The engine and gear ratio, together with the engine power, are critical for any
propeller sizing.
When choosing an engine &/or propeller for your boat, it is important to
consider the effect of the gearbox ratio on the propeller. Most marine diesel
engines have a reduction gearbox, which makes the shaft turn slower than the
engine, at a speed more suitable for the propeller.
The gear box ratio determines the rotation speed of the propeller in
revolutions per minute (rpm):
Propeller speed (srpm) = Engine rated rpm ÷ Gearbox reduction ratio
With an existing engine there were generally several different gear ratios
available to choose from at the time of install depending on shaft angle. It
is best that you get your actual ratio off the gearbox tag. It is generally
stamped on the top or side of the gearbox expressed as a ratio or number
(example: 2.64:1 or A–2.64).
The best method to get the actual ratio is to use your cell phone camera & take
a photo of the plate on the transmission (beats hanging upside down reading it
with a mirror and a flash light).
Before selecting a propeller type you should look at the advantages, features,
benefits & differences in performance relative to your sailing style/boat use
so that you can make an informed decision. I know there is a lot of information
below but I want to make sure you choose the correct propeller to suit your
sailing style/boat use.
I personally am looking at the Variprop Feathering or Gori Folding
The Variprofile feathering propeller is a unique feathering propeller and the
first to incorporate the hi-tec GAWN/KAPLAN profile. The only feathering
propeller with a shaped blade which is 7-11% more efficient than the std flat
bladed feathering propeller. This profile has been used primarily for high
performance power boat applications, but also for rudder design and in the
aircraft industry.
Normally associated with high-speed power boat and aircraft propeller blades
the Gawn/Kaplan blade profile & slim hub delivers very low drag, excellent
Hydro-dynamics while running extremely more quietly and efficiently under power
than other propellers with the result that the VariProfile delivers speed
normally associated with fixed propellers or the best folding propellers in
both fwd. & reverse with its continuous independently variable adjustable
external pitch control (patented). Forward and Reverse pitches can be changed
independently in just seconds, even underwater.
The Gori is a folding propeller with two(2) pitches in fwd & full 100% reverse
thrust, no prop walk. The Gori does not auto-rotate/spin under sail as does a
feathering propeller at higher speeds, or have one blade drop down and increase
drag or not open when coming into the slip.
The drag for the Gori 3-blade propellers is the lowest of any fixed, feathering
or folding propellers (even less than any 2-blade) giving increased sailing
speeds & less turbulence over the rudder, enabling you to point higher. The
unique folding design prevents seaweed etc from fouling on the propeller giving
increased speed potential. You will see an increase in sailing speeds over your
fixed 3-blade propeller. The drag reduction will amount to 0.95 + of a knot for
a 36 footer.
The blades of the Gori propeller are designed with both camber, curve & twist
to them, which gives increased efficiency (thrust) throughout the rpm range,
which will always give you better motoring performance (speed/thrust),
especially in heavy seas, or when motor sailing with the "Overdrive" Function.
The Gori 3-blade "Overdrive" function, by increasing the blade pitch angle,
gives increased thrust and speed - at the same rpm or when motoring sailing, or
the same boat speed at much lower rpms. The result is less engine noise, less
vibration, increased fuel economy & therefore a greater cruising range.
The Gori 3-blade propeller is the only propeller that will give you exactly the
same performance in reverse as in forward due to the unique blade folding
action of presenting the identical leading edge in reverse as in forward -
resulting in no 'prop walk'. See http://ab-marine.com/folding/gori/ &
http://gori-propeller.com/
Rodney Meryweather
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