Question about sea toilet and siphoning

[Cspirit]

As part of major refit on my new-to-me LM27, I have fitted an new Jabsco toilet and a holding tank. Both the toilet and the holding tank are above the waterline. The inlet to the toilet has a secondary ball valve by the toilet as a precaution. The discharge hose from the toilet rises further above the waterline then goes to the holding tank. The system is ‘straight through’ I.e. when the toilet seacock is open everything is discharged to sea. When the cock is closed discharge remains in the holding tank and is then emptied by gravity when well offsshore. The holding tank has a 25mm breather vented to the hulll just below deck level.

My question is this. The belt and braces side of my brain tells me that I should install a gate valve on the discharge side of the toilet in the unlikely event of any siphoning. (I have not installed a vented loop in the discharge pipe because there’s no room). However, since the holding tank has a vent, it seem to me that this seacock would be unnecessary since this vent will prevent siphoning in the discharge system.

Is this correct?

Bob

 

[vyv_cox]

My arrangement is similar to yours. The discharge from the toilet goes via a dip pipe to the bottom of the tank. With the boat level seawater just wets the bottom of the tank. When heeled to port (in my case) seawater will partly fill the tank and cover the end of the dip pipe, so the fact that the tank is vented is immaterial.

In my case the discharge loop reaches beneath the deck, thus the boat would need to be heeled to an extreme angle to commence a siphon, a risk I am prepared to take.

Gate valves should not be fitted to a boat. They tend to fail closed when the pin driving the gate fails by corrosion. You could fit a NRV but that is what the joker valve does.

 

[johnalison]

If I understand it, you don't have a seacock where the discharge pipe passes the hull. If this is the case, regardless of a syphon, there should be a seacock there. If you are just concerned about the loop, I shouldn't worry. My HR is like that and I only rely on the seacock and the vented holding tank. even with the tank not in circuit I have neve had a syphon. A syphon break on the inlet is useful though.

 

[vyv_cox]

If I understand it, you don't have a seacock where the discharge pipe passes the hull. If this is the case, regardless of a syphon, there should be a seacock there. If you are just concerned about the loop, I shouldn't worry. My HR is like that and I only rely on the seacock and the vented holding tank. even with the tank not in circuit I have neve had a syphon. A syphon break on the inlet is useful though.

I think he says there is a seacock at the exit from the tank. He refers to closing it for emptying offshore.

 

[Tranona]

You are correct. The vent in the tank serves the same purpose as the vent in the outlet pipe in a non holding tank system. On the first system I installed I put a ball valve in the outlet from the tank to avoid having waste in the hose between that and the seacock but it really was not necessary and factory installations don't have that. most important thing is to flush enough water through so that you don't have any waste left in the hose from the toilet to the tank.

 

[VicS]

You are correct. The vent in the tank serves the same purpose as the vent in the outlet pipe in a non holding tank system. On the first system I installed I put a ball valve in the outlet from the tank to avoid having waste in the hose between that and the seacock but it really was not necessary and factory installations don't have that. most important thing is to flush enough water through so that you don't have any waste left in the hose from the toilet to the tank.

When installed as the OP describes the Instructions say that you must fit a vented loop in the toilet discharge pipe to ensure that the contents of the tank do not syphon out through the bowl but unless the pipe dips down inside the tank I cannot see how this can happen.

See https://www.sparesmarine.co.uk/_web...duct_Pdfs/Jabsco 29090-3 & 29120-3 Toilet.pdf
Section 5.3, Outlet pipework. Option 3

 

[Tranona]

It says YOU MAY fit a vent. None of the installations I have had were vented in this way.

 

[Cspirit]

Thanks everyone. As I guessed, I should be fine not fitting the extra ball valve.

Bob

 

[LittleSister]

I think you will find that in an LM27 the top of the toilet bowl is not above the waterline, but about level - perhaps very slightly above or slightly below - when the boat is level. It certainly isn't above the waterline when the boat is heeled.

I have an LM27 and when I had a syphon leak (due to worn toilet valve and no anti-syphon fitted) it spilled slightly over the top of the toilet. My LM27 may be lower in the water than yours (oversized engine etc.), and your toilet may perhaps be slightly taller, but there won't be much difference, I believe.

I would be interested to know where you ended up installing your holding tank in the LM27. It is a challenge!

I am uncertain whether the advice above that you should not should not have a problem with siphoning back from the holding tank necessarily holds true if either (a) the tank is full, or (b) the holding tank (or part of it) is above the toilet (which it must be if the whole of the holding tank is above the waterline). I think one would need to know more details about the relative positions of the waste inlet into the tank, the breather outlet from the tank, the waterline both level and heeled, etc. to be sure.

You don't mention an anti-syphon valve on the water intake side. The secondary ball valve you mention between the inlet and the toilet is a not a full substitute for an anti-syphon valve (though the height for that anti-syphon valve is not easy to achieve on the LM27, I know). If you've made a conscious choice to have it that way, then so be it. Note that if fitted the inlet anti-syphon valve should go between the Jabsco's pump and the toilet bowl (in place of the short pipe supplied), not between the hull inlet and the pump.

 

[VicS]

It says YOU MAY fit a vent. .

That is when the toilet discharge elbow is above the tank top. Option 4 in the text. I guess you have just looked at the picture which is incorrectly labelled

 

[johnalison]

I think he says there is a seacock at the exit from the tank. He refers to closing it for emptying offshore.

I suppose so. When he described it as the toilet seacock I imagined it as coming direct from the bowl rather than from the tank. Personally, I find having a Y or T junction with the option of bypassing the tank makes things easier.

 

[LittleSister]

I suppose so. When he described it as the toilet seacock I imagined it as coming direct from the bowl rather than from the tank. Personally, I find having a Y or T junction with the option of bypassing the tank makes things easier.

Finding space for such elaborate pipework and fittings around the heads on an LM27 is the not 'easy' bit!

 

[vyv_cox]

I suppose so. When he described it as the toilet seacock I imagined it as coming direct from the bowl rather than from the tank. Personally, I find having a Y or T junction with the option of bypassing the tank makes things easier.

I would have preferred a Y but utterly impossible in a Sadler 34, on which the whole heads compartment is as small as it could be.

 

[Cspirit]

Many thanks to you all for such useful comments and in particular to Little Sister for thoughts on the LM7.

As Little Sister correctly states, there's not much room in the LM27 heads, and elsewhere for that matter as the designer has crammed an awful lot into the boat and as a consequence, maintenance and adding stuff can be difficult. I managed to fit a small holding tank - 30 litres -in the locker under the sink. I copied this idea from another LM27 owner on the LM owners' website. I have measured numerous times and I'm convinced that the tank and the toilet bowl are both above the waterline. I admit that on port tack this may not be the case but she's not a boat that is sailed on her ear.
When I bought the boat she was largely in her original 1984 state - she is apparently one of the Scanyachts UK built boats I guess from hulls and parts bought from the original builder. She had an RM69 toilet with no anti siphon valves in the heads plumbing and the seller stated that he had experienced no problems. In one season's ownership and sailing before refitting the heads, I found the same.

So, my conclusion on all this is that I will probably be fine without an additional ball valve located near the toilet in the discharge pipe. However, I'm now going to fit one 'just in case' so that in the, I think unlikely, case of a syphon back into the bowl, I can simply close this seacock.

Thanks again all.

Bob

 

[LittleSister]

I believe Scanyachts (who had previously imported LMs to the UK) bought the LM27 moulds after LM had ceased production of them, having moved on to the later LM26/28/30/32 series.

Scanyacht's interior fit-out styling was quite different to LM's, and they offered various options including deeper keel, larger rig, different layouts.