confess to being very surprised at the number of people
who have read my article on gas versus diesel engines
and applied it to sail boat auxiliaries. I thought
that it was clear in the article that it only applied
to power boats. In any case, most of the information
contained that article does NOT apply to sail boats,
except the repair cost factor, which is significantly
higher than a comparable gas engine.
As to whether gas or
diesel is preferable in sail boats, as far as I know,
there are no longer any gas inboard engines available
for sail boat auxiliaries. I could be wrong about
that, but I havent seen any, and a net search
hasnt turned up any. So, as near as I can tell,
its a moot point.
Since Ive been
asked this question innumerable times, Ill give
my viewpoint here, though its strictly a matter
of personal opinion.
If I were primarily
a day sailor, Id be quite happy with the Atomic
Four, as it is very quiet and smooth running. With
its old valve-in-block design, it hasnt
proved particularly reliable, and doing a valve job
on it can be quite expensive. Of course, there is
a major carbon monoxide problem with gas engines in
sailboats so that you must keep the exhaust system
in tip top shape at all times or risk killing someone.
But I dont think
Id want that engine were I involved in extensive
long-range cruising. It has a very weak cooling system
that is prone to overheating because of the incredibly
small size of the water pump. It doesnt take
much to bring the whole system down.
Auxiliary diesels are
often a real head-scratcher to me. Frankly, I dont
know how boat owners can tolerate the horrific noise
and vibration these engines make. Here we have sailors,
most of whom profess to detest the internal combustion
engine, and yet have the noisiest engines of all in
their boats. Part of the reason for this is that sail
boat builders have never put much time or thought
into engine installation. Small diesels, by virtue
of their few cylinders are poorly balanced engines.
That means that they have to have even better mounting
systems, which they usually don't. The simple rule
is that the more cylinders there are in an engine,
the more those cylinders balance out the natural vibration.
Thus, the 4 cylinder engines run quite a bit smoother
than 2 or 3 cylinder engines. So, if there is an option
for 4 cylinders versus two, I'd opt for four, even
if it is more power than I need.
A two cylinder engine
has so much vibration that its almost impossible
to dampen it except by the most extreme and costly
mounting methods. Basically, that is cost prohibitive
to most people.
You may be aware that
Yanmar once supplied an engine mount with their engines
that was very good at reducing vibration, but, unfortunately,
the mount was so flexible that extreme engine movement
caused serious engine alignment problems and were
ultimately abandoned, many of which had to be refitted
with other mounts.
There are three ways
to minimize the vibration problems of sail boat diesels.
The first, if youre buying new, is to opt for
at least a four cylinder engine, regardless of whether
you need that much power. The second is to upgrade
the engine mounts. The Bushings, Inc. Mounts supplied
with most diesel installations are just not adequate.
There is a new mount out, with which Im very
impressed with the design, and is supplied by Caterpillar
on all their engines. Unfortunately, I dont
recall the name of the manufacturer, but you can contact
any Cat dealer to find out.
new Caterpillar mount. Note that this engine
is not properly aligned and is distorting the
mount. The top plate below the second nut should
be level but is cocked at an angle.
Naturally, they are
not cheap, but if you cant stand to live with
that horrible vibration, its really a small
price to pay.
I find many sail boats
fitted with all manner of fancy and expensive shaft
couplings. For the most part, these are of little
value since, if your shafting is set up right, they
are not needed. The vibration does not come from the
shaft or propeller (unless those items are out of
whack) but the engine itself. And the way to deal
with that is in the engine mounting system. Unfortunately,
most sail boat mounting systems are poorly designed.
Often, theyre little more than a small raised
foundation glassed to the hull. Unlike power boat
stringers, which run the length of the hull and distribute
the vibration over a wider area, these engine mounting
pads concentrate the vibration at a single point.
Thats not helpful.
Considering the low power of the engines, you should
never experience transmission failures. That they
do occur is usually the result of two problems. Excessively
loose, weak or worn out engine mounts allow the engine
to move. This throws the shaft out of alignment and
puts strain on the output shaft bearings. Plus, sailors
are prone to ignoring the condition of shaft cutless
bearings. When these wear out, the shaft then jumps
around, also causing strain on the gear box. Beware
that if the bearing is installed in the keel, it is
going to wear at a higher rate than one on a free-standing
strut due to less water lubrication available.
Engine movement should
not be perceptible. If you can see the engine moving
or shaking from vibration (and many of them do), then
you've got an engine mount problem, and damages of
one sort or another will surely occur. If the top
of the engine is rocking back and forth, so will be
the bottom part, where the shaft connects to the gearbox.
The other common problem
is an oversize prop, either in diameter, pitch or
both. To get a handle on this, you need to consult
a propellers load chart and see whether the load delivered
by the prop exceeds the torque rating of the transmission.
If it does, you needn't look any further for why you
gear box failed. You can get the box torque ratings
from dealers or the manufacturer.
Factors That Affect
Diesel Auxiliary Engine Life
Under ideal circumstances,
sail boat diesel auxiliary engines should last the
life of the boat. That they rarely do is usually the
result of two things: (1) Failure to maintain properly
and, (2) faulty exhaust system installation.
As with power boats,
exhaust riser failure is the number one cause of major
engine damage. Second is excessive exhaust system
back pressure caused by creating risers that are too
high. Sail boats often have two risers, a riser being
any upward direction of the exhaust piping intended
to prevent water backflow through the exhaust system.
First there is the riser at the engine, usually a
cast iron or stainless steel part. After this there
may be a secondary riser consisting of the exhaust
hose -- usually after the muffler -- that makes a
high upward sweep before turning downward and out
through the hull. In larger sailing yachts, Ill
often find these to be in excess of four or five feet
What we need to realize
here is that the exhaust pressure has to push large
amounts of cooling water UP that four or five foot
height. This causes pressure to build up in the engine,
with the result that the engine runs far less efficiently,
burns more fuel and runs hotter, often causing overheating
and premature valve wear or even value burning problems.
The illustration above
shows the right and wrong way to design an exhaust
system. The top illustration shows how the dry riser
eliminates the problem of exhaust pressure having
to move a lot of water. In the lower illustration,
not only does exhaust pressure have to move water
through the muffler, but nearly straight up through
a water cooled pipe or hose. In addition, there is
often a a section of hose that lays along the bottom
of the hull that also fills up with water, so that
exhaust pressure has to move that water, too. And
since water weights 64 lbs per cubic foot . . . .
well, you get the picture. Remember the school kid
trick of stuffing the potato up the school bus exhaust
pipe? Pow! Instant artillery piece. Ohmygod, Johnny
was shot with a potato. But nowadays the kiddies don't
mess around; they use real guns.
Getting the right balance
between minimal exhaust pressure and reducing the
risk of water entering the engine takes a bit of expertise
of the sort that comes with years of experience in
simply knowing what works and what doesnt. There
are no nice design books that anyone can go to for
a quickie solution, since exhaust systems tend to
be designed around available space to put them. That,
of course, is having the horse push the cart: a good,
reliable system should be designed first, regardless
of space requirements, but that is not the way things
are done these days. Precious interior design always
comes first. After all, warranties will long run out
before you, the owner, are called upon to pay the
price of these design faux pas.
When it comes to riser
design, its best that the riser section not
be water cooled, but dry insulated to the point where
water is injected into the cooling system on the down
side of the rise. This completely eliminates the potential
for riser failure wrecking the engine, but it also
poses the hazard of setting the boat on fire if the
insulation is not maintained. Due to vibration, insulation
does not last forever, but begins degrading on day
one. Eventually, if not replaced when it is degraded,
it will likely start a fire. The cost of insulation
is not high, but many boat owners overlook this hazard.
If you have picked
up on the fact that dry insulated risers will eliminate
the exhaust back pressure problems too, hooray for
you, youre right, it does. If youve got
one of those exhaust systems that pushes water up
three feet before pushing out the exit port, you should
seriously consider having your exhaust system redesigned,
for the ultimate benefit of increased engine life
will likely be well worth the cost.
Another factor that
gives rise to reduced engine life is the fact that
most of these engines are poorly ventilated, if at
all. Many people fail to understand that much of the
engine cooling derives from the air around it, so
that if the engine is closed up in a little box without
an adequate air flow, the ambient temperature around
the engine rises dramatically.
Most of the engine
manufacturer installation spec sheets I've seen say
that ambient engine room temperature should be no
more than 125 degrees as an absolute maximum. Even
at that, reduced performance and cooling is occurring.
In most sail boats that I've seen, the temperatures
run well over that, so it's no surprise to me that,
combined with high exhaust back pressure, that so
many boat engines don't run for the mythical thousands
of hours before overhauls, often not even reaching
If this describes your
engine installation, I'd suggest improving the air
flow to the engine. This is best done with a forced
air blower, bearing in mind that blowers should always
pull air out, not in.. Why not? Well, because if you're
pushing air in, those stinky engine smells get spread
throughout the boat. Adding a forced air blower is
most recommended for those that will be doing long-range
cruising and a lot of motoring, particularly in the
hotter climates like the southeast US.
The trick to force
air blowers is finding a way to bring in air that
is not wet and full of salt crystals, otherwise this
ends up going through the engine. Unfortunately, auxiliary
diesels are rarely ever fitted with air intake filters,
so the better way to deal with this is to fit the
blower inlet duct system itself with a filter. That
will prevent salty air from rusting up everything
within the engine compartment.
Usually the best place
to install an intake duct is within the box formed
by the sheet winch mount foundation. Or, less good,
would be a scupper on the aft deck, bearing in mind
that you'll be able to hear the blower from the cockpit
when installed under the winch pod. But, if you create
a good filter box with a foam filter, this will cut
down the noise considerably. Of course, there are
good blowers and the noisy cheap ones. The ones that
operate smoothly and quietly are never cheap. Plus,
you want a blower that is rated for continuous duty
operation, which the usual kind found at the local
marine store are not. If you're looking for a good,
quiet but powerful blower, check with the industrial
supply houses like Granger or manufacturers like Hartzel
What, the engine has
no air intake filter? Have you checked yours, because
it probably doesn't. Guess what happens when you have
a rubber drive belt that is chaffing and fraying.
You know how all that black stuff goes all over the
engine compartment? Of course, the engine isn't sucking
that stuff in. Is it? This gives new meaning to the
term "burning rubber," and why all engines
should have air intake filters.
So there you have yet
another reason why I say that sail boat engines are
usually poorly installed and why they often don't
last as long as they should.
: See Marine
Engines at www.yachtsurvey.com