On Fri, 29 Oct 2004 08:44:03 -0400, nafod40
wrote:

My semi-informed opinion is...not much. Water is incompressible, so the
pressure change across the tunnel opening will not be a factor. Here's a
good site on bow thrusters.

==========================

Absolutely incorrect. The aperature represents a discontinuity in the
hull form and will create a great deal of turbulence which translates
to drag.

Wayne.B wrote in
:

On Fri, 29 Oct 2004 08:44:03 -0400, nafod40
wrote:

My semi-informed opinion is...not much. Water is incompressible, so the
pressure change across the tunnel opening will not be a factor. Here's
a
good site on bow thrusters.

==========================

Absolutely incorrect. The aperature represents a discontinuity in the
hull form and will create a great deal of turbulence which translates
to drag.



The thruster or torpedo tubes or whatever if open are always on a
curvature, no matter how slight(so the theoretical non-compressing stable
water surface at the juncture of hull and water does not exist). This
causes the water flowing over them to produce eddies at the opening lip,
which in turn causes turbulance above and downstream from the opening,
thus disrupting the stream along the hull with resulting increased drag.

However, the question is really more one of how much effect and not
whether it exists or not. What may be undesrable amounts in say an
Americas Cup racer, would not even be measurable in a heavy 40' cruiser.


For my part I have no problem at all with internal thrusters, but those
ungodly clunky externals will never find a berth on any boat I own.

--
MonteP
"Against stupidity, the very gods themselves contend in vain." -
Friedrich von Schiller
"Ignorant voracity -- a wingless vulture -- can soar only into the depths
of ignominy." Patrick O'Brian
-=The answer is simple...send pretzels to the Whitehouse!=-