What? Another engine smoke question?

[Skipper]

Yep. But I've never seen this one asked before.

Universal M18. When I'm motoring during the day, not a trace of smoke comes out the back, not even at startup. I'd think it was an electric motor, but for the pleasing rhythm.

However, at night there appears to be billowing smoke, illuminated by the stern light. And it's not just all at once; it's been consistent -
Day: clear. Night: visible smoke.

No issues with the motor.

Has anyone had this?

 

[JoeWhite]

It is not smoke - it is steam due to cold air.

 

[Skipper]

It is not smoke - it is steam due to cold air.

I thought about temp change, but the air is in the high 60's low 70's here after dark.
Maybe, with lower dew point it changes something.

Or is it a trick of the light.

I rarely ran at night in Annapolis. -We moved back to CA and took the boat with us- Now back in So Cal I can sail at night again. So I've been noticing the smoke. I never saw it here before because all my boats had gas outboards.

Just wondering if others see this.

 

[Benny17441]

Could never visualize properly the temperature inversion that causes fog but could be similar to what you describe.

 

[WayneH]

I'm going to go with steam, also. Warm exhaust over cold water, relative humidity way up at night and bingo.

I've seen that in Galveston only during the winter months when the water temperature finally drops to "too cold to go swimming" range.

 

[odaydokay]

Did you put your hand down on the exhaust water to see if cool or just luke warm?

 

[Skipper]

Did you put your hand down on the exhaust water to see if cool or just luke warm?

I can't reach it safely while under way. Did it a couple years ago at the dock. Was cool. --I'm glad you brought that up though, am going to hit it with the laser thermometer this weekend while at operating temp, and compare the output with sea surface temp, just out of curiosity regarding the difference.

The motor consistently runs at its designed operating temp, 165f.

I think steam has to be the answer, as others suggested. It's the only way to explain the day/night change. It just looks a whole lot like smoke under the stern light.

 

[Alan Gomes]

I can't reach it safely while under way. Did it a couple years ago at the dock. Was cool. --I'm glad you brought that up though, am going to hit it with the laser thermometer this weekend while at operating temp, and compare the output with sea surface temp, just out of curiosity regarding the difference.

The motor consistently runs at its designed operating temp, 165f.

I think steam has to be the answer, as others suggested. It's the only way to explain the day/night change. It just looks a whole lot like smoke under the stern light.

Does it dissipate quickly or does it seem to just hang there for a bit? If it dissipates quickly that points to steam.