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-   -   Cavitation burn on rudder (http://www.mastercraft.com/teamtalk/showthread.php?t=9938)

viabill 06-28-2006 10:44 AM

Cavitation burn on rudder
 
I recently bought an '89 Barefoot 190 with the 454HO engine. THe boat has 280 hours and the rudder has a cavitation burn on the upper right (looking at it from the rear) side that is over 1/4" deep. Is this a common occurrence with some boats? Should I expect to have to replace the rudder every 300 hrs?

bigmac 06-28-2006 11:00 AM

Quote:

Originally Posted by viabill
I recently bought an '89 Barefoot 190 with the 454HO engine. THe boat has 280 hours and the rudder has a cavitation burn on the upper right (looking at it from the rear) side that is over 1/4" deep. Is this a common occurrence with some boats? Should I expect to have to replace the rudder every 300 hrs?

I had this kind of conversation with my propeller guy a couple of weeks ago. On inboards, the interplay between propeller and rudder is complex, but he tells me it's mostly a function of propeller geometry. He showed me a Ski Nautique he had in the shop with a serious cavitation burn on the hull between the prop and rudder, and he strongly contends that such burns on the hull or rudder are mainly due to a prop thats out of spec, badly designed, or simply the wrong prop for the boat.

His trick is to custom tune the prop. He paints the prop blades with mechanic's paint, puts it back on the boat and runs it. He will then periodically check the prop for the areas where the paint is NOT wearing off and says that represents an area where there is inefficient prop blade flow and therefore a clear source of cavitation. He grinds that part away, which he says will fix the problem.

If I were you, I'd consider getting some mechanic's paint and paint your prop blades. According to this guy (Mike, at Precision Propeller Works , this will give you a lot of information about how your propeller is working on your particular boat. Mike's a great guy, and will talk your arm off about propellers. I'm confident he'd be happy to discuss your situation with you if you want to give him a call.

Cloaked 06-28-2006 11:00 AM

Quote:

Originally Posted by viabill
I recently bought an '89 Barefoot 190 with the 454HO engine. THe boat has 280 hours and the rudder has a cavitation burn on the upper right (looking at it from the rear) side that is over 1/4" deep. Is this a common occurrence with some boats? Should I expect to have to replace the rudder every 300 hrs?

I have never seen any burn on the rudder from cavitation.

Honestly, this makes no sense to me, but I may not clearly understand you're point.

Sounds like a side-grinder gone wild with a 1/4" gouge in the nibral rudder. :eek:

Cloaked 06-28-2006 11:05 AM

Quote:

Originally Posted by bigmac
I had this kind of conversation with my propeller guy a couple of weeks ago. On inboards, the interplay between propeller and rudder is complex, but he tells me it's mostly a function of propeller geometry. He showed me a Ski Nautique he had in the shop with a serious cavitation burn on the hull between the prop and rudder, and he strongly contends that such burns on the hull or rudder are mainly due to a prop thats out of spec, badly designed, or simply the wrong prop for the boat.

His trick is to custom tune the prop. He paints the prop blades with mechanic's paint, puts it back on the boat and runs it. He will then periodically check the prop for the areas where the paint is NOT wearing off and says that represents an area where there is inefficient prop blade flow and therefore a clear source of cavitation. He grinds that part away, which he says will fix the problem.

If I were you, I'd consider getting some mechanic's paint and paint your prop blades. According to this guy (Mike, at Precision Propeller Works , this will give you a lot of information about how your propeller is working on your particular boat. Mike's a great guy, and will talk your arm off about propellers. I'm confident he'd be happy to discuss your situation with you if you want to give him a call.

Hmmmmmmmmmm.....

Not to dispute anything I know little or nothing about but this is a bit extreme for me to clearly grasp. I'd have to ask right away, why not put the correct prop on the boat and eliminate all of this underwater-galactic fine tuning and spray painting?

bigmac 06-28-2006 11:28 AM

Quote:

Originally Posted by Sporty
Hmmmmmmmmmm.....

Not to dispute anything I know little or nothing about but this is a bit extreme for me to clearly grasp. I'd have to ask right away, why not put the correct prop on the boat and eliminate all of this underwater-galactic fine tuning and spray painting?

Obviously, cavitation burns are far less likely on an outboard or I/O, but on an inboard, the prop wash goes from the prop back along the hull and across the rudder. If the prop is putting out a stream of bubbles (cavitation), THAT will cause cavitation burn. The most likely source is a ding on the prop (which mechanics paint will point out). But it might be that the prop flow hits the hull or the rudder in a certain way that ALSO creates that stream of bubbles, or it might be there's a ding on the hull, or on the rudder that's doing that too.

All of these inboard manufacturers redesign their hulls rather frequently, but the prop that the boat manufacturer puts on there is an off-the-shelf Acme (some mfgrs use OJ). IIUC, the boat manufacturer selects a prop for appropriate RPM at WOT using the best/cheapest prop available from their selected manufacturer, but they're constrained by what props are available from that prop maker. In point of that, I note that the stock prop that came on my 230VRS was an OJ 14x18. I replaced it with an Acme 847 and have better acceleration, better top speed (by 1.5-2 mph), and less cavitation. Since cavitation problems may take time to show themselves, the boat maker has no immediate way of knowing if that particular prop will create some kind of flow issue that results in cavitation across the hull or rudder - they can only look at performance (WOT and acceleration).

In other words, the boat maker may have several reasons for not being able to put the correct prop on the boat - mostly, I suspect that they can't afford to test and have a prop custom made for every boat model they make - they have to take what's available.

east tx skier 06-28-2006 11:57 AM

Quote:

Originally Posted by Sporty
Hmmmmmmmmmm.....

Not to dispute anything I know little or nothing about but this is a bit extreme for me to clearly grasp. I'd have to ask right away, why not put the correct prop on the boat and eliminate all of this underwater-galactic fine tuning and spray painting?

Propellers are never perfect. My hunch is that the propeller on his older boat is hand finished. It was not uncommon to get a hand finished prop NIB that had some flaws to it. I think BigMac is assuming that he has the recommended prop on there as far as size/pitch goes.

bigmac 06-28-2006 12:03 PM

Quote:

Originally Posted by east tx skier
Propellers are never perfect. My hunch is that the propeller on his older boat is hand finished. It was not uncommon to get a hand finished prop NIB that had some flaws to it. I think BigMac is assuming that he has the recommended prop on there as far as size/pitch goes.

Yeah - based on my limited knowledge, imparted to me by a (IMHO) real smart guy who's living is propellers, I suspect that viabill's cavitation burn is either a small ding in the prop, or a small ding in the part of the rudder just forward of the gouge he mentioned.

viabill 06-28-2006 08:54 PM

Thanks for the inputs, guys. I'm going to take some pictures of the rudder and post them so you can see what I'm talking about. It's pretty strange!
For now, I'm attributing it to something bigmac mentioned about the mfgr not having the time or resources to choose the best prop.
If I'm not mistaken, MC put this 425 HP/ 500 ft lb engine in these boats without making any real changes to the hull or rudder designs to accomodate it. The prop looks like it is original, and is a 4-blade stainless version. Another twist is that the prevoius owner had a ballast bag in te boat and was trying to wakeboard behind it. I suspect the attitude of the boat while pulling was much more bow-up than MC intended, and the prop was slipping more than it normally would with skiers in tow. Maybe these factors could cause what I'm seeing.

TMCNo1 06-28-2006 10:23 PM

In other words, the boat maker may have several reasons for not being able to put the correct prop on the boat - mostly, I suspect that they can't afford to test and have a prop custom made for every boat model they make - they have to take what's available.[/quote]


You should see the pallets of experimental props in boxes that Mastercraft has built up over the years, that MC has had made special for all the boats they have built. The prop manufacturers are always working with boat builders in this manner for their business. The R&D department is always trying out new ideas and combinations to provide the best all around performance in the boats they design for sale. The most amazing thing is, most if not all of these props that were not acceptable for use are then sold to a scrap dealer or given back to the prop manufacturer to melt down and recycle into other props.

Why do OB's and I/O's have sacrifical zinc anodes on the engines and lower units? Electrolysis! Why do most inboards other than salt water boats not have them. Any boat powered by a engine using electrical current as part of its function releases static electricity into the water to ground while running and this causes the softest metal in the water that has a direct ground to the engine to eat away at that metal. In OB's and I/O's the softest metal in the water is the engine or outdrive of aluminum, therefore a softer metal to be exposed is the sacrifical zinc anodes that they install on the transom and on the adjustable tab above the prop and have to be replaced over time. On an inboard the softest metal with a direct ground to the engine is the prop, which in turn discharges the static electricity to the rudder thru the water and then into the water causing the rudder to burn or eat away because it is softer [brass] than the prop [nickel/brass/aluminum alloy] or S/S which is harder. You may even see some erosion of the prop blades near the hub, but that is usually caused by cavatation and maybe some very small amount of electrolysis. The strut does not erode because it is isolated from the shaft by the bushings plastic surfaces and even though the steering cable is attached to the strut it is not attached directly to the electrical system via a ground. The New Saltwater Series boats that MasterCraft builds has sacraficial zinc anodes on the S/S Swim platform brackets and the shafts because salt water has a higher erosion factor because of the salt and electrolysis.
This was explained to me by a Mercury/Mercruiser engineeer several years ago at a boat show and in numerous article in magazines published throughout the marine industry.

JohnnyB 06-28-2006 10:28 PM

Never seen a cavitation burn....can you post a pic?


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