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| Tony's Tips |
ARTICLE DATE: 10/01/2005 |
| Installing shafts, logs,
struts & rudders with your favorite engine / transmission |
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Replacing a propeller shaft, shaft log,
strut, or rudder, one at a time is a fairly straight forward job as you
have a "given" as to exactly where one of these pieces is supposed to end
up and probably what it looked like in its original form. If a shaft log
is in place or still part of the vessel, its angle is fixed and both the
engine and strut must be in line with it. If the strut is in place, then
the log must be installed in-line with it. Pretty simple, when you think
about it, as there isn't much to think about; it's all been done for you
before. What this article is about, is when you are starting with NOTHING
but a bare hull/engine room stringers that generally will only tell you
about where the engine / transmission will go or be situated in the vessel.
Now you are scratching your head.
The images here are really the bulk of this article, as "they say" what
is hard to put into words, by showing the basic sequence of events that
need to take place. All of these types of jobs must start on the drawing
board though, but this does not mean "AutoCAD" or ?, just a basic drafting
board and the most basic drafting tools along with manufacturers drawing
Armed with this now, the person overseeing the work must also possess the
knowledge to select not only the right engine/trans combo, he needs to know
the diameter of the prop, shaft size, the general location and dimensions
of the rudder. Basically, these major components must be decided BEFORE
you hit the drawing board. Yes, it is during this initial layout and planning
period that you may find that an initial selection of a particular transmission
may not allow the engine to clear the underside of the deck, or it may hit
the bottom of the boat.. It is much easier to "erase" and re-think a selection
now than after your purchase. Where I see most of the problems in this area
of the planning is with purchasing an engine/trans combo with the lack of
(enough) down angle, or whether it's an older style coax-type gear (like
a Twin Disc MG 506 or 507) that typically need more clearances to get them
to fit. Because of the many varieties in transmission design, this has made
it more complex as to choosing the right gear, but also has given us the
flexibility to do a "better fit" job. I think the biggest innovation in
marine gears was the "down angle" design, initially introduced w/ the early
Twin Disc MG502A, but highly popularized by the IRM220A w/ a built-in 10
degree down angle (now called the ZF220A) about 20+ years ago. To this day,
this basic design has been adapted to most installations and continues to
evolve into marine gears with ratings to well over 1000 HP. I'm still waiting
for a 12 degree down angle gear as this would even be better in some installs/designs.
But, that's why we are still in this "drawing stage".
I use a small 30+ year old drafting machine (you know the ones with arms
and a protractor angle "Vemco" type) on a regular table. I draw at 1 to
2" per ft scale and have found that one can easily get everything in place
to within 1/2" or so, including the final shaft length. As the actual install
progresses, some parts of the job are more easily figured out at that time
instead of attempting to draw them. The "drafting work" is more of (what
I call) a POINT drawing, in that it tells me the major positions and clearance
I need to make it all fit. This is not supposed to be hi-tech, just common
sense using basic high school skills mixed in with past experience. Also,
keep in mind that this is not a production boat - These are all "ones-offs",
as what applies to measurements on this boat will most likely never apply
again. You also find on twin engine boats, that "adjustments" may need to
be made as to "final" position on some components due to the irregularities
in vessel hull construction, between port and starboard sides.
Doing this work is not for most "first timers" as good skills in glass work
and / or metal fabrication work are needed along with the foresight as to
how each small position change will affect another part of the entire shaft/log/strut/final
engine-trans position. But I hope the pictures presented here will help
someone do this with a better understanding of the over-all job involved. |
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