|
> 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 -----
|