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Old 24 November 2002, 10:36   #1
Country: Greece
Town: Gloucetsreshire
Boat name: GATO DI MARE
Make: MAR.CO
Length: 9m +
Engine: Yamaha 200Vmax
MMSI: 235027678
Join Date: Sep 2002
Posts: 3,339
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Some technical stuff

I did not know where to post this thread. I was doing some research on props and came accross some usefull info. Is a lot of staff and just took a few notes down. If you are interested please let me know and will post the rest. AND MORE INTERESTING


Anything you wanted to know about them but no one wanted to tell you

The workings of a prop
To find whether a prop is right or left screwed one should take a prop and bring it at eye level, when the pro is rotated with a down force on the right one should always see the blades at the rear side of the prop. This is right screw prop which means that the blade pushes the water backwards, a similar action of a swimmer when pushes the water back with the back of his hand.
During the same time the prop performs this movement, water rushes to the forward side of the blade to take the place of the water that already has been pushed away. This action creates a pressure differential between the two sides of the blade.

a. a POSITIVE PRESURE where the water is pushed on the lower side of the blade and
b. a NEGATIVE PRESURE where the water is sucked in on the top side of the blade

Both of those actions take place simultaneously on all blades during a full rotation of a prop.
Therefore, during a prop rotation water is sucked in from the front side of a blade and is pushed backwards from the rear side of the blade, thus creating an imaginative tunnel which a little bit wider than the diameter of the prop.
While the revs increase at the prop, the water flow is accelerated via the blades thus creating water current behind the prop. This current is moving in water-made tunnel, which is a little bit smaller than the actual diameter of the prop.
This movement of the water shucked in from the forward side of the blades and the simultaneous flow of water from the rear of the blades with grater speed adds to water high acceleration.
This change in water acceleration is the force that we call thrust.


A prop has a number of characteristics. Whereas the average user is interested with the pitch, is useful to know what all these numbers that characterize a prop mean.

1. PROP DIAMETER is the diameter of the circle between the tips of the blades. To find the exact diameter of a prop an engine the manufacturer calculates the max revs per min that the prop will rotate, in conjunction with the power that will be transmitted to the prop via the gears, the inclination (pitch) of the blades in the various sailing conditions and the desirable speed of the hull. In a series of similar props the diameter is usually larger when the prop is designed for low speed hulls and it becomes smaller for faster hulls. When all other prop characteristics remain the same. Diameter increases as the BHP of the engine increases. Also the diameter increases when prop revs /min decrease and finally when the surface of the blades increases. However, prop diameter is mostly for the interest of engine manufacturers rather than the individuals and outboard users. However, for inboard (slower) engines changing the prop diameter may increase engine performance.

Pic No3 shows that a prop with smaller diameter has a higher rate of slip.
Pic No4 shows how one measures a prop diameter

Anyway as I said if you find this info usfull please let me know and I can write a few more pages about pitch and pitch calculation.


PS JK shall we have a technical forum?? or this may make things more complicated. I suppose life is complecated enough Forget I even suggested it
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