Cavitation

[Geoffs]

I have well over 500hrs on my D4/260 - DPH outfit, and all has been faultless.

In the last 10hrs, or so, I have been seeing what I think is cavitation pitting on all blades.

The boat is dry berthed, so I don't think it is corrosion, and I check all running gear after each use, so I know this has only appeared in the las 10hrs. It is on the forward surface of the blades, so can only be cavitation, I think.

I polished lightly with wet 'n dry to show up in photo.

The question is, why should I start seeing this so suddenly after 500hrs? Nothing has changed in boat loading or trim. Bottom is totally clean and polished, no antifoul, even. There's no sign of any slipping or ventilation when running. It doesn't get hard use, 2500rpm and 23 knots is the norm, and performance is normal. No tight high speed turns or hard acceleration.

My local VP dealer has been to have a look, and checked boat geometry, cavitation plate level with bottom of boat etc., but they have no answers.

I know that some have experienced corrosion on G series Nickel/Ali/Bronze props, but I don't think that's my problem.

Any ideas what can cause this, and remedial action? Are there any known cavitation issues with G series props, any one experienced this sudden activity? I'll drop a line to VP, but don't expect a very encouraging response.

 

[[2068]]

Some change in the shape of the blade? leading edge damaged, too sharp, or too cupped?
Or it could be a problem with the casting/alloy of the prop itself.
Prop expert needed!

 

[RobWales]

Could it be perhaps porosity below the surface in the initial casting process and general wear is now revealing the said problem?

 

[pissativlypossed]

I'm not a prop expert but I am a mettallurgist and that looks suspiciously like porosity in the casting, not sure why it would manifest itself now, after 500 hours, but the most likely cause is as per Robs suggestion.

 

[Geoffs]

Thanks for these observations, Dave and Rob, with some expert backing from Paul.

Must admit casting problems had not occurred, I shall raise this, though doubt it'll 'cut much ice'. If I have to spend over a grand on new props, it'd be nice to be assured the problem will not re occur.

 

[RobWales]

I think (but stand to be corrected) that cavitation probs if evident would manifest on the trailing blade face?
Where a vortex type thingymybob occurs around the lip into the cupped face!
May be wrong tho...........

 

[jimmy_the_builder]

[ QUOTE ]
If I have to spend over a grand on new props, it'd be nice to be assured the problem will not re occur.

[/ QUOTE ]

You might not have to. Hamble Propellers can refurb Nibral props like these - worth a try?

Cheers
Jimmy

 

[Geoffs]

Now that is interesting, thanks for that.

I was told by my dealer that they've tried to get Nibral props repaired, but have not been able to find anywhere. I'll have to tell 'em.

 

[CHI]

I had my Nibral props for my d4 260,s done at hamble propellers. Fantastic job, better then giving Volvo massive amounts of money!!! /forums/images/graemlins/ooo.gif

 

[Geoffs]

Propeller Timing.

Has anyone come across this before?

Apparently the positioning of duo props is critical. There should be no more than one blade passing under the cavitation plate at any time, and props should be correctly positioned on splines to achieve this. If not correct cavitation can be caused. There is no mention of this in the Volvo procedures for removal and fitting of props.

I removed my props today and took them to various places for advise. The general opinion is that the damage is cavitation burning, and as pitting is near blade roots, repairs can not be made, as heat would damage the bushes. /forums/images/graemlins/frown.gif

As I said, my damage occurred very suddenly,after 500hrs of trouble free running. As it happened, it started after a recent service, when the props would have been removed, I didn't mention this as I couldn't see how a service could possibly affect this. I'm now beginning to wonder.

I didn't learn about this supposed timing issue until after I'd removed the props, so I'll never know if this could be related.

The person that told me, knows his subject, and said If I Googled Prop timing I'd learn all about it. I've done such a Google, but there is nothing about it.

So is this an engineering fact, or am I being misled?

 

[DAKA]

Re: Propeller Timing.

Hi Geoff,
I didnt reply before as my volvo penta duoprops were aluminium and I didnt think it was relevant but I used to suffer damage that was identical to your pitting.

It was without doubt cavitation damage from poor prop design.
I must have got through 8 outer props over 4 seasons ( did more miles back then).

Props never failed due to the cavitation damage, they would fail as a blade would just sheer off but no where near the cavitation damage.

In my case the cavitation damage was from edges on the blades being too sharp.

I am not saying your damage is not from 'timing' but mine certainly was not as the inner set was 3 blade prop and the outer 4 blade prop , I cant see how you could time that ?

I had aluminium props repaired many times, in order to repair the prop had to be put in an oven and heated to a very high temp before the welds could be done, this did not seam to harm the boss.

I would take your props to Hamble props, if they can not fill the damage , they will at least be able to race balance and edge your props to reduce the cavitation damage in the future, they could even move the damage to a different area, in an attempt to rest the damaged area.

If they can not fill them I would use an epoxy glue in an attempt to slow the damage.

Good luck, I never did resolve my VP duoprop issue, I used to carry 6 spares !

Red = blade snap off here

Green= sharp unfinished blade area, that is too difficult for lazy hand finishers to get to, Hamble props will race tune for you

Yellow = worst area of cavitation damage I used to get

Any damage or sharp area on any one of the 7 blades would cause cavitation damage as you have mid blade.(damage=brand new careless hand finishing)

 

[Geoffs]

Re: Propeller Timing.

Thanks for your comments, Pete. I can't believe there is any fundamental problem with the props, as they've given 500hrs of trouble free service, might be wrong.

Something must have changed for such a sudden change in condition, but was at a loss to think or see what this could be. Never come across this timing thing before, just wondering if the props could have been re fitted incorrectly after the service. Will never prove it now, but it would be a relief if this were true.

I did visit Hamble, very helpful and gave an honest assessment. Be about £300 for a refurb, but, as I said, they can't do the pitting near the root, a fairly critical area from your comments. So do I go ahead, £300 a lot cheaper than new ones, but still a good way towards the cost, £995 + VAT from Motortech. Maybe less from Coastal Rides. I did see an advert for third party stainless G series props somewhere, but can't remember where.

I wondered about epoxy, as you suggest, but didn't know if that is a good idea. /forums/images/graemlins/confused.gif

 

[DAKA]

Re: Propeller Timing.

The cavitation damage did not affect the performance.
The props always snapped off before the cavitation damage got to bad and the snap off was nothing to do with the cavitation damage.

Volvo Penta HO eventually took an interest in the damage about 3 weeks after I sold the boat so I let it drop, they were aware of an issue.

I gave this some thought over night and our use is possibly the same and unusual in that my props were only in salt water for 3 months of the year and then covered a lot of tough cruising.
As such the props had to work hard without suffering any corrosion damage which most props suffer from.

I was close to getting stainless props but as I carried 6 spares I would have had a real pain when changing any props as 3 blade stainless were not interchangeable with aluminium 4 blades.( as they snapped I used to change them offshore, hanging over the bathing platform)
At that time A6 stainless props were not available for diesel engines .
And I was concerned about stainless causing pitting to the legs.

As you dry berth I think stainless is the answer for you and there are now some non volvo penta parts available, Steel developments used to sell them.

Some brand new props looked like yours after just 500 nm and one blade snapped off after about 200 nm.
The good props lasted for years so it paid to have a good prop repaired as a second hand prop would out last a new prop .

I had one very old(12 years) short hub prop that looked a bit odd cosmetically but it never did suffer any damage.

Props are rough cast and then finished by hand, it is the quality and care of this hand finish that gives the performance, having an old one repaired benefits from this careful hand finish, otherwise the economics dont make a repair sensible.