Prop Cavitation problem

[DW]

I believe the definition of cavitation is boiling water on the surface of a prop due to extreme partial vacuum created by a revolving prop. Based on this definition, cavitation may occur only on the back side of the blades where a partial vacuum exists. This condition can lift tiny pieces off the prop. Since the wear is on the pressure side of a blade. I dont think cavitation is the cause.

[Wiscowalleye]

Originally Posted by DW

I believe the definition of cavitation is boiling water on the surface of a prop due to extreme partial vacuum created by a revolving prop. Based on this definition, cavitation may occur only on the back side of the blades where a partial vacuum exists. This condition can lift tiny pieces off the prop. Since the wear is on the pressure side of a blade. I dont think cavitation is the cause.

I didn’t think so at first either but then I read this. It goes a lot more in depth then I probably need to know but it was pretty interesting non the less. I’m almost positive it’s cavitation after reading this and the information from others. According to the article I believe it’s sheet cavitation which makes sense because it started showing up after sustained high speed runs to some of my fishing spots.
https://www.iims.org.uk/introduction...er-cavitation/

[brigeton]

I have a 115 merc on my 1750 Crestliner. It's a command thrust so I have a 21p instead of a 17 because of the different gear ratio. It is also a 4 blade spitfire and I have paint off the prop in the same pattern as yours. I don't know if this is normal or not. I don't know if the x7 stainless prop cures the problem or if you just can't see it because the stainless prop is not painted.

[REW]

If you wished, you could take the prop to a sand blasting shop and have them bead blast the prop to remove all paint. Then you can polish the aluminum and not worry about what you are seeing, because there would be nothing to see with no paint on the prop.

Aluminum does not corrode in fresh water so there is no need for paint, if you don't wish to have paint on the prop.

You will not have solved your issue, but you will no longer see any issue to be concerned with.


Good luck

[staylor]

Going into this topic a bit deeper should take into consideration the fact that the Spitfire props are progressive pitch- ie, the pitch at the leading edge of the blade is 1-1.5 inches less than the rated pitch, and at the trailing edge just before the cup the pitch is 1-1.5 inches higher than the rated pitch. The average pitch of the prop is what Merc stamps on it. The damage to the OP’s prop is at the trailing edge just before the cup- right where the pitch is highest- and this is the region where cavitation on the pressure face of the blade would be expected to initiate.

REW is correct in that you could strip the paint from the prop so that you wouldn’t see the damage, but that would not cure the problem which is the prop simply running inefficiently since it is being overloaded from its intended design. In some cases the prop may also exhibit excessive slip

For reference, before I retired I did a lot of work eliminating cavitation problems from equipment added to the external hulls of various warships. On these applications we managed to measure the shock loads caused by cavitation bubbles collapsing on the external equipment. The pressures recorded were in the range of 25,000 psi- more than enough to strip paint, and in the range where the continuous bubble pressure would also erode cast aluminum. Steel/stainless steel are much stronger than cast aluminum, and hence were not affected by the 25,000 psi loading, but the idealized design would be such that cavitation would not occur.

As for the OP’s question about running the 17 pitch Spitfire at a full throttle load of 5600 rpm- this won’t cause any engine problems, but I suspect the rig would run better propped to the high end of the rpm rating band.
Doug

[Wiscowalleye]

Originally Posted by staylor

Going into this topic a bit deeper should take into consideration the fact that the Spitfire props are progressive pitch- ie, the pitch at the leading edge of the blade is 1-1.5 inches less than the rated pitch, and at the trailing edge just before the cup the pitch is 1-1.5 inches higher than the rated pitch. The average pitch of the prop is what Merc stamps on it. The damage to the OP’s prop is at the trailing edge just before the cup- right where the pitch is highest- and this is the region where cavitation on the pressure face of the blade would be expected to initiate.

REW is correct in that you could strip the paint from the prop so that you wouldn’t see the damage, but that would not cure the problem which is the prop simply running inefficiently since it is being overloaded from its intended design. In some cases the prop may also exhibit excessive slip

For reference, before I retired I did a lot of work eliminating cavitation problems from equipment added to the external hulls of various warships. On these applications we managed to measure the shock loads caused by cavitation bubbles collapsing on the external equipment. The pressures recorded were in the range of 25,000 psi- more than enough to strip paint, and in the range where the continuous bubble pressure would also erode cast aluminum. Steel/stainless steel are much stronger than cast aluminum, and hence were not affected by the 25,000 psi loading, but the idealized design would be such that cavitation would not occur.

As for the OP’s question about running the 17 pitch Spitfire at a full throttle load of 5600 rpm- this won’t cause any engine problems, but I suspect the rig would run better propped to the high end of the rpm rating band.
Doug

Thanks again for the excellent information Doug. I talked to the dealership today and they said if anything I want to go up in pitch or stay the same with the stainless version. From what you explained and from what I read looking at it from a physics standpoint I definitely want to go down if anything in pitch to a 15 degree not up to in this case the next pitch is 19 degree. What am I missing? I see it as a larger bite at the water will drop my WOT RPM and will create more disturbance on the back side of the blade not less, whereas less pitch would cause the prop to more effectively slice through the water and increase my RPM at WOT. And since I’m already at 5600rpm WOT I definitely don’t want to drop it down very much at all.

[DW]

Originally Posted by Wiscowalleye

Thanks again for the excellent information Doug. I talked to the dealership today and they said if anything I want to go up in pitch or stay the same with the stainless version. From what you explained and from what I read looking at it from a physics standpoint I definitely want to go down if anything in pitch to a 15 degree not up to in this case the next pitch is 19 degree. What am I missing? I see it as a larger bite at the water will drop my WOT RPM and will create more disturbance on the back side of the blade not less, whereas less pitch would cause the prop to more effectively slice through the water and increase my RPM at WOT. And since I’m already at 5600rpm WOT I definitely don’t want to drop it down very much at all.

I am intrigued by your situation.

Doug’s explanation of the pitch variations on your prop, might explain the paint erosion. Usually painted aluminum props lose paint in a uniform pattern along the edges. With a variable pitch surface the paint may be vulnerable in an unique pattern. So, I still suspect a paint issue. Since you are running a 4 blade prop the prop surface pressure extremes must be less than a typical 3 blade prop, and that dissuades a cavitation theory somewhat.

I would be tempted to get a new SS prop, too. I think a 2” drop in pitch may be too much but reaching 6000 RPM WOT would be the goal. A 1” pitch reduction may be the right number. If you dont get it just right plan to have a new prop reworked by a good prop shop.

[Wiscowalleye]

Update to my post. I went to a different dealer he suggested we try the stainless X7 in the same pitch of 17 if it’s still cavitating he said bring it back and we can swap it out. He also mentioned which I did notice that this boat motor combo has a very small trim window so to be mindful of the trim angle. Something made completely worthless by Mercury using an analog gauge and a digital trim switch/sensor. Which makes the trim gauge completely worthless. Ugh. However after testing the new prop I was surprised to find that the boat gets on plane even faster! And I gained 2mph. Max rpm now runs right at about 5900rpm thats pretty much trimmed up as far as I’d like to go topping out at right around 44mph. The vessel view app says 0-20mph time was 4 seconds.

The only hub kit they had was the flo-torq ssr solid. Not too thrilled about the “solid” part but I’ll run it carefully until the 2 piece Flo-torq SSR hub kit becomes available. Being stainless I have noticed a slightly higher vibration at the lowest speed in gear but I’m hoping this with be solved by a 2 piece hub kit. As for the SSR portion of the kit it’s a ton of rubber basically, and it really does make for almost zero clunk when shifting.

Thanks again everyone for all the suggestions and feel free to give me anymore advice. I do have one question. How would I be able to tell if this new prop is even cavitating since I won’t see any paint. Is there a certain noise to listen for? It almost seems to be operating perfectly now.

[DW]

That is a snazzy looking prop.

If you experience cavitation, the surface of the prop erodes and may pit.

I think 44mph on that boat with a 4 blade prop is pretty good. You will probably pick up another 3 or 4 mph with a 3 blade, but lose some low speed control.

[Boondock]

Originally Posted by DW

That is a snazzy looking prop.

If you experience cavitation, the surface of the prop erodes and may pit.

I think 44mph on that boat with a 4 blade prop is pretty good. You will probably pick up another 3 or 4 mph with a 3 blade, but lose some low speed control.

Nope
Nothing over a mph to pick up speed wise. Merc killed it with the x7 spitfire. That’s why people like myself rave about them. Only way to pick up speed is to go with a 19 but holeshot will suffer.

Felt like a different boat huh