Black exhaust smoke in reverse..

[CaptnPugwash]

Howzit going my fellow yachties and boaties!

After arriving back to my sailing boat in the uk from being away 6 months in south africa, I happen to notice that when I have my ten hp perkins perama diesel fired up, it emits a suspicious black substance mixed with the cooling seawater out the exhaust when in reverse gear, but all fine in neutral or forward gear. Is this anything to be worried about or is my engine telling me something? Oil levels are spot on, belts are fine, cooling is all pumping and circulating ok, exhaust manifold has slight carbon buildup last time I checked.

 

[GrumpyOldGit]

Normally it would be caused by a fouled prop or hull but, would be more likely to smoke in forward gear. This should get you back to the top of the list where someone of huge knowledge may be able to help you out.

 

[scottie]

Gearbox ratios?

They may be different ratios for fwd reverse if hurth box giving an overload in reverse ie unburnt fuel = black smoke?

 

[DanTribe]

No idea really, but a few hypothesis;
1/ Because the prop is less efficient in reverse you are giving the engine a bit more welly.
2/ The boat is going astern so is collecting the smoke rather than running away from it.
3/ You're looking astern so are more likely to notice the smoke.
4/ The prop is aerating the cooling water intake so the engine is taking in froth. No idea why that should result in more smoke though]

 

[nigelmercier]

Gearbox ratios?

Yes because they can easily slip

 

[CaptnPugwash]

Thankyou for all your suggestions, I had a closer look again and their seems to be a black oily looking substance swirling around on top of the water from the exhaust cooling. I'm stumped that it only occurs in reverse, the engine doesn't rev as fast in astern as it does in forward which I'm thinking is normal. But it does seem to chug slightly more in reverse gear. Maybe it is unburnt diesel as scottie suggested, is that good or bad?

Thanks a million for your help! I really appreciate it.

 

[VicMallows]

I assume this is a conventional drive shaft installation? Does sound as if the engine is coming under excessive load in reverse. Is something in the drive train moving forwards/backwards and therefore binding with resulting friction? ..perhaps due to a flexible coupling? I don't know whether an internal gearbox fault might similarly cause excess loading in reverse?

Vic

 

[Tranona]

Thankyou for all your suggestions, I had a closer look again and their seems to be a black oily looking substance swirling around on top of the water from the exhaust cooling. I'm stumped that it only occurs in reverse, the engine doesn't rev as fast in astern as it does in forward which I'm thinking is normal. But it does seem to chug slightly more in reverse gear. Maybe it is unburnt diesel as scottie suggested, is that good or bad?

Thanks a million for your help! I really appreciate it.

You are part way there with your thinking. You probably have Tecnodrive box where the reverse is 2.13:1, but the forward is 2:1 or 2.6:1 - probably the latter. If you have a fixed prop then it will be pitched for maximum speed in forward at full revs. In reverse you will not be able to revs as high. If you try and run flat out in reverse you will overfuel the engine - hence the black smoke. Pull the throttle back and the smoke will go. You can double check by checking the engine revs in forward and reverse while tied up in a berth. You should be able to get around 3400 in forward, but probably only 2400 in reverse.

 

[CaptnPugwash]

Hi Vic,

Thanks for your reply.
The drive train seems ok, only slight movement when switching from forward to astern. Everything is as lined up as it can be on a conventional setup with flexible mounts, flexible shaft coupling, volvo seal etc. I'll double check the gearbox.

The black substance spews out in slow astern and at full steam. Not huge amounts though.

 

[Bilgediver]

Thankyou for all your suggestions, I had a closer look again and their seems to be a black oily looking substance swirling around on top of the water from the exhaust cooling. I'm stumped that it only occurs in reverse, the engine doesn't rev as fast in astern as it does in forward which I'm thinking is normal. But it does seem to chug slightly more in reverse gear. Maybe it is unburnt diesel as scottie suggested, is that good or bad?

Thanks a million for your help! I really appreciate it.

Do you have one of thos propellers where the blades are free to feather and can be set to different pitch angles for ahead and astern. Maybe incorrectly set.

Usually the ratio of astern means that the prop turns slower for the same revs so is it possible you have the wrong handed propeller and are using ahead for astern and astern for ahead. Might be worth checking the rations by turning the engine by hand both ways in gear and seeing if the ratios are correct way round.