Prop. Question

[DJ]

I've never professed to be a prop. expert. All I know is to match the prop. to the engines peak operating (rpm) range.<br /><br />My question is:<br /><br />What phenom. makes the paint, for the lack of a better word, "burn" off of the blades? Particularly the "aft" side.

 

[ob]

Re: Prop. Question

djohns,As I understand it ,the trailing edge ,or discharge edge of the blades will experience this even on a newly factory teflon painted SST prop in relation to suspended particulates in the water that the prop is running in.Sort of a sand blasting effect.I run in saltwater exclusively and have found no coating that will resist this.

 

[Franki]

Re: Prop. Question

I think I may be able to answer that..<br /><br />Water in low pressure areas (like the top of mount everest for example) boils at much lower temperature then normal....<br /><br />In front of the prop is a very low pressure area, believe it or not, a small amount of the water in front of the prop actually boils superhot...<br /><br />And that burns any paint off eventually.<br /><br />Believe it or not..<br /><br />Check it out,, you will find out I am right..<br />Thats why there is nothing much you can do about it...<br /><br />regards<br /><br />Franki

 

[ob]

Re: Prop. Question

Franki,With all due respect I have a fairly good understanding of pressure and temperature relationship as it pertains to the boiling point of fluids.The "absence or decrease of pressure" as it pertains to water does not increase its temperature to a degree that would burn paint off of a propeller but rather lowers the temperature at which it boils(changes state).<br /><br />The term boil does not refer to necessarily what humans or propeller paint would consider to be hot ,but rather a given temperature and pressure that a specific fluid will change its state.

 

[FlyBoyMark]

Re: Prop. Question

Personally....I think it jus' sucks it off after a period of time because of the lower pressure and maybe exasperate by bubble abrasion because of low pressure. The water is not hot....(unless your driving thru a volcanic lake..

 

[Franki]

Re: Prop. Question

ok, so maybe burning off the paint is the wrong wording... but the low pressure causes water boil and that energy wears off the paint..<br /><br />My father is a marine engineer, it was he that told me.<br /><br />To back it up.. I found a couple of snippets:<br /> http://www.barnacleblok.com/mi.html <br /><quote><br />Cavitation - water pressure fluctuation that causes the water to boil and the resulting energy to hammer at the propeller<br /></quote><br /><br /> http://www.lenscove.com/newsletter.asp?hl=40 <br /><quote><br />Cavitation is probably one of the most misunderstood and used words in boating. Here is a crash course in the basics. When a propeller “breaks free” in a sharp turn it is actually ventilating, not cavitating. The prop breaks the surface of the water and sucks air in and loosens the grip on the water. The plate above your prop is actually an anti-ventilation plate, not a cavitation plate.<br /><br /> Cavitation actually transpires when a fully submerged prop creates a low-pressure area on the backside of the blade. The prop is turning so fast that the “hole” it cuts in the water has not had a chance to fill. Low-pressure in this area allows the water to actually boil at the lower temperature and bubbles are formed that can create a feeling of an unbalanced prop. Those bubbles also eventually implode under the increasing water pressure and can actually “eat” away the paint and metal on the prop. This not only makes it look bad but can weaken the blades. Having a prop with too much cup, nicks or a sharpened leading edge caused by filing the surface in the blades, can increases the possibility of the condition. The next time you see a swath of removed paint form the blades of your propeller, this is likely what you are seeing.<br /></quote><br /><br />I can find some more sources if you like.. there were heaps of pages on it in the google search I did..<br /><br />regards<br /><br /> <br />Franki

 

[JB]

Re: Prop. Question

Hmmm. Lots of good explanations of hydrodynamic theory, but some misunderstanding. <br /><br />Vaporization, which is cavitation, is not the same as boiling and has no abrasive effect. It occurs on the leading (front) side of the blade.<br /><br />Paint or other "soft" coating on the trailing, or discharge side and leading edge of a prop blade is abraded by the debris and grit in the water it is forcing to move. Turbid, sandy or muddy water can do this in one day's running. In clean, clear water it takes a lot longer.

 

[ob]

Re: Prop. Question

Amen JB.Frankie,I understand exactly what you speak of pertaining to cavitation and its subtractive effect on rotating equipment components that have been subjected to it for extended periods.The overhaual and maintenance of boiler feed pumps and the like have been my occupation for years and I've seen my share of metal displacement directly resulting from cavitation.I don't feel that this is the phenomena that is causing the subtractive art on my prop or the one in question.It's particulates in the water being moved.