considerations for a water tank design

[hd002e]

water tank design:

After my bladder type water tank burst and filled my bilge with 200L of water as I was preparing for a trip, I decided to build a plywood+epoxy tank. The plastimo bladder type tank had only two fittings, one connected to the deck fitting and the other to the water pump, and there was no vent connection. Which I believe led to its demise as the plastic bladder could not handle the pressure. So for the tank I am building I want to incorporate a vent line.

The tank is about 150L, it has a baffle in the middle. it is on the port side of the boat lengthwise under the settee. I am using the the inner face of the false hull as one side of my tank, the front side is the v-berth bulkhead and the rear side is a partial bulkhead I built about two feet foreward of the galley. The inlet is towards the top of the tank and the outlet is towards the bottom of the tank, where I use delrin thru hull fittings with barbs. I would like to think I have things figured out until this point, however I need some expert advice on where to attach the vent fitting on the tank and how to route it to the thruhull and if I should use antisiphon etc..

I am guessing (and I could definitely be wrong about this) that the vent should be placed on the highest part of the tank where air gets trapped as the water tank fills up. Should I have two vents to make sure I can fill up the tank to its maximum capacity? I am guessing the vent line should be routed straight up, without creating any kind of neck. I am thinking to add a thruhull close to the hull/deck joint (as high as possible) amidshiprs but then I will not be able to add an antisiphon. I can install the vent a bit lower and add an antisiphon (not sure if this is necessary?) or I can route the hose all the way to the back of the boat and let it exit aro und the transom where it it less likely to get dipped underwater. I have also seen boats where this vent line is routed to the galley and that also makes a lot of sense to me. If I use a thruhull, should there be a checkvalve in case the boat heels over and dips the thruhull in the water? Should I add a seacock? So many questions

So, I would love to hear expert advice from you fine folks. If there is a resource (like - considerations for watertank design on sailing vessels) I would be very interested in reading more about this before I start drilling holes in my water tank and in my boat. Thank you!

[hd002e]

[Pete7]

Keep it simple, just put the vent in the highest point you think exists and don't worry about it. A couple of litres of air is neither here nor there. You might add a tank gauge and sender unit. It won't read true towards the bottom of the tank but its nice to be able to just glance at the gauge to see how much water is in it.

The vent can be in the cockpit or saloon topsides if it makes it easier.

[CassidyNZ]

Venting to outside is really done to prevent odours in the boat so diesel and blackwater tanks must vent to outside. But a fresh water tank vent does not have to go out of the boat. Take it up to the highest point inside the boat as close to the centreline as you can and in a place where the odd drops of water won’t really matter. Even at really steep angles of heel, a vent on the centreline will rarely spill any water.

If you are hell-bent on venting to the outside, it is really advisable to take the vents to the transom. Even if a vent on the side is right up at the gunwhale, there is a good chance that salt water will enter your fresh water supply whenever you dip the rail and a syphon break won’t prevent that. It doesn’t take a lot of salt water to turn fresh water brak.

There’s a reason why all production boats vent all tanks to the transom.

[CassidyNZ]

Seriously, I would put more than one baffle in. Half a tank = 75kgs of water which can build up a lot of momentum in the space you’re providing and as an amateur carpenter I know how hard it is to make really strong joints in the edges of plywood boxes

You also need to have at least one large hole (2”) in the bottom of the baffle(s) to allow the water to migrate from one side of the tank to the other. Just allowing water to flow over the top of the baffle may not be that effective.

[hd002e]

Hi Cassidy,

Point well taken of the forces on the baffle. I do not expect any bad odors as this is my fresh water tank and it will be much easier to keep the vent inside the boat. As for the opening in the baffle, I have a large opening in the bottom as you pointed out, it is not easy to see in the photo because of the fresh paint.

Thank you!

Originally Posted by CassidyNZ

Seriously, I would put more than one baffle in. Half a tank = 75kgs of water which can build up a lot of momentum in the space you’re providing and as an amateur carpenter I know how hard it is to make really strong joints in the edges of plywood boxes

You also need to have at least one large hole (2”) in the bottom of the baffle(s) to allow the water to migrate from one side of the tank to the other. Just allowing water to flow over the top of the baffle may not be that effective.

[hd002e]

Interesting idea to vent it to the saloon topsides. Thanks for your comment.

Originally Posted by Pete7

Keep it simple, just put the vent in the highest point you think exists and don't worry about it. A couple of litres of air is neither here nor there. You might add a tank gauge and sender unit. It won't read true towards the bottom of the tank but its nice to be able to just glance at the gauge to see how much water is in it.

The vent can be in the cockpit or saloon topsides if it makes it easier.

[Pete7]

Originally Posted by CassidyNZ

There’s a reason why all production boats vent all tanks to the transom.

No they don't. If the vent is in the cockpit or sides of the cabin top and you get salt water in any quantity in there, then you have other more pressing problems to worry about.

Not sure I would want a vent inside the yacht. All you have to do is forget the hose for 5 minutes and you have a bilge full, after its splashed over the furniture and cushions. I like my bilges dry and dusty.

One other thing to think about and a reason for keeping the vent short. Most of us clean the tanks and lines out regularly with bleach or pots of stuff from the chandlers. This process should also include the fill and vents pipe work. If the pipe is clear plastic all the more better for being able to inspect it.

Pete

[Uncle Bob]

Originally Posted by hd002e

water tank design:

After my bladder type water tank burst and filled my bilge with 200L of water as I was preparing for a trip, I decided to build a plywood+epoxy tank. The plastimo bladder type tank had only two fittings, one connected to the deck fitting and the other to the water pump, and there was no vent connection. Which I believe led to its demise as the plastic bladder could not handle the pressure. So for the tank I am building I want to incorporate a vent line.

The tank is about 150L, it has a baffle in the middle. it is on the port side of the boat lengthwise under the settee. I am using the the inner face of the false hull as one side of my tank, the front side is the v-berth bulkhead and the rear side is a partial bulkhead I built about two feet foreward of the galley. The inlet is towards the top of the tank and the outlet is towards the bottom of the tank, where I use delrin thru hull fittings with barbs. I would like to think I have things figured out until this point, however I need some expert advice on where to attach the vent fitting on the tank and how to route it to the thruhull and if I should use antisiphon etc..

I am guessing (and I could definitely be wrong about this) that the vent should be placed on the highest part of the tank where air gets trapped as the water tank fills up. Should I have two vents to make sure I can fill up the tank to its maximum capacity? I am guessing the vent line should be routed straight up, without creating any kind of neck. I am thinking to add a thruhull close to the hull/deck joint (as high as possible) amidshiprs but then I will not be able to add an antisiphon. I can install the vent a bit lower and add an antisiphon (not sure if this is necessary?) or I can route the hose all the way to the back of the boat and let it exit aro und the transom where it it less likely to get dipped underwater. I have also seen boats where this vent line is routed to the galley and that also makes a lot of sense to me. If I use a thruhull, should there be a checkvalve in case the boat heels over and dips the thruhull in the water? Should I add a seacock? So many questions

So, I would love to hear expert advice from you fine folks. If there is a resource (like - considerations for watertank design on sailing vessels) I would be very interested in reading more about this before I start drilling holes in my water tank and in my boat. Thank you!

For what it's worth a bladder only requires the two connections mentioned, no vent required as the bladder expands and contracts with the amount of liquid added or withdrawn.

[dfelsent]

I’ve got two tanks and two vents. One is a neat fitting with a stainless cover that the PO made. It sits on the outside of the doghouse. The other is a spout into the galley sink.
Not sure which is better.
Do recall that no matter the diameter of the vent hose the pressure of the water in the vent hose when overfilled is the same.

If your vent is a few feet above the tank that is an extra few psi. About two feet per psi.

[Chotu]

Originally Posted by Pete7

No they don't. If the vent is in the cockpit or sides of the cabin top and you get salt water in any quantity in there, then you have other more pressing problems to worry about.

Not sure I would want a vent inside the yacht. All you have to do is forget the hose for 5 minutes and you have a bilge full, after its splashed over the furniture and cushions. I like my bilges dry and dusty.

One other thing to think about and a reason for keeping the vent short. Most of us clean the tanks and lines out regularly with bleach or pots of stuff from the chandlers. This process should also include the fill and vents pipe work. If the pipe is clear plastic all the more better for being able to inspect it.

Pete

Exactly.

I’ll also add if you want to vent a fresh water tank inside the cabin, they are routed to the galley sink typically.

[Matt Johnson]

The vent end just needs to be higher than the fill point. If you overfill, it comes pouring out the fill fitting before it comes out the vent.


Maty

[01kiwijohn]

You need a vent, to allow the water to be pumped out; otherwise, you'll get a vacuum and you'll have trouble filling the tank. For that size tank, I'd use a minimum 3/8" dia, vented as high as possible, not near the bow (where the boat motion, in heavy seas, can cause salt water to be driven into the tank)

[peghall]

Originally Posted by CassidyNZ

If you are hell-bent on venting to the outside, it is really advisable to take the vents to the transom. Even if a vent on the side is right up at the gunwhale, there is a good chance that salt water will enter your fresh water supply whenever you dip the rail and a syphon break won’t prevent that. It doesn’t take a lot of salt water to turn fresh water brak. There’s a reason why all production boats vent all tanks to the transom.

Actually, most don't except for a few OEMs who vent black tanks out the transom in a misguided attempt to keep odors away from people.

Most boat builders use a vent thru-hull designed to keep sea water out of the fuel and water supply, coupled with a high arch in the vent line.


--Peggie

[peghall]

Before you commit to building a tank, check out Ronco Plastics. They make TOP quality thick-walled water and waste tanks for a very reasonable price and have more than 400 shapes and sizes, over 100 of which are non-rectangular, and they install fittings in the sizes and locations specified by the customer when they make the tank. There are retailers who sell Ronco tanks, but Ronco sells direct for a much lower price...and they're great to work with.

Their current marine catalog is here Ronco Plastics marine Tanks Because so many people told 'em that their previous catalog is MUCH easier to search, they put the section with all the tank drawings in it back up Ronco Plastics old marine catalog

Whether you build or buy...when you spec the fittings location the vent fitting should be on the top of the tank...vent out the hull right below the toe rail using a specifically labeled "vent" thru-hull. And a high loop in the vent line if possible. If not, locate the vent thru-hull on the edge of the tank closest to the centerline of the boat. If you select a tank designed to sit on the centerline, put all the fittings on the centerline. Discharge thru-hull at (NOT on) the bottom of the aft end of of the tank.


--Peggie