Prop Pitch and Cavitation
After all this time, I’m still having prop problems. When the engine reaches about 800 RPM, I can hear cavitation. How do I know that’s what it is? Well, one of my regular crew is a Navy boy and he said that’s what the sound was. Not only that, when we revved it up more, there what a strong vibration to the boat – more than I’d expect to be normal.
So I asked Scott at Electric Yacht what he suggested. He said to try decreasing the pitch on the prop.
Pitch is the distance a propeller will advance with one turn. So, a 10″ pitch means that the propeller will move forward 10″ with one revolution.[1]
The prop is a 13″ 2-blade Max Prop. If you check the specs on the thing, you can see that it’s fully adjustable: pitch and direction of movement. I had based the pitch I set it at to what it was before I took the old inboard off. Or so I think. I didn’t know that I could look inside and find the settings. So I measured the angle and found a formula online to calculate what the pitch was. The pitch was 12″. So we decreased it to 10.
What’s so bad about cavitation? What is it? Well Wikipedia defines it as
… the formation and then immediate implosion of cavities in a liquid – i.e. small liquid-free zones (“bubbles”) – that are the consequence of forces acting upon the liquid[2]
The problem is, those bubbles can cause pitting on the prop. Enough of this and you have a non-functional prop.
After decreasing the pitch to 10″, the vibration at high RPMs seemed to go away. But the cavitation sound was still present. So I turned to the wisdom of the CAL boat and electric boat forums. The comments attached are the responses I received.
What now? Well, we’ll try decreasing the pitch to 8″ and see what happens.
[1] http://www.propellerpages.com/?c=articles&f=2006-03-08_what_is_propeller_pitch
Doing it Wrong
In our family, the story of the farmer and the mule is near and dear to our hearts:
A farmer is heading out of town. While he’s gone, his neighbor is going to take care of his mule.
“He’s a good mule, ” says the owner. “Treat him well, say kind words, and he’ll not give you a lick of trouble. He works hard, that’s a fact.”
So the owner leaves and the neighbor decides to make use of the mule. So he hooks him up to a plow and tries to get him to move. The animal won’t budge. So the neighbor tries speaking kind words to him.
No good. The mule won’t move.
He tries bribing the animal with apples, with carrots, with hay. It doesn’t help. The mule won’t budge.
The next week, the owner returns. His neighbor relates his frustration.
“I tried everything, just like you said. Kind words, treats. Nothing worked. That damn mule wouldn’t budge!”
“Hmmm,” said the farmer. Then he picks up a 2×4 and smacks the mule on the side of the head. The mule looks up, startled, and slowly starts to move.
“First,” said the farmer, “you have to get his attention.”
So, in our family, we talk about 2×4 mentalities.
This brings me to my ‘engine’ problems. Because the engine was the new piece on the boat, I blamed all the problems I was having on it. Everything wobbled. It had to be the engine. The boat wouldn’t move. It had to be the engine. In each case, that 2×4 came and smacked me hard on the side of the head to make me see it wasn’t the engine; it was other problems instead.
The serious vibration problem was a bent shaft. It had been suggested to me that I have the shaft checked before I put it back on the boat. I tried rolling the thing on a flat surface. It rolled just fine. I didn’t think it was an issue. But, after exhausting all other possibilities, when I took the boat out of the water again, I disassembled everything and took the shaft into a machine shop to have it checked. It was bent – at the conical section that attaches to the prop. So roll it all I want, I wasn’t going to see the issue.
Hooray! So we put everything back together, and checked it before putting it back in the water. I was told that might not find all the problems, because most of that vibration came when trying to move the boat through the water. But I figured it would point out any serious issues as I had been seeing.
Everything looked fine, so we launched again.
And the boat wouldn’t move. I mean, if it wasn’t a calm day, I would never have made it back to my slip!
Again, I blamed it on the engine. I wasn’t getting nearly the RPMs I should have. So I figured I’d connected something up wrong on the engine to make it less efficient.
Bless his heart! Scott McMillan said I was certainly getting enough amps out; I should be getting some speed. So he mailed me a new controller.
In the meantime, Irene happened. And, since we were expecting 100 MPH gusts in the area, I had the boat pulled out of the water. Luckily, those never materialized. In fact, Williamsburg and Richmond, more inland than we are, got the brunt of the storm in this area.
But I digress.
While we had the boat out of the water, I wanted to check to make sure I’d put that Max Prop together correctly. The damn thing has a hundred pieces and, having had inexperienced help to put it together, I figured we should recheck it.
While we were pulling it apart, someone else in the boat yard came by:
“Not moving the boat, huh?”
Yes, I answered, can’t move at all.
“I had the same problem. I put the prop together wrong, and couldn’t move at all.”
He offered to help us put it back together right. So we got out the installation instructions that I pulled off the web, and, very carefully, put the thing back together.
What had happened was that the blades were spinning flat. There was not corkscrew effect. So the thing wasn’t pushing the boat through the water at all. It’s also why the thing sounded like a blender in the water, rather than making the sound I was used to hearing.
As it happens, the guy who helped us out started explaining about the wobbling he was now experiencing. I strongly suggested he check out whether his propeller shaft was straight or not. 🙂
We put the boat back in the water again. This time, it ran like a dream. It did everything I expected it to. It is the end of the summer, but I finally have a boat that is a pleasure to use again. But, as a friend pointed out, I missed the hot miserable summer and I have a boat just in time to enjoy some beautiful fall weather!
[Top]Aligning an Engine
OK so. I thought the simpler coupler would make it much easier to tell if my engine was aligned to the shaft. After all, the coupler wouldn’t fit together right if the engine wasn’t aligned correctly.
But it doesn’t seem to have worked out that way. The first time we took the boat out, it ran like a dream. I heard a slight knocking from the engine compartment, but that didn’t seem to be too bad. When we got back to the marina, however, and put the boat in reverse to back it into the slip, we didn’t move: the prop had slipped out of the coupler!
The engine sits on a bracket, with bolts and spacers to attach from engine to bracket. There are three different slots on the bracket to fit onto two different screw holes on the engine. You select the slots and holes that will fit your engine. Mine was an ‘in between’ issue: too low for one location, too high for another. We selected the ‘too low’ one and raised the bracket on the rail to adjust. By doing so, we put it almost to the top of the bolt on the rail.
There are two bolts on the bolt that attaches to the rail: one under the bracket, one above it. The idea is to adjust the lower bracket upward, then tighten it down with the top one. Moving the attachment to the ‘too high’ one, I had to remove the underneath bolt. I put in washers instead to try to bring it to the correct level.
Try again. Get it all attached and take it out again. Same problem. But this time I actually lost it while we were out in the water. Terrible knocking started. When we got back, we found the prop shaft had slipped out again.
I called Scott at Electric Yacht again. He said that they’d started sending another bracket along to stabilize the engine, once it had been adjusted correctly. I received the bracket after getting the boat down to Portsmouth. So I installed it before heading back up to Yorktown. It shook less, but the knocking was still there. So I made use of the outboard instead to get the boat home.
And I’ve given up. I’m turning to the professionals at the marina to get it aligned. Hopefully, they can fix what I couldn’t.
Follow up: Seems the problem wasn’t the engine. And I need stop blaming everything on it. I had a bent shaft. Once that was replaced, all the knocking stopped.
Once that was fixed, all I had to do was loosen the bolts holding it from rotating and run the engine at low speeds. I then rocked it, checked it for motion, then rocked it a bit more. When I was satisfied that it was seated properly, I tightened everything down.
Oh, by the way, I figured out the prop shaft problem myself. The marina, in this case, wasn’t much help.
[Top]Boat Repair Pictures
I took copious pictures throughout the repairs this spring/summer. Rather than post them all here, here are links for you to follow to find them:
General Repairs
Cutless Bearing Replacement
Topside Painting
Motor Installation
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Kersplash!
Woowee! After 2-1/2 months, the boat is back in the water! Hooray! Unfortunately, it wasn’t an easy launch. I was most concerned with the new stuffing box. But I guess I should have paid a bit more attention to the new knotmeter as well.
The boat was put into the lift and taken over to the water. Mike, from Wormley Creek, let me check everything out as soon as I dropped it in. I found a dry engine bed, so I put the stuffing box, hose and clamps on correctly. But there was water coming in where we’d put the knotmeter in.
Paul put the male threaded coupler in, gluing it in with 4000 that I had. The directions said to wait for it to dry before screwing it in place, which we did. It also said to add more sealant and hand tighten it into place. I did so. But I guess this wasn’t good enough.
Mike pulled the boat back out of the water. He pressed on the fitting and showed that there was alot of play. The ‘nut’ that I hand tightened didn’t put enough pressure to seal it in place. I suspect, too, that we should have put it in while the sealant was wet, instead of following the directions.
We undid the pieces and scraped off all the old sealant. Mike found a tube of 4200 in his shop and placed that on the bottom of the boat, where the fitting would seat. He then handed it up to me and I put some down on the inside, where the nut would seat. He reinserted the fitting and I tightened down the nut, using a huge pair of pliers that he gave me.
But I couldn’t tighten it down very far. The hole in the shell on my boat wasn’t cut much larger than the hole through the bottom, leaving little room to manipulate. So Mike came up and, while Chris held the fitting in place, he applied the giant pliers and tightened the nut. I then screwed in the knotmeter and took the zinc spray down to respray the fittings.
Mike said that luckily, the sealant cured faster in the water than out. Thank goodness!
Mike then lowered the boat back into the water. This time no leaks!
I started the new electric motor and drove the boat over to her slip and tied her up. Tomorrow, we test out the new motor.
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