Hydrostatic conundra

[bobnewbury]

Have just been away for two weeks of torrential rain & high winds, so no change there.

Observed an interesting little phenomenon in the heads this year. We have a lavac installed on the starboard side with the top of the bowl just about on the waterline. I plumbed in a holding tank with a fiendishly complicated set of diverters and loops last year and it worked perfectly, much to my surprise.

This year, however, we ended up being starboard side to pontoons in a heavy blow on several occasions. Under these conditions the bowl kept filling. On close observation (the joys of boating) it appears that each time the boat rolls to starboard a small amount of water enters the bowl from the inlet pipe.

The inlet seacock is on the port side about 70 cm below the waterline. As it's a Lavac I decided that I couldn't use an anti-siphon loop in the inlet, but I fed it through a loop that reached about 1 M above the waterline at its highest point (highest I could get).

What I suspect is happening is that the wind induced water pressure on the port side is pushing water up the inlet pipe and the rocking of the boat is sufficient to slosh a small amount over the top of the loop each time.

What confuses me though, is why doesn't the bowl overflow, even if this has been left to happen overnight? If I pump out the bowl with the lid up then it starts to fill with about a cupfull every 5 minutes, but it still never overflows.

Any ideas?

<hr width=100% size=1>

 

[Colin_S]

I would guess the reason it doesn't overflow is that, once the relative water levels have equalised the water stops flowing.

Could the water flow be caused by a syphon effect? You say you've got a loop about 1M high. If that is filled with water, water would flow from the highest water level, that being the outside, until the water on the inside reaches the same level. Don't know if that will make sense and, never having looked at the workings of a sea loo, may be totally incorrect.

I guess you either need an anti syphon device or the feed pipe to the loo needs to let air into the loop to stop the syphon effect.

<hr width=100% size=1>

 

[snowleopard]

the problem with anti-syphon valves on the inlet side is that they are designed to let air in to break the syphon. that means that when the pump sucks, it gets air from the valve rather than water from the seacock so defeating the exercise.

maybe you should just shut the seacock after use - the simplest solutions are usually best!

<hr width=100% size=1>

 

[PaulJ]

I have recently fitted a Lavac but I'm afraid I haven't tried it yet. Oddly enough you are SUPPOSED to have an anti-syphon valve in the inlet pipe. It states it clearly in the installation instructions but like you I thought it would interfere with the working of the system so I checked with Blakes-Lavac-Taylors who supplied it and they said that the hole is so small compared with the volume of the pump that it makes no discernible difference to the suction. They actually supply TWO different "valves" with the kit, one has a larger hole than the other and they say that the size of hole will determine the level of the water in the bowl....

If you want to check with them they are on 01489 580580.

HTH,
Paul.

<hr width=100% size=1>I used to have an open mind but my brains kept falling out.....

 

[jerryat]

Hi Paul,

I too have a Lavac and I strongly recommend you insert one of the anti-syphon plugs in the top of the loop for two reasons: first, the tiny hole will prevent the water syphoning back into the bowl and possibly flooding the boat and second, you need to try both sizes (as you say, they have tiny holes) because it is these that determine the amount of water that returns to the bowl after pumping. The larger the hole, the smaller the amount of water, and we use the larger so the water never slops out of the bowl onto the floor in bouncy weather. The smaller hole allows around a third full bowl.

As you probably know, it's not rocket science to fit, just drill a hole (near the OD of the plug) in the top of the plastic pipe at it's highest point - then push it in. Doesn't need mastic or anything like that.

Hope this helps

Cheers

Jerry

<hr width=100% size=1>

 

[ex-Gladys]

Just a note, the Lavac valves are not.... They are purely a plug with a hole in them to prevent siphoning

<hr width=100% size=1>Larry Botheras

Anderson 26 "Amber"

 

[jerryat]

Hi Botheras,

As I stated in my post!

Cheers

Jerry

<hr width=100% size=1>

 

[PaulJ]

Further to my last posting, there is also a website www.blakes-lavac-taylors.co.uk which gives full details and off which you can download the owners manual. On page 13 of the manual is a diagram which clearly shows the "Air Inlet Valve".....

<hr width=100% size=1>I used to have an open mind but my brains kept falling out.....

 

[jerryat]

Hi Paul,

Haven't checked my manual (or looked on their website) but I suspect the 'proper' anti-syphon valve is probably recommended/required where the bowl is sited below the waterline.

Could be wrong of course!

Cheers

Jerry

<hr width=100% size=1>

 

[jerryat]

Hi Paul,

Haven't checked my manual (or looked on their website) but isn't the 'proper' anti-syphon valve recommended/required where the bowl is sited below the waterline.

Could be wrong of course!

Cheers

Jerry

<hr width=100% size=1>