The Streamlining Issue


Outer Bamnks diving on the Great Escape Southern California Live-Aboard Dive Boat

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Posted by tleemay on August 02, 2001 at 16:23:20:

I am copying this from the Nat'l Techdiver list. The guy who wrote it
does engineering and fractual design work for Boeing. I hope the formatting holds up.

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I ran some calculations on the Drag Force (lbs) [DF/lbs] which would be generated by a diver in the water as a function of the current.

These are ROUGH numbers. I am also assuming this test subject has
perfect trim and is not moving.

The test subject was 220 lbs and 76 inches tall. I ran 4
configurations. I used PST 104 tanks and an EE Pro 6 cannister light.

Streamlined means Basic DIR setup with canister light and legs
straight. Bent Knees means Basic DIR setup with canister light and
legs bent.

Current Velocity (kts) 1 2 3 4 5 6 7

Streamlined -single tank 9 36 82 146 227 327 446
Bent Knees - single tank 16 67 151 269 420 605 823
Streamlined - dbl Tank 10 42 94 168 262 377 513
Bent Knees -- dbl Tank 18 73 164 291 455 655 891

These are ROUGH numbers. But they are good illustrations and talking
points.

-tom

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As a reference,

1 kts equates to 1.15 mph
2 kts equates to 2.30 mph
3 kts equates to 3.45 mph
4 kts equates to 4.60 mph
5 kts equates to 5.75 mph
6 kts equates to 6.90 mph
7 kts equates to 8.05 mph


The DF/lbs number is perhaps a bit of an un-assuming value to
most of you, but the real value to the numbers is in the differences
between being streamlined and having bent legs at any of the given current speeds. This give you a good example of how drag can increase
and by how much with something as simple as hinging your knees. Of
course the formulae Tom used reflected a trim DIR diver. How much
difference would there be if it were calculated with a diver who dove
in a not so trim position? The initial streamlined DF/lbs number would
most certainly be higher.

Now put these divers in the water with no current and lets see how efficient they can be of minimal drag while swimming. Again compare
the trim and streamlined diver to one not so trim and streamlined.


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