Air Inside Sea Chest Filter

I would check the strainer for junk or anything that could be obstructing the flow through the system, and check the pump assembly, the impeller could be bad, it should be able to self-prime...check the pressure gauge on the water line when performing this maneuver to see if pressure is being lost...if this doesn't resolve the issue, call a mechanic...

Sir,The valve on top should be fitted with a pipe venting well above the water line. In most cases this will vent to the main deck on a vessel this size. If it is of ample size, open and un-obstructed you should automatically purge any air that enters your sea chest filter.

If the problem persists, and this is a new build I would think the "wash" was not considered carefully upon design and placement of the sea chest.

If it is not a new build, I would lean heavily on the vent valve being closed or vent blockage

Tim

I'm guessing the port DG sea chest inlet is in line with the hydrojet when it is set at a 15° stbd angle, but fail to see how the reverse thrust could starve the inlet of water. The sea chest filter or the air vent could be clogged. You could try various manoeuvers where space permits; backing to port and stbd at various angles and rudder amidship and monitor the temperatures of each DG.

The other thought that comes to mind is; the sea water pump. You mentioned it had a plastic rotor (impeller?). Could it be cavitating? I'm also assuming the sea water pump is self priming, but 2 meters above the WL might be excessive. You might need another pump located closer to the WL, if priming is the problem.

Is the seachest strainer the same type used on conventional ship hulls. There may be a type that is shaped better for water jet use. At any rate, the seachest intakes are probably too far back and should be relocated further fwd or moved further to the side, out of the thrust path of the jets. This would improve water flow through the strainers.

What about fitting a header tank above the engines which can supply cooling water for the brief time that air is being ingested by the pump. An automatic small bore air bleed from the top of the tank would purge the air when the pump could ingest clear water again.

The pump would feed to the top of the tank and the motor cooling draw from the bottom.

This proven solution comes from a method for removing residual ozone from a water treatment system. Install a vertical section comprising two concentric pipes. The pipes have a common base height and the inner pipe is shorter than the outer pipe. The top of the outer pipe is sealed but has a float operated vent valve. The inlet is connected to the base of the central pipe. The outlet is connected to the base of the outer pipe. The water flows up the inner pipe and over the top creating a weir that de-gasses the water. The de-gassed water flows down the annulus between the inner and outer pipes and on the the DG chest. When the removed air accumulates sufficiently, the float vents the access to atmosphere and then re-closes, allowing the whole system to remain at the pump head pressure.

sorry,

the problem was just because of used wear plate inside sea water pump!!!

sorry to disturb...

pa