Posts: 3479
Location: Elk River, Minnesota | Hiya,
+1 on Steve Worrall's response.
Here's the goofy thing...in order for a prop to work, it has to have some slip, otherwise it could not do it's job.
In most cases, slip mainly happens with too high of a motor height or the motor is trimmed out too much. The proper definition of trimming too far out would be ventilating, where the prop is pulling air into the prop from the water's surface. Many times a person will feel this when they are trimmed up and trying to carve a turn...the prop just cannot keep a bite on the water, rpms go way up and speed drops off.
Many times, an aluminum prop will slip more than a steel prop due to thicker blades, which are more blunt on the front cutting edge. They also flex more in the water under load. With less cupping capable on the trailing edge of an aluminum prop, the prop cannot grab the water as well either.
Motor height plays a role as well as if the motor is mounted too high, air gets into the blades (like ventilating) but is happening from turbulent water coming into contact with the blades As water starts to fill back in from the hole the boat made, the prop runs through that turbulence, thus cannot grab the water as well.
If the blades on a prop are bent, dinged, blunted, etc....this will also cause slip due to creating turbulence within the water as the prop cuts through it. In cases such as this, the prop is turning fast enough that when it passes through the water, it can actually cause the water to vaporize behind the cut or ding, which over time will actually erode the propeller in that area. This is referred to as cavitation...the prop is literally creating a cavity of low pressure along the blade's surface...most definitely a bad thing to have happening.
Steel props can have a bit of slip as well, depending on design, venting, etc. Mercury makes props in larger hub diameters that have plugs that come with different sized vent holes. Go too large on the holes, and exhaust gasses continue to escape through the vent holes, even at higher rpms, thus introducing exhaust gasses to the blades.
Steve
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