Fitting a holding tank

[Martin_J]

Wow.... Looks pretty indestructible and easy to get to all parts of it...

I guess the thing that worried me was adding extra connections that are effectively below water line... In your case though, even if a big leak happens and sea water comes in, it's in its own compartment with sides above water line so double safe...

In the s38, the 'at the bottom of the companionway steps' location for the big flat tank is effectively open to all the under floor area... Good pictures though and yet another suggestion!

 

[knuterikt]

Wow.... Looks pretty indestructible and easy to get to all parts of it...

I guess the thing that worried me was adding extra connections that are effectively below water line.

In any case the only one part of the pipework that will let water in and that i the seacock/throughull?
That will be the same for all installations.
In my gravity install the seacock is opened to empty.
In this nice installation http://www.ybw.com/forums/showthread.php?493990-Fitting-a-holding-tank&p=6321852#post6321852 I would open seacock before pumpout and close after.
Don't think there is much difference in risk of seawater leaking?

 

[Tranona]

The potential downsides of a pumped system are first cost, second length of sewage filled pupe, third, number of connections, fourth. need for pump ans or macerator. Advantages are easier to provide bypass and might be able to use some dead space.

Gravity tank is much simpler (and cheaper) and usually has only two additional connections but arguably is more prone to blockages. However this can be minimised with careful management. difficulty for existing boats is that there is rarely room to fit one.

 

[Arcady]

Gravity tank is much simpler (and cheaper) and usually has only two additional connections but arguably is more prone to blockages. However this can be minimised with careful management. difficulty for existing boats is that there is rarely room to fit one.

The TekTank gravity tank (http://tektankslimited.com/series-a-vertical-waste-tanks-455-c.asp) has the advantage of the pump out fitting directly above the bottom discharge. With a vertical pipe to the deck fitting, that allows one to rod through the bottom discharge fitting if needs be, from the deck. So far I have not found that to be necessary, despite a final 90 deg bend in my outlet pipe.

 

[BabaYaga]

Gravity tank is much simpler (and cheaper) and usually has only two additional connections but arguably is more prone to blockages. However this can be minimised with careful management.

To which might be added the problems with run back into the toilet bowl and need for frequent cleaning of joker/clack valves.
However, the tendency to blockage and requirement to be able to 'rod out' is the more important reason for considering alternatives, in my view.

 

[Tranona]

The TekTank gravity tank (http://tektankslimited.com/series-a-vertical-waste-tanks-455-c.asp) has the advantage of the pump out fitting directly above the bottom discharge. With a vertical pipe to the deck fitting, that allows one to rod through the bottom discharge fitting if needs be, from the deck. So far I have not found that to be necessary, despite a final 90 deg bend in my outlet pipe.

Yes, that is how they made the custom tank I fitted into an old wooden boat. That is one of the advantages of having a custom tank, you can configure it to meet your exact requirements. Same thing with the tank I had made in Corfu by the same process for a Bavaria 37.

 

[Tranona]

To which might be added the problems with run back into the toilet bowl and need for frequent cleaning of joker/clack valves.
However, the tendency to blockage and requirement to be able to 'rod out' is the more important reason for considering alternatives, in my view.

No reason you should ever get any flowback on a well designed tank unless you fill it to overflowing. That is just bad management of the system. Rodding is not an issue if you design the tank and installation as described above.

On the Bav 37 referred to above the factory system had a tank the other side of the boat from the loo. Imagine having nearly 8m of sewage filled 1 1/2" pipe running through your boat to deal with if you ever have a blockage. Guess who did not order that option but fitted a gravity tank. Fortunately newer boats like my 33 have a simple accessible gravity tank with an inspection port for rodding.

 

[BabaYaga]

No reason you should ever get any flowback on a well designed tank unless you fill it to overflowing.

I don't think the risk of flow back has so much to do with the design of the tank, more with the location of it. To me it seems likely that the risk increases if there is a metre or more of vertical pipe filled with water pushing on the valve as compared to the pipe leading off more or less horizontally.

That is just bad management of the system. Rodding is not an issue if you design the tank and installation as described above.

Don't know which description you are referring to, but the often mentioned importance of locating the deck fitting in relation to the tank discharge pipe (to facilitate rodding) suggests to me that there is a something that is not quite right with the entire concept.

Not say that a gravity tank cannot function well, I'm sure it can, if managed properly.

 

[Tranona]

Not say that a gravity tank cannot function well, I'm sure it can, if managed properly.

The features i described are designed to achieve that - plus users ensuring that the tank is emptied before the level reaches the top and flows back down the pipe. You are right that the vertical pipe (whether it be external or an internal stack pipe) will contain fluid, but it should not be waste, but clean seawater. This is true of any pumped toilet and there is no reason the joker valve should let fluid flow back if it is well maintained.

The potential for blockages is little different in a pumped tank, just that the pump or macerator is perhaps better able to shift it than gravity. however if the blockage does need clearing manually i would rather do it manually through rodding than dismantle a pumped system to find out first where it is and then deal with all the mess.