On Sun, 9 Nov 2003 18:50:24 -0500 (EST), (Ron
Thornton) wrote:

Jim Woodward's comment on hydraulic bow thrusters resurrected a thought
I had a long time ago and never followed up on. If you are going to pump
a fluid, why not pump seawater to nozzles in the hull for thrust.

Regards, Ron

This is a jet versus prop question.
A jet can do well at low speed if
1) It can move a column of water as wide as a prop diameter.
2) it can move that column of water as slow as a prop does.

This highlights the prospect of 20 inch tunnels of slow moving sea
water as thrusters.
Or...you could use three half inch pipes of hydraulic fluid to a
motor/prop combination.....

Brian Whatcott Altus OK

On Sun, 9 Nov 2003 00:35:06 -0800, "Evan Gatehouse"
wrote:


"Glenn Ashmore" wrote in message
news:JDPqb.10756$62.1728@lakeread04...
I don't think it is quite ready for prime time yet but here is an
interesting idea. A small retractable Voith-Schneider drive like those
used on modern tugs. Retracts flat into the hull and when extended will
deliver thrust in any direction. http://www.spw-gmbh.de/en/hb/

Wow. I'm not a great fan of VS drives ('cause they have lower efficiency
than ducted props) and they are hideously expensive. I'm surprised he has
got around the VS patents!

The new Volvo 70 rule allows for retractable propulsion systems - using a
Volvo engine and shaft drive. Not sure how designers will make this work
but it will be interesting to see if can trickle down to more mainstream
designs. Now I want to sit down with a sketch pad and start thinking about
this!!

I have long thought that a vertical saildrive could be made in a
retractable form. A non-folding prop could be keyed so it could be
gotten vertical from inside, and the whole thing lifted through a
tubular well. A diisk on the bottom could retract into th ewell far
enough to allow a faired surface to fit the particular hull to be
added with microballoons and epoxy.

It could be made in azimuthing form, but that would add a lot to the
price.

It couldn't be ducted without makig the well much larger, but for a
sailboat efficency isn't as important as getting out of the way.

I believe it would come down to which is cheaper, if they both take up
as much room inside.

We don't really know if he has gotten around the VS patents, do we?
They may just not have noticed him, or don't feel threatened.




Rodney Myrvaagnes NYC

Let's Put the XXX back in Xmas

I asked the same question, as we expect to have a fairly high capacity fire
pump aboard. Turns out that aside from Glenn's and Brian's explanation
(which are correct, you want low speed, very high volume for a thruster and
a fire pump does the reverse) that a "standard" hydraulic bow thruster is
off the shelf and relatively cheap (to the extent that anything in our world
is cheap), particularly since we have the prime mover and want hydraulics
aboard anyway for windlass, docking capstans, crane, and get home chain
drive (that's a bonus which was cheap).

Also, one of the advantages of the bow thruster is that it eliminates all
the single point of failure issues in the steering system -- the bow
thruster is proportional electric controlled, and can be run, if necessary
from the autopilot, continuously, at sea. That eliminates the tiller head,
tiller arm, two heavy blocks and tackle, and the two hearties to heave on
them if the steering failed. And, of course, even that doesn't save you if
the rudder itself breaks or jams.

It is possible to buy venturi jet thrusters that might work from the fire
pump, but they won't solve the steering issue, and they are generally for
larger vessels.


--
Jim Woodward
www.mvFintry.com


..
"Ron Thornton" wrote in message
...
Jim Woodward's comment on hydraulic bow thrusters resurrected a thought
I had a long time ago and never followed up on. If you are going to pump
a fluid, why not pump seawater to nozzles in the hull for thrust.

Regards, Ron