Raw Water Pump Replacement (C36 Mk II M35B)

by Larry Brandt

When the raw water pump seal begins to fail, it offers a clear signal to the skipper. And in this case, it properly flagged an impending failure. This article describes the evidence of a failing raw water pump, and documents its replacement with a new pump.

C36IA Member Mike Turek writes to me that on his boat the raw water pump had smaller weep holes than you see pictured here on my M35B. He experienced a blockage, with salt, of the weep holes, which allowed water to ingress into the crankcase of his engine through the raw water pump's power take-off seal on the block. He writes, "It does not take long for sea water in the crankcase oil to emulsify and cause possible engine failure. This actually happened on my boat. The weep holes plugged over time...[and] once the infusion of salt water emulsified in the oil, my fuel injector pump froze open causing a 'runaway diesel.'" He says, "This was a very harrowing experience." I agree with Mike that inspection of these weep holes should be on the maintenance list.

The first photo shows the location of the two pumps, the pump on the left in the photo is the closed system coolant pump, which is almost identical to the water pump in an automobile engine. The pump on the right in the photo is the raw water pump, which is the pump that utilizes the rubber impeller that boat owners are always so concerned about. The impeller should be changed frequently (eg, annually, in High Flight’s case) because it is somewhat fragile (being rubber) and its failure could result in engine damage or might preclude the use of the engine, perhaps at a moment when it is sorely needed. This photo looks aft toward the companionway location of the engine, so I note that the raw water pump (right in photo)is on the engine's port side; the closed-system coolant pump is the belt-driven pump on the engine's starboard side (the left one in photo).

Pump Locations

I began the pre-ocean-voyage maintenance with my typical engine attention: changed the oil and filter, changed the heat exchanger zinc, checked and tightened the belt, and did an overall power-plant inspection. That careful look caused me to note the rusted condition of the raw water pump casting.

In the photo, the casting has obvious rust and saltwater corrosion. Rusty drips were visible even on the hose and clamps immediately below the casting. These are signs of impending seal failure.

Evidence of Seal Leakage

The casting has several holes (weep holes, or “witness” holes) that are designed to allow water that leaks past the pump’s seal to leave visible evidence, exactly as appears in these photos. This photo shows saltwater evidence, with a lot of readily apparent rust. I am not sure what fresh water evidence would look like, if different; but being less corrosive, it probably would take longer to become visible, and may not be this rusty.

Evidence of Leakage

I began the task by closing the engine seacock. A person is unlikely to forget this important step more than once, because the moment one of the hoses on the pump is loosened an impromptu shower will commence, drenching the maintenance person and probably the galley and salon as well. This could be considered one of those once-in-a-lifetime educational experiences.

Removing Hoses

Removing the pump is reasonably simple: two hoses to be loosened, and two hold-down attachments on the pump casting. The starboard hose is a bit difficult to remove due to poor access, but I found that pushing on it with a broad-bladed screwdriver made the job easy.

Here is what the pump's mounting flange looks like with the pump removed.

With Pump Removed

Just in case, I took the removed pump with me to the Westerbeke dealer to purchase a new pump and a new flange gasket.

This is what I got for my money.

Westerbeke Pump P/N 42175

Universal Gasket P/N 302678 and Pump

Thanks to previous discussions on our great C36IA Forum, I now know where to send my old pump for overhaul, so it can serve as a back-up for the next time I need one.

I did a cursory clean-up of the engine, particularly the flange, and began the installation of the replacement pump. I lightly lubricated the paper-like gasket before I placed it into the flange.

And then, I forgot something very important! I forgot the rotational direction of the rubber impeller blades!

Impeller blades are “curled” in one direction, and they should not be made to rotate “against the curl.” So, I opened the old pump and found out that they curled “clockwise” when viewed looking aft. This means that the pump body, once in place on its pad (or flange), should be rotated counter-clockwise only, viewed towards aft, in order to not force the impeller blades against their “curl.” Sure enough, there is sufficient space to rotate the pump on its pad in a complete circle.

Pump Mounting Hardware

Once the pump is in place, the two mounting hold-downs are installed and snugged down. This photo shows that the mounting hold-downs are not identical, one – the starboard-side-one (left one in this photo) is wider (thicker) than the portside. I daubed these with anti-seize and reinstalled them, snugged them a bit, and then via CCW direction only, I re-clocked the pump to match with the hoses. Then I tightened down the pump hold-down.

Starboard Attachment

Portside Attachment

Then I reattached the two hoses and tightened the hose clamps.

Tom Sokoloski reminds me that while the hoses are off is a good time to place a spare belt over the outboard hose, tied in place, out of the way. Then, if and when the primary belt fails, the spare will be ready to easily install, without having to break in to the raw water plumbing.

New Raw Water Pump Installed

This photo shows everything in place.

I opened the thru-hull and started the engine. As when starting any marine engine, the first thing you should do is validate engine cooling. When starting the engine I always ask myself aloud, “Do I have engine cooling?” and I make the deliberate turn about to look aft for ripples and splashes, and listen for the sounds, indicating that the raw water flow is present. In this case the answer after about 10 seconds was “Yes”.

I ran the engine for over an hour, continuing my inspection and coolant hose clamp tightening. All seemed OK, so I shut down, closed the engine compartment, and organized the boat for next morning’s push back.

Engine Test Run

This photo shows the tools that were needed.

Tools Used

Disclaimer: This is the procedure the author used to do this task. Neither the author nor the C36IA are attempting to tell others how to do this task, but rather the author is simply telling his/her story. Others intending to do a similar task must make their own judgments and decisions as to the suitability of any of the information presented herein. The author and the C36IA make NO guarantees, and take NO responsibility, for the decisions and actions of others. In all cases, each individual installer is responsible for his/her own actions, own skill sets, and own work practices.