raw water strainer mystery

[Ken Heyman]

After replacing a blown raw water impeller (Sherwood pump), I unscrewed the palstic bowl that housed the strainer (at the thru-hull) and cleaned the mesh strainer. As a curiosity I then reopened the seacock and of course got a healthy return of sea water. I then screwed the element back in  - reopened the seacock- yet the bowl didn't fill. I repeated the test a couple of times with the same result. Once I started the engine all was normal and the raw water element(bowl) filled.

The question is why doesn't the bowl doesn't fill without the engine running--suction break ? back pressure ?---just curious.  Ken

[Gary]

I think I understand your question. My response would be that the whole raw water system should be a sealed link...water as well as air. After cleaning and replacing the strainer and re-installing the bowl, un-twist the bowl a bit. That breaks the seal and allows the raw water to fill the bowl. You will get a small amount of raw water in your bilge. I like to do this when cleaning the screen to make sure that the pump does not have to overcome very much air to re-prime. This all works when your strainer is close to the sea cock and below the water line. Of course, if your strainer is mounted above the water line this method will not work.

Gary

[Ken Juul]

Gary beat me to it, this is what I was typing.

Nowhere for the trapped air to go.

I get nervous when the strainer isn't full of water.  When I clean it I don't tighten it all the way.  Open the sea cock and let the bowl fill.  When full, fully tighten the bowl and clean up any water that dripped.

Since you did the impeller, here is another way.  Remove the input line from the pump, after cleaning the bowl, leave it slightly loose and pour water into the engine end of the input hose until the bowl is full.  Put hose on pump without clamping, tighten bowl, then finish clamping hose on pump.  

Doing it the second way ensures you won't get an air bubble in the system, but is probably overkill.

[Gary]

Ken...great description !!  Your suggestion works for both above and below the water line stainer installations.

Terrific !!

Gary

[Ken Heyman]

Great info and advice Gary and Ken--consistent with Stu's comments per a recent discussion we had.  -----------great minds think alike.

Thanks much,

Ken

[Stu Jackson]

Can I object to being included in the "great minds" category?

Ken had a great analogy when we chatted today:  finger on a drinking straw.  Think about it:  the pump, when all sealed up, is the finger on the straw.  Water may come in if you take the cover plate off or remove the intake hose, but since it's an "open system" (engineering speak), the hose and strainer pressure drop and height above the waterline between the intake seacock and the rw pump is more than the atmospheric pressure can overcome.  Everybody confused now?   

[Ron Hill]

Great Minds : Most of the early MkIs that I've seen have the raw water intake hose from the thru hull, have ties that hold that line well above the engine height of the raw water pump(crossing from port side to STBD).  So to use Ken J's method work, the end of the intake line must be higher than than where the line is tied up to and don't spill too much water reattaching - or you just introduced an air bubble!!

At one time I used to used the little Handy Par pump to suck up the raw water into the line after both  the strainer or impeller were changed.  That way the line is filled with water, shut off the thru hull and then attach the hose - it's a two person operation unless you have a 10/12 ft arm reach!!

If only the strainer was cleaned the easiest method that I found was/is:
Turn off the thru hull, take off the hose clamps, remove the hose from the thru hull, tie it straight up and fill the hose if you spilled some out. Then unscrew the strainer, clean it and screw it back on.  Open the thru hull until you see the water starting to come out of the top barb of the thru hull (& strainer is full).  Shut off the thru hull and put your thumb/finger over the intake hose to keep the prime from flowing out and reattach to the thru hull.

A thought - anyway, it works for me.

[Ken Heyman]

The foregoing does seem like a good idea. Over the years, I have generally allowed the system to "bleed itself" by starting the engine and then observing that all is well by the discharge at the stern exhaust thru hull. This does deprive the impeller of water for a few seconds longer than would be the case by adding water first to the strainer while keeping air out of the system. If I have changed the impeller I lubricate with vaseline before starting. If I'm just cleaning the strainer, I don't. I guess I may be reducing the life of these impellers.