Can't get max rpm after fitting a 3 blade Flexfold prop

rtmccue · Jun 15, 2015

Old prop

discovery said:
Help,
3 yrs ago we replaced our 3 blade fixed prop on our Hunter 456, with a Flexfold 3 blade folding prop. About a yr into the new prop I noticed a bit of blue smoke when we were motoring into our slip, long story short when I questioned my Yanmar guy he asked if I could get the 3700 rpm that I was rated at, I said no just 3100-3200, he says I'm over proped, called Flexfold and they blamed it on the engine, anyone know how I can resolve this? Hate to have the prop re pitched if it's not the problem? What engine diagnostics would I do to determine if the engine is not putting out enough power? I can get 3800 rpm at neutral, I don't want to damage my engine, but it runs fine now except for a bit of smoke, doesn't overheat starts easy, uses maybe a half quart of oil every 150 hrs. I change oil and filter every 150hrs. Had the injectors replaced last yr, engine has 1600 hrs.its a Yanmar 4 jh3-te 75 hp turbo.

I think I would try the original prop .are you shut it is rated at 3700 RPM ? It has a tag on the engine . I remember 3400 RPM but I could be wrong. A lot of the boats I moved have tachometer that are a little off

ALNims · Jun 15, 2015

You mentioned blue smoke and engine rpm. I have a couple of questions. Where you able to get 3700 RPM after the fitting of the new prop? Was there an RPM drop after you noticed the blue smoke? Do you remember what the RPM was with the previous prop installed?

CarlN · Jun 15, 2015

The most frequent cause of reduced RPM is probably fouling on the prop or shaft.

If clean then, when you are running full speed, do you get a lot of black (not blue) smoke and possibly overheating? That would be a classic over-propping. Or a lot of other things too. You really can't overload an engine coming into a slip.

Blue smoke is burning oil that's getting into the cylinders - not caused by over-propping. Are you burning more oil than usual? Can you get the blue smoke after running for a while in neutral and then revving the engine? Can you get to 3700 in neutral? (could just be an out of adjustment throttle linkage)

Finally, a little over propping is not like running an engine with no oil. If the boat speed is good, the engine sounds fine, doesn't smoke at cruising speeds (which I assume is less than 3100), and isn't overheating then it's unlikely you are doing any real damage from the prop.

coolista · Jun 16, 2015

Smoke could be the turbo unwinding its bearing seals. The issue is, any diesel engine losing only that much should have no detectable smoke, but if it is discharged from the turbo bearings, the tiny rate has the greatest opportunity to show up in the exhaust unburned. The question is, does your turbo slow in response to bearing wear to the point of reducing HP, and is bearing wobble impinging the turbo seal's diameter, which is the first sign of near-future turbo bearing failure. If turbo is slowing, the damage is done, but in most cases, this type of wear to reduce HP is short-lived and the turbo should have lunched itself by now. Blue smoke and power loss could be unrelated.

capta · Jun 16, 2015

I'm a little confused. Did you get the full 3700 rpms for the first year after the reprop? If not, then the prop pitch is wrong. If so it is an engine problem. I wouldn't trust the prop manufacturer on pitch for your boat. MaxProp has it all wrong for the Pearson 530 with the Perkins 4-236 engine, even though a number of them have MaxProps.
What do you get for no load rpms? Is that within spec? If the engine gives you full no load rpms, then you can move on to the drive train. Do you have the blue smoke at no load full rpms?
I'd also look into the turbo if you are not burning oil and everything else is OK.[/quote]

Mikem · Jun 16, 2015

I have a H466 with the same engine. Three years ago I replaced my 18 x 17 fixed three blade for a 20 x 14 three blade flexofold. I still get the rated 3600RPM that I got with the fixed prop. Same hull speed under power as before. Do you have the 2.63-1 tranny?

BTW, flexofold wanted to sell me a 22 x 14 but I thought that a bit much, especially since and identical H460 down the pier had a 20 x 14 flexofold with the same results as I noted above.

What size prop do you have?

Mikem · Jun 16, 2015

If the prop is clean you should get the rated RPM if there are no engine problems. What is your hull speed at wide open throttle?

At 3000 rpm my temp is about 185 and hull speed 8kts. At 3600 rpm the temp begins to climb to about 205-210 with a GPS speed of 8.4+ kts. Theoretical hull speed is 8.3kts

CarlN · Jun 16, 2015

So you probably are a little over-propped but there's no sign the extra load is doing any damage to the engine - no smoke, no overheat at high RPM. I assume you are going at least as fast under power as you were under the old prop?

The smoke in the marina is something else. If blue, it's oil leaking somewhere. Again, no real problem until the oil consumption gets a lot worse.

You will do damage if you worry and start to baby the engine. Run at full throttle for 10 minutes every once in a while.

uncledom · Jun 17, 2015

discovery said:
Help, 3 yrs ago we replaced our 3 blade fixed prop on our Hunter 456, with a Flexfold 3 blade folding prop. About a yr into the new prop I noticed a bit of blue smoke when we were motoring into our slip, long story short when I questioned my Yanmar guy he asked if I could get the 3700 rpm that I was rated at, I said no just 3100-3200, he says I'm over proped, called Flexfold and they blamed it on the engine, anyone know how I can resolve this? Hate to have the prop re pitched if it's not the problem? What engine diagnostics would I do to determine if the engine is not putting out enough power? I can get 3800 rpm at neutral, I don't want to damage my engine, but it runs fine now except for a bit of smoke, doesn't overheat starts easy, uses maybe a half quart of oil every 150 hrs. I change oil and filter every 150hrs. Had the injectors replaced last yr, engine has 1600 hrs.its a Yanmar 4 jh3-te 75 hp turbo.

If your RPMs are accurate, you are "lugging" the engine, which is bad. I bought an optical/mechanical tach that checks rotation speed at the crankshaft pulley. Lets assume your tach is close, you are lugging by about 12% ALL the time. That's like trying to start off from a red light in third gear. Your dumping fuel into the cylinders and not allowing the engine to rev up to the RPM for the amount of fuel entering. The excess is going to show up in the exhaust. Talk to a Yanmar authorized repair shop, someone big like Mack Boring in NJ, they have multiple people at the Annapolis boat show, and they will confirm lugging is very bad. If this didn't exist before the new prop and if you swap out the prop, which is a fast test, and it goes away, repitch. Good luck

Ron20324 · Jun 17, 2015

discovery said:
..., so I would assume I weight at least 2 thousand pounds more than spec

As said, whatever rpm speed you get in idle, you should get underway in calm waters. So, you need to get a prop that will get the max rpms with the load that your boat actually carries, not what the designer had in mind.

When I go out, I run the engine up to top throttle. If I can NOT get the expected 3600, I know something is not right. Like last weened, I was 200 rpm low, but it has been two years since haulout and I know the bottom and prop are dirty and slowing me down.

uncledom · Jun 17, 2015

discovery said:
The thing that concerns me is that I have now had two boats with the same engine and hull, say they have the same prop and do get 3600 rpms, so would a extra 1000 pds or so make the difference due to us loading the boat up with stores, a arch, dink etc?

I don't know how much a thousand pounds would shave on a 45 foot boat. I'm pretty sure not much. I would start by putting the original prop back on and reading the RPMs. Remember if your using the tach on the boat it can be off considerably.

CarlN · Jun 17, 2015

Remember, you have no evidence that the prop is a problem because you don't know if the RPM went down with the new prop. The smoke is not from overpropping. The slow RPM could be many things. If that new prop has been on for three years, swapping for the old one is going to take a haul and some effort with a prop-puller. And then another haul to swap it back if the prop is fine. Expensive.

Keep looking for inexpensive, easy things to rule out. Have you checked the throttle cable?

Your Yanmar manual has a section about setting the throttle cable. It could have slipped and not be opening the throttle all the way.

Mikem · Jun 17, 2015

First check to see the prop and shaft are clean. We have the identical hull, engine, tranny and prop size. You should get 35-3600 rpm no problem. Are you sure the prop is a 20 x 14?