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Yes the amount of force you must produce to overcome the drag. Someone else wrote to me and asked me to back up my numbers. Please let me post a suto reply here to answer any prospective comments, hereafter. I was only trying to provide some numbers which would be approximate of a diver in the water, nothing exact, nor really good. An engineer would have interpolated the results as showing a trend. The tend being was how water resistance varies as a function of speed. A while ago some of the members asked comments about how high of a current one could dive in; well I decided to show just how much approx force would have to be reacted to hang on to a line or wreck under water or swim against. This in part is a function of the frontal blockage area. I have a background in aerodynamics as well as a little in hydrodynamics. But not enough in hydro to claim anything other then to give you "back of the envelope" numbers. But then again that is what engineering is about a lot of the time. I would expect the reasonable diver would look at the numbers for lets say (1) knot, note it in his mind. Then go out diving in a (1) knot current. While doing that how much force does it feel like. Try to be honest. Then take these "feely experimental" numbers and see how they vary compared to my standard numbers. My numbers are scaleable. Drag force use a non-dimensional constant, Cd, to determine the force. The bigger the constant the bigger the force. The smaller the constant the smaller the force. I used a drag constant of 1.4. This is a rather large factor. Most really clean water borne shapes, I would suspect to be 1/2 this or much less. But a scuba rig is what I would not call clean. Hence the number I chose. I don't know of any better known numbers, hence the number I chose. I based my calculations on the experimental hydrodynamics of Sighard F. Horner, the father of aerodynamics. He has many many examples of real numbers which were derived from experi! mental configurations. I had many to choose from. None of them were for scuba, but the approximate body shapes and the "worst case" shapes were noted as such. It would be hard for my to explain over the net why I chose the number I did. One might say that I used 16 years of experience and then guessed. If you had an aerodynamic background then we could set at the desk together for the next several weeks and bounce things back and force and it would probably not get any better without any real data to base our assumptions on. Hence my suggestion to actually get in the water and see how they feel and then "scale" them accordingly. This would be a valid method in my humble opinion. I am sure there are a few other hydro/aero types out there who do have much more experience in this discussion then I do and if they care to produce more accurate Cd values for each "installation of gear" then we can move further on. Until then this will have to serve as a stating point or not e! ven discuss it and choose to remain in the dark. These were my baseline numbers and assumptions. I used a rig which I assumed was cleaner then most although not clean. The DIR rig. The assumption would be approximately of a diver who was not hanging on a line or by any surfaces and the water was smooth flowing. A line for example could cause the drag force to actually increase or decrease greatly depending on how it was configured-- true statement. I assumed the diver had a body shape of 220 lbs and was 6 ft 4 in. And he used a 104 cu-ft tank. A partially inflated wing which produced a frontal profile of 20 sq in. The approximately areas for each configuration are (they could be more they could be less): Streamlined -Single Area= 330 sq-in Bent Knee -- Single Area= 470 sq-in Streamlined -Double Area= 380 sq-in Bent Knee --Doubles Area= 420 sq-in Coefficient of Drag = 1.4 -----Original Message----- From: Jim Cobb [mailto:cobber@ci*.co*] Sent: Friday, August 03, 2001 8:25 AM To: Charlie Allen; techdiver@aquanaut.com; Rogers, Thomas A Subject: Re: Drag force generated by a diver in the water Thomas- So you are saying here that at 3 knots the force that you would exsert holding yourself stationary in DIR config would be 90lbs? And at 7 knots 500lbs? This analysis seems to be correct as with a scooter putting out 40lbs of thrust you should only be doing 1.5kt or so, at least that's what it feels like. This should put the final nail in this "7 knot current" thing, as I have not seen a pickaxe yet that you can hold onto with 500lbs dragging you downstream. Jim ------------------------------------------------------------------- Learn About Trimix at http://www.cisatlantic.com/trimix/ > From: "Charlie Allen" <charlie.allen@us*.ne*> > Date: Thu, 2 Aug 2001 21:30:55 -0700 > To: <techdiver@aquanaut.com>, "Rogers, Thomas A" > <Thomas.Rogers@PS*.Bo*.co*> > Subject: Re: Drag force generated by a diver in the water > > Do you have any comparison of the drag forces of DIR setup vs. > non-DIR jacket BCD setup?? > > Charlie Allen > > ----- Original Message ----- > From: "Rogers, Thomas A" <Thomas.Rogers@PS*.Bo*.co*> > To: <techdiver@aquanaut.com> > Sent: Thursday, August 02, 2001 1:39 PM > Subject: Drag force generated by a diver in the water > > >> >> I ran some calculations on the Drag force (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 a 6 in EE light >> >> Streamlined means Basic DIR setup with canister light and legs > straight. >> Bent Knee means Basic DIR setup with canister light and legs > bent. >> >> Current Velocity (knots) 1 2 3 4 5 6 7 >> Streamlined -single 9 36 82 146 227 327 446 >> Bent Knee - single 16 67 151 269 420 605 823 >> Streamlined -Double Tank 10 42 94 168 262 377 513 >> Bent Knee --Double Tank 18 73 164 291 455 655 891 >> > > > -- > Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. > Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'. > -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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