Cavitation Marks on Prop??

[TopEnder]

Hi All,

I have searched everywhere to find a fix to my problem but just can't seem to find the answer. I have recently noticed small arc shaped wear marks on the back side of the prop about 10mm from the tips of all three blades (cup side) which I believe to be caused by cavitation. What I don't know is how to stop it. The boat and motor are only about 30 hours old and I have not noticed any performance issues at all(ie ventilation etc) The boat is a Quintrex 450 TopEnder with a Yamaha 60hp EFI 4 banger. Any advice would be appreciated.

Regards
Dave ;D

[NQCairns]

I understand it is water according to boyles law (thanks Uni ::) )exceeding in pressure. boiling and turning to steam in a microsecond with a violent pop that excavates some of the surface material, is the mark like it has been sandblasted through the metal? or more a scrub mark. I have heard of scrub marks being mistaken for cav marks, but dont know much about how to make a 'for certain' ID. Hope someone does.
A pic would be interesting. cheers nq

[TopEnder]

Thanks for the info NQ. I know how it is caused like you explained. What I don't know and what I'm after is how to stop it. I have attached pic so you can all see what I mean.

Regards
Dave ;D

[TopEnder]

And another one.

Regards
Dave ;D

[mackmauler]

[smiley=book2.gif] [smiley=helpa.gif]

had really bad probs with that!

[TopEnder]

And how did you fix the problem? I know I will probably need a new prop or need to get the current one repaired. I just don't want it happening again.

Regards
Dave ;D

[Spaniard_King]

Topender,

IMO you have too much slippage and I noticed you have no cupping on ya propeller.

If you are at the top of the rpm range for your engine you should take it to a propeller repairer and ask them to cup the propeller. This will drop the top- end rpm a little 1-300rpm but will increase the grip that your proppelor has and should rectify this problem.

You can paint the prop again with spray can and check after a few trips to seeif problem still exist. Re-cupping should not cost more than $50

Garry

[dazza]

just drive it up a few sandbars, then you won't notice it ;D ;D ;D ;D
rob,
looks like you went to the same nav school that i went to ???
cheers
dazza

[NQCairns]

I would go the other way from Garry but I am really just talking out my low tide blowhole, wouldn't cup cause a pressure increase and therefore the cavitation?.
By those marks it cannot be anything else but cavitiation, look forward to hearing if the pro's decide to pound more or less cup into the prop :-/. Hope it's less most people could do with a few extra rpm. nq

[Spaniard_King]

nq,

Cupping a prop is like adding pitch to a propellor.

Cavitation occurs when a prop is overpowered for the situation for eg turbulant water (prop slips in water)

The idea with cupping is to increase load on engine so as not to overpower propellor.

Garry

[NQCairns]

Thanks Gary, I can understand that on a prop with as much rake as Mackmaulers (of a ski boat?) but the prop is on a 60 hp 4 st! would it have the guts to push the correct pitch as hard as you say? I dunno just curious, might be just a poorly designed prop? what brands is it Dave?

[TopEnder]

Thanks for all the replies people. I am starting to think that a bit of cupping would not hurt. The prop is the standard Yamaha one that came on the motor. The motor is tiller steer and I am without a tacho so I am not sure exactly what she revs out to. Also being 4 stroke it is so quiet that even at WOT it doesn't seem to be working real hard. Sounds like it could rev heaps more. (might just seem that way I guess ???). WOT she will do 38knots on the gps so it isn't exactly slow but one could always go faster. :o
Once again thanks for all of your help and I will keep you posted once the silly season is over.

Regards
Dave ;D

[jimnbob]

Cavitation occurs when the small steam bubbles collapse. The theory is that when water is subject to low pressures, the boiling point temperature drops. (To test this you can set up a flask with a couple of cm of water and apply a vacuum - chem lab is the best place to find the necessary gear. You can get water to boil at 20oC)

Back to the propeller (or pump impellor), water entering the propellor zone initially sees a negative pressure and some water will 'boil' forming small steam bubbles, now when these bubbles start to see to higher pressures, the bubbles implode which usually occur expending a large amount of energy, hence the damage to the paint and metal.

Is the cavitation damaging paint only or is it also pitting the aluminium. If paint only I would not be too concerned. If the aluminium is being pitted, it will eventually 'wear out' your propeller. Judging by photos, it's definately cavitation and appears to be pitting the aluminium. I would talk to some of the speciaist prop manufacturers/repairers to get their advice. Ask them for and email address and forward your photo's, they will then be able to advise a fix.
The long term solution would be to repace the prop with a stainless steel one which is a more suitable material in combating cavitation.

Jim

[Fed]

I don't want to put a damper on all the theories but in my experience, prop cavitation has always appeared on the other side of the prop... the er, 'low pressure' side.
Think about it. ::)

[Fed]

An after thought, my missus want to know if you've been doing wheelies in reverse? :D

[jimnbob]

FED

Who's in reverse? The photos shown depict cavitation pitting and it is on the 'high pressure' side of the prop. Do you have a photo of your prop damage?

Jim

[Spaniard_King]

Fed I think you have cavitation mixed up with ventilation :o

[gunna]

One of the guys over on Fishraider (Rick T) had the exact same problem with his Hornet. It was caused by cavitation. He also had a porpoising problem. He changed the engine height and it fixed both problems. His pictures were identical to yours.

[Fed]

jimnbob.
Sorry mate but I don't have any pictures but it looked the same as the previous pictures except the pitting was on the other side.
By that, I mean the side facing the front of the boat.
At the time I was advised it was caused by water boiling at extremely low pressure and it made sense to me that it would occur on the low pressure side.

Spaniard.
Plenty of names for this stuff but I'm talking about boiling water, not sucking air down from the surface.

[jimnbob]

Fed

I'm a mech engineer and have worked in the mining and chemical industries for many years. Pumping problens invarably involve cavitation. Doesen't matter if it's a froth pumping (flotation cells) or cooling tower pumps, or a propeller, cavitation occurs because of the implosion of a steam (assuming the fluid is water) bubble.

As alredy posted, I would suggest you look at the ventilation aspect if the pitting is on the front side of the prop.

I'm not and expert on props, and suggest the best people to talk to are the specialist prop manufacturers/reparers.

Jim

[NQCairns]

I found this on the net I think P/D is Pitch/Diameter;

Cavitation occurs when pressure surrounding the propeller dips below the water's 'vapor pressure'. This causes the water to produce bubbles or cavities of water vapor - typically at the face, tip, or back of the propeller. Face cavitation usually occurs only on propellerswith uncharacteristically low P/D ratios at high vessel speeds. A major threat to propeller corrosion, excessiveface cavitation is caused by a negative blade angle ofattack. This is generally indicated when the P/D ratio is less than , or close to, the advance coefficient, J.Theoretical performance is not greatly affected byblade area. In general, a lower blade area produces highefficiency (and better performance). So, the goal inselecting a blade area ratio is to use as little blade area as possible, but enough to control cavitation. A majorityof the commercial (stock fixed-pitch) propellers comein predefined blade area ratios, so the choice of blade area is usually easy to make. Tip cavitation is typically indicated by excessivetip speeds. Tip cavitation generally does not affect thrust, but can produce noise and contribute to blade corrosion. Finally, back cavitation appears in heavily loaded propellers, and is the principal cause of blade corrosionand thrust loss. Back cavitation is indicated byexcessive blade pressure (too much lift) or cavitationpercentage, as well as a blade area ratio less than the recommended.

CURES The cure for cavitation depends on the type. For face cavitation, often the P/D ratio is too low for the RPM and ship speed. With tip cavitation, typically the RPM is too high. Most importantly, back cavitation generally indicates that there is too much power being pushed through the given propeller. So, reducing power,increasing diameter or blade area, changing air-foil to flat-faced blade sections (low RPMs only), or reducingthe P/D ratio (possibly with cupping) may all help.


>:(
Looks to me that the standard Yam prop are just a rubbish design, how can a 60 make too much power or spin too fast relative to any other outboard?
Topender IMHO turn that prop into a spare and buy another (mercury?) prop that follows basic prop design principles. Who on a fishing rig wants to deal with this stuff :o

[blaze]

reseach like that makes us more aware NQ, makes me wonder if thats the reason we have so much of a problem with dealers these days. Once upon a time we would go to a dealer and take what they said as gospil and these days the average boatie is able to research and is much more savey and aint prepared to take any excuse as an answer. Good find NQ
cheers
blaze
Ps
Hows the project going

[Spaniard_King]

If he wanted to save $$$ he could just cup it like the article suggests.

Wasn't that suggested before :-X

Garry

[NQCairns]

Yeah Garry but without info to the contrary we have gotta assume he is happy with his current WOT rpms and performance, I understand extra cupping reduces RPM,- rice puddings and 4strokes :P.nq

[Kerry]

Most alloy props are really a nasty piece of work as they have such thick shoulders and depth in the blades to achieve strength and stop blade flexing they tend to cut through water like cutting butter with the back of an axe. All sorts of wonderous things can/do/will occur with alloy props. Alloy props make good backups and that's about there rightfull place on many boats.

SS props on the other hand can be made thinner and are much more streamlined when it comes to tracking through the water.

As the water flow over an alloy or SS prop would certianly be different, would a SS prop on the same rig exhibit the same type of cavitation problem?

In any case if the prop is not matched to the boat/motor then wierd things will generally occur.

Cheers, Kerry.

[TopEnder]

Hi everyone,

This picture is for Kev who has the same setup as I do. I wanted to know how high his ventilation or cavitation plate is in relation to the keel. Any feed back from anyone else would also be appreciated.

Regards
Dave ;D

[Kev_McC]

sorry about the delayed reply. Some pics of mine are attached, I think my motor might be mounted a touch higher than yours?
kev

[Kev_McC]

showing mounting height

[Kev_McC]

deck layout - is yours the same? I have minn kota batteries under the foremost hatch, storage under the larger hatch on the rear of the foredeck, and a 60l nally bin for dry storage under the hatch in the centre. I'm thinking of converting this into an insulated icebox/kill tank plumbed through to the transom.
Hope the pics help
cheers, kev

[Kerry]

Topender, 1" higher at least probably a little more even. The keel (proper) shouldn't even be a consideration when setting the motor height. The keel has nothing to do with motor height but the cav plate a fraction below the hull (proper) is generally a little better than above the hull proper on a non pod setup.

These type of keels and especialy keels that are tapered up for the last few feet go a long way to throwing dirty water at the prop and especially in a turn.

Cheers, Kerry.

[Dignity]

Kev_McC I am looking to getting a small ally and really don't know much about trailer set up but that last roller of yours doesn't appear to have any weight on it, always thought that the keel rollers took most of the weight and there really should not be any weight on the side rails. Do you have trouble winding the boat onto the trailer.

Sam

[Kev_McC]

Sam I'm no guru on trailer set up either....that first double roller is there to guide the boat on to the keel rollers and rails further up the trailer. It floats there at a lower height than all the rest, and centres the boat as you winch it on - seems to work well. There's no weight on it once the boat is properly settled on the trailer. It's no problem to winch on or to push off, although on a very shallow gradient (like a beach launch) it can be a big effort when by myself. The side rails are there to take a lot of the weight.
cheers, kev

[TopEnder]

Hi Kev,

Thanks for the pictures. Yep mine is the same layout minus the small hatch in the foredeck (I'm going to put one in myself when I can find some carpet to match) and also the hatch in the main floor. Spoke to my dealer today in length and we have decided to leave it un till my next service in acouple of weeks. It is the first time he has seen this happen on a boat this size. He thinks it could also he caused by bubbles trailing back off the keel area into the prop causing a pressure difference and also some turbulance. They are going to look at it then and do some testing. I will keep you posted. Could you measure the height between the top of the transom and and the motor bracket from the motor height 2 picture.

Regards
Dave ;D

p.s. what model minn kota did you get and how does it go? some pics of the mounting bracket would be greatly appreciated as I failed to think of that one at the time of ordering. ??? :'( Its gonna cost heaps more now and the thought of the poor paint work :'( :'( :'()

[Kev_McC]

the gap between bracket and transom is 17mm measured from outside the hull. I ended up going with the 55lb minn kota with the long shaft and it has worked out well. I've fished all day with it in breezy conditions and it's been good. There have been occasions when wind and big tide have conspired to get me to throw the anchor out, but in those conditions the anchor makes life easier no matter how big an electric you've got. Quintrex do a really good job of the bow mount itself. It's well bracketed in and the plate is about 8mm. From memory I think it was $100 on my quote.
kev

[Kev_McC]

another one, sorry about the pic size