Post
Engine Envy - Propeller Pragmatism Part 2 of 2
Pitch
Diameter
is only one part of the prop equation.
Pitch
is the term for the distance a propeller
would move forward in one revolution if
there were no slip.
Slippage
or slip is the term for the
difference between the distance that the
propeller actually advances passing
through water, compared to the distance
a similarly pitched screw would advance
passing through a solid medium. If it
were screwing through wood, an
11-inch-pitch prop would travel forward
11 inches in one revolution due to the
angle of the blades. In water, because
of slippage, it might only go 8 inches
due to the slip in the liquid medium. A
slip of 20 to 30 percent is common for
an H28.
Slip is
needed to produce thrust. A propeller
with no slip will generate no thrust.
As the
vessel travels faster the less will be
the slip.
The
objective for any propeller installation
is thus not to optimise slip but to
optimise efficiency where efficiency is
measured as the shaft thrust x forward
speed (which equates to the power from
the shaft) / power from the engine.
A very
different and difficult equation in
which slip will obviously be an
important variable.
Props
often have the diameter and pitch marked
on them on the hub. A 14 x 11 prop has a
14-inch diameter and 11 inches of pitch.
The
sailing prop
An
alternative for reducing drag is to fit
a sailing prop. These props have some of
the blade area removed in a way that
doesn’t affect the effective thrust
from the prop too much. There are two
and three bladed sailing props.
The
effect of prop size on engine RPM
If the
prop has too much pitch the engine will
labour and not be able reach full speed.
Hence the engine won’t be able to
deliver the full horsepower when needed.
An indication of an engine being over
loaded is black smoke coming out the
exhaust. This is unburned fuel and can
result in damage to the exhaust valve(s).
If the
prop is under pitched the boat won’t
reach full speed.
When you
really need the engine chances are the
wind is on the nose and the hull is
dirty. Under these conditions you want
all the horsepower you can get to get
you where you want as fast as possible.
This means being able to get the engine
going at full revs without black smoke.
The pitch
of the prop can be changed by taking the
prop into a "prop shop". A
rule of thumb is a change of 1 inch of
pitch will result in a change of 200
rpm. So if your engine is not reaching
full revs then its possible to take an
inch of pitch off the prop. This will
cost around $60 plus haul out fees.
Keeping
it clean
No matter
how good a prop is the efficiency of the
prop reduces with fouling. All sorts of
attempts have been made to reduce
fouling including polishing, painting,
coating with grease, and coating with
lanolin. However as soon as the prop
spins the coating starts to come off.
But wait
all is not lost! Our technical co-ordinator
Kerry Blaymires and Monty Python
Skipper Richie Williams have tried
coating their props with Teflon and it
appears to work. It costs around $200
and the marine growth doesn’t stick as
badly. However watch out for the travel
lift operator with the big scraper as he
may try to get all that black stuff off
for you.
Typical
prop sizes fitted to an H28.
An 18~20
hp engine installed on an H28 will
typically be fitted with a 14 by 9 inch
prop. The pitch and diameter will differ
from engine to engine depending on the
gearbox ratio and the maximum rated RPM.
A 20-hp engine should be able to
comfortable push along at six knots.
A 10~12
hp engine would typically have a 13 x 7
inch prop and should be able to reach 5
knots in moderate conditions. Again the
pitch and diameter will differ from
engine to engine depending on the
gearbox ratio and the maximum rated RPM.
In my
case I have a Yanmar YSM diesel rated at
12hp. Unlike most engines that use a
left hand prop mine is the other way
around. Some of the 12 horses in my
engine are a bit sick so I have the prop
pitched to 6 inches. I can comfortably
reach 5 knots in moderate conditions.
Richard
|
