How to Install a Raw Water Pump

454 water pump

These two Shurflo live well pumps replace the belt driven raw water pump on my small block Chevy (OMC) boat engines.

The engines on the Rock ‘n Row are both OMC Cobra’s, which are really just marine versions of the Chevy 350, the same engine in the Corvette. Instead of a radiator, like a car engine has, these engines have a heat exchanger. The heat exchanger is really just a copper container. Engine coolant flows through one side and cold sea water is pumped through the other. The liquids are not allowed to mix, but heat from the hot engine coolant is transferred to the cold sea water, which keeps the engine from overheating.

Without a water pump, an engine will overheat. I ended up replacing my original pumps with a 12 volt inline water pump.

How Much Does a Water Pump Cost?

The original pump for the sea water (called a raw water pump) was driven by a belt coming off the flywheel of the engine. The pump was made by Jabsco, and was proprietary to OMC. That means that when OMC went out of business, the pump went out of existence. When I first purchased the Rock ‘n Row, one of the pumps were seized and needed to be replaced. I was able to find a brand-new pump on eBay for $600. It was the only one I could find despite several days looking on the internet and calling around to different part suppliers.

As painful as it was, I forked over the cash and got the replacement pump. When I went to install the new pump, I noticed that the mount for it on the motor had broken. I jimmied a fix, but the belt was just slightly off center. About two hours into the running of the engine, the pump starting whining terribly and it stopped pumping water. I quickly shut down the engine and brought the boat into the marina on one engine. After removing the pump, I found that the impeller had shredded itself; presumably because the pump had been mounted slightly off-center.

How to Change a Water Pump

how to change a water pump

I had to get a little creative with the plumbing, but everything uses standard sizes.

Never being a very mechanically minded person (I’m an electrical engineer), I decided these belt driven pumps were archaic and too frustrating and expensive to work with. Besides, I had no clue how I was going to go about fixing the mount on the engine. I started doing research and discovered Shurflo 12 volt pumps.

The origonal spec for the Jabsco pump was 900 gallons per hour (GPH) at 4000 RPM. The Shurflo Piranha livewell pump is rated for 1100 GPH, which exceeds those specs. So I knew it *should* work. Everyone I talked to advised me against it however. I couldn’t find anyone who had done an electric retrofit or knew how to install a water pump in this way.

There was one main difference that I discovered the hard way. The Shurflo pumps are designed for high volume, but very low pressure. The first year I ran the engine on the electric pump, it worked great, but the wet exhaust consistently had more steam than the other engine with the belt driven pump. The engine also consistently ran 10 degrees celcius warmer. I figure out later that I didn’t have enough pressure coming from the 12 volt inline water pump. Over the course of that first year, I had to replace the pump twice. I believe this was because I was simply running it too hard. However, at $50 a pop and only 15 minutes to replace in the field, I didn’t mind replacing it. I keep two spare pumps on board at all times now.

On a recent trip to Rosario Resort, I noticed that the Jabsco pump on the other engine was leaking profusely. With a year of successful trials and lessons learned, I decided to go all electric. I also learned my lesson about the pressure and put two 12 volt inline water pumps on each engine.

shurflo 12 volt pumps

Here is the installed 12 volt inline water pumps. This solution should could also replace a 454 water pump or a mericruiser sea water pump.

By putting the pump in series, I was able to double the pressure and maintain the same water volume (1100 GPH). At first I tried putting them in parallel, which should have doubled my water flow but kept the pressure the same. That didn’t work very well and I didn’t notice that the engine ran any cooler. I mention it here in case anyone tries it. In-series is definitely the way to go.

At this time, Ken is also experiencing issues with the raw water pump on his boat. Being a mechanically minded person, he is hesitant to go with an electric solution. We looked at the specifications for his raw water pump, which is 900 GPH, so this solution should work just fine. In fact, this type of 12 volt inline water pump should work as a replacement for most boats, including the 454 water pump, a mercruiser sea water pump, and many others.

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4 Responses to “How to Install a Raw Water Pump”
  1. Donnie Abbass says:

    I have the same problem with my twin 6.5 lt US Marine Diesels, they are equipped with johnson pumps that have been discontinued . I like your idea but after talking to a marine tech he told me that there is a chance that it could flood the engine with sea water.
    are you still having success with your electric pumps.
    Thanks, Regards, Donnie Abbass

    • Chris says:

      I sold the boat about six months ago, but yes, they were working just fine for over a year. I had done the same retrofit on both engines and they were both running just great.

      I don’t understand how they could flood the engine. On mine, the raw water went through a heat exchanger and then through the wet exhaust manifold. There is no way they could flood the engine any more than the original pumps could have.

      • josh says:

        I like your idea was curious what your thoughts on this would be i have a 1979 volvo penta AQ131B 2.1 liter 4 cylinder engine in my boat going thru the same type of thing raw water pump is leaking and a new one is 300 or 150 for rebuild kit so im gonna try your set up being that my motor is only a 4 cylinder would you still think id need two electric motors?

        • Chris says:

          I ended up needing two pumps to get the pressure I needed to pump enough water. Adding a second pump in series increases pressure, but not volume. If you can get the volume of water flowing through your engine to simulate the old pump, you should be fine with just one.

          The old pump should have a specification for volumetric flow, in gallons-per-minute or gallons-per-hour. Just make sure the rating on the electric pump is equal or greater to that. After that, it’s just making sure everything runs right. I noticed that my pumps were burning out after a couple months because I was working them too hard. Adding a second pump in-line meant they weren’t working as hard, and made both pumps last much longer.

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