I am planning to mount a bow thruster on a 41' Sailboat and i wonder if
someone
knows the influence of the tunnel opening at the bow. Namely how much
the created
turbolence will affect the performance of the sailboat.
In the Vetus Web site, as small as possible tunnels (possibly more than
one) are suggested just to keep the resistance under way low;
unfortunately this very statement
makes me worry!
Thank You. Claudio.

On Fri, 22 Oct 2004 17:06:07 GMT, igavioli wrote:
I am planning to mount a bow thruster on a 41' Sailboat and i wonder if
someone
knows the influence of the tunnel opening at the bow. Namely how much
the created
turbolence will affect the performance of the sailboat.
In the Vetus Web site, as small as possible tunnels (possibly more than
one) are suggested just to keep the resistance under way low;
unfortunately this very statement
makes me worry!
====================================

It's difficult if not impossible to quantify the drag but it will
definitely be significant. Racing boats go to great lengths to either
plug up, or put mylar flaps over, through hull fittings that are far
smaller than a bow thruster aperature. The bow thruster aperature is
also in just about the worst possible place as it will disrupt
attached flow over the entire length of the hull. If you are serious
about sailing performance, I would not install a thruster.

wrote:

I am planning to mount a bow thruster on a 41' Sailboat and i wonder if
someone knows the influence of the tunnel opening at the bow. Namely how
much
the created turbolence will affect the performance of the sailboat.
In the Vetus Web site, as small as possible tunnels (possibly more than
one) are suggested just to keep the resistance under way low;
unfortunately this very statement
makes me worry!
Thank You. Claudio.

igavioli wrote in message ...
I am planning to mount a bow thruster on a 41' Sailboat and i wonder if
someone
knows the influence of the tunnel opening at the bow. Namely how much
the created
turbolence will affect the performance of the sailboat.
In the Vetus Web site, as small as possible tunnels (possibly more than
one) are suggested just to keep the resistance under way low;
unfortunately this very statement
makes me worry!
Thank You. Claudio.

Claudio, normally bow thrusters are mounted on power boats such that
they are out of the water when the boat is on plane. I suspect they
create a lot of drag if they are not out of the water. In your case,
with a sail boat, it would always be in the water. I would talk to a
bow thruster manufacturer (they often have people at the boat shows)
before I installed one, they would know best or they may have models
made for sail boat applications.

John

Wayne.B wrote:
It's difficult if not impossible to quantify the drag but it will
definitely be significant. Racing boats go to great lengths to either
plug up, or put mylar flaps over, through hull fittings that are far
smaller than a bow thruster aperature. The bow thruster aperature is
also in just about the worst possible place as it will disrupt
attached flow over the entire length of the hull. If you are serious
about sailing performance, I would not install a thruster.

I understand what you write but I happened to encounter quite a few
sailboats with bow thrusters; usually 40" or longer.
I remember all of them very well: entering with nonchalance her narrow
berth while I am struggling with her backing that makes her wander to
all places except the right one!
I wonder if it is possible to put in front of the tunnel openings
something like a flap valve or a venetian blind device. This would
reduce the impact on the flow pattern.

Daniel

igavioli wrote:
I am planning to mount a bow thruster on a 41' Sailboat and i wonder if
someone
knows the influence of the tunnel opening at the bow. Namely how much
the created
turbolence will affect the performance of the sailboat.
In the Vetus Web site, as small as possible tunnels (possibly more than
one) are suggested just to keep the resistance under way low;
unfortunately this very statement
makes me worry!
Thank You. Claudio.

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.

http://www.diy-boat.com/Pages/Archiv...e/upgrade.html

On Fri, 29 Oct 2004 08:51:35 GMT, Daniel
wrote:
I remember all of them very well: entering with nonchalance her narrow
berth while I am struggling with her backing that makes her wander to
all places except the right one!

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

I've found that the secret to backing a sailboat is to give it a quick
burst of reverse power while still in open water. Get enough speed in
reverse, usually 1 or 2 knots will suffice, that you have water
flowing over the keel and rudder. At that point, take the engine out
of gear and let the boat coast backward under its own momentum. It
should now respond to steering just as if you were moving forward.
Practice, practice, practice...

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!=-

Wayne B says:

I've found that the secret to backing a sailboat is to give it a quick
burst of reverse power while still in open water. Get enough speed in
reverse, usually 1 or 2 knots will suffice, that you have water
flowing over the keel and rudder. At that point, take the engine out
of gear and let the boat coast backward under its own momentum. It
should now respond to steering just as if
you were moving forward.

I find the best way is to get the "going forward" part just right, so that
reverse is used only to take way off the boat so you can step gently onto the
pierhead with your bow/stern line at the perfect moment. An admiring audience
of curvaceous females who appreciate the art never hurts here.

It doesn't always work like that in practice, however... ;-)

Steve