Yanmar blowing black, then eventually stopped

[Wotname]

Jimmy_G, how's the fault finding progress going on your 2GM20?

[Jimmy_G]

Hi Wotname,
thanks for asking. I have added fuel doctor, and after another look at my prop, have engaged an in-water (dive) prop cleaner for sometime in the next few days (hopefully). Then I'll test run.
I'll also re-look at my oil level after reading another cruiser forum story this morning to check I'm not 'making fuel'. Probably a red herring, although I noted possible overfill of the sump at last service and corrected that as my first thing several weeks ago.
Probably worth noting this side issue. After breaking down a couple of weeks ago (and requiring a short tow) I changed the fuel filter and found a possible blockage in the tank exit (the fuel bowl didn't fill straight up when the new filter was installed). But blowing back through the tank hose appeared to unblock and the bowl filled. I had let the fuel tank get to less than 25% full and now realise this is not smart and to keep it full. I think this probably cause the breakdown, and probably not related to the black smoke.
Back to black smoke: Still have not checked the outlet south of the exhaust elbow, but will do if no luck with the aforementioned.
So still going one thing at a time, but other life issues have meant I'm not at it every day
cheers
Dave

[Wotname]

Thanks for the update Dave, hope you get it sorted when you get time.

As an aside, the dipstick on these GM engines are a pain. IIRC, Yanmar suggests measuring the oil level 10 mins after shut down.

Plenty of owners have noticed different readings on the dipstick depending if the engine is cold or measured after 10 mins or after 45 mins or after 2 hours of shutdown. Crazy I know.

Me, I used to check mine it when cold but only on the second insertion of the dipstick. It used to be a bit lower cold than when measured after 10 mins after shutdown whereas a mate had the same engine and his was bit higher cold - go figure. I never understood why the slight variations but there is plenty of threads of the internet about it if you have a day or so to waste reading about it.

I just made sure it never measured above full or below the minimum and was happy to accept varying reading provided they stayed somewhere in the middle!

[Jimmy_G]

Useful to know! Thanks again. I'll try and check when cold to get as consistent-a-picture that I can.
I will report back to the forum following my attempts at resolving the issue(s).
all the best.
d

[Jimmy_G]

We seem to have resolved the issue!
I has a diver clean the prop on the weekend and out test run revealed no black smoke up to WOT. The prop must have been worse than I thought and as you can see in one of the photos taken.The hull was still quite clean, despite the prop and hull being anti-fouled at the same time, albeit the prop not done well.
She blew some grey/white smoke at the higher revs, but not very much. Happy to take any advice on that, although my stress levels about it are low and hopefully can stay that way
I also added a fuel doctor treatment and will keep that up now, but suspect the knife-edge change was due to the prop clean.
I collected the following data with the clean prop (Yanmar 2gm20 - Hartley sister twin-bilge keel, glass over marine ply hull):
1500 rpm - 4 kn
1850 rpm - 4.5 kn
2100 rpm - 5.1 kn
2250 rpm 5.35 kn
2500 rpm - 5.8 kn
2700 rpm - 6.1 kn (WOT)
I was not getting much above 3.5 with the dirty prop.

Thanks to all who took the time and effort to help me through this one. I hope to be able to help others in future once I build some more knowledge and experience.

Very kind regards!
Dave/Jimmy G

[Wotname]

Congratulations!

Be aware that as you are only getting 2700 instead of the rated maximum of 3600 at WOT (with a clean prop), any prop fouling will be more noticeable compared to a correctly pitched prop (i.e. one which allows the engine to reach 3400 to 3600).

[misssherry]

You mentioned that you blew into the fuel line hose to clear debris from the pickup tube. That situation will likely reoccur and could cause an engine shutdown. Now would be a good time to remove the pickup tube from the tank and pull out the screen wire mesh that is located up in the bottom of the tube. You don't need it if you have a good primary fuel filter like a racor.

[Jimmy_G]

Yes, I will need to do more with the prop. I will have it cleaned and prepared with more care next time She's out of the water. Probably using 'prop speed', then see what speed at revs she can do. Then re-assess the props fit for purpose.

And thanks misssherry for the tip with the fuel line etc. I have not looked at that yet, so more discovery to come. Interestingly when I first bought the boat and had the engine serviced the 1st stage fuel filter was very dirty, but the subsequent two were not. But your idea should shed more light onto the situation and I may be able to clear a substantial amount of any solids from the tank. I assume I should then replace the mesh once the 1st stage filter starts showing up as clean.

d

[Compass790]

Originally Posted by Jimmy_G

Yes, I will need to do more with the prop. I will have it cleaned and prepared with more care next time She's out of the water. Probably using 'prop speed', then see what speed at revs she can do. Then re-assess the props fit for purpose.

And thanks misssherry for the tip with the fuel line etc. I have not looked at that yet, so more discovery to come. Interestingly when I first bought the boat and had the engine serviced the 1st stage fuel filter was very dirty, but the subsequent two were not. But your idea should shed more light onto the situation and I may be able to clear a substantial amount of any solids from the tank. I assume I should then replace the mesh once the 1st stage filter starts showing up as clean.

d

I'd doubt prop speed is going to get you to max revs. You'd likely need repitching.
Glad you inproved it & that was an ugly before pic. My guess the gray smoke is a mix of ovefuelling & oil smoke. Overfuelling is from oversize or overpitched prop assuming injectors compression etc ok & oil smoke is from wear.
Can run for a long time overpropped but it's not ideal.

[Jimmy_G]

OK, thanks Compass790.
I will go back to the prop guy and have a good chat about options re pitch etc.

[Jimmy_G]

Not sure if this helps assess the prop performance or fit-for-purpose, but the relationship between RPM and speed looks like a fairly straight line.
Dave

[Wotname]

Originally Posted by Jimmy_G

Not sure if this helps assess the prop performance or fit-for-purpose, but the relationship between RPM and speed looks like a fairly straight line.
Dave

It sort of is... but only until you reach the maximum hull speed of your boat. This is the maximum speed a displacement boat can achieve regardless of how much additional power is applied.

Without knowing all the details of your boat (LWL & hull shape etc), I think your max speed will be about 6 knots.

If you are reaching 6 kts at 2700 rpm (calm conditions, clean bottom and prop), then you are using about 8 hp.

If the engine rpm was 3400, then you are generating 16 hp but still only be doing ~6 kts.

So why bother changing anything you ask...

Because when the wind is up and the waves are big, you will need more than 8 hp to overcome the additional resistance (drag) to reach 6 kts. The only way to get more power from the engine is to increase the rpm.

If you are over propped, you don't have that option!

If you are correctly propped and the weather is calm, the engine will be running easy at say half throttle, turning at say 2700 with the boat moving a 6 kts. When the weather turns bad and you want to maintain 6 kts, simply increase the throttle, the engine turns faster and develops more power. At WOT, the engine will be at 3400 and develops 16 hp.

See the power curve for the 2GM20 from the service manual.

[Compass790]

You will only get over hull speed which is probably about 6knt by burining lots of fuel & trying to tirn a bilge keeler into a planing hull.
Check the vicprop calcualtor on Google. I reckon it's ok for us,but feed the parameters for your boat into the calculator.
I'm sure you can use your current prop but you don't have any reserve power.
We have a hugely overpitched prop on our YSM8, it only reaches 2400 rpm in flat water with no headwind, clean btm etc.
I'm a cheap/poor bstd so accepted a free prop that isn't correct but I'm ok with how the engine copes with the too big prop. EXCEPT when it matters. We have been stuck in hideyholes on offshore islands for diving purposes & have had to motorsaill out of anchorages with wind change because the engine didn't produce enough hp at it's restriced revs by the too big prop.
We coped because my father made me sail from maybe 6 years old but if you are a novice it would be much better to have a matched prop & engine
Should reach 3200 rpm for max hp,
The problem is we are limited to 6hp at best. Trust me this is a real issue at winds over 25knts. Its' just like being stuck in overdrive in your car.
If that doesn't correspond to how you are going to use your vessel ,don't worry about it.
Just my 0$ worth

[Jimmy_G]

Thanks very much for those insightful responses Compass790 and Wotname. I have a few things to think about there.
1st i'll work on getting my head around it, then try the vicprop calculator and go from there.
greatly appreciated,
Dave/JG

[Compass790]

Sorry for repeating what Wotname had already said but I was writing whilst he was posting but it doesn't hurt to get a 2nd opinion.
Your prop grew a lot of stuff in a year. Do you know what antifoul you used? I want to avoid it.
Vicprop calculator would probably not allow for a bilge keeler so you would probably need either a smaller diameter or less pitch than they recommend. The pitch & dia should be stamped on the prop boss. If you tell the prop bloke who is going to do the repitching what revs it can reach in flat water with a clean btm & prop at WOT & the max revs you want to reach he would probably get it pretty close anyway/
Make sure you tell him it's a bilge keeler.
Hope that helps