Bow Thruster
 

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

On 29 Oct 2004 21:31:09 GMT, ospam (Stephen Baker)
wrote:
An admiring audience
of curvaceous females who appreciate the art never hurts here.

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

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

That's true.

Docking Disaster Coefficient (DDC) = N_spectators^3

There IS something very satisfying however about backing a 50 foot, 20
ton sailboat smartly into a slip and stopping the whole thing with a
quick burst of forward power just as you're about to take out the main
pier. It's the element of suspense that really gets them on their
toes.

Stephen Baker wrote:
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

When docking a boat, head into the dock, a kick ahead on the engine, is
used for:
1. To increase/decrease rudder effectiveness.
2. To increase headway

...... a kick astern is used for:
1. To make use of prop walk in steering
2. To reduce headway

When docking a boat, stern first into the dock, a kick astern on the
engine is used for:
1. To make use of propwalk to increase/decrease rate of turn.
2. To increase sternway

...... a kick ahead is used for:
1. To make use of the rudder to move the stern opposite to the direction
of the propwalk.
2. To decrease sternway.

Notice, I'm saying a "kick" ahead/astern. Any use of the throttle, means
you screwed up the docking, 90% of the time, for one reason or another
(misread wind/current/turning rate/speed).

Are these rules written in stone? HELL NO!!! These are basic rules, to
work WITH, to think about as you maneuver, so you have a basis to judge
how well your maneuver went.
If you are moving astern, fast enough to have steering with your rudder
(even on a sailboat) you are probably moving astern too fast (NOTE, I
said PROBABLY).
The most important line you will PROBABLY use when docking, is a spring
line (either the "fwd after spring" or the "aft forward spring") which
should be run from a point 1/3rd of the way aft of the bow, to a point
midships, or 1/3rd of the way fwd of the stern, to a point midships, but
only if necessary, right AT the bow or stern.

:) Let the games begin!!

otn

BTW, I forgot to mention:

When handling a boat, there's a Right Way, a Wrong Way, and Your Way.

For the vast majority of boat handlers, they have a specific boat that
they handle at (generally) specific locations, under specific
conditions, and they learn a way to handle this boat, that works for
them under those conditions.
The point being, most of us don't get to do too many dockings or
undockings during a short season, so that we have to rely on what we
experience for our particular boat and conditions.
There is nothing wrong with this. The BEST way to handle your boat, is
the way which works best for you...... :) my comments just tend to delve
deeper into the subject.

otnmbrd says:

snip

If you are moving astern, fast enough to have steering with your rudder
(even on a sailboat) you are probably moving astern too fast (NOTE, I
said PROBABLY).

You're a stinkpotter, aren't you? ;-)


Steve

On Sat, 30 Oct 2004 00:09:08 GMT, otnmbrd
wrote:

If you are moving astern, fast enough to have steering with your rudder
(even on a sailboat) you are probably moving astern too fast

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

That's what everyone always says right up until I put the boat right
where I want it, the first time. (sailboats only please, big rudder,
big keel, etc.)

Stephen Baker wrote:
otnmbrd says:

snip

If you are moving astern, fast enough to have steering with your rudder
(even on a sailboat) you are probably moving astern too fast (NOTE, I
said PROBABLY).


You're a stinkpotter, aren't you? ;-)


Steve


G Nowadays, but not always, so that argument don't fly

Wayne.B wrote:
On Sat, 30 Oct 2004 00:09:08 GMT, otnmbrd
wrote:


If you are moving astern, fast enough to have steering with your rudder
(even on a sailboat) you are probably moving astern too fast


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

That's what everyone always says right up until I put the boat right
where I want it, the first time. (sailboats only please, big rudder,
big keel, etc.)


If it works, use it ..... any comments I make, are directed more to the
person having a problem or not understanding why something may be giving
him trouble.

otn

If you are moving astern, fast enough to have steering with your rudder
(even on a sailboat) you are probably moving astern too fast


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

That's what everyone always says right up until I put the boat right
where I want it, the first time. (sailboats only please, big rudder,
big keel, etc.)


This is simply not true. The problem with backing single screw or
non-counter rotating twins is often interprted as have the ability to turn
only one way or the boat won't go straight back. This is caused by Prop
Walk. To gain or maintane stearage when backing your drive line must be in
neutral. Don't try to power backwards instead think of the power as a push.
Short spurts of power in reverse then retuning to neutral and glideing on
the push will give you control in either direction. Try it the next time
your on the boat.

John

On Mon, 01 Nov 2004 00:42:11 GMT, "John Cassara"
wrote:

Short spurts of power in reverse then retuning to neutral and glideing on
the push will give you control in either direction. Try it the next time
your on the boat.

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

Absolutely correct, at least for sailboats with a big rudder.

Sorry. Kinda long

John Cassara wrote:
If you are moving astern, fast enough to have steering with your rudder
(even on a sailboat) you are probably moving astern too fast


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

That's what everyone always says right up until I put the boat right
where I want it, the first time. (sailboats only please, big rudder,
big keel, etc.)



This is simply not true. The problem with backing single screw or
non-counter rotating twins is often interprted as have the ability to turn
only one way or the boat won't go straight back. This is caused by Prop
Walk. To gain or maintane stearage when backing your drive line must be in
neutral. Don't try to power backwards instead think of the power as a push.
Short spurts of power in reverse then retuning to neutral and glideing on
the push will give you control in either direction. Try it the next time
your on the boat.

John

One quick point before the main thrust .... There are very few twin
screw inboards that are not "counter rotating", and the term "counter
rotating" does not tell you what you need to know, because twin screw
with "inboard turning", generally handle totally differently than twin
screw with "outboard turning" props (both are "counter rotating").

To the main thrust .....

Overall, I don't disagree with what you're saying, but the speed you
will need to use, will vary greatly between various power boats, sail
boats, and powerboats/sailboats and this can cause problems.
For powerboats, because of various rudder sizes and shapes, you may get
good steering at slow speeds, to, no steering at any speeds.
When I go aboard a single screw powerboat, I'll know what direction the
prop turns (let's say right hand) and what to expect there, but I won't
generally know what kind of rudder power I might have, so, to begin with
the rudder stays midships or hard left (G sometimes I have a feeling)
so that when backing, a kick astern turns my stern to port and a kick
ahead (with the hard left) turns my stern to stbd. Once I figure out the
rudder power astern, I can alter this to suit the boat, which could, and
has, easily concurred with what you say above.
On a sailboat, I normally figure I will have reasonably good rudder
power, and one thing I seriously look at is ... tiller or pedestal steering.
G My Contessa had tiller steering. Get that boat moving astern and you
almost immediately had steering, but get it moving too fast and it took
two men and a boy to hold the tiller so as to not "over steer", i.e.,
very little rudder for big results ..... pedestal steering tends to
alleviate this... at least, that's MY experience.
All this brings up a point. Steering astern, tends to be different than
steering ahead. In many cases, if you have enough stern way to steer,
you can easily get to a point where more than minimum rudder will give a
strong shear which may be hard to correct or counter (phew, I'll use my
Contessa as a prime example) and simply shifting the rudder may not be
sufficient to counter this shear or turning rate fast enough to solve
the problem, without the use of the engine..... hence, keep the speed
down to a point where you can easily use kicks ahead (with
rudder)/astern, to steer.
Once again, boats vary, operators vary, conditions vary. My main point
is that you need to learn your boat, your own, and the conditions
variables . Use what works for you, but make sure you have a Plan B,C,
etc., and don't be afraid to experiment.

otn