Q: black water tank outlet - above or below WL?

Fairline fit swan necks where the black tank is below w'line but I always de-spec this and fit an electric seacock instead. My reasoning is that I want the absolute minimum metre-age of shiite-lined pipe on the entire boat, and eliminating the swan neck can save several metres. The electric seacock stops any back-filling with seawater

I have the outlets below the w'line. No scientific reason; i just prefer them there and don't want stinky pipes to spoil my daily swim around the boat when I clean the waterline
 
Wouldn't fancy swimming near the outlet, a gently roll of the boat and the chances are that something held in the swan neck is going to come out. Mine are under the boat with a stop cock.
 
KevB said:
Wouldn't fancy swimming near the outlet, a gently roll of the boat and the chances are that something held in the swan neck is going to come out. Mine are under the boat with a stop cock.
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Chances are that the boat will have rolled much more underway after the pump have run well offshore, than it will do if you are at anchor..would be same if outlet was above or under water by the way and of course you would not go swimming after just pumped even off-shore... and if you want to prevent accidental happenings, you could always close the valve ...
 
Deleted User said:
Definitely above the waterline as far as I'm concerned for a number of reasons. .......................and third, you can observe the flow ...........
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That sounds delightful ! The portapotti on my cuddy boat sounds quite civilized in comparison, but maybe not as "entertaining" !
 
A couple of further thoughts. Vas, you have a diaphragm pump right? Those dont half squirt on the downstroke. Are you going to fit a baffle to stop a visible jet of blackwater, or what? Sounds pretty awful... My vote remains in favour of underwater discharge!

The other thing is, if you fit a seacock, I bet you a pint that sometime in the first season a helpful crew person will run the pump with the valve shut. Then you have to get the rubber gloves on and fix the inverted duckbill valve, yuk, and usually with a full ish tank. However, if you fit an electric seacock you get a +24v signal switched by valve shut and another switched by valve open. You can use the latter to power a relay coil, with the relay contacts interrupting the pump 24v motor circuit. The pump is then disabled till the valve is open. Then you NEVER have to fix inverted duckbill valves. This is what I have done on my last 3 boats and I swear by this set up. An alternative is a manual seacock with an inbuilt open/shut microswitch
 
On my Bayliner, I had the outlet mounted at the bottom of the swim platform which is around 2 inches above the normal waterline, close to the transom and pumping "into" the water. It means one cannot see it, the output ends up under the surface so does not leave a whiff behind, there are no unexpected surprises when swimming (then again, we tend to pump out well offshore and at speed) but one has above-waterline safety.
 
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My system is Vetus and exit below waterline.The pong still can be detected when discharging if moving at low speeds. :)
 
KevB said:
Wouldn't fancy swimming near the outlet, a gently roll of the boat and the chances are that something held in the swan neck is going to come out. Mine are under the boat with a stop cock.
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I regularly swim around the back end of my boat cleaning the exhaust soot off and I never see or smell anything from the blackwater outlet. Actually, apparently a much greater danger when swimming at the back of your boat is the generator exhaust causing a build up of carbon monoxide under the bathing platform http://www.boatingsidekicks.com/cobro/cobrochure.htm so I do keep away from the generator exhaust outlet when swimming and I advise my guests to also
 
it appears that all Italian makes have the outlet above waterline,
on BA, on the transom, below the platform, 20cm above waterline,
all in one row, bilge pump-, second bilge pump-, airco-, water maker-,blacktank -, grey tank- outlets,

in all my other boats the oulet was below WL

one small advantage below waterline;
every now and then, fe when stucked in a marina for bad weather, or in winter, you have to dump in the marina,
(yes I plead guilty)
when below WL you can do it more discrete,

so on BA, when I need to, I do it at evenings
 
BartW said:
one small advantage below waterline;
every now and then, fe when stucked in a marina for bad weather, or in winter, you have to dump in the marina,
(yes I plead guilty)
when below WL you can do it more discrete,
Click to expand...
I was too polite to mention that;)
 
thank you all,

as always a wealth of information to consider.

after careful checks, it's simply impossible to have an elegant, short length, unobtrusive solution with above waterline exit! Outer bunk is too close, the open cabinet under it is not sized right for the extra hole of the seacock lever, so effectively it's a no go :(

blackwatertank_1.jpg


blackwatertank_3.jpg



Then as I was taking a couple of pics to post here I realised that as the tank sits on the frames above the keel and is 150mm offcentre to port I can mount the pump (as planned) on top of the tank and lead a very short hose to the stbrd side of the hull and fit a seacock there (by the keel obviously underwater)
This has the following pros and cons (comments appreciated!)

PROS
v.short run
easy access to seacock by the corridor hatch
could even start the hose with a gooseneck just above the pump and under the bed (there's enough space)

CONS
seacock will be 400mm ahead (as in towards the bow) of the aircon intake and 800mm ahead of the aircon outlet.
seacock in direct line with genny and stbrd engine water intake...


Means that:
A. I better not run the aircon when I empty the tank :rolleyes:
B. when motoring and emptying, some of the black tank contents may end up "cooling" or soiling... the stbrd engine :D

JFM, 1% of the craft value is a bit too steep for a 1.5inch electrovalve!
But, thanks for the idea, I can add a microswitch on the valve (unless I can get one from NZ with the switch embedded as the local dealer hasn't imported any of the microswitched valves) and route it to the BMS. The black water pump is operated via the BMS, so a dead simple if routine to check the switch is on before closing the relay ;)

Mike, regarding the venting pipe I'm still thinking where I'm going to place the outlet... As you say, got to be far away from cabins, but then it's going to be aft towards the salon windows or even worse aft deck :( Good compromise seems to be just aft of the cabins at helm level.
However I need to have decent access to swap the filter (I've got one, will buy more...) and I've no idea if it's best to have the filter towards the outlet or closer to the tank. I guess it wont make any difference, but good to get a confirmation. I have decent access halfway through, so might do it like that.

cheers

V.
 
jfm said:
The other thing is, if you fit a seacock, I bet you a pint that sometime in the first season a helpful crew person will run the pump with the valve shut. Then you have to get the rubber gloves on and fix the inverted duckbill valve, yuk, and usually with a full ish tank.
Click to expand...

John,

just dismantled fully one of the pumps to find the non-return valves.

I have one at the outlet that looks like the ones fitted on my small gulpers and an odd, twisted, wraped/squashed/whatever on the suction side. Is that the one you call duckbill?

I found a repair kit on sale for my Rheinstrom M50E RG pump that has the following bits (from GER-EN translation)

0350009 Membrane 1
0350012 Flap valve 2
0350011 Seal 1
0350008 Suction valve 1

there are no pics in the site so I've no idea which one is the flap and which one is the suction. Can you help?
Admittedly not much of an issue as I have to order a couple of these kits anyway, but would be nice to know what I have at hand :D

both (outlet on the left, inlet on the right)
rheinstrom_valves_1.jpg


rear side of the inlet one, all nicely twisted and I guess non functional!
rheinstrom_valves_2.jpg


cheers

V.
 
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