Oil in water around stern

[AnnieRose]

Is this normal! Turned on the engine today and noticed a fair bit of oil in the water at the stern of the boat…

[Tonka]

Where does the exhaust discharge?

[AnnieRose]

At the stern to the left side 

[pete.i]

Mine used to do that especially if the boat had been stood for without starting the engine for a while. My boat had a BMC 1.5 that also smoked a bit on initial startup. I no longer have a boat so this was a while back. 

[magnetman]

If you have just had the Diesel topped up it might be leaking from under the top rubbing strake. This looks like a Diesel leak. Sometimes the welds at the top there are slightly porous. 

 

 

Our is that just water. 

 

 

[Tacet]

Has the bilge pump recently been at work?

[magnetman]

 

I had a Boat once which leaked from a weld in exactly this position. 

The muck in the water does look more like bilge than fuel though as it appears to have some grease in it.

4 minutes ago, Tacet said:

Has the bilge pump recently been at work?

It's dry under the bile pump outlet in the picture provided.

 

 

I would rub that black damp patch with a tissue and sniff it. Not for psychoactive reasons just to see if it's oil. 

Edited January 12 by magnetman

[Tony Brooks]

Check around other boats, it might be from another boat that has pumped oily bilges, wind will blow the waters surface about.

[AnnieRose]

On 12/01/2026 at 09:49, magnetman said:

If you have just had the Diesel topped up it might be leaking from under the top rubbing strake. This looks like a Diesel leak. Sometimes the welds at the top there are slightly porous. 

 

 

Our is that just water. 

 

 

So it’s been a while since filled with deisel, it’s been raining and so that will be why it’s wet there. When cruising I noticed it in the water intermittently 

On 12/01/2026 at 10:04, Tony Brooks said:

Check around other boats, it might be from another boat that has pumped oily bilges, wind will blow the waters surface about.

It does in fact seem to be my boat. I thought this at first but currently moored near no other boats and can still see it 

On 12/01/2026 at 09:50, Tacet said:

Has the bilge pump recently been at work?

It hasn’t been pumped this morning and o can still see a light film of it on the water. More confusingly, I noticed it on the floor at the stern and around the sides of the boat yesterday while cruising.

[David Mack]

Does your boat have the conventional built in fuel tank in the half-round of the stern, with the rudder tube passing through it? Derusting and blacking the inside of the rudder tube is not often done, and it is possible for this to rust through unseen, allowing diesel out into the canal, especially on an older boat.

[AnnieRose]

I believe so yes. I will be sure to check this tomorrow once it’s a little bit dryer out

[magnetman]

Just a thought. Did you check the black mark on the side of the hull and is it definitely water?

 

My thought her and it is hopefully completely wrong is that someone may have stolen some of your Diesel and in the process spilt a bit on the deck and surrounding areas. 

 

 

 

 

4 hours ago, AnnieRose said:

 More confusingly, I noticed it on the floor at the stern and around the sides of the boat yesterday while cruising.

 

Not wanting to alarm you and like I said hopefully not but Diesel theft has been known to happen. If there is oil on the deck what other explanations are there?

 

You need to clarify what you mean by "i noticed it on the floor". Are you referring to the stern deck area or inside the engine bay area. If the second of these then @David Mack  could be right but this would usually not involve oil "on the floor".

 

 

When fuel is pumped from the tank there will be a hose or pipe which is inserted but when this is taken out it will drip fuel onto the deck around the filler. Diesel does not evaporate like petrol so when it rains this will end up going down the sides and into the water. 

 

[alan_fincher]

On 13/01/2026 at 14:20, David Mack said:

Does your boat have the conventional built in fuel tank in the half-round of the stern, with the rudder tube passing through it? Derusting and blacking the inside of the rudder tube is not often done, and it is possible for this to rust through unseen, allowing diesel out into the canal, especially on an older boat.

Yrs, I was thinking this, but David beat me  to it.

How likely this failure is will depend on the age of the boat.

[Arthur Marshall]

Mine sometimes seems to squeeze grease out of the stern tube. Looks a bit like that.

[Tony Brooks]

54 minutes ago, Arthur Marshall said:

Mine sometimes seems to squeeze grease out of the stern tube. Looks a bit like that.

 

It certainly looks like something more viscous than fuel or typical engine oil to me because it has blobbed rather than spread out. Partially emulsified engine oil via a bilge pump or, as you say, grease broken up by a revolving shaft.

[magnetman]

On 12/01/2026 at 09:50, magnetman said:

 

. 

The muck in the water does look more like bilge than fuel though as it appears to have some grease in it.

It's dry under the bile pump outlet in the picture provided.

 

 

Another possibility here is that another Boat (lots of Boats around in the urban environment n the picture) discharged a lot of dirty bilge oil and some of it has simply hit and stuck to the side of the OP Boat.

 

No sign of recent dribbles.

 

The black mark to the left looks too me like oil not water. 

 

 

If you look carefully at the original pictures there are at least three other vessels in close proximity and a multi storey block of flats. 

 

Wet weather.

Being double moored.

Vessel alongside has auto bilge pump and cruiser stern..

 

Splat. This also explains the oil 'on the floor'.

 

It is someone else's bilge.

 

And that black mark on the side is oil. 

 

 

Edited January 14 by magnetman

[blackrose]

On 14/01/2026 at 18:06, Arthur Marshall said:

Mine sometimes seems to squeeze grease out of the stern tube. Looks a bit like that.

 

That was my initial thought. It doesn't look like the usual iridescent rainbow effect of diesel on water to me.

 

On 14/01/2026 at 19:07, magnetman said:

 

It is someone else's bilge.

 

 

You're sure about that are you? Just from a few pictures.

 

On 13/01/2026 at 14:09, AnnieRose said:

 

It does in fact seem to be my boat. I thought this at first but currently moored near no other boats and can still see it 

 

 

Edited January 16 by blackrose

[magnetman]

I still think so yes. Oil sticks to the side and can still show on the water later on.

 

Note that the post in your last quote was made shortly after the discovery. I think it was after a cruise.

 

@Tony Brooks was referring to someone else's bilge -on the water- whereas I am referring it having hit the side and deck of the OP's vessel.

 

They did say there was oil 'on the floor' by which I think they meant the deck. 

 

That black mark down the side is oil I am sure of it. 

 

It completely makes sense because the first photo shows other Boats in reflections, greasy bilge water in the canal and a black oily mark on the side of the counter. It also appears to show that the bilge outlet on the Boat has not discharged recently.

 

They then go for a cruise, moor where nobody else is and see oil marks on the water. This is the oil which stuck to the side, from when the other vessel discharged it's bilge, creeping downwards and into the water.

 

 

 

 

 

 

 

 

This is the most likely explanation. 

It has been raining. Other Boats moored very close. Bilge pump actuation. Loads of people have oily greasy bilges.