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Cavitation Confusion (21-6)
by Steve Horsfall
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 I
have an unusual boat-motor combination that's
causing some problems.
I bought a 29-foot Chris Craft-Grew that weighs 7,000
pounds. The previous owner removed the two four-cylinder
engines and replaced it with a 232 hp single diesel.
The engine is made by a German company called Steyr.
It's unfortunately utilized with a OMC King Cobra outdrive.
I am told Mercury builds a larger outdrive that would
work better.
The prop is a Mercury 16x16 with a conversion kit to
fit the OMC outdrive. I believe the previous owner went
this route as OMC does not make a prop aggressive enough
for this engine. The problem is I can easily red line
the engine but my top end at the red line is only 35
kilometers, which is pretty poor.
I have gone to local marinas and they are stumped. They
say that if I go with a more aggressive pitch, I have
to go with a smaller prop diameter and that will just
cavitate. They contacted the manufacturer and they are
stumped. Do you have any suggestions?
T. Simmonds
(e-mail)
With MerCruiser props, it's true that as the pitch goes
up, the diameter goes down, but not by very much. A
two-inch increase in pitch usually means a 400 rpm reduction
at top end and going with cupped blades will help reduce
the rpm even more, while helping eliminate cavitation.
If cavitation is still a problem you could try enlarging
your cavitation plate, using a plastic fin or homemade
aluminum plates bolted to the existing cavitation plate
of the outdrive.
FINDING THE RIGHT CHARGE
I purchased a 1986 Johnson outboard engine.
On the transom bracket, the model number is J140TLCDC.
On the engine block it says CJ120TLCOS, followed by
G7025226. It has no oil injection. I am not sure what
type of spark plug I should use. Can you help?
P. Vande Velde
(e-mail)
The recommended spark plug is a Champion QLV4 with a
gap of .030. This is used on all 140 hp Johnsons from
1985 to 1997.
COLD CONCERN
I am a new owner of an older Doral Citation
25-foot cruiser. We did a lot of research and determined
this type of boat would be the perfect starter for our
family and we are loving it so far. The boat has had
a lot of major modifications in the past three years,
so what little work we have left to do is mostly cosmetic.
I will undertake most of this myself, but I am wondering
if you have any recommended discussion groups where
fellow boaters can chat to use for future reference.
I also need to replace the fridge. That being said,
I was recently speaking with a friend who raised the
question of possibly using a standard mini fridge, instead
of using one for marine use. I know this would only
run off of direct power and not be AC/DC such as some
marine fridges.
Is there a valid reason for not using a standard mini
fridge as opposed to using a fridge specific for marine
use, with the understanding that it would only have
power when we were connected to shore power?
J. Sinclair
(e-mail)
The problem with using a camper or a household mini-fridge
in a boat is they don¡¯t usually meet the requirements
for spark arresting electrical devices. In the enclosed
space of a boat where there is the possibility of fuel
vapors in enclosed spaces, this can be a very real safety
concern. I don¡¯t think it is worth the risk.
I do not know of discussion groups on the internet,
but I am sure they must be there. Perhaps you could
try the Canadian Power Squadron for interaction with
other boaters. They also have courses for DIY repairs
to boats and engines.
Got a question, a problem? Send inquiries to Steve
Horsfall at stephen.horsfall@sympatico.ca Fax: 905-624-6764.
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