Increasing the capacity of your water tank

[Jambo]

The particulars from the sale of our boat to the previous owner suggested that the water tank had a capacity of 250 gallons. I'd long been suspicious of this because with two of us on board we were getting through the tank in about 9 days which would imply a consumption of something like 120L per day which is clearly way more than we use.

 

I just got round to measuring the flow rate of the tap and timed the refill from empty and it works out that our tank actually has a capacity of 100 gallons which I believe is pretty small for a fairly large widebeam. This comes out to about 50L per day which for two people sounds much closer to the mark (although I'm sure we could get this down a bit).

 

It would be very useful if our water tank lasted two weeks so that we didn't have to move every week to refill so I was wondering how feasible it might be to add tank capacity. The tank is located under the foredeck with the filler just in front of the foreward cabin bulkhead on the port side. I was thinking we could potentially sit an additional tank on the deck over the existing tank. Ideally it would be directly connected to the filler of the existing tank so that we fill through the extra tank straight into the main tank. We could construct then bench seating to cover it.

 

Has anyone heard of something like this being done before?

 

Also out of a matter of interest, how much water do others use per day?

 

[GUMPY]

50l a day is very little

We use just over 100l a day between us and all we do is shower daily and wash up.

On the barge when we had the washer/dryer we added another tank in a cupboard with its own filler

[springy]

100 gallons does seem small, you say its under the foredeck - i presume its a separate tank as opposed to an integral

structure - I would have expected there to be room for a far bigger tank under a widebeam foredeck - is there something else

under there or is there room for a bigger or additional tank under the foredeck.

 

If you add a tank on the foredeck remember that you will increase the "head" slightly and any fittings - particularly

the link between the additional tank and existing tank will have to cope with this. - a leak in the top of the existing tank

(what is it made of and does it have an inspection hole?) - would result in the contents of the additional tank escaping

into the bilge.

 

springy

Edited January 14, 2014 by springy

[BEngo]

What you propose is feasible but there a couple of things to think about.

 

100 gallons is 16 cubic feet in round numbers. Can you afford to lose that much space?

 

Water is heavy- the extra 500Kg or so of water will affect the longitudinal trim and when the tank is full may do things like submerge the gas locker drains, or cause prop ventilation and lack of GO in the propulsive area. Both are easily cured, at a cost, but best done when the new tank is being fitted.

 

There will be a bigger trim change from full to empty - this might mean the GO is poor when nose up because the tank is empty

 

Your new tank will have to either be spilt Port and Starboard or lie across the centreline of the boat so as not to cause a list.

 

The extra free surface is likely to make the boat more tender but with the likely tank size, and in a widebeam, I'm not sure this matters.

 

N

[mattlad]

Prop ventilation? 8-/

[Mike Tee]

Can you actually get to the existing tank to measure it? That would give a much more accurate idea of size in gallons - measuring from a tap flow can't be that accurate. 100 galls does seem small for a widebeam, I had a 250 gallon tank on a narrowboat and it didn't affect the trim or handling to any noticeable

level (no pun intended!).

If you can measure it, conversion is 6.2 gallons to 1 cu ft.

[BEngo]

Prop ventilation? 8-/

Erroneously called cavitation- by some. It's when the blade draws in air as well as water, because it is not immersed deeply enough. There is then a significant loss of thrust.

 

N

[Jambo]

Thanks for all the comments.

 

I need to have a good look at how the tank is integrated and will get back to you.

 

100 gallons is what we already have (I think... experimental sample size n = 1 and more research is definitely needed Mike Tee!) and I was thinking a further 250L (50ish gallons) is what we would need to see us through two weeks.

 

With some quick googling I found a 250L plastic tank which measures 0.95m by 0.5m by 0.6m which should fit and would be about the right size for a bench I think.

 

We already have a bunch of massive semi circular weights on the starboard side of the foredeck to counterbalance our generator which is located on the port side of the engine room. So I could possibly reposition some of the weights to the starboard side of the engine room to deal with the longitudinal trim change and add a bit more there to address the lateral trim deviation which 250kg of water would induce. The difference in trim between full and empty would certainly be more pronounced than it is currently but given the size of our boat (65ft x 11ft) I would hope it would be within limits?

 

The integrity of the connections is definitely important. I guess that's why I wanted to know if anyone's done this before - I don't particularly want to be the first to stick 250kg of water right by the entrance to the cabin and then spring a leak!

[springy]

With a tank like that you could just have a tap and hose and use it to top up the internal tank after a week or so - It certainly would

make a good interim solution as well as a bench !

 

springy

[Jambo]

That might well be easiest.

 

Will have a think about this as it would be very nice not to have to move every week if we don't want to.

 

Thanks all, you're as helpful as ever!

[mattlad]

Erroneously called cavitation- by some. It's when the blade draws in air as well as water, because it is not immersed deeply enough. There is then a significant loss of thrust.

 

N

I'm with you. Thanks!

[Jambo]

Recalling the Tom Clancy novels I read in my youth.... isn't cavitation when you get bubbles forming around the prop due to sharp pressure changes e.g. when you slam it into reverse? I think you can feel it when you do an emergency stop. The collapsing bubbles cause a lot of noise for submarines.

[Ray T]

 

With some quick googling I found a 250L plastic tank which measures 0.95m by 0.5m by 0.6m which should fit and would be about the right size for a bench I think.

 

Jambo we have done similar to your suggestion, although we fitted our auxiliary tank in the front cratch locker.

 

It was relatively simple as our main water tank is in the bow where in many boats the gas bottles are.

Water automatically fills from the main tank. After experimenting with a Schraeder valve, which didn't work I managed to get a central heating pressure relief valve which solved the problem of venting air from the tank when filling. I did have to take the valve apart and fit a softer spring though.

 

I have fitted stop cocks on the both inlet and outlet on this tank.

 

Edited January 15, 2014 by Ray T

[mattlad]

Recalling the Tom Clancy novels I read in my youth.... isn't cavitation when you get bubbles forming around the prop due to sharp pressure changes e.g. when you slam it into reverse? I think you can feel it when you do an emergency stop. The collapsing bubbles cause a lot of noise for submarines.

 

That's correct. It does not necessarily have to be when going hard astern, it can happen when going ahead as well. However prop ventilation is when the propeller starts drawing in air as well as water. This can be either by accident or by design.

 

How did we get from water tanks to prop cavitation / ventilation?

Edited January 15, 2014 by mattlad

[Mukiwa]

I added a 50 gallon auxillary tank some years ago on Sarah Kate. it was connected to the main tank and had a balancing pipe. Filling took place at the main tank and the take off was also on the main tank. the main effect I noticed that it was slow to fill the last few gallons as the air was not able to escape easily.

There was no discernable difference to trim or boat handling. The tank was placed so that the weight was evenly distributed from port to starboard and it had internal baffles to diminish the effect of "sloshing".

Hope this helps.

[Jambo]

Interesting Ray T. So if I understood you correctly you fill the main tank and then water passes to the auxiliary tank and then in to the boat? Or do you have to turn the stopcock in the auxiliary tank once the main tank is dry?

 

Thanks Mukiwa. Good to hear that trim wasn't an issue. In our case it would be located to port so we'll have to add something but I suspect given our size it won't be too big a problem.

[Ray T]

Interesting Ray T. So if I understood you correctly you fill the main tank and then water passes to the auxiliary tank and then in to the boat? Or do you have to turn the stopcock in the auxiliary tank once the main tank is dry?

 

 

Yes the water passes from the main tank (in the bow) into the auxiliary tank due to gravity and water levels, then into the shower, sinks loo or whatever when the pump is used. When the levels in the two tanks are equal the water transfer stops of course. I then know that the bow tank is only a 1/4 full.

 

I have also fitted a loo type ball valve in the auxilliary tank to shut the water flow off when the aux tank is full.

 

I then the shut the pressure relief valve in the aux tank, and the vacuum effect formed in the auxiliary tank, when the water pump is used, sucks the water from the front tank.

 

The stop cocks are there so I can isolate either tank if necessary.

 

It is actually quite useful to isolate the auxiliary tank from the main one as it gives us a visual indication of the amount of water we use per day.

Edited January 16, 2014 by Ray T

[Jambo]

Well we bought a 125L tank (this one) and it's working well. With careful usage we can make our water supply last two weeks now which has made life *a lot* easier.

 

We wanted a slightly bigger one (about 200L was our aim) but the dimensions just didn't quite fit with our well deck. This one tucks under the gunwales so it doesn't take up much space. It's chocked up on some wood and the outlet is right by the inlet for the main tank so a short length of hose for filling works well.

 

I've covered it with a bike cover to keep the light off it but I'd like to build some sort of bench arrangement to cover it and provide seating (although the tank seems sturdy enough to sit on).

[Paul's Nulife4-2]

Consider fitting a Watermaker, They take up Much less space than an extra Tank, They Weigh Much less, and Produce as much water as you need, I've had mine 6 years now,As a livaboard with Dishwasher, Washing Machine 2 Adults Dailey Showers and a non stop boiling Kettle. it's Obviously in Daily use, makes anywhere around 30 Gallons a day,depending on the state of the water sources,and I have never had any trouble with it since I fitted it in 2007/2008.

I have to be honest,I don't know the make of mine,it's American,but it's permanently plumbed in via it's own Sea cock,got a Strainer,Then 4 progressively finer Filters down to 5 micron. Then a Charcoal Filter,then a 2 micron Filter then a Reverse Osmosis Membrane, then a Food Grade Ultraviolet Light Zapper. I do Monthly Water Tests via a kit,and it's been a fit & forget bit of kit for me. I wouldn't be without one again.

I'm thinking of replacing mine to a newer well known make,that I can get Spares for easier this year.

Edited March 30, 2014 by Paul's Nulife4-2

[RLWP]

Are you sure your existing tank is filling properly? If the vent is in the wrong place it may be half full of air

 

Richard

[Jambo]

Interesting idea. It's quite difficult to see but there might be a slight tilt upwards in the direction of the bow and the filler is at the rear left of the tank. It's quite modest though so it doesn't look like it would make a huge difference but it probably is doing something. I don't think there's much we'd be able to do about it without major work though so the extra tank will have to do for now.