now use.

 

[MichaelMc]

@lowviz, Just so I keep it straight, you are not finding your center of mass, but rather the point that downward force must be applied to keep you in some trim. Think of a body consisting entirely of heavy legs, lungs, and neutral arms. You must apply downward force to the arms, but the center of mass is the legs. If you add lead at the point you find, you are shifting your center of gravity to be in line with the waters, but we do not know where that center is.

When adding gear, you must balance out their forces to equal the down force and location needed to keep your base body horizontal.

 

[lowviz]

... Just so I keep it straight, you are not finding your center of mass, but rather the point that downward force must be applied to keep you in some trim. ...

Correct. You get into position (frog or flutter) and find the line across your back where you will balance in horizontal trim without moving. The parts of you that hang over the bar are equally buoyant. That line will be your "center" of buoyancy, not a center of mass. ("center" used loosely. It is really a plane, not a point.)

When adding gear, you must balance out their forces to equal the down force and location needed to keep your base body horizontal.

Yes. And if you picked, say, ten degrees for your wing when you balanced it, your wing numbers should put you pretty close to ten degrees.

 

[MichaelMc]

Thanks. I need to think about adding gear that is not in trim, and buoyant tanks, but can't right now.

On terms, Wikipedia defines 'center of buoyancy' as the centroid of the displaced water.