Bow thruster drag
 

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

Bow thruster drag
 

"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