[gurps] underwater sails

[Anthony Jackson]

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.