Raw water pump flow

[ErikN]

We're on an extended cruise and when we started the engine this morning the exhaust wasn't spitting water at its usual rate. We've checked the raw water line from the through hull through the strainer up to the (Oberdorfer) pump, and that is all clear. We also replaced the impeller (even though it looks fine). We then pulled the hose off the pump outlet and started the engine. The pump is producing water, but it seems a bit anemic. However, I don't know how much it should produce, i.e., a fast stream or just a modest flow?

I'm guessing we need a new pump, but any thoughts would be appreciated. I apologize if this has been covered elsewhere on the forum, but we have very limited bandwidth at our current (beautiful) anchorage.

Erik

[Stu Jackson]

Erik,

Two things come to mind when dealing with raw water and its pump:

1.  FACEPLATE, FACEPLATE, FACEPLATE - Yes, good impellers help, but without a flat flat flat faceplate, they may not.  Fine sandpaper on a block.

2.  Blockage AT THRU HULL -Use a dinghy foot pump and blow back, easiest I've found via the hose at the rw pump.

Good luck.

[scgunner]

Erik,

The important question would be is the motor running hotter? The volume of exhaust water is determined by motor RPM. Even though right now you don't think it's producing the normal volume does it still increase when the RPMs do? Also the exhaust water should be warm to the touch, not hot. Is this a recent development or has it been coming on?

I wouldn't be in a big hurry to replace the pump, they're pretty simple as long as the impeller is spinning it's going to pump water, plus those little buggers have gotten expensive. I replaced my Obie once but that was because the seal was leaking. The pump was working fine it was just leaking seawater in the engine compartment.

If your temp gauge reads normal and the exhaust water isn't getting hot then your motor is getting enough water.

[Ron Hill]

Erik : Besides checking the faceplate (prime suspect!!), the impeller is intact, make sure that there aren't any old broken off blades in the exhaust elbow of the exhaust hose going to the HX and make sure that the exhaust hose from the pump to the HX is clear!!  Also make sure that you are using s paper thin gasket between the pump and the faceplate!!

A few thoughts. 

[ErikN]

I hadn't taken a close look at the faceplate. With a bit of sanding, a couple of shallow grooves became obvious. I'm hoping this is the culprit. Based on the photo, can anyone suggest how these grooves might have happened? Maybe it was just something that got past the strainer, but if it is a sign of a larger problem, I would like to prevent a future occurrence.

Thank you for steering me in the right direction.
Erik

[Stu Jackson]

I wrote a Tech Note about just this issue, very recently.  If you are a C34IA member, you get Mainsheet magazine.  The article is so recent it hasn't been posted on tech notes online yet.  It's in the Spring 2022 issue.  I ended up having to have the old faceplate "faced" at a  machine shop.  Depending on the age of your pump, you can get replacement faceplates.  If your pump is as old as my 1990 pump, Oberdorfer managed to "modify" the hole spacing just enough to make the new faceplates incompatible with the old pumps. 

[Ron Hill]

Erik : On your extended cruise I'm sure you may not be able to get to a machine shop to have that plate refaced.  The wear pattern looks normal to me!!

You need to get some wet/dry 300 grit sand paper - wet it and start sanding on a flat surface. You need to sand that inside until it is smooth.  Then make the final sanding with some 400 or higher grit paper. 

Don't think that you want to go to a finer strainer as it is more apt to clog and reduce the water flow!!! Just one of those WEAR things you need to take care of when you change impellers or like I did every spring before the splash!!

A few thoughts

[dfloeter]

I recently "refaced" a Groco pump cover that is about the same as our water pumps and it took about 20 minutes.  Starting with 400 grit paper on a sheet of glass I took it down at least .024" and then went to finer paper stopping at 1500 which was overkill I know.  The Flitz polish then was really over the top but it was very pretty. The point is that the whole project took less time than a good machinist would take and the results were perfect.

[scgunner]

Erik.

Those grooves look like normal wear caused by the compression lobe and impeller vanes. While a nice flat face plate is a good thing I'm not sure it's going to solve your problem. In fact I'm still not sure what your problem is. Other than the exhaust water flow seems diminished are there any other attendant problems? Is the motor running hotter than normal? Is the water flow still increasing when RPMs are increased? Are you seeing a higher than normal amount of steam from the exhaust port?

I can only see two ways pump face plate could be the problem. If the face plate seal was so poor that 1) the pump lost it's prime in which case you'd be getting no exhaust water at all or 2) it was leaking so bad it had the effect of diverting water from the H/X which would be spraying all over the engine compartment.

It sounds like your pump is working fine. If it was me I'd be looking for a in line blockage.

[ErikN]

I appreciate all the interest that our rookie problem has garnered. We finally figured it out. We isolated the pump by running the inflow hose directly into a bucket of water—the pump works fine. Then we ran the strainer inflow hose into a bucket of water and the pump still worked fine. It had to be the thru hull. The first thing we had tried, before we even looked at the water pump, was to blow out the thru hull with the dinghy pump, which produced lots of bubbles under the hull. Nevertheless, I pushed a plastic rod (aka a drain cleaner) through the thru hull, and after working it back and forth for a few minutes a geyser erupted under the sink.  Apparently the dinghy pump failed to fully dislodge whatever was obstructing the thru hull.

The spring issue of Mainsheet is probably in our mailbox back home, and I'm curious to read Stu's article. In the meantime, we still have a couple of weeks in the Broughtons.
Erik

[Ron Hill]

Erik : I've had three obstructions in that raw water thru hull.  The first one was from hydrilla (sea weed) on the Potomac and two other from sea grass in the Keys.  I learned to keep a 3/8" wooden dowel under the head sink to poke out that kind of stoppage.  Easy to diagnose - just remove the hose to the RW pump and slowly open the thru hull.  You should get a geyser of water - I got NOTHING!!   

After we got back to the Chesapeake and the boat was out of the water I installed a small round bronze strainer over the outside of the RW inlet.  I installed it with a couple of dabs of caulk - so if I ever wanted to get rid of the strainer I could put the wooden dowel on it and drive it off !!  That was back in 1997? and the strainer is still on the boat and I've NEVER had a stoppage since that then!!   

A few thoughts

[KWKloeber]

Remember that, although it wasn't the cause in this case, diminished exhaust flow can be due to a blocked injection wye above the muffler.  "Quite" common in salt water.

[ErikN]

I just gave the short version—we checked the injection into the exhaust and every other piece of the raw water system before we went back to the thru hull.