GL: Bow Thruster vs. Stern Thruster?

[John Ventura]

I'm a strong proponent of a stern thruster, having used one quite successfully
on the loop 2003-2004 (41' single screw trawler).  Here's why:

As has been mentioned earlier, every vessel has its pivot point, around which
it turns left or right.  On a monohull, that point usually is about one-third
aft of the prow.  To visualize, when you turn to starboard, the forward
one-third of your vessel will swing to the right, and the after two-thirds
will swing to the left.

(We used to teach in shiphandling school that the helm station is usually
located at or near the pivot point -- a useful tip in gaining the "feel" for a
vessel while conning her.  I have found that tip to be valid on the bridge of
a destroyer as well as at the helm of my trawler.)

Because of the physics related to this pivot point, the stern thruster is
inherently far more efficient than a bow thruster:  the lever-arm upon which
the bow thruster works (horizontal distance between the thruster and the pivot
point) is very short --  only a few feet on your Monk.  The stern thruster on
the other hand has a very long lever arm -- perhaps 25 or more feet from the
transom to the pp.

This means that the stern thruster acts much more quickly and requires
considerably less power.



There are other considerations:

Some have said you can swing the stern in a single screw vessel by judicious
use of the engine and rudder.  While that is certainly true,  there are
disadvantages to maneuvering thus in close quarters:  among them, there will
always be fore and aft motion which can be undesirable;  next, there will be
additional strain and wear on the transmission and linkages; finally it is
inherently inefficient and clumsy compared to use of a thruster.

Stern thrusters are usually mounted  on the transom and require no underwater
hull penetrations, which suggest that they may be less expensive to install
and repair.


I bought my vessel for the loop and it had a stern thruster already installed.
I had had no previous experience with such a thruster and was, frankly,
skeptical.   I grew to love the thing.

On the Loop there will be scores, if not hundreds of occasions  -- other than
docking and undocking -- where you'll appreciate a thruster:  Waiting for
drawbridges and locks to open, lining up prior to anchoring, etc.   Simply,
you are in tight quarters very frequently, and a thruster -- whether bow or
stern -- will provide you with comfort, safety, peace of mind and flexibility
innumerable times.   I wouldn't consider doing the loop in a single screw
without one.

Best Wishes,

John Ventura








  ----- Original Message -----
  From: Michael Linden<mailto:lzfairwinds@earthlink.net>
  To: great-loop@lists.samurai.com<mailto:great-loop@lists.samurai.com>
  Sent: Sunday, February 11, 2007 7:52 PM
  Subject: GL: Bow Thruster vs. Stern Thruster?


  I am thinking of adding a thruster to our boat.  I am hoping the
  "List" can provide some good pros and cons of each.  Thanks
  Mike Linden
  Monk 36
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