[gurps] underwater sails

[Jeff Wilson]

From: Susan Koziel <kataryna_dragonweaver at yahoo.com>
On Tue, 1/13/09, David Scheidt <dmscheidt at gmail.com> wrote:
>>> > > Well if you were an underwater creature you could use
>> > a sail like object to maneuver via placing a sail in a under
>> > tow or counter current, as there are water organisms that
>> > basically move like that....
>>> > >
>>> > > But not being personally a sailor type what the heck
>> > is an under water sail; and why do you need a density
>> > difference for a sail... doesn't a sail work because the
>> > wind fills it an pushes a vessel along a surface - and wind
>> > has a velocity that you are harnessing, and the surface has
>> > less resistance then the force the wind exerts over the area
>> > of the sail. Density of the wind or the water has nothing to
>> > do with it.
>> > 
>> > No.  You don't (generally) sail by letting the wind
>> > push the sail.
>> > The sail is an airfoil.  It generates lift.  (even downwind
>> > sails,
>> > like spinnakers generate more lift than drag)  The keel
>> > resists the
>> > sideways component of the lift's vector, and what's
>> > left is where you
>> > go.   With out a keel, you can only go straight down wind. 
>> > Real boats
>> > can go much faster with the wind on their beam.  (Actual
>> > angle depends
>> > on the boat, and how fast depnds on the boat, too.)
>> > 
> 
> See... It just shows I'm really not a physicist.
>  :) 
>> I still don't see how the density (g/L) has anything to do with it
> though. 

Strictly speaking it is resistance to motion outside the direction of 
the keel that is important, and the inertia per unit volume of the 
medium is one source of resistance that depends on the density. Another 
source of resistance is viscosity,  which depends on the medium's 
molecular structure being conducive to flowing, but the number of 
molecules flowing past one another also depends on density, etc.


> -Sue-damn it Jim I'm a Biologist not a Physisist
> PS: wind on their beam? (don't sail either - being rather land locked
> here) I thought that you turned the sails to catch the wind... but would
 > like to know better.

With wind blowing perpendicular to the "plane" of the sails, you are 
limited to the speed of the wind; once you are equal in speed, there is 
no more relative push and you stop accelerating.  With wind blowing over 
the curve of the sails at an angle to the direction of travel, the lift 
can diminish but remains significant, continuing to accelerate the craft 
to the limits of resistance from water and air, and can easily go faster 
than the prevailing windspeed, in an oblique direction. It is also 
possible to sail *upwind* by zig-zagging at obtuse angles.

("Lift" here is used to mean the same force that lifts aircraft in a 
similar situation because airplane wings are concave upward; sails are 
concave horizontally.)

-- 
Jeff Wilson - jwilson at io.com
< http://www.io.com/~jwilson >