MerCruiser 13, Service Manual

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1A - 6 - GENERAL INFORMATION 90-816462 2-695
Weight Distribution
Weight distribution is extremely important; it affects a
boat’s running angle or attitude. For best top speed,
all movable weight - cargo and passengers - should
be as far aft as possible to allow the bow to come up
to a more efficient angle (3 to 5 degrees). On the neg-
ative side of this approach is the problem that, as
weight is moved aft, some boats will begin an unac-
ceptable porpoise.
Secondly, as weight is moved aft, getting on plane be-
comes more difficult.
Finally, the ride in choppy water becomes more un-
comfortable as the weight goes aft. With these factors
in mind, each boater should seek out what weight lo-
cations best suit his/her needs.
Weight and passenger loading placed well forward in-
creases the “wetted area” of the boat bottom and, in
some cases, virtually destroys the good performance
and handling characteristics of the boat. Operation in
this configuration can produce an extremely wet ride,
from wind-blown spray, and could even be unsafe in
certain weather conditions or where bow steering
may occur.
Weight distribution is not confined strictly to fore and
aft locations, but also applies to lateral weight distri-
bution. Uneven weight concentration to port or star-
board of the longitudinal centerline can produce a se-
vere listing attitude that can adversely affect the
boat’s performance, handling ability and riding com-
fort. In extreme rough water conditions, the safety of
the boat and passengers may be in jeopardy.
Water in Boat
When a boat loses performance, check bilge for wa-
ter. Water can add considerable weight to the boat,
thereby decreasing the performance and handling.
Make certain that all drain passages are open for
complete draining.
Elevation and Climate
Elevation has a very noticeable effect on the wide-
open-throttle power of an engine. Since air (contain-
ing oxygen) gets thinner as elevation increases, the
engine begins to starve for air. Humidity, barometric
pressure and temperature do have a noticeable effect
on the density of air. Heat and humidity thin the air.
This phenomenon can become particularly annoying
when an engine is propped out on a cool dry day in
spring and later, on a hot, sultry day in August, does
not have it’s normal performance. (See chart.)
Although some performance can be regained by
dropping to a lower pitch propeller, the basic problem
still exists. The propeller is too large in diameter for
the reduced power output. The experienced marine
dealer or a Quicksilver Propeller Repair Station can
determine how much diameter to remove from a low-
er-pitch propeller for specific high-elevation loca-
tions. In some cases, a gear-ratio change to the drive
unit to more reduction is possible and very beneficial.
It is a known fact that weather conditions exert a pro-
found effect on power output of internal combustion
engines. Therefore, established horsepower ratings
refer to the power that the engine will produce at its
rated RPM under a specific combination of weather
conditions.
72006