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GORI PROPELLERS
Gori has provided high-quality folding propellers
for 20 years. Gori propellers are know world-wide for their quality of design,
casting, machining, balance and ease of maintenance.
Folding propellers are widely recognized as having the minimum drag when
a vessel is under sail, next to no propeller at all. There are three Gori
models
Standard Gori 2-Blade Folding Propeller
The standard Gori 2-blade folding propeller is a geared propeller shown
below in the "under sail", folded orientation. When the auxillary
engine starts and the transmission engages in forward, the propeller starts
to turn. Centrifugal force pushes the tips of the blades outward, causing
them to open and, as the blades begin to bite into the water, positive pressure
develops on the blade face. This pressure pushes the vessel forward and
holds the blades rigidly open. A correctly sized folding propeller can deliver
as much thrust as a regular, fixed 2-blade propeller in forward.
When the engine is put into reverse, the blades rotate in the opposite
direction. Again, centrifugal force opens the blades but in reverse, as
the blades cut into the water, the positive water pressure develops on the
back of the blades as it pulls the vessel backward. Now the water pressure
trys to close the blades and the only thing holding the blades open in reverse
is the centrifugal force. In reverse it is very important to develop high
RPM's, or "lay the whip to it".
The standard Gori 2-blade propeller has several important design features
that distinguish it from other folding propellers: they are more massive,
they are geared and there is a gap between the blades when folded. The greater
mass means that there is more centrifugal force for the same RPM than other
propellers. The gears make the blades work in unison, keeping both blades
closed when under sail in light air (no matter what the propeller's orientation),
as well as improving efficiency when under power. When there is no or little
gap between the blades, an area of low pressure can develop when underway
and the blades can be reluctant to open because the low pressure area creates
a "virtual seal" under some conditions. The gap in the Gori propeller
ensures that the blades open freely when the shaft starts to turn. The Gori
2-blade enjoys an excellent reputation for performance in reverse, forward
and under sail.
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Gori 3-Blade Folding Propeller
Sailers, like power boaters (banish the thought), are putting more horsepower
into their vessels. With more horsepower comes the need to run a 3-blade
propeller. Also, many of these vessels are long-range motor sailers that
can benifit from a propeller with two different pitches: a lower pitch for
standard operation and a higher pitch for motor-sailing in down wind conditions.
The Gori 3-blade can be oriented for sailing, reverse operation, standard
forward operation and in overdrive forward operation.
The Gori 3-blade is folded, as pictured below, when under sail offering
a minimum drag configuration. When the propeller starts to turn under power,
water pressure develops immediately on the exposed portion of the blade.
The water pressure determines if the blades open to the right or to the
left... the propeller opens one way when turning clockwise and the other
way when turning counterclockwise. The face of each blade has one pitch
and the back of each blade has a higher pitch.
When the vessel is put into forward, the propeller opens so that the face
of each blade is the positive pressure side. The water pressure pushing
on the blade face pushes the vessel forward. When the vessel is put into
neutral before putting the vessel in reverse, the blades fold immediately
under the current caused by the forward motion of the vessel. In reverse,
the propeller rotates in the opposite direction, flipping the blades over.
The water ressure is again pushing on the blade face and pulls the vessel
in reverse.
When under sail in favorable conditions, the skipper can decide to motor
sail using the back face of the propeller blades, which have a steeper pitch
than the front face of the blades. This is accomplished by starting the
engine and backing down to put the propeller in the correct orientation.
While putting the vessel in neutral, the current keeps the blades open.
When the transmission is put into forward, the propeller blades do not flip
over, because the positive pressure that develops when the prop starts to
turn holds the blades in place. The sails are raised and the vessel is now
overdrive, achieving higher speeds for a given RPM, or better feul economy
by running at reduced RPM's at a given speed.
The Gori 3-blade is made for engines up to 150 HP. Because the blades are
naturally fixed, the vessel will see the same performance under power as
with a fixed blade propeller but will have minimum drag under sail.
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Racing Folding Propeller
The Gori racing propeller is for sailers who want to go FAST and is the
propeller of choice for many of the Whitbread Round the World racers. The
primary design consideration for the racing propeller is that it offer the
absolute minimum drag possilbe when under sail. The propeller is geared
to ensure that both blades remain closed even in light air, and that they
work in unison when opening in reverse and in forward. The blade area is
very large to deliver the maximum thrust during acceleration and when backing
down.
The blades on the racing propeller close tightly; any site for turbulence
would increase drag. To ensure that the blades open every time the engine
is engaged, there is an exposed "thumb" on the top blade that
is not covered by the bottom blade, and vice versa. When the propeller turns,
the water pressure pushes on the thumb to pop the propeller open.
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Maintenance
The Gori folding propellers require no special maintenance. Like all propellers,
they should be kept clear of marine growth to maximize performance. A gentle
rubbing with scotch-brite or like material will eliminate most growth. Severe
growth can be cut with a brass-wire brush (never stainless steel on a bronze
propeller!) or a fine grade sandpaper. Over time, the stops can become worn
or the pin hole elongated. These items can be repaired readily in most cases.
The gears are water lubricated and are not a place where significant damage
usually occurs.
As with all propellers, care should be taken when going from forward to
reverse or reverse to forward. It is best to hesitate in neutral for a moment
when changing directions. Careless handling can result in damage to the
underwater gear (transmission coupling, shaft, bearings and strut) or even
tear an engine from its mountings and over on its side. Old Gori propellers
are often worth repairing; there are many of the original Gori propellers
still in use today after 20 years of faithful service.
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Alameda Prop & Machine Home Page
Alameda Prop & Machine
2021 Alaska Packer Place
Alameda CA 94501
(510)522-7899
(800)774-7899
FAX: (510) 522-4984
alaprop@marinepropelles.com