Watermaker sucking air in

[geoffr]

We have a PowerSurvivor 40 that is working well for us at anchor, but once we are underway it regularly stops working as too much air is being picked up.

The reason for the air is that the thru-hull is only about foot below the waterline and when we are underway in any waves the churn is causing air bubbles. Unfortunately I can't think of a place where we can lower the thru-hull as we only draft 3 feet, but the lower ~2 feet are the keel (and water tanks).

Does anyone have any suggestions on how to "remove" the air from the intake so the watermaker is happier?

The photo below is the intake set-up (Left thru-hull is the watermaker, right is the engine). The top of the water filter is the waterline. The smaller hose off the filter goes to the watermaker.

[AKA-None]

I wonder if some sort of day tank like design might work

Pump water into a tank with overflow and draw from the tank

I guess it will all depend on how often the through hull is drawing air

So like damping but in reverse

[dougweibel]

I think there is always a question of how much is too much chop. If your intake is only 1' below your waterline then it is not surprising that you can tolerate little chop. But even with a deeper intake you will hit conditions where you take in too much air, so moving your intake can improve the situation but not alleviate the issue entirely.

In addition to going deeper, going further forward can help. A lot depends on your hull shape.

[team karst]

I tried to run our RO in 5’ seas today. Fail…. Pickup is just 2’ down.

[Scubaseas]

Use a feed pump and an "air scoop". I suppose you could coat the inside of a cast iron one with epoxy. or make one out of bronze or plastic tee fittings. Pump the excess air to the salt drain or overboard.

Look at what they use for hot water base board heating to see how an air scoop works. A 2" tee necked down to hose would probably do it or a 2-3-2 tee with the air scoop one the 3 side

[geoffr]

AKA-None,

I had thought of a day tank and pump.. but am not keen on trying to find space for the tank, piping and pump, ontop of the extra power for the pump..

Originally Posted by AKA-None

I wonder if some sort of day tank like design might work

Pump water into a tank with overflow and draw from the tank

I guess it will all depend on how often the through hull is drawing air

So like damping but in reverse

[geoffr]

Scubaseas,

I like the idea. I do see that the ones used in hot water heating require a positive (above atmosphere) pressure to work. So I would need a feed pump to keep the pressure up

Can you explain your 2" or the 2-3-2 tee - I'm not following, but am intrigued.

Originally Posted by Scubaseas

Use a feed pump and an "air scoop". I suppose you could coat the inside of a cast iron one with epoxy. or make one out of bronze or plastic tee fittings. Pump the excess air to the salt drain or overboard.

Look at what they use for hot water base board heating to see how an air scoop works. A 2" tee necked down to hose would probably do it or a 2-3-2 tee with the air scoop one the 3 side

[Scubaseas]

Tee with the middle facing up. 2 " fitting each side and the top one is a 3" fitting. similar to below. Not sure there is anyway around a feed pump though.

[Searles]

As it’s a smaller cat the best intake position would be on the centre line just aft of the stub keel , not much of a choice .⛵️⚓️

[Tellie]

Originally Posted by geoffr

We have a PowerSurvivor 40 that is working well for us at anchor, but once we are underway it regularly stops working as too much air is being picked up.

The reason for the air is that the thru-hull is only about foot below the waterline and when we are underway in any waves the churn is causing air bubbles. Unfortunately I can't think of a place where we can lower the thru-hull as we only draft 3 feet, but the lower ~2 feet are the keel (and water tanks).

Does anyone have any suggestions on how to "remove" the air from the intake so the watermaker is happier?

The photo below is the intake set-up (Left thru-hull is the watermaker, right is the engine). The top of the water filter is the waterline. The smaller hose off the filter goes to the watermaker.

Do you have a salt water head intake?

[geoffr]

Searles,

Yup - small cat and the intake is just aft of the keel.

Originally Posted by Searles

As it’s a smaller cat the best intake position would be on the centre line just aft of the stub keel , not much of a choice .⛵️⚓️

[geoffr]

Tellie,

yes we have a salt water head, but diagonally across the boat.
(WM = STB Aft, Head = Port Fwd)

Originally Posted by Tellie

Do you have a salt water head intake?

[Tellie]

Originally Posted by geoffr

Tellie,

yes we have a salt water head, but diagonally across the boat.
(WM = STB Aft, Head = Port Fwd)

You might be best served by adding a small boost pump to this to help the flow reach the input to the watermaker. These small systems can take a long time to prime with an intake line like you seemed to have described. They have to draw water to themselves, which they can do, but it is such a small amount that it can take a while to get to the input especially if the intake line pulls back air into the system. A small boost pump can be installed with a SPST switch to turn it on and off. You might find that once the boost pump delivers the water to the input that you can turn it off and the system will take care of it from there. This opens up then option of using the intake on the other side of the boat.

[geoffr]

Tellie,

I'm not sure I understand how the boost pump would help (although it sounds interesting!).

I am assuming with my set-up right now that as air enters the system the PowerSurvivor 40 just can't suck up the water when there is too much air in the input side. (Although it is able to eventually overcome this air in calm water if I put the bypass on (not making any fresh water) to clear the air before going back to normal operation.

1. Would the boost pump help the pump overcome this air (that can expand easily)?
2. Would this pump then push the air into the PowerSurvivor? (I assume this wouldn't damage it?)
3. I assume you are thinking the boost pump would only be needed when there is air in the system (or the possibility (ie at sea)) ?
4. Once I have fresh water output I could turn the pump off and water still flows through it?

What pump would you recommend? (I've read the March pump should be at least 1 food below the waterline which will be a challenge..)

Thanks!

Originally Posted by Tellie

You might be best served by adding a small boost pump to this to help the flow reach the input to the watermaker. These small systems can take a long time to prime with an intake line like you seemed to have described. They have to draw water to themselves, which they can do, but it is such a small amount that it can take a while to get to the input especially if the intake line pulls back air into the system. A small boost pump can be installed with a SPST switch to turn it on and off. You might find that once the boost pump delivers the water to the input that you can turn it off and the system will take care of it from there. This opens up then option of using the intake on the other side of the boat.

[paulajayne]

Fit an external drop pipe pickup - may have to change the through hull to fit one.