Raw water strainer filling with air

[PabloPicasso]

Is the engine overheating?
Is it bubbles, or filling with air interupting water flow?

Is it a serious problem, or are you just anxious about it becauae you coukdbt see it before

Most likely an air leak in the new fitting. Change it, or refit it.

 

[LittleSister]

Isn't it just air getting into the hull intake, either bubbles streaming along under the hull, and/or the hull intake sometimes coming above the waterline in bigger waves' troughs? (How far below the waterline is the water intake skin fitting?)

The presence of some air in the top of the filter bowl doesn't matter - the pump will still be drawing water - except if the air continues to increase in volume. Does it get to a certain point and stabilise? How far down (i.e. what proportion of) the bowl have you found the air going?

If air continues to increasingly fill the bowl
(a) it will gradually reduce the working area of filter available, and
(b) potentially eventually to the point where it entirely fills the filter bowl.
But at the latter point the water pump will draw in air momentarily (without ill effect on the engine) until it water is drawn into the bowl again.

It's possible that a worn pump might affect how much air it can shift (being lower viscosity than water), but I'm a bit sceptical about the likelihood of that being the root problem, and a blocked exhaust even more so.

 

[PaulRainbow]

The filter is sitting on top of the intake through hull, it's before the pump, the engine and the exhaust etc. The only way to get air in the filter is before the filter, not after it. It must be drawing it in from outside or the lid isn't sealing 100% (low demand could be OK but for higher demand it might draw air in around the seal).

 

[LittleSister]

The filter is sitting on top of the intake through hull, it's before the pump, the engine and the exhaust etc. The only way to get air in the filter is before the filter, not after it. It must be drawing it in from outside or the lid isn't sealing 100% (low demand could be OK but for higher demand it might draw air in around the seal).

The facts that
a) opening the lid eliminates the air, and the bowl fills with water, even when running; and
b) he has not reported any water leaks when the engine is off;
seem to suggest a leak is not the problem.

 

[MikeBz]

Is the engine overheating?
Is it bubbles, or filling with air interupting water flow?

Is it a serious problem, or are you just anxious about it becauae you coukdbt see it before

Most likely an air leak in the new fitting. Change it, or refit it.

The engine temperature remains rock steady at 80C even when the level is at the bottom of the basket (it goes no lower).

I’m just slightly anxious because one would normally expect the strainer to be full of water. Discussions on other forums have suggest that sucking air could shorten the life of the impeller.

I don’t see how it can be a leak given that when I introduce a leak it expels the air. Also I don’t see why a leak would be there when under way (sailing) but not when anchored/berthed.

Isn't it just air getting into the hull intake, either bubbles streaming along under the hull, and/or the hull intake sometimes coming above the waterline in bigger waves' troughs? (How far below the waterline is the water intake skin fitting?)

The presence of some air in the top of the filter bowl doesn't matter - the pump will still be drawing water - except if the air continues to increase in volume. Does it get to a certain point and stabilise? How far down (i.e. what proportion of) the bowl have you found the air going?

If air continues to increasingly fill the bowl
(a) it will gradually reduce the working area of filter available, and
(b) potentially eventually to the point where it entirely fills the filter bowl.
But at the latter point the water pump will draw in air momentarily (without ill effect on the engine) until it water is drawn into the bowl again.

The skin fitting is about as low as it could go, just above the hull/keel intersection.

It stabilises when the water gets down to the top of the intake pipe, bottom of the basket. This can take 1.5 to 3 or more hours, more or less in proportion to sea state so I’m sure you are correct about it being an accumulation of bubbles streaming under the hull.

I do wonder whether a scoop style intake fitting would be better.

 

[LittleSister]

Discussions on other forums have suggest that sucking air could shorten the life of the impeller.

Only if it goes on for a prolonged period, in my view. Otherwise, the remaining water in the wet pump will lubricate the impeller blades while it's pulling water through again.

 

[PaulRainbow]

The facts that
a) opening the lid eliminates the air, and the bowl fills with water, even when running; and

Because the filter is below the waterline.

b) he has not reported any water leaks when the engine is off;
seem to suggest a leak is not the problem.

I didn't say it was leaking. It could be sealed enough so that water doesn't leak out and air does not get in when demand is relatively low, such as it would be if the engine is running at low speed, but high demand, such as running the engine faster causes enough suction to draw air in.

I've seen it before with these filter when fitted above the waterline and also seen it several times with diesel problems, a connection doesn't leak fuel, but allows air to be drawn in when demand is high.