Prop cavitation

[Mike.Gritten]

I have a '75 E35 Mk II fitted with a Westerbeke W30 4 cylinder diesel, Paragon 1:1 transmission, and a three blade MaxProp feathering propellor. My problem is as follows. The boat came with the current drive configuration. When we hauled the boat last summer, we did a bang-up job cleaning and polishing the prop and the boat bottom and, of course, giving her a fresh coat of bottom paint. During the sea trial I had queried the PO about the prop "cavitating" - the prop spins in an air cavity created around the blades. He had alluded that the prop may require a pitch adjustment as his brother had "been messing around with the settings". I haven't made any changes to the setup since, and the boat (with a clean bottom) would power along quite nicely at hull speed as long as you applied the throttle in small increments. If you just hammered it onto full throttle, the prop would "cavitate", I would have to back off the throttle until the prop "caught" and then continue to increase the throttle until hull speed was reached. No real problem, just a bit of a hassle. Well, now (with a less-than-spotless bottom) achieving even 4.5 knots is becoming difficult without the cavitation monster rearing its ugly head. It seems obvious that cleaning the bottom will have a positive effect, but while it is out I would like to make a pitch change to eliminate/reduce the problem all together. Before I start pouring time/cash into a series of haulouts to make the adjustments, I was hoping to get some feedback from you "experts" out there! Comments?

 

[Geoff Johnson]

I have a two bladed MaxProp and was achieving 6 knots at 2200 rpm (engine speed - I have a Hurth 2:1 transmission). This resulted in lugging the engine and soot on the hull. So I had my yard change the pitch to get the engine speed up to about 2400 rpm (BTW my Universal tachometer reads slow by a consistent 200 rpm). According to the PYI web site, on a two bladed MaxProp a 2 degree decrease in pitch will result in a 10 to 15% increase in engine speed. Same for the three bladed model:

http://www.pyiinc.com/index.php?section=max-prop&action=three-blade-classic-installation&sn=5

Seems to me you need to do the reverse, i.e. figure out how much you need to increase the pitch to get your rpm's into the optimum range of your engine.

 

[Martin King]

Mike,
I don't have all your particulars like prop diameter but
I am guessing your problem isn't pitch so much as that
direct drive transmission. You really need at least 2:1
or even more to swing a wheel with decent blade area.
Many of the early Ericson's were equipped with an A4
mated to that Paragon and swung a little "bow tie"
of a prop. It would get you out of the harbor and out
to the starting line ok, but was hardly the ideal setup.
Increasing pitch may help your cavitation problem, but
may be a band aid and not a cure. You might want to
talk to PYI, they seem pretty good at dialing in the
props they sell.

Martin

 

[Mike.Gritten]

Martin,
If I have to increase pitch, then for a given boat speed the RPM of the engine would be lower, if I understand what's happening there. That sounds like the opposite of what I would like to have happen! When I was reaching 6.5-7 knots under power, 2200 RPM would be about all I had on the tach. The max rpm for my engine is 2500. I agree that ideally, a new reduction gear transmission would be the way to go. Unfortunately, That's not really in the cards (i.e. no money for that right now!). Can used transmissions be found for this boat/motor combo these days? If we still have this boat a couple of years from now, maybe we'll replace motor and transmission as a unit, and then, hopefully, this problem will be gone! Aaaahhhh.....time and money....that's all it takes......

 

[Geoff Johnson]

Remember that 2200 on your tach is 2200 rpm at the prop, whereas as my prop would be turning at 1100 rpm. So that, presumably, is why you are experiencing cavitation. For your prop to turn at 1100 rpm, you would have to increase the pitch greatly, but this would put you way down on the power curve for your engine and would not be good for it. However, I am no expert at this. See what PYI has to say.

 

[Mike.Gritten]

Thanks, Geoff. I will try PYI and see what they say.

 

[Martin King]

Mike,
You can cast around for a used transmission, but you
may also have to fab up an adapter plate. I'd rather buy
a new one with a warranty that is going to work with
your engine rather than mess around. Also, I wouldn't
be overly concerned with reaching your rated engine
rpm's. You want to be up on the powerband when
cruising and so long as you can achieve hull speed,
it's not critical in a displacement sailboat. More on point
is a slower turning prop with more available torque. I'd
worry about that before cavitation ruins that expensive
three blade.

Martin

 

[Mike.Gritten]

Contact with PYI

I just wanted to update everyone after my conversation with PYI. That's right, a real, live conversation with a human being! As suggested, I went to PYI's website and sent them a message regarding the discussion we have been having here. Much to my astonishment, Fred from PYI phoned me at the office the next day! Remember, I have never been their customer. I just bought a boat that a PO had installed a MaxProp on. Anyway, Fred was very helpful and we discussed the issue for about 15 minutes. This is a summary of the results of that call. First, he says that he is not surprised that the prop is cavitating. The 1:1 transmission ratio was originally designed for the A4's and is not the best for diesels. He feels that increasing pitch is the wrong way to go. He wants me to decrease the pitch by 2 degrees, therefore, increasing the rpm of the prop(at a given boat speed), but, and here's his reason, it will reduce boat speed when manoevering at or just above idle speeds (docking) and it will increase the range of rpm's before boat reaches hull speed. This will mean that the prop will still cavitate, but, it will allow more room for throttle adjustment before max boat speed is reached. Of course, the BEST solution is to trash the 1:1 tranny and get a reduction gear transmission. All these problems go away. Anyway, he was genuinely interested in my problem and wants to know how the "fix" goes. He also wants me to check for prop-to-hull clearance as he can "build-up" the existing blades to be longer, which will help as well.
First rate service from a great supplier!
I will let you know in a couple of months how it all went.

 

[Geoff Johnson]

I'm interested to know if he suggested that cavitation would destroy the prop.

 

[Mike.Gritten]

Geoff,

He didn't seem to think that any damage would result. I did ask him this as someone had alluded to it earlier in this thread.

 

[Martin King]

Allow me to quote from Dave Gerr's excellent book on
props: "Effects of cavitation. Contrary to what most
people think, cavitating propellers can still generate plenty
of thrust. The problem is that the vacuum bubbles implode
against the propeller causing vibration and pitting. The
vacuum bubbles form and implode irregularly, causing uneven
pressure both along the blades and between them. This creates vibration identical to having unbalanced or unequally pitched
blades. What's more, the force of the imploding bubbles is so great that it actually sucks metal right off the surface of the
propeller. The resultant pitting leads to uneven wear, bad
balance and even more vibration. "

I've seen the pitting he's talking about first hand, and it's
not pretty. The best way to combat it, is keep the RPM's
down.

Martin

 

[Mike.Gritten]

Good one, Martin! We are still discussing the alternatives and will add this to the pile of data we are using determine the best course of action.

 

[Mike.Gritten]

Followup

Okay, it appears that my problem has been solved - and in a very cost effective manner! I hired a diver to come and dive my boat to clean the bottom and have a look at the prop and see if anything was apparently out-of-sorts. Well, after a few minutes he came up and reported that apart from soft growth and a few small mussel farms under the keel, the bottom wasn't too bad - but - the prop, shaft and strut were literally covered in hard growth (barnacles)! He said every blade on the MaxProp had lots of them and that he could tell that it wasn't operating properly. It was failing to come all the way to the stops, at least in the "forward" position. He spent quite a time cleaning them off. The difference has been a revelation. We were able to achieve hull-speed easily and recorded a max speed under power of 7.8 knots with the GPS. Even backing (making sternway?) improved significantly. The power delivery now seems smooth and linear. I am able to accelerate normally with no fear of cavitating the prop. My world has changed!!!
So, what have I learned?.....First, PYI has excellent service even to those of us who aren't the original purchasers of their excellent MaxProp product.
Second, check the obvious stuff first. I know that it seems simple after the fact, but, it might be the easy (read: inexpensive) things
that are causing your problem. I clean my boat with a brush below the waterline every week or two, as far as I can reach from the dock or dinghy, but, mother nature finds the weak spots.
Three, I will investigate ways to protect my prop, shaft and strut so that when we haul out in the fall, we can take appropriate action.

 

[captainic]

I thought I would reopen this old thread with a question about cavitation. I recently re-powered my E30-2 from a Yanmar YSM12 (10 HP) to a Yanmar 3GMF (20 HP). This included replacing my original 3-blade 12LH10 prop with a 3-blade Federal 12RH14 cupped prop (1" shaft), and also pushing the prop shaft back a bit, resulting in about 3 inches between the cutlass bearing and the prop (compared to ~1 inch with the previous engine). The new engine runs great, but I notice a sort of "swirling/whining, maybe lightly grinding?" noise above 2400 RPM, which definitely is not coming from the transmission. The engine is aligned within specs. Would anybody know of an easy way to test whether this noise is caused by shaft vibrations or cavitation and, if the latter, whether increasing pitch could fix the problem? Thank you. Nic

 

[Loren Beach]

Do you have a picture of the finished prop/shaft installation you could post?

 

[captainic]

Unfortunately I do not, but I will try to take one. The tip of the prop is about 1 inch from the hull, and the prop about 3 inches from the cutlass bearing.