Tamaroak wrote:
I'm thinking of putting a bow thruster in a 36" trawler. Has anyone read
any studies or done any research on what this hole and the subsequent
tunnel might do to its fuel economy?

Capt. Jeff

Jeff

Bow thrusters are supposed to be mounted such that, at crusing speed,
the unit is out of the water. This may not be practical on some very
slow moving boats that do not sit bow high at cruse.

John

Come on.

To be effective, a tunnel thruster needs to be 2 - 4 diameters below
the surface. A lot of boats are less but their thrusters are usually
toys that don't have enough thrust when you really need the help.
There's only so much thrust you can generate with a certain diameter.

Put these factors together with a boat shallow enough that it's going
to be able to go fast enough to lift its bow very much and lifting the
thruster clear of the water to reduce drag becomes petty improbable.

How do you like these thrusters?

http://home.maine.rr.com/rlma/D124-7.PDF

--

Roger Long



"Capt John" wrote in message
oups.com...

Tamaroak wrote:
I'm thinking of putting a bow thruster in a 36" trawler. Has anyone
read
any studies or done any research on what this hole and the
subsequent
tunnel might do to its fuel economy?

Capt. Jeff

Jeff

Bow thrusters are supposed to be mounted such that, at crusing
speed,
the unit is out of the water. This may not be practical on some very
slow moving boats that do not sit bow high at cruse.

John

"Roger Long" wrote in message
news


Put these factors together with a boat shallow enough that it's going to
be able to go fast enough to lift its bow very much and lifting the
thruster clear of the water to reduce drag becomes petty improbable.

How do you like these thrusters?

http://home.maine.rr.com/rlma/D124-7.PDF


All retractable?

otnmbrd wrote:
"Roger Long" wrote in message
news

Put these factors together with a boat shallow enough that it's going to
be able to go fast enough to lift its bow very much and lifting the
thruster clear of the water to reduce drag becomes petty improbable.

How do you like these thrusters?

http://home.maine.rr.com/rlma/D124-7.PDF



All retractable?

Nope. Probably Schottel or Gill Pump Jets. 360 degree thrust in
shallow water.

Rather poor efficiency though.

Evan Gatehouse

They are actually a Schottel type that is now out of production. If
the project ever gets funded, we'll have to go back to Schottel's.
Efficiency is low but the operational advantages outweigh that, even
for a ship like this that used them for long periods on scientific
station.

This is a diesel electric vessel so can divert an enormous amount of
power the thrusters. In this case, they also double as propulsion
devices in areas where there are marine mammals, divers, or sensitive
bottom habitat.

--

Roger Long



"Evan Gatehouse" wrote in message
...
otnmbrd wrote:
"Roger Long" wrote in message
news

Put these factors together with a boat shallow enough that it's
going to be able to go fast enough to lift its bow very much and
lifting the thruster clear of the water to reduce drag becomes
petty improbable.

How do you like these thrusters?

http://home.maine.rr.com/rlma/D124-7.PDF



All retractable?

Nope. Probably Schottel or Gill Pump Jets. 360 degree thrust in
shallow water.

Rather poor efficiency though.

Evan Gatehouse

Roger

I spent a week on a boat with this kind of setup once, and it worked
very well, easily moving the bow . The boat manufacturer, Viking (58),
was very specific about the mounting location, out of the water at
crusing speed. I haven't seen a factory mounted bow thruster on a power
boat yet that didn't sit out of the water at cruse, where it should be.
Obviously, all thrusters do have their limitations with respect to
their ability to counter the effects of wind and tide. And I do agree
that they are probably over kill on smaller boats (less than 55 feet or
so), but they often help a captian get in and out of places that they
might not normally be able to.

John

In the commercial world, I mostly deal with people who can dock fine
in light conditions. They need the bow thruster for those tough days
that many yachtsmen probably wouldn't be out in anyway. They want the
full potential power that the manufacture says they can get. The
thruster on the Viking probably wasn't putting out but a fraction of
what it could have if properly immersed.

With proper tunnel fairing there would have been no need to have it so
high and it would have been much more effective with the identical
unit.

--

Roger Long



"Capt John" wrote in message
oups.com...
Roger

I spent a week on a boat with this kind of setup once, and it worked
very well, easily moving the bow . The boat manufacturer, Viking
(58),
was very specific about the mounting location, out of the water at
crusing speed. I haven't seen a factory mounted bow thruster on a
power
boat yet that didn't sit out of the water at cruse, where it should
be.
Obviously, all thrusters do have their limitations with respect to
their ability to counter the effects of wind and tide. And I do
agree
that they are probably over kill on smaller boats (less than 55 feet
or
so), but they often help a captian get in and out of places that
they
might not normally be able to.

John

Tamaroak wrote:
I'm thinking of putting a bow thruster in a 36" trawler. Has anyone read
any studies or done any research on what this hole and the subsequent
tunnel might do to its fuel economy?

We're preparing for the same project. If it's faired &
flared correctly, the aperture should add very little drag
at our operating speeds (below 8 knots).


Roger Long wrote:
To be effective, a tunnel thruster needs to be 2 - 4 diameters below
the surface. A lot of boats are less but their thrusters are usually
toys that don't have enough thrust when you really need the help.
There's only so much thrust you can generate with a certain diameter.


Correct. Although I'm wondering about design of the tunnel
to incorporate a nozzle to improve flow and reject air. A
problem with that idea is that the most effective
converging/diverging nozzle designs are not symmetrical for
input & output... you need two tunnels, one for each
direction... or a steerable unit... or a less efficient
symmetrical design. Compromises compromises!

The problem with our boat is that while it does have a
relatively deep forefoot, there is a limit and it doesn't
include an option to put the tunnel 4 diameters below the
LWL. And I don't want to give up the interior room to put in
multiple tunnels, either.



Put these factors together with a boat shallow enough that it's going
to be able to go fast enough to lift its bow very much and lifting the
thruster clear of the water to reduce drag becomes petty improbable.

How do you like these thrusters?

http://home.maine.rr.com/rlma/D124-7.PDF


Not as much as I like the daggerboard!

Fresh Breezes- Doug King

"Capt John" wrote in message
oups.com...

Tamaroak wrote:
I'm thinking of putting a bow thruster in a 36" trawler. Has anyone read
any studies or done any research on what this hole and the subsequent
tunnel might do to its fuel economy?

Capt. Jeff

Jeff

Bow thrusters are supposed to be mounted such that, at crusing speed,
the unit is out of the water. This may not be practical on some very
slow moving boats that do not sit bow high at cruse.

John


Maybe on some boats, but most I've seen are on cruising type hulls that
don't lift the bow enough to get the thruster tube out of the water (other
than in higher sea states). As someone else mentioned, a proper
installation will include a flair on the tube opening (both sides) that
diverts the water away from the tube while underway at speed. I have a
Navigator 4800 Classic (52' LOA) and had a bow (and stern) thruster
installed by the dealer. Made absolutely no difference in the boat's top
speed (23 knots) or cruise performance.

RCE