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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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With our rudder log/lower bearing in place,
we can now use our "dummy" shaft to reconfirm clearances, angles, etc.,
before moving on to our next step-the strut.
After the dummy shaft is close to EXACTLY where it needs to be, held on
one end with the transmission/coupling, and a brace located just in front
of where the strut will be, we slide up our fully machined stainless lower
tube with a bearing or very accurately machined bushing inside of it. |
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| Start
of strut building |
Tee
slotting base plate |
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| Some measuring, and we now have it in final resting
position in perfect alignment as it's on the shaft. Next comes some basic
drawing and design of what we want this strut to be like (single leg, V-type,
extra HD etc.) and then after that decision, we make the base plate. We
temporarily screw it up to the boat in its correct position, and now design
and fabricate the "leg" between the base and the heavy wall tube. |
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| Pattern
for center leg |
Pattern-setup
center leg |
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| Center
leg fit |
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On single legged struts, we employ two methods of construction
and this is dependent upon size and duty. Most common is a solid center
leg made of ½" -1" thick 316L stainless plate that has been cut and shaped
to fit. This is usually what we do for props up to about 21" diameter and
1.75" shafts using close to minimum prop clearances. For heavier duty applications
when we still want to use a single legged strut, we use a 3 piece leg tapered,
curved and shaped to form a very strong somewhat diamond shaped cross section.
This is the only way we feel we can get the strength needed for single legged
fabricated struts using props up to 30" diameter and sometimes bigger. In
both cases we always "slot & key", and weld the center leg to the base allowing
for welding attachment form both sides. When the application requires a
much heavier/stronger strut, then the "V-Strut" design comes into play.
Back to "OUR" strut. All 3 pieces have now been made and it's time for the
"tack-up". .A portable TIG makes life easy now, so we tack it up in place,
take to the welding bench, and we weld it up so it becomes ONE. We already
know that the final welding will distort it some from this exact shape/angle.
No problem, as we are going to float our finished strut in place w/ epoxy
to compensate for these types of imperfections. |
| With our finished strut in hand, it's back
to the dummy shaft that is exactly where we want it. Some prepping on the
hull, a pre-fit / slide-up of our new strut to see if when in position,
it is not interfering with the hull. |
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| Patching
& prepping out holes |
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| Actually we do not even want it to touch,
if possible-maybe 1/16" to 1/8" clearance all around, as this will make
floating much easier. A few more checks and we mix up a cup or so of our
favorite epoxy that is suited for this purpose-this is also a judgment call
based upon the size of the fill, the base plate dimensions, ambient temps,
and your experience as to what works for you. Gray "Marine Tex" will never
let you down, although we don't use it.. Our favorite of late are the "Simpson
Strong-Tie" 2-part epoxies that come in "twin tubes", having compression
yields above 10,000 PSI. Most any high quality 100% solids epoxy non-sag
paste sold thru industrial or marine outlets will do the job. Using West
System, Systems III , or ??, will also work if used with hi-density fillers. |
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| Strut
- float and cure |
Strut
pre-fairing with epoxy and
hi-density filler |
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| Strut
pocket |
Faired
strut pocket |
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| The strut has now had a 24 hour soak in it's epoxy
bed, so now it's time to drill. We have planned well so we punch thru the
hull w/ a drill 1/64" larger than are bolt size -- (always sloppy) as we
want clearance for our sealant that will be used for bolt-up and backing
plates. After drilling, we measure and fabricate an applicable backing plate
(1 piece or 2 pieces), temporarily screw them in place, and mark them for
final drilling. Backing plate material can be a thick section of fiberglass,
Stainless or bronze. If SS (our preferred choice) usually 3/16" to 1/4"
is more than sufficient. Lots of 5200 or other type of underwater sealant
is what you do not want to skimp on when doing the final bolt up-We are
looking for a 100% fill/bed of the bolts and backing plates. |
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| Fully
faired |
Strut
backing plate |
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| Strut
install |
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| We now install the bearing(s) permanently. Our own
SOP for this is "NEVER" install a bearing that will not slide easily into
the strut, stern casting, shaft tube, etc.. It's just "our thing", and we
always machine as needed for a "slide" fit-maybe .015 to .020" clearance
for a bearing OD of 2" to 3". Slide the bearing in, mark for set screw drillings,
remove and drill full depth/diameter for these, refit the bearings, put
in set screws and seat lightly, but then back off ½ turn or so - and now
you have a MECHANICAL interlock that will never fail or distort the bearing
shell - remove for the last time, coat w/ 5200 or a thinner epoxy, slide
in, remove excess - add sealant to the set screws, seat lightly, then back
off ½ turn - and voila-you now have a "floated" bearing that will not do
anything stupid. Many times we will machine the "bore" of our strut or machine
.060" to .125" off the bearing OD to allow a "Full" float of the shell.
This will allow a more perfect bearing to shaft alignment to compensate
for other types of misalignments we find doing this type of work. We install
the bearing with lots of epoxy and the set screws holding it, but allowing
it to wiggle as needed, slide in the shaft, and let it cure as a shaft holding
the bearing in perfect alignment. Many repowers that have poorly aligned
struts and fixed shafts logs need this type of "realignment". Of course,
your next question is how do your remove a "glued-in" bearing - simple,
same as normal-the right tools and techniques sometimes using a small of
heat. |
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| Bolted-up
struts |
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