Alpha Raw water pump pickup

[martynj19]

Hi All, is there a strainer or any type of filter between the pickup holes and the impeller blades on the stern leg? Also, if the blades of the impeller broke up due to failure , would the "bits" be captured somewhere upstream or would they enter the cooling system?

Thx

Martyn

[mmwjr]

No strainer. The holes in the side of the drive go into a cavity that leads right up to the input of the raw water pump housing.

Depending on the size of the pieces they can travel all the way through the system. Common for then to end up at the t-stat housing.

[martynj19]

Would you recommend fitting a strainer inline from water pump to engine? I know routine maint should prevent most impeller failures , but I am new to boating and I am thinking its only a matter of time that the sterndrive will end up in the sand!

Martyn

[mmwjr]

Strainers will stop large objects such as sea grass however sand will pass through the mesh. Also the biggest issue with sucking in sand is it will trash the impeller and score the pumps housing. A strainer can only be installed after the raw water pump on Alphas.

[martynj19]

Thx , my thoughts were that a post pump strainer will catch the impeller bits if it was as you say trashed or broken by the sand etc. I know the trip would be ended without the raw water pump in operation. Will consider installing it to save stripping and flushing cooling system.

Martyn

[mmwjr]

Sand will not cause the vane to break off it will wear the edges and score the body so you do not get a good seal and the result is low pressure.

Typically vanes only break off if the impeller is years old so if you follow the 2 year maintance by mercruiser this should not happen.

[martynj19]

Hi Mike , thx again , I will certainly replace the impeller as part of PPM schedule as identified , also I know we spoke on another thread about the blower system on my 1900 and I carried out a "smoke" test , ie generated safe smoke in the bilge lower extract/blower hose and could see it exiting from the port vent , and as suspected the port vent box is being pressurised , most of smoke exiting , but some coming back down the "fresh" air ducting. I will re-route the "fresh" air duct from s/side vent/inlet to the bilge area. This has to be a design flaw surely?

Martyn

[mmwjr]

Good test. I know the old 3200 had an issue with generator being exhausted out the back of the boat under the platform. This got retrofitted due to the station wagon effect that causes turbulence that allows exhaust gases to enter the boat.

So yes design flaws exist and by many manufactures.