Outboard – Long Shaft Conversion

By Cory Carpenter

Article taken from Good Old Boat magazine: Volume 4, Number 4, July/August 2001.

A 27-year-old outboard starts a newer, deeper life

7-hp Eska outboard

I bought Brushfire, my 1975
San Juan 24, from the Sea Scouts. Before I agreed to the deal, the
Sea Scouts offered to throw in an outboard as a sweetener. The motor
they dug up for me was a 7-hp Eska of 1973 vintage, a two-cycle with
an integral three-quart fuel tank.

It ran well enough once I did some research and discovered that it
wanted a 24:1 fuel-to-oil ratio, but I found that even with my 180
pounds as far aft as possible and Brushfire’s scissors-style
motor mount at its lowest setting, the Eska’s cavitation plate
was barely below the surface. The whole prop came out of the water
with any kind of wave action and the motor would race madly with a “www-AH,
www-AH, www-AH,” like a hydroplane running through chop. My original intention was to use the Eska just long enough to get
Brushfire about 3-1/2 miles down the Columbia River from the Sea Scouts’ base.
Outboard engine out of the water
I figured I would return it to the Scouts and treat myself to a new
long-shaft Honda four-stroke. Then I started checking prices . . .
and concluded that it wouldn’t be such a bad thing to have an
outboard from the same era as the boat! In the course of researching long-shaft outboards on the Web, I happened
upon Bay Manufacturing of Milan, Ohio, which makes shaft-length conversion
kits for Mariner, Mercury, OMC, and Yamaha outboard motors, but not
for 27-year-old Eskas. Looking at the pictures on their Web page
caused me to wonder what it would take to do my own long-shaft conversion.

The motor

My Eska is a model 1747-C. It uses a Power Products/ Tecumseh recoil-start,
two-cycle, air-cooled power head, much like a lawnmower engine’s
except that the exhaust port is on the bottom of the cylinder, rather
than the side. I’ve seen discussions on the Web that indicate
this motor was also sold under the Sears Gamefisher and Ted Williams
trademarks. It has a spring clutch to engage the driveshaft, providing
neutral and drive gears. (Mine was frozen with rust, and the motor
had to be started in drive. Soaking the clutch in kerosene for a week
or so helped free it up.) For reverse, you swing the motor all the
way around and, while hanging precariously over the transom, you try
to remember which direction to twist the throttle. In drive, a small
pump provides cooling water to the exhaust manifold/adapter plate that
mounts the engine to the lower unit of the motor. I obtained a service manual from Certified Parts Corporation, which
purchased the outboard and trolling motor divisions of Eska in 1988
and still provides most parts for these units. Some study of the
exploded diagrams in the manual showed that, given the proper tools,
it would
be fairly straightforward to construct a shaft extension. Regular shaft, long shaftConveniently,
the housing containing the propeller shaft, gears, and water pump
separates from the main section of the column just above the cavitation
cooling-water tube merely connects to the pump with a friction fit
into an O-ring, while the top end of the driveshaft fits into the
clutch on the engine’s output shaft in much the same way, torque
being transmitted by splines. Because of this simple design, all I
needed to do was add length to the cooling-water tube, fabricate a
new driveshaft, and build an extension fairing, a shell that would
conform to the cross-section of the column where it joined the gear
housing. I decided that an additional six inches would be about right. For the best corrosion resistance and electrolytic compatibility
with the rest of the motor, the new parts should have been fabricated
aluminum and stainless steel. Because I wasn’t willing to go
to the trouble and expense of obtaining the ideal materials, because
my MIG welder won’t handle aluminum wire reliably, and since
this project was experimental anyway, I used mild-steel stock that
was lying around my garage for the major components of the extension.

The long driveshaft

The piece to add to the shaft

To avoid doing unnecessary work in case the project proved infeasible,
I started with the driveshaft, the most critical piece of the extension
in terms of tolerances. The original driveshaft is a 7/16-inch
steel rod approximately 24 inches long, splined at each end and keyed
at the lower end to drive the water pump. (I note that the original
shaft will deflect the compass aboard Brushfire, so it may not be stainless
steel.) What I had on hand was 1/2-inch rod, but since the center
of the shaft runs free inside the column, the diameter is only critical
at each end. After cutting it to length, I turned the ends of the 1/2-inch
stock down to 7/16-inch. Then after fabricating a flycutter
and index latch for my lathe, I cut the splines in each end of the
shaft. Since I’d removed the pinion gear to disassemble the
old driveshaft from the gear housing, I had it available to test the
fit of my new splines. Once the splines were finished I matched the bottom end of the new
driveshaft with the original, marked the locations for the snaprings
that retain the pinion gear, and used the lathe to cut seats for
them. The keyway for the water pump impeller is just a flat spot on
the driveshaft.
It was simple to rough it out with a hacksaw and an angle grinder
and then to clean it up with a file.

Extending the column

Identifying the trailing edgeWelding

The HotCoat system

From top: identifying the trailing edge, welding, and the HotCoat system.

Reassured that this whole exercise might actually work, I tackled
the extension fairing for the column. What I had available was 14-gauge
sheet steel (approximately .075 inch thick). I quickly determined
the curve at the leading edge of the column fairs back into a straight
line, so I could match it with just five sections of material as
shown at right: two curved for each side of the leading edge, two straight
for the trailing edges, and one flat piece for the back edge. I first obtained the outline of the required section by clamping
a sheet of heavy paper between the column and gear housing and tracing
around the joint. This also punched holes in the paper, providing locations
for the fasteners that connect the gear housing to the column. I translated
the tracing of the fairing cross section inward by the thickness of
my sheet stock, then cut templates from 1/2-inch plywood. Laying
the template on a flat surface, as shown in the top photo on the opposite
page, showed me where the trailing edge would begin. I marked this
location, then wrapped a strip of paper around the curve to the tip
of the profile, marking it to establish the length of sheet metal that
I would need to bend in order to form each side of the leading edge.
Later on, I used the templates to align the pieces of the extension
fairing while welding. Creating the curved sections for the leading edge of the fairing
involved lots of hammering with the sheet metal supported against cylindrical
objects of various diameters, such as an empty propane-torch tank and
a length of 2-inch iron pipe. I checked my progress frequently against
the plywood templates. I found that it worked best to create a curve
that was tighter than what I needed, then flatten it out progressively
by hammering against the anvil surface of a machinist’s vise.
(It took a while to form acceptable curves, but I got there eventually.) Once I had all the pieces of the fairing worked to their proper lengths,
I ground a chamfer of about 45 degrees on the outside of each edge
to be joined. This was to ensure good weld penetration of the entire
thickness of the sheet metal. This was important because I wanted
to build up a bead that I could later grind down flush with the outside
surface of the extension fairing. Assembling the pieces around my plywood templates, I tack-welded
them together with a MIG welder, then removed the forms. I completed
welds a bit at a time, allowing them to cool between passes in order
to minimize distortion of the sheet metal. I built each seam up into
a nice, heavy bead as shown in photo.

Once the shell of the extension was welded together, I used an angle
grinder to clean up the top and bottom edges, checking often with
a framing square to make sure the surfaces were parallel with reference
to the back edge of the fairing. I ground the edges until the height
was uniform and slightly more than six inches, then did the final
with a large mill file across the entire top and bottom edges of
the shell to get the mating surfaces as straight and square as possible.
The holes that allow water to drain from the column after the motor
is hauled out were cut with a hacksaw and shaped with a file.

Finished result of the baking

Don’t try this at home, kids, without your mom’s permission: fairing after coating and the finished result of the baking (done in a home oven).

When I was satisfied with the fit of the shell against the gear housing
and the column, I fabricated alignment tabs from 1/8-inch by 1-inch
steel strap and welded them into the top and bottom of the fairing.
These provide lateral rigidity as well as ensuring that the extension
stays centered against the inside surfaces of the column and gear

I fabricated mounting flanges from 1/8-inch by 1-inch strap
to provide the connection points at the top and bottom of the fairing.
I first drilled the holes in them and then lined them up with the holes
in my templates and traced the outline that would become the edges
to be welded to the inside of the fairing shell. As the drawing above
shows, the flanges at the rear of the fairing are threaded for the
mounting bolts. Here I cheated: instead of spending a lot of time messing
around with taps to thread holes in the flanges themselves (in which
case they would have needed to be made of thicker material), I installed
a nut and bolt on each flange, with the nut on the inside face, then
I tack-welded the nuts to the flanges and removed the bolts. This provided
for a strong connection in a situation in which it’s impossible
to get a wrench on the nut. The bottom flange at the leading edge is constructed in much the
same way, but it uses a section of threaded rod to mirror the stud
on the
leading edge of the column. The upper flange merely has a plain hole
that the column stud passes through. It’s secured by a nut and
lockwasher from inside the extension fairing. (The original trailing-edge
mounting bolt was frozen; I was forced to drill it out. I elected to
install its replacement from the outside of the column, otherwise both
top flanges would have had plain holes.) Once the construction was complete, I ground the welds down flush
with the surface of the fairing, then polished the whole outside surface
with a 3M Scotch-Brite surface conditioning disc installed on an angle
grinder. This is the best way I’ve found so far to remove rust
and mill scale from steel components. It also left a very smooth, shiny
surface, which was important for the next step. Had I been thinking
ahead, I would have polished the inside surfaces of the fairing before
welding it together. Since I hadn’t polished the inside, I did
as much cleanup as I could afterward with wire brushes and a sand blaster.
Live and learn.

Powder coating

Joining the cooling water tubeReassembling the gear housing and driveshaft

Finished project

From top: joining the cooling water tube, reassembling the gear housing and driveshaft, and the finished project.

Because my materials were very susceptible to rust, it was important
to protect them. Epoxy-based or even enamel spray paint would probably
have been adequate, although getting good coverage on the inside
of the fairing would have been tricky. Luckily, I had a better alternative
available. Powder coating is a process that leaves metal parts with a smooth
protective layer that is harder and more durable than most paints,
covers irregular
surfaces more readily and uniformly, and is non-toxic to boot. The
process involves applying an electrostatic charge to a finely powdered
plastic that is then sprayed at low pressure over a grounded metal
part. The powder sticks to the metal surfaces just like dust to a
TV screen. Once it’s been coated, the part is baked in an electric oven
until the plastic melts, flows, and cures to a durable covering that
is resistant to abrasion and to most solvents. (The only thing I’ve
found to date that will attack powder coating is methylene chloride,
found in products such as spray-on gasket remover. Acetone dulls the
surface slightly; gasoline and oil run right off.) This was an industrial
process that required expensive equipment until a couple of years ago
when The Eastwood Company started selling a hobbyist’s powder-coating
package. Their basic HotCoat system sells for about $150. The nicest part of using the powder-coating process for this project
was that the charged powder was attracted to and covered every surface
of the fairing, inside and out, even the irregular surfaces of exposed
welds. To avoid fouling the threads, I installed bolts in the threaded
holes and masked off the trailing-edge stud with high-temperature fiberglass
tape (also from Eastwood) before coating everything with the shade
of powder I had on hand that came closest to matching the Eska’s
paint. Photos on Page 12 show the fairing after coating and the finished
powder-coated fairing after it was baked for 15 minutes at 400 degrees
F. I also powdercoated my new driveshaft after taping off the splines
and the surfaces that would bear on the bushings and seals. (Any
color would have done for the driveshaft; I happened to have black
in the
HotCoat gun at the time.) The final piece of the project was the extension for the cooling-water
tube. In this case I actually had to break down and go to the hardware
store to buy aluminum tubing that matched the 3/8-inch outside
diameter of the water tube, as well as vinyl hose with a 3/8-inch
inside diameter to join the extension to the original tube. (Since
I was there anyway, I also purchased stainless-steel cap screws, nuts,
and lockwashers.) I cut the ends of the new 6-inch extension to the
same angle as the end of the original water tube so they would fit
together flush, then smeared the outsides of the two aluminum pieces
with RTV silicone and joined them with a length of vinyl tubing as
shown in photo. (Epoxy might have been a better choice for this, but
so far the RTV seems to be working fine.)

Assembly is the reverse of removal

To reassemble the motor, I first coated all the bare metal surfaces
on the bottom end of the new driveshaft with a generous layer of
waterproof grease (meant for the wheel bearings of boat trailers) to
protect them
from rust, then reassembled the gear housing and driveshaft as shown
in photo.

With the motor supported upside down on an empty 5-gallon bucket,
I applied RTV silicone along the entire upper surface of the extension
fairing, then fitted the extension in place and bolted it on. The
part of this operation was getting the nut started on the threads
of the stud inside the leading edge of the fairing. I found that a
parts-retriever, a tool like a walkie-talkie antenna with a magnet
at the tip, was invaluable for this. Once the nut was finally engaged
with the threads of the stud, I tightened it with a socket on the
end of a long extension. With the motor still upside down, I coated the bottom of the extension
fairing with RTV, applied grease to the exposed metal at the top of
the driveshaft, and carefully lowered the gear housing assembly onto
the column until the driveshaft splines meshed with those in the clutch.
Working through the exhaust port and the small gap that remained between
the gear housing and the extension fairing, I used a long screwdriver
to jockey the end of the water tube into location in the water pump.
Once the driveshaft and water tube were started, I simply pushed the
gear housing down until it was seated on the extension fairing and
bolted it loosely into place. Before tightening the fasteners down
for good, I put the clutch selector in the “drive” position
and gave several pulls on the starting cord to make sure the driveshaft
wasn’t binding and in the hope that it would tend to align itself
by shifting the gear housing around slightly. The assembled result
is shown on the opposite page in bottom photo. After the RTV had cured, I refilled the gear housing with SAE 90
lubricant as specified in the Eska service manual and, using the time-honored
outboard-motor test stand, a sawhorse and a 55-gallon trash can full
of water, fired up my motor. With the gear selector in “neutral” I
was gratified to see that the clutch was now working properly. The
propeller remained motionless while exhaust racketed from the relief
port at the top of the column, sounding like a leafblower on steroids.
Once the motor warmed up and the idle settled down, I cautiously moved
the selector to “drive,” and . . . it worked!


I’ve been very pleased with this conversion’s performance
aboard Brushfire. With the motor mount in its lowest position, the
entire extension is underwater, giving the propeller plenty of bite,
and I can move forward in the boat without it becoming a menace to
low-flying birds. The tip of the gear housing’s fin is just
barely in the water with the motor mount in the topmost position. With
the motor rocked forward in its tilt bracket, everything is high and
dry. One odd side effect of the extension is that with the motor idling
and the gear selector in “neutral,” the exhaust resonates
inside the comparatively thin-walled extension fairing with a funny “bloop-bloop-bloop-bloop” note
that puts me in mind of The Secret Life of Walter Mitty.

If you don’t have a well-equipped metalworking facility in your
garage, you should be able to find a machine shop willing to fabricate
all or part of a similar shaft extension “kit.” Some
research in my area turned up two shops that were capable of the job,
one specializing in high-volume CNC production and not interested in
one-off jobs; the other a gear-making business that estimated $225
just to make the driveshaft. Though I haven’t had to take work
to a machine shop for nearly two decades, I find it hard to believe
that there’s nothing left but specialists.

Fabricating the driveshaft obviously requires some moderately specialized
equipment, but the operations involved are straightforward and shouldn’t
be too costly in terms of labor. (It took me about six hours all told,
but 80 percent of that was in building a flycutter.) If you don’t
have a welding outfit, any decent machine shop should be able to construct
the extension fairing as well. If you don’t know of a shop already,
talking to a few old-timers around your marina should turn up someone
who can do the job. Anyone who can cope with simple hand tools can
handle the fabrication of the cooling-water tube extension and the
final assembly. I’ve noticed that even a reconditioned long-shaft outboard starts
at $900; new ones are going for $1,500 or more. If you have an older
motor available, this project could be cost-effective even if you have
to farm out the actual fabrication work. (Not too long ago, I noticed
a 7-hp Eska identical to mine offered for $350 at my local marine exchange.
Now if I were to buy that and modify it I might be able to clear about
$500. Hmmm . . . )

Potential problems


While I hadn’t noticed any signs of rust on
the exterior of the extension assembly, it still concerned
me that the whole thing is potentially susceptible to corrosion.
I’m relieved to report that, after four months and
about four-and-a-half hours of accumulated running time (including
two extended motor-only runs of an hour each), there was
no sign of a problem when I disassembled the extension to
take photos for this article. Brushfire sails in fresh water.
I would definitely go to the expense of using inherently
corrosion-resistant materials for saltwater operation, or
if the motor were semi-permanently installed in an outboard


The through-the-prop exhaust system depends on a
good seal between the mating surfaces of the column, the
extension fairing, and the gear housing. The walls of the
column casting are about 1/8-inch thick, and the sealing
surface of the gear housing is a nice milled area that varies
from 1/4-inch to about 1/2-inch wide. The walls of the extension
fairing are roughly 1/16-inch thick, providing only minimal
sealing area. Almost immediately after I put the converted
motor into service I noticed bubbles of exhaust gases forcing
their way between these junctions. The RTV silicone obviously
wasn’t enough. I later made gaskets from 1/32-inch
automotive-gasket material and bedded them in hardening-type
gasket sealer. These worked for a while, but they eventually
started leaking also.

I’ve since welded lengths of 1/8-inch steel rod along
the inside of the extension fairing at the top and bottom,
making a generous bead that I ground down flat to provide
a wider sealing surface. I also filed the powder coat off
the mating surfaces. I suspect that the coating is so slick
that the sealants I’ve tried have not been able to
adhere to it very well. I have yet to put enough run-time
on the motor to tell whether this change will be effective.

Drain holes:

When I originally designed and built the extension,
I neglected to do anything about the original drain slots
at the bottom of the column’s leading edge. These
slots are right at the waterline, and when I first tried
the converted motor aboard Brushfire I noticed quite a bit
of exhaust escaping from them. My remedy wasn’t pretty,
but it does work: I threaded stainless-steel sheet metal
screws into two scraps of wood about 1/8-inch thick and,
with the extension fairing removed, filled the drain slots
with thickened epoxy, using the wood on the inside of the
column as backing. I tightened the screws down to hold the
wooden backing blocks in place while the epoxy cured, then
filed everything down flush with the mating surface of the
column. With the extension reassembled, the epoxy seals the
original drain holes. (The screw heads are visible in the
bottom photo on the opposite page, just above the top of
the extension fairing at the leading edge of the column.)

Shaft alignment and vibration:

I have no way to evaluate
this, since there’s so much incidental vibration when
the motor’s running. I’m reasonably confident
that the drive shaft itself is true, since I could check
that while I was machining it; I don’t know how accurately
it is aligned between the motor and gear housing but there’s
not a lot that I can think of to correct such a problem.
(I try to ignore the possibility, apart from making sure
I have fresh batteries in my hand-held VHF transceiver in
case the gear housing should fall off in mid-river and I
need to call for a tow.)


Bay Manufacturing
P.O. Box 1250
Milan, OH 44846

Parts Corporation

1111 W. Racine Street
P.O. Box 8468
Janesville, WI 53547-8468
orders 800-356-0777

Eastwood Company

263 Shoemaker Road
Pottstown, PA 19464

Sea Scouts Ship 601

Cory’s uncle taught him to sail when he was in high school. After 20 years, he’s
relearning those skills with the help of
Brushfire, his 1975 San
Juan 24. Most of his sailing is done as a singlehander on the Columbia
River in Portland, Ore., but the right 30-something crewwoman could
change all that.

Join Our Sailing Community. Subscribe Now!

Complimentary monthly supplement

  • featured articles
  • news from the helm
  • mail buoy
  • book reviews
  • sailor photos
  • and much more

Good Old Boat News Flash!

Our website is getting some long overdue improvements! Audioseastories.com has merged with Goodoldboat.com.

Thanks for your patience while our website is under construction.