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Re: [PSUBS-MAILIST] fairings drag vrs entrained mass



> Inertia and momentum, both mass related, versus severe parasitic drag.  Ask Joe about that.
 
It just occurred to me:  You're a pilot aren't you?  All this stuff is probably right up your alley.
 
Rick
Vancouver
----- Original Message -----
Sent: Tuesday, November 29, 2005 4:08 AM
Subject: Re: [PSUBS-MAILIST] fairings drag vrs entrained mass

> how much saving in hydrodynamic drag do i have to save to justify the the entrained mass that i will carry in traped water? especially with a speed of only 3.5 kts. diplacement about 6000 lbs
 
Wow - now there's a head scratcher.  I hate qualifiying my answers but - it depends.
 
Allow me to rephrase the question for a moment: Why should I drag around a few tons of useless water inside a supposedly hydrodynamic fairing if the gains aren't worth it?  Why not let my pipes and appendages just hang out?  Valves, too, while we're at it.
 
Maybe this will help: a simple method, not empirical by any means, would be to compare your own mission specifics with those of existing subs.  This is where Busby comes in REALLY handy.  This is actually the technique I used in my early days when my interest started to take root.  How many horse power or lb. of thrust do I need?  Well, what did Nekton use?  Deep Star, Alvin and Aluminaut were all inappropriate vehicles to model after.  But, Nekton wasn't bad.  General shape, payload, etc.  HOWEVER . . .
 
I'm operating strictly on intuition here.  Magical Child will be a tourer.  I know that most of my energy losses in terms of entrained water mass will come from acceleration, not cruising speed.  My mission specifics demand battery and air preservation.  When I leave the dock I will be accelerating MC slowly to cruising speed and will keep her there.  I will have neither surface support nor cachees of fuel and air along my route.  I have to minimise drag.
 
MC's hull form will be a monocoque with little in the way of drag structures like fleet-boat style decking.  No grates for walking, no gimbeled planes with struts.  The hull will BE the deck - which means that I will be dragging around more water than I would ordinarily with a high speed "sport" boat.  My decks have to be wide for lounging in chairs, attending to dock duties, and driving golf balls from.  So wide decks mean a wide hull which means more entrained water.  Fine.
 
Rather than sitting down with all the math I prefer to compare my mission demands with those who have walked (swum?) before me.  A speed of "only" 3.5 kt. is comfortable in good vis.  Dragging around entrained water is preferable to hanging everything out in the current if any long distance touring will be involved.  Why?  While drag is forever (more so at speed - the increase is logrithmic), entrainment is prevalent mostly while accelerating (and decelerating).  Inertia and momentum, both mass related, versus severe parasitic drag.  Ask Joe about that.
 
You will have to waste precious time, not to mention drive yourself nuts, by calculating drag coefficients of various parts like valves, pipes, air cylinders, motor casings, empennage, the sail/canopy, etc.  Once you have dealt with surface composition, fluid viscosities at various salinities, temperatures and boundary layers, you will have to deal with angles of attack for various 3D geometrics like cones, cylinders, toroids, etc.  To what end?
 
I regretfully refer you to the following link:
 
 
I specifically refer you to 2.1 Total-Head Loss Measurement of the Drag of a Body.  Bring a change of underwear.
 
I have as much interest in calculating drag as I do in propeller sizing.  None.  I hope my "streamlined" technique works for you.  It's a real time saver.
 
Rick
Vancouver
 
 
----- Original Message -----
Sent: Sunday, November 27, 2005 6:03 PM
Subject: [PSUBS-MAILIST] fairings drag vrs entrained mass

hi guys
 
    Rethinking fairings and have a question. how much saving in hydrodynamic drag do i have to save to justify the the entrained mass that i will carry in traped water? especially with a speed of only 3.5 kts. diplacement about 6000 lbs
 
 
rick m