Posts: 3500
Location: Elk River, Minnesota | Hiya,
This is a classic example of what cavitation really is. You definitely have a cavitation issue as what you are experiencing is an actual pocket (or cavity) forming on the back side of the blade as it goes through the water. It is an extreme drop in pressure between the blade and the water which causes the water to acutally vaporize, creating the pits you are feeling on the blade surface. That is also why you don't have it on the edge of the blade but rather on the fin itself.
In looking at your second picture of the transom, I have a feeling it could be a few different things that can lead to the issue you are having. Cavitation is a normal thing to happen, and most props will have some sort of cavitation..but not enough to actually erode the prop. Your case is excessive. What I see are a couple of items that can potentially create disrupted water reaching the prop: The transducer, and potentially those EZ pumps. In order for the EZ pump to work, it has to stick down into the water just a touch in order to create pressure and force-feed water to your livewell (much like many of the speedometers on the boat work...water pressure). In creating this, the water flow to the prop is disrupted (introduction of air to the water before hitting the prop...it is in the path of the prop, thus creating the pressure drop on the rear side of the fin. The Transducer could also potentially cause this as well.
The difference in ventilation vs cavitation is when you ventilate, you are actually introducing air into the water from either vent holes, or from piercing the surface of the water directly on the prop, whereas cavitation is dealing with air that is already trapped in the water and is passing over the blade surface. This is why when you turn quickly you actually are ventilating...not cavitating as you are creating a pressure drop on the water's surface drawing air into the prop fins. Or...if your motor is set too high, the prop can draw air in at the water's surface.
Think of ventilation like mixing a glass of chocolate milk. you spin in one direction creating a flow of milk in that direction. Then stop the spoon and you create a drop in pressure on the back side of the spoon because the direction of the milk will continue in the direction of stirring, and you are now changing the direction of flow.
My suggestion to consider: Remove the EZ pump covers, AND the transducer. Then, if you can polish the spots where the cavitation is happening, paint (with a zinc base so it etches into the aluminum...otherwise it will just flake off) and go for a spin...see if the cavitation occurs again. If not, put the transducer back on and retest... If no cavitation, then put the EZ pumps back on and remove the transducer...retest.
It could be the combination of transducer and the ez pumps creating a very wierd flow of water to the prop...you have water being forced in a couple of directions (sideways and down) from both parts, which could potentially create a flow of water that is actually spiraling between the transducer and ez pump....thus a low pressure area...leading directly to your prop.
Steve
Edited by VMS 10/24/2013 9:30 PM
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