Sink water and skin fitting

[Sid Charles]

I am a little concerned about my galley sink water exit from the boat, having restored the boat in full from the wreck I purchased, I have located my kitchen sink close to the original skin fitting hole, I put a new fitting in, and it complies with the regs regarding where below the gunnels to hole sits, but now that I am fitting a sink I find that the hole in the side of the boat is slightly higher than the sink drain, so water cannot go uphill and I am wondering if this is something others have discovered.

Edited September 27, 2025 by Sid Charles

[Russ T]

Raise the sink

Pump the water.

 

As far as I remember, the regulations state a skin fitting min height above the water line, not below the gunnels.

Or, find a shallower sink.

[BEngo]

The requirement is for the waste to be watertight up to the specified distance above waterline, not for the outlet to be a specified distance above waterline. 

 

In practice, pipework which has all joints screwed or olive type joints or push on pipe secured to proper nozzles or skin fittings by two hoseclips will satisfy the requirement and then the watertight limit is the top of the sink, basin or whatever.  Low level stuff like shower trays is usually pumped and the outlet pipe taken up to gunwale level and back down to the outlet.

 Avoid the pump in a box serving several wastes to pump to one outlet.  They are in the same usefulness bracket as chocolate teapots.

[Ronaldo47]

I found that the trap outlet of the bathroom sink in our house was slightly lower (by about half an inch)  than the waste  pipe where it went through the external wall to discharge water into an external hopper. It wasn't a problem, it just meant that the short uphill length of pipe between trap and wall  had the effect of making the sink trap deeper by half an inch. As long as the bottom of the bore of the pipe that goes through the wall, is lower than the bottom of the sink, it will just have the effect of deepening the trap. 

Edited September 27, 2025 by Ronaldo47

[blackrose]

17 minutes ago, Ronaldo47 said:

As long as the bottom of the bore of the pipe that goes through the wall, is lower than the bottom of the sink, it will just have the effect of deepening the trap. 

 

The OP said that "the hole in the side of the boat is slightly higher than the sink drain" so it depends whether he means the top of the sink drain (the plug hole) or the bottom of the sink drain. If it's the top then as well as a full waste pipe (which isn't a problem), he'll always have water in the plug hole or even at the bottom of the sink. 

[Kingdom Isambard Brunel]

If you use solvent weld waste pipes and anchor any screwed up rubber ring compression traps with a stainless screw, they are considered water tight and not going to pull apart so the effective height is the rim of the bowl.

[Sid Charles]

21 minutes ago, blackrose said:

 

The OP said that "the hole in the side of the boat is slightly higher than the sink drain" so it depends whether he means the top of the sink drain (the plug hole) or the bottom of the sink drain. If it's the top then as well as a full waste pipe (which isn't a problem), he'll always have water in the plug hole or even at the bottom of the sink. 

Yes by the time I put the sink drain fitting on to reduce to 19MM it takes it lower than the skin fitting, I wish I had taken more interest when I ripped out the old boat, so would it just mean I have a waste pipe full of water nearly all the time, what about me fitting a non return check valve?

[Peanut]

This happens on a friend's boat. In time, the pipe gets clogged up with sink detritus, and you have to pump it out with a sink plunger, and the sink gets scummy with the slow flow.

I would fit a shallower sink, as mentioned above, or jack the work top up with battens all round. Sorry about your tiling.

[David Mack]

57 minutes ago, Peanut said:

This happens on a friend's boat. In time, the pipe gets clogged up with sink detritus, and you have to pump it out with a sink plunger, and the sink gets scummy with the slow flow.

I would fit a shallower sink, as mentioned above, or jack the work top up with battens all round. Sorry about your tiling.

Or fit a lower level skin fitting, and block the existing hole.

[Ronaldo47]

On the heritage railway where I volunteer, they had a problem with a double-sink vanity unit in the workshop toilet block which had been installed with two traps connected by a horizontal piece of pipe with  no fall  that was always getting blocked. As the sinks had 1 1/2"  (bath/kitchen sink) traps, I was able to provide some  fall by fitting a shallow trap to the upstream one so the connecting pipe would have a fall. Shallow traps, where the axis of the centre of the horizontal outlet is at the height of the face of the union connector for the trap rather than at least an inch or so below it, only seem to be available for the larger bath/kitchen sink wastes, and not for the smaller 1 1/4" handbasin wastes. If you have the larger diameter waste, it might be possible to fit a shallow trap. 

 

However, looking at the latest (September) Toolstation catalogue, they do also sell a P trap that does appear to have the horizontal outlet at a similar height to the shallow bath trap, and that is available in both sizes. (VIVA Swivel P trap 75mm seal, code 67720 for small @£4.24, code 76185 for large @ £5.88), and this does appear to have more scope for  adjustment than a straight bath trap. Toolstation (and Screwfix)  are good at returning unused stuff for a refund if it doesn't fit. 

 

Edited September 27, 2025 by Ronaldo47
Typos

[blackrose]

2 hours ago, Sid Charles said:

 so would it just mean I have a waste pipe full of water nearly all the time, what about me fitting a non return check valve?

 

There's no problem having a waste pipe full of water other than it tends to allow any particulates in the water to settle in the pipe which means you need to clean it out more often using a springy pipe cleaner.

 

Don't fit a NRV, that will just make matters worse.

[Sid Charles]

15 hours ago, Russ T said:

Raise the sink

Pump the water.

 

As far as I remember, the regulations state a skin fitting min height above the water line, not below the gunnels.

Or, find a shallower sink.

I can't find anything on the BSS sheets stating how many inches above water level a skin fitting should be, I did once know but have forgotten, is it 7 inches? I should know because when I put 2 new holes through the hull for bilge pump exits, I raised them to just below the rubbing strake, does anyone  know the correct safe distance ?

[Wafi]

Mine has a pump; works very well, the only issue is that it's operated by a button on the floor, which gives me a hell of a surprise when I tread on it accidentally. If it needed replacing I'd go for one of the automatic Whale pumps with a separate yellow manifold. I used one to replace the manky sump box in the bathroom, and it works perfectly.

[Russ T]

14 minutes ago, Sid Charles said:

I can't find anything on the BSS sheets stating how many inches above water level a skin fitting should be, I did once know but have forgotten, is it 7 inches? I should know because when I put 2 new holes through the hull for bilge pump exits, I raised them to just below the rubbing strake, does anyone  know the correct safe distance ?

I think my last surveyor suggested a figure of 10" above the waterline, its not part of the BSS . My advice above was a bit misleading though. See @BEngo response below my one. 

[David Mack]

29 minutes ago, Sid Charles said:

I can't find anything on the BSS sheets stating how many inches above water level a skin fitting should be, I did once know but have forgotten, is it 7 inches? I should know because when I put 2 new holes through the hull for bilge pump exits, I raised them to just below the rubbing strake, does anyone  know the correct safe distance ?

There is no BSS requirement for private boats. For hire boats the BSS requires a minimum downflooding height of 250mm above normal water level.

Edited September 28, 2025 by David Mack

[ditchcrawler]

On 27/09/2025 at 16:38, Sid Charles said:

I am a little concerned about my galley sink water exit from the boat, having restored the boat in full from the wreck I purchased, I have located my kitchen sink close to the original skin fitting hole, I put a new fitting in, and it complies with the regs regarding where below the gunnels to hole sits, but now that I am fitting a sink I find that the hole in the side of the boat is slightly higher than the sink drain, so water cannot go uphill and I am wondering if this is something others have discovered.

First question I would ask is which regulation. Id this RCR compliance on a refit 

[Momac]

I don't know why people are particularly concerned about the height of a skin fitting above the water line.

A skin fitting for a sink waste or bilge pump or shower sump pump  could be just above water line and that would be fine if the waste hose  inside the boat rises upwards inside the boat to a suitable height.

 

 

 

 

 

[Alan de Enfield]

35 minutes ago, Momac said:

........don't know why people are particularly concerned about the height of a skin fitting above the water line.

  

My reading of the OP is that the hull fitting is higher than the sink - water will not run uphill and so he cannot empty the sink.

 

(Hence the suggestions he raises the sink or makes a new (lower) hull fitting.

[Momac]

Just now, Alan de Enfield said:

  

My reading of the OP is that the hull fitting is higher than the sink - water will not run uphill and so he cannot empty the sink.

 

(Hence the suggestions he raises the sink or makes a new (lower) hull fitting.

Yes I know but folks are drifting off and misunderstanding the requirements for the height of skin fittings above the waterline.

 

[ditchcrawler]

Well I am glad my skin fittings are high as in 25 years I have had two share off, Doing it again I would have welded guards on the leading edge 

[Alan de Enfield]

19 minutes ago, Momac said:

Yes I know but folks are drifting off and misunderstanding the requirements for the height of skin fittings above the waterline.

 

 

There are no requirements in the BSS (for private boats) just recommendations - I have several skin-fiittings that are actually below the water line.

[Tony Brooks]

41 minutes ago, Alan de Enfield said:

 

 

There are no requirements in the BSS (for private boats) just recommendations - I have several skin-fiittings that are actually below the water line.

 

Strictly true, but I am sure that your boats conform to the RCD down flooding stipulations, in the way shown by Momac's image. Otherwise, I think that you have very hooky RCD documents - which I do not believe. However, I don't think your post is particularly helpful to those trying to understand what is required. In any case, this topic is about a misinterpretation of what the BSS show and the hull fitting being too high for gravity draining. I would lay odd, without having seen the boat, that the OP can lower the hull drain and still meet the down flooding height if that is what he wants to do. Just like your boats.

[ditchcrawler]

48 minutes ago, Alan de Enfield said:

 

 

There are no requirements in the BSS (for private boats) just recommendations - I have several skin-fiittings that are actually below the water line.

Which is why I asked which regulation some time ago 

 

[Alan de Enfield]

42 minutes ago, Tony Brooks said:

Strictly true, but I am sure that your boats conform to the RCD down flooding stipulations,

 

No they don't, but they don't need to - (they also passed several BSS examinations) in that they have seacocks fitted.

[Tony Brooks]

10 minutes ago, Alan de Enfield said:

 

No they don't, but they don't need to - (they also passed several BSS examinations) in that they have seacocks fitted.

 

Even if the boat was a hire boat, which it is not, then as far as I can see the hull opening height above the water lien still does not apply as long as the down flooding height is maintained by the pipework inside the boat - as has already been explained to the OP.