Why won't my Yanmar reach redline?

[mckenzie.keith]

And check the bottom at the same time.

Just got off the phone with divers. They will clean the whole bottom and prop and get the numbers off of the prop and report on what they found.

 

[mckenzie.keith]

MK,

Disregard my previous suggestion of running it "as-is".

I did not properly read your original post. I read it as you being able to get 3400 rpm, not the 3100 rpm that it appears you are actually getting. My mistake.

3100 seems to be more than "a little bit" over-propped.

Don't worry, I was never considering that anyway. Your explanation of why you suggested it in the first place makes perfect sense. I probably could have found a way to present the information more clearly. Thanks for all the anchor videos.

 

[mckenzie.keith]

Prop was clean. They cleaned it (and the whole bottom) but it was not very foul at all they said. They replaced the zincs. The numbers on the prop are 1713L. So I guess that is 17 inch diameter and 13 inch pitch.

According to my calculations, the no-slip speed is 13 inches / 12 inches per foot / 6076 feet per NM * 2900 rpm / 2.23 * 60 min per hour = 13.9 knots.

So the slip would be about (13.9 - 7.8) / 13.9 = 0.44 or 44 percent. Does that sound plausible for a two-blade flexofold on a boat like this? Just looking for a rough reality check.

Would you just choose the new pitch by assuming the slip stays the same and try to get, say, 9 knots at 3800 RPM engine speed? Hull speed is a bit over 9 knots.

I mean, I will ask flexofold about this but I feel like some of you are actually pretty knowledgeable about this stuff so I am asking here, too. And I am also just curious how the process works.

The published spec is 17000 lbs but for various reasons I believe it is more like 20,000 lbs (saw an actual weight from a sistership at one point and I believe it was about 20,000 lbs IIRC).

 

[DDW]

Slip for low solidity props of 40 - 50% is plausible. How's the tip-hull clearance? Incrementing from what you got (measured performance) is probably the most accurate way. It'll be interesting to see what Flexofold says it should be.

If the published spec is 17,000 lbs, even 20,000 is probably light. They never seem to be anything like the designer imagined, and then gain weight through out life.

 

[Zonker]

Hmmm... without doing any math that's a small prop for 55 HP with a 2.3:1 ratio. Doesn't explain why you can't get to rated RPM but gut feel is "not overpropped"

The pitch is in the ballpark of what I would expect for a 17" prop with that sort of power inputs too.

The fact that previously it was reported that the boat could do 3400 RPM says "something has changed".

- easy check: tach is reading low because V-belt is slipping (get an optical tach for sure)
- easy check: RPM in neutral
- pain in the ass check: exhaust elbow is carboned. Remove hose and from block. You'll need a new exhaust gasket. This is one not to re-use.
- another easy but unlikely check: pull off the air filter. Is there a sock in the intake?

Check that the throttle lever (circled) is hitting the stop at full throttle. The stop is that little screw with the safety wire with red arrow pointing to it. The safety wire is factory to stop people from tampering with it, so make sure it's there. If it is not somebody may have messed with the full throttle setting. (really getting into less likely territory here)

 

[Zonker]

By the way Yanmar is still using their steel thin tube oil lines. See that thin silver tube running horizontally under the throttle lever and then right above the oil filter in a nice curve? Keep an eye on them. They rust and because they are thin and carrying engine oil, it is not nice when they fail. Don't know if this engine has more of them on aft end or other side. They are tricky buggers and a pain in the ass.

 

[mckenzie.keith]

Slip for low solidity props of 40 - 50% is plausible. How's the tip-hull clearance? Incrementing from what you got (measured performance) is probably the most accurate way. It'll be interesting to see what Flexofold says it should be.

If the published spec is 17,000 lbs, even 20,000 is probably light. They never seem to be anything like the designer imagined, and then gain weight through out life.

Not sure about the tip-hull clearance.

I don't doubt you are 100 percent correct about the weight. Some day I will have to weigh it.

 

[mckenzie.keith]

Hmmm... without doing any math that's a small prop for 55 HP with a 2.3:1 ratio. Doesn't explain why you can't get to rated RPM but gut feel is "not overpropped"

The pitch is in the ballpark of what I would expect for a 17" prop with that sort of power inputs too.

The fact that previously it was reported that the boat could do 3400 RPM says "something has changed".

- easy check: tach is reading low because V-belt is slipping (get an optical tach for sure)
- easy check: RPM in neutral
- pain in the ass check: exhaust elbow is carboned. Remove hose and from block. You'll need a new exhaust gasket. This is one not to re-use.
- another easy but unlikely check: pull off the air filter. Is there a sock in the intake?

Check that the throttle lever (circled) is hitting the stop at full throttle. The stop is that little screw with the safety wire with red arrow pointing to it. The safety wire is factory to stop people from tampering with it, so make sure it's there. If it is not somebody may have messed with the full throttle setting. (really getting into less likely territory here)

View attachment 580691

Thanks Zonk, and not to beat a dead horse, but since the 3400 rpm reading, the saildrive was changed and the throttle cable was changed. I will do as many of these things as I can as soon as I can and report back. Probably some time next week. Really appreciate the feedback.

 

[Zonker]

The saildrive ratio change is about 3300 RPM if nothing else changed.

Checking that the throttle cable makes the throttle lever move through the full range is such an easy check that you do it anyway.

 

[DDW]

The saildrive ratio change is about 3300 RPM if nothing else changed.

Curious how you arrived at that. My thinking comes more from airplanes but the principles are the same. In the power curve, 3400 is about 51 hp, 3100 about 47 hp. I figured the ratio change good for 11% in power, based on rpm^3. (As I remember, the torque to drive a prop is rpm^2 but the power is rpm^3). The ratio of 51 to 47 is about 8%. So going from 3400 to 3100 seems possible on ratio alone. But I could certainly be wrong.

 

[mckenzie.keith]

The saildrive ratio change is about 3300 RPM if nothing else changed.

Checking that the throttle cable makes the throttle lever move through the full range is such an easy check that you do it anyway.

I will absolutely do all the easy stuff. It may seem I am getting ahead of myself. But that is because I am not on the boat now. So I can think about changing props and whatnot but I can't test anything. Don't worry. I am taking your advice to heart. It would be really stupid to buy a new prop if it is not necessary.

 

[mckenzie.keith]

They allegedly removed and cleaned the exhaust mixing elbow when they serviced the engine (and replaced the saildrive). So I think we can move that to the "less likely" list.

 

[slug zitski]

Prop was clean. They cleaned it (and the whole bottom) but it was not very foul at all they said. They replaced the zincs. The numbers on the prop are 1713L. So I guess that is 17 inch diameter and 13 inch pitch.

According to my calculations, the no-slip speed is 13 inches / 12 inches per foot / 6076 feet per NM * 2900 rpm / 2.23 * 60 min per hour = 13.9 knots.

So the slip would be about (13.9 - 7.8) / 13.9 = 0.44 or 44 percent. Does that sound plausible for a two-blade flexofold on a boat like this? Just looking for a rough reality check.

Would you just choose the new pitch by assuming the slip stays the same and try to get, say, 9 knots at 3800 RPM engine speed? Hull speed is a bit over 9 knots.

I mean, I will ask flexofold about this but I feel like some of you are actually pretty knowledgeable about this stuff so I am asking here, too. And I am also just curious how the process works.

The published spec is 17000 lbs but for various reasons I believe it is more like 20,000 lbs (saw an actual weight from a sistership at one point and I believe it was about 20,000 lbs IIRC).

Yah

A boat is always heavier with more windage

Normally the prop guys can only make a good guess

 

[Zonker]

DDW I was strictly thinking of the difference in gear ratios, not where the engine was on the power curve. You could well be right.

 

[mckenzie.keith]

I received an initial reply from flexofold to my inquiry. They think I should move up from two to three blades and from 17 to 18 inches in diameter. But they also asked a few questions so this is still a bit tentative. As for pitch, let me wait until I hear back from them after answering their questions. But their initial estimate was 11 inches (but that is based on a three-blade with 18 diameter, so not directly comparable to the two blade 17 inch as slip will probably be different).

I told them the waterline is 47 feet because the boat has a plumb bow and when motoring at speed, the waterline extends all the way from stem to stern. But they looked up the sailboatdata.com waterline which is much less. It seems to me that any prop calcs of hull speed should be based on 47 feet of waterline. Looking for opinions on that.

 

[climenuts]

If you can fit the appropriate 2-blade pitch and diameter to load up the prop I don't think going to 3-blade is the right call. My understanding is the two-blade is the most efficient and you should only add blades if you don't have the real estate to get the diameter you need.

 

[kinardly]

If you can fit the appropriate 2-blade pitch and diameter to load up the prop I don't think going to 3-blade is the right call. My understanding is the two-blade is the most efficient and you should only add blades if you don't have the real estate to get the diameter you need.

I think I also heard somewhere that the three blade was smoother, less vibration.

 

[DDW]

I'd go for the three blade, as big diameter as would fit. 2 blade in theory is more efficient - one blade even more so - but in this case you have a blade loading problem running 55 hp through a fairly small 2 blade prop. The lower the blade area the higher the angle of attack has to be to absorb the power.

My opinion on the waterline is that you only get the additional waterline after you've made a big enough hole in the water to sink the stern. Of course the lines plan makes a difference, how flat the run etc. Also how much cruising gear on board sinking the stern even at rest.

I had a vibration problem with my first prop on the saildrive. It was 4 blade, the engine was 4 cylinder, and the reduction ratio close to 2. Some pundits said this was a problem, exciting resonance. High end German Variprop feathering prop. I replaced it with a 3 blade feathering Autostream, which looks a lot less high tech. That reduced the vibration substantially. This is a bit of a black art I think. One other difference was the hub design placed the Variprop closer to the drive leg. If you are going to 3 blades, that is a whole new prop so you can go any direction you want. The Autostream has been great, but sadly they are no longer made.

 

[Fah Kiew Tu]

I'd go for the three blade, as big diameter as would fit. 2 blade in theory is more efficient - one blade even more so - but in this case you have a blade loading problem running 55 hp through a fairly small 2 blade prop. The lower the blade area the higher the angle of attack has to be to absorb the power.

My opinion on the waterline is that you only get the additional waterline after you've made a big enough hole in the water to sink the stern. Of course the lines plan makes a difference, how flat the run etc. Also how much cruising gear on board sinking the stern even at rest.

I had a vibration problem with my first prop on the saildrive. It was 4 blade, the engine was 4 cylinder, and the reduction ratio close to 2. Some pundits said this was a problem, exciting resonance. High end German Variprop feathering prop. I replaced it with a 3 blade feathering Autostream, which looks a lot less high tech. That reduced the vibration substantially. This is a bit of a black art I think. One other difference was the hub design placed the Variprop closer to the drive leg. If you are going to 3 blades, that is a whole new prop so you can go any direction you want. The Autostream has been great, but sadly they are no longer made.

Rumour has it that they're going back into production. At least that's the story I got off a friend of mine who has one and wanted some spares.

I have a saildrive 18" Autostream on the bench ATM. I stripped it down to give it a full service before I decide what I'm going to do with it. I'd like to get a 20" one for a shaft drive so I'm hoping production restarts. Otherwise maybe I'll machine a shaft with the splines for the saildrive prop.

FKT

 

[mckenzie.keith]

I'd go for the three blade, as big diameter as would fit. 2 blade in theory is more efficient - one blade even more so - but in this case you have a blade loading problem running 55 hp through a fairly small 2 blade prop. The lower the blade area the higher the angle of attack has to be to absorb the power.

I will give this some thought.

My opinion on the waterline is that you only get the additional waterline after you've made a big enough hole in the water to sink the stern. Of course the lines plan makes a difference, how flat the run etc. Also how much cruising gear on board sinking the stern even at rest.

Video of a sister ship:


I think once the motor is much above idle, the water stays attached all the way to the stern. Doesn't need to squat. It just needs a bit of a wave forming.