From: "Arild Jensen" elnav@uniserve.com
Subject: TWL: different propulsion drives

These are floating engines encased in  watertight shell  just big enough to
provide flotation.  The driver sits on top and steers with a wheel.
The boom boats can move in any direction because of the swiveling drive pod
underneath.

I remember at Expo 86 in Vancouver they had a feature called the "Boom Boat
Ballet."  They recruited some of the best boom boat drivers in BC and
choreographed a dance to a famous piece of ballet music.  Sounds corny, but
it was actually an extremely impressive display of the maneuverability of
this type of boat, to say nothing of the skill of the drivers.

The questions that could be asked about the practicality of this type of
drive in a recreational, trawler-type boat are:  1. What does it do to the
draft of the boat?  It sounds like it could be more than the conventional
shaft and prop design.

2. What is the cost compared to the conventional engine(s)-aligned-to-
   shaft(s) layout used in our boats today?  For a commercial, wage-earning
   boat like a boom boat, tug, or cruise ship, the added maneuverability can
   make the extra cost well worth it.  But would the value remain the same for
   a recreational cruising boat?

3. What sort of maintenance and repair considerations are there?  The more
   moving parts you stick in something, the greater the chance of failure, and
   the higher the requirement for maintenance.  The Z-drive has a couple of
   transmissions or gearboxes devoted simply to re-directing the power flow.
   So there are two more "items" to lubricate, cool, adjust, etc.  Finding a
   mechanic capable of working on a conventional drive system is pretty easy,
   as is getting parts, at least for the more common engine and transmission
   types.  If your Z-drive goes out in Mexico or in Petersburg, Alaska, how
   easy will it be to find someone who knows how to work on it, and how easy
   will it be to get parts?

4. What is the cruising efficiency of this type of system?  Efficiency
   isn't high on the list of requirements for things like boom boats and tugs.
   Yes, you want as much efficiency as you can get, but maneuverability and
   360-degree power are much more important.  Every time a drive system turns
   a corner, there is some loss of efficiency.  With the conventional straight
   drive system, there is some efficiency lost in the forward-neutral-reverse
   transmission, but not all that much I suspect.  But with two right angles
   in the drive, plus the gear arrangement required to let the final drive
   rotate 360 degrees, plus the forward-neutral-reverse gearing, it would seem
   there is much more potential to lose efficiency with the Z-drive.

That said, one doesn't have to watch a little boom boat at work very long,
or a huge Foss Z-drive tug, to realize the terrific maneuverability this
drive system affords.

C. Marin Faure
GB36-403 "La Perouse"
Bellingham, WA

Marin Faure  asked:

2. What is the cost compared to the conventional engine(s)-aligned-to-
   shaft(s) layout used in our boats today?

would the value remain the same for a recreational cruising boat?

REPLY
I have asked for pricing of these.
I feel that to make a fair comparison the cost of a bow thruster should be
included  in the conventional  design.

3. What sort of maintenance and repair considerations are there?  The more
   moving parts you stick in something, the greater the chance of failure, and
   the higher the requirement for maintenance.  The Z-drive has a couple of
   transmissions or gearboxes devoted simply to re-directing the power flow.
   So there are two more "items" to lubricate, cool, adjust, etc.

REPLY
How long can a car or truck differential run before requiring service?
The gearing required to change  power direction by 90 degrees is as simple as
what you have in your car.

oil valves etc. they are not exactly a simple gear on a shaft.
If these boom boat drives were  prone to breakdown or  frequent maintenance, I
doubt they would be as popular as they are.

Since this drive also eliminates the need for a separate shaft log and
associated seals  etc this must also be factored into the equation.
Both from a maintenance perspective and a cost comparison.

Concerning the draft, it is equal to the prop diameter plus the 10% tip
clearance  required from a normal  drive. Therefore the draft of this kind of
drive would be comparable to a conventional drive mounted in the same hull.

Boom boats are deliberately  deep draft to give them stability and mass to
bulldoze  the logs around, not because the drives dictate it.
Since these drives are also used for water taxis that  operate in  waters
similar to what  recreational boats use.
Why the would the draft be any different?

One drawback to recreational boats is the  shallow draft  often requires the
prop shaft to be  placed at an angle.
This angle produces some efficiency losses in propulsion.

I deliberately mentioned that I did not think these drives  would be a good fit
for long distance cruising boats.
However during past threads we have already established that most boats  operate
in coastal  waters and many operate in confined waters and require frequent
maneuvering  such as docking, approaching locks in the river systems and  so
forth.

This attendant maneuvering  will most certainly affect overall propulsion
efficiency  much more than any drive configuration would.

Finding a mechanic capable of working on a conventional drive system is pretty
easy,
as is getting parts, at least for the more common engine and transmission
types.  If your Z-drive goes out in Mexico or in Petersburg, Alaska, how
easy will it be to find someone who knows how to work on it, and how easy
will it be to get parts?

REPLY
The exact same  argument and concern has been stated on this list  with regards
to conventional transmissions.
Seems to me that several list members posted requests in the past asking about
finding a qualified mechanic in their neck of the woods.

These units are  comparable in weight and complexicity  to a normal marine
transmission.
Any reason why  you can't handle a replacement in the same way.
Certainly in the PNW  where Marin cruises finding  mechanics  familiar with this
equipment is not an issue.

Reversing his argument, it could be said that we should never  have a boat
equipped with something not made by a local manufacturer and supported by local
service techs.
That kind of thinking would pretty much eliminate  90%  of exploration type
cruising and  rule out most brands except for Caterpillar and Twin disc.
who claim a global service network.

Cheers

Arild

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