[gurps] underwater sails

[David Scheidt]

On Tue, Jan 13, 2009 at 3:06 PM, Susan Koziel
<kataryna_dragonweaver at yahoo.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.)

>
> I've "sailed" using roller blades and a kite like object while in my "sounds crazy lets try that" years.
>



> -Sue-definately not a physics person nor a sailor
>
>
> --- On Tue, 1/13/09, David Scheidt <dmscheidt at gmail.com> wrote:
>
>> From: David Scheidt <dmscheidt at gmail.com>
>> Subject: Re: [gurps] underwater sails
>> To: "The GURPSnet mailing list" <gurpsnet-l at sjgames.com>
>> Received: Tuesday, January 13, 2009, 11:08 AM
>> On Tue, Jan 13, 2009 at 1:00 PM, Anthony Jackson
>> <ajackson at iii.com> wrote:
>> > David Scheidt wrote:
>> >>
>> >> You can only sail at the interface of two media.
>> That's normally the
>> >> interface between air and water, but it can be
>> other things.  Water
>> >> and water doesn't work; the density difference
>> between the two isn't
>> >> high enough.
>> >
>> > Density difference is unnecessary; it helps preserve
>> the boundary, but if
>> > the boundary exists for other reasons, you can sail as
>> long as you're on the
>> > boundary between two regions with different
>> velocities.
>> >
>> > In practice, this isn't likely to be very useful
>> underwater, but it's
>> > theoretically possible.
>> >
>>
>> The density difference is what makes it possible to
>> transfer energy
>> from one medium to the other.  it's how keels work.
>>
>>
>> --
>> David Scheidt
>> dmscheidt at gmail.com
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-- 
David Scheidt
dmscheidt at gmail.com