Operation Manual

Diesel fuel flow sensor kit Installation and Operation Manual
15
NAVMAN
The propeller is the final link in the chain that
decides if your vessel is a peak performer or just
another boat. The best hull and the strongest
engine will be wasted unless your propeller is
the right size and shape.
From the fuel consumption curve, you can
very simply work out how well your propellers
are working. The performance of a propeller
is measured by its slip factor, and this section
describes how to calculate the slip factor of
your propeller.
Propeller size
First of all, a few basics on propellers. Propellers
are described by a series of numbers - nearly
all propeller people seem to prefer inches to the
metric measurement. For example if a propeller
is described as 24 x 21 then:
The diameter is 24 inches,
The pitch is 21 inches. The pitch is the
theoretical distance the propeller would
move forward through the water for each
full turn.
Slip factor
In reality, because water is a liquid and some
of it will sneak around the edge of the propeller
blades, the boat will not move forward by the
pitch for every turn of the propeller. This is called
the “slippage” or slip factor. The amount of slip
depends on many things - shape of hull, any
rudders, struts, keels etc near the propeller.
Calculating the slip factor
It is very useful to work out the slip factor for
your propeller.
You will need to know:-
A boat speed and RPM setting. Use the
boat speed at maximum rated engine
RPM:
a Either use figures you have taken
from the fuel consumption table (see
section 3).
b Or run the boat at the maximum rated
engine revs and note the revs and
boat speed through the water.
The ratio of the gearbox(es) on your boat
- this will be a number typically between 1
and 3.
The pitch of your propeller in inches.
First calculate theoretical speed:
1 Take the engine RPM and divide by
the gearbox ratio. This will give you the
number of revolutions per minute the
propeller turns.
2 Multiply this number by the pitch of your
propeller in inches. This gives the number
inches forward per minute, if there was no
slip.
3 - Multiply this number by 60. This gives the
number of inches forward per hour.
4 - Divide this number by 72912. (72912 is
the number of inches in a nautical mile)
As an equation it is:-
Theoretical speed =
RPM x pitch x 60 .
Gearbox ratio 72912
The answer is the theoretical speed, for the
chosen RPM, if there was no slippage. The
units are nautical miles per hour (knots).
Because of slippage, the actual speed you
were able to achieve is less than the theoretical
speed.
To find the slip factor (as a percentage):
1 Take the theoretical speed you worked
out above.
2 Subtract the actual speed of the boat to
get the difference in speed.
3 Divide this by the theoretical speed.
4 Multiply by 100 to express it as a
percentage.
As an equation it is:-
Slip factor (percentage) =
Theoretical speed - Actual speed x 100
Theoretical speed
4 Measuring your propeller’s performance