Prestige 36 - Fresh Water Tanks

[hpwhite]

I wonder if anyone can answer whether the two fresh water tanks starboard & port are self levelling with each other and are joined by pipe work to allow them to fill/drain equally? The fact they require independent filling ports would indicate they are completely separate. If so, which one drains first? as this does effect the boat slightly listing over to the port side on the plane!

[rene460]

Hi hp,

Most likely both tanks feed the one water pump and are thus joined somewhere in the line to the pump suction.

When pumping, the flow will tend to favour the tank with the shorter total line length (including an equivalent of about 13 times the pipe diameter for each elbow fitting), but as soon as you turn off the tap, the pump stops after the few seconds it takes to re pressurise the accumulator.  Then so long as the boat is level the tanks will tend to equalise.

If the boat is slightly off level for some other reason, for example batteries, fuel tanks or holding tanks off centre, or passenger or luggage loading, water will tend to go to the low one, which of course increases the unbalance.

The tank lines should each have a shut off valve somewhere along their length.  This allows you to isolate the tanks and use one at a time and gives clear notice when one is empty.  

You you can get an idea of what weight is causing your list by a simple inclining test.  Rig a plumb bob on centre line and see how much it moves when you shift your body weight from one side of the boat to the other.  If you suspect water moving, water is 1 kg per litre.  But water tanks and fuel tanks empty, fuel tanks fill, food is consumed so if you want to stay really level you will always have adjustments to make.

i just noticed that you said you were listing when on the plane.  Do you have adjustable trim tabs that can level everything up.  Remember with the engines working hard you have a significant engine torque tending to cause a list that is not present when stationary.  Unless twin engines counter rotate.

Rene460

 

[hpwhite]

Hi Rene460,

Thank you for your information which is a huge help.
In answer to your question regarding adjustable trim tabs, unfortunately my boat doesn't have them, although my last one did and they were a total pain to get right as there were too many changing variables with every trip!

The only strange evidence to the theory of even drain is that I noticed with the boat moored up and both tanks completely full, I used a reasonable amount of water on board over a few days and then attempted to top the tanks back up.
The port side tank needed hardly if any water to top up, but the starboard tank took 5/6 mins to fill back up with a fast hose?

This might indicate an uneven draw from the starboard tank first?

I'm sure you must be right as the manufacturer wouldn't allow such weight imbalance from one full and one empty tank of this size!

Thanks again, Howard.

[rene460]

Hi Howard,

A difference in line length or an extra fitting or two would not cause the difference you are noticing.

I had had one further thought that might be relevant.  An air bubble in a line can be equivalent to a blockage in a gravity line as the flow has a reduced cross section, much the same as a smaller diameter section of line.  Many of us have noticed this effect in our sink drain line.  It would make it very difficult to balance the flow in two lines to a single pump suction.

Check that both lines are as near as possible to running flat, or an even slope all the way from the tank to the pump inlet with no high points.  If the line from your port side tank runs over a beam or other obstruction before the two lines join, the resulting air bubble would cause a big difference in the resistance of each line and could explain the symptoms.  A low point does not matter if the line slopes up continuously each way from the low point.

If you suspect an air bubble in the line from the tank that does not empty, try isolating the other one while you pump a a bit of water to see if you can purge the air through, then you can open to both tanks again, though it may be more satisfactory to draw from one tank at a time.

rene460

[napoleon]

Jan 7, 2014 17:45:27 GMT hpwhite said:

I wonder if anyone can answer whether the two fresh water tanks starboard & port are self levelling with each other and are joined by pipe work to allow them to fill/drain equally? The fact they require independent filling ports would indicate they are completely separate. If so, which one drains first? as this does effect the boat slightly listing over to the port side on the plane!

Hi HP,
Been away for a while so only just read your post.

The tanks are not self levelling but they do have independent on/off valves located on the inner bulkhead adjacent to the hot water cylinder.
The only problem is that they are a little difficult to see properly as to if they are on or off as the control knobs are only slightly elliptical and at first glance appear to be circular. They are open when the greatest diameter is vertical and off when horizontal. I made labels for each valve and stuck them close to each unit.
If both valves are left open then the water usage is pretty much even and we have never had a problem with too much on one side or the other.
Regards,
Napoleon