Originally Posted by GILow
What you describe is EXACTLY the situation that can cause cavitation. Running at speed, lots and lots of turbulence around the outboard foot, high power output. Lift
from being on the plane brings the prop closer to the surface.
Not wanting to be belligerent about this one, just pointing out that cavitation situations are not always obvious.
It may well be a slipping hub, I liked the idea of making the two marks by DougR
Okay, so I have been thinking deeply about this problem.
I think there can only be two causes for what I am experiencing. Either (a) cavitation or (b) propeller
slippage at the hub.
If what I call a "hub slippage" is actually cavitation, then that is the only experience I have ever had with cavitation. Never experienced it at any other time in my little dinghy
with normal running.
The possible 'cavitation' events
happened when I was up on the plane. I cruise
at about half throttle which gives me about 13 knots. Just hums along with little fuss. Those are the times (about a half dozen or so in the past dozen trips) when the slippage has momentarily happened when I have struck something just under the water
. At WOT, the boat
does about 18.5 knots and there is much more turbulence and the most power going through the prop. However, I have never had the slippage thing happen at WOT when the water around the leg is most turbulent.
The transom is low and the motor
is supposed to sit a maximum of 20 mm below the bottom of the boat. At the moment it is at 40 mm which is 20 mm (approx 3/4 inch) below what it should be and I have just raised it up a bit a couple of weeks ago (but the slippage thing was happening before I reduced the prop depth). So the propeller
is way further under water than it should be (which reduces engine efficiency). I tend to think seeing as the prop is deeper than normal that cavitation is less likely than if the prop was too high.
Just thinking about it, prior to the prop doing its odd slipping thing, I had done hundreds of kilometres with narry an issue.
So, could be cavitation as I am no expert with it, but I think it likely not to be as: the prop is running deep, the leg design is the same as Yamaha (and I have never heard of cavitation problems from their leg design), and I had years of using it before any slippage problems.
Prop slippage due to hub failure:
I have had several hits of the propeller onto underwater obstacles such as the top of a submerged tree, which didn't seem to cause any problem. However, one time coming in to a concrete ramp
with flood and dirty water, the propeller running just above idle grounded hard. It was after that happened that I ran into some half-floating weed mass just under the surface that the motor
did a bit of a surge in revs that made me wonder what was going on.
The propeller is set up with a rubber hub so is designed, just like similar Yamaha and other motors, as a protection for the motor in case of the propeller hitting something hard. The tricky thing with my motor is that if it is the rubber insert which has done its job to protect the motor, then why is it still functioning most of the time?
What is going on?
I think that possibly the occasional slippage of my propeller (with momentary reving up of the motor) is due to several factors. (a) the original hub slippage being a 'low energy' accident
, (b) the rubber hub being tough and (c) my boat set-up being light.
I think first caused the propeller to slip on the hub was not a catastrophic one. I think it was just enough to break the seal to allow the prop to slip on the rubber but not cause the rubber to seriously fail. Consequently, with the way the rubber is hugely compacted into the hub, it does grip enough to continue to be surprisingly effective. The are made to grip after the seal is broken as a safety
feature to limp the boat home.
My 9.8 hp motor goes on the back of a Savage Gull 11 foot aluminium dinghy
with max motor rating of 10 hp. All up weight is under 200 kg (440 lbs) including skipper
. The forces to get my boat going are not huge. Maybe if my boat were heavier or I had a few more passengers, the load on the prop may be enough to start it slipping more than it has been.
Today I took my boat out for a run to try and get the prop to slip. I punched two marks on the prop and leg so I could tell if it slipped.
On the river for a half hour, I did numerous zero to max full throttle starts, I ran at WOT and did some turns. Wind
gusts were up to 20 knots and when I saw them coming across the water at me, I nervously held on and braced as they would punch the boat around. Eventually I slowed when I saw 'em coming to keep in better control. The chop was not big enough to be a problem. The propeller never played up once. I decided against trying to run it through weed beds or mud to load it up a little and see if it would slip.
At home I inspected the propeller and the marks were still aligned. No slippage (and I also never had any of the slippage/cavitation things).
I am convinced that it is the hub that has spun. However, they are designed when they spin to give a 'limp home' mode. That is the big thing about them. With this little Parsun outboard on a lightweight set-up, I think the 'limp home' mode is allowing me to do more than limp. For now. Will only get worse in time.
I have ordered another original prop from Parsun. Only cost $80 and is cheaper than any after-market prop or similar prop from another outboard company that I could find. I will chase up places that replace the rubber hubs but I did find an Australian shop that not longer does the small props like mine as not economical for them. So I am not hopeful of finding somewhere that is cost effective. Perhaps I should follow Bruce K and put some heat on it. My hot-air gun can do 600 degrees C so that might do the trick.
I am interested to use the new prop to see if, without hitting anything substantial, it does this prop slippage thing. Whatever the outcome, I intend to write it up here.