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BoatBITZ #3 – Replacing a Raw Water Pump Impeller
September 27th, 2011 @ 12:32 PM EST by admin

By Sean McKee

For those of us here at NYC who have boats with inboard water-cooled engines, this month’s column will be of interest (and use) to you. For the rest with outboard motors, or the cool dry-sail kids who sail on and off the sea wall, read on and learn – you might need to know someday when you strike it rich and start campaigning your new all-carbon sled on the IRC circuit.

How it Works
Water-cooled engines, as they imply, use water, sucked up from the sea and run through a cooling manifold to prevent overheating and eventual catastrophic failure of your “iron genny” when you need it most. Integral to your engine is the raw water pump, which draws the water from the sea via a through-hull and “squirts” it through the manifold with the help of an impeller.

The impeller itself is a star-shaped rubber wheel that spins inside the body of the raw water pump and pushes water into the manifold. Over time, the impeller like any moving part, will wear out and need to be replaced. In the event that your raw water intake becomes clogged with assorted growth or other lake schmutz (or in the event you’re a complete fool and forget to open the raw water intake sea cock when you launch in the spring) the impeller will run dry. Without the lubrication and cooling effect of the raw water inside the pump body itself, the impeller will heat up and self-destruct within a very short period of time. In any case, a worn or damaged impeller will start to come apart, often forcing chunks of corroded rubber into your cooling system – VERY BAD! If this happens, you face a costly tear-down of your engine to find the offending bits (that’s big $$$, folks).

Changing out your impeller is one of the simplest jobs you can do on your boat, allows you to be seen tinkering on your engine (so your fellow yachties can think you know what you’re doing), and is a great excuse to putter on the boat with a beer after work. The first step is to refer to your engine’s owner’s manual (if it still exists) to understand where the raw water pump is located on your engine. It should be easy to locate as it will have a flat plate secured by several screws; if in doubt, follow the raw water intake hose from the through hull to the point where it connects to the big hunk of metal called the engine which will be the pump.

Before you go stampeding to the water pump like a bull at the gate, it’s best to refer to the owner’s manual to confirm the model of your engine and the part number for the impeller and gasket or O-ring required to effect the repair. If your owner’s manual was lost by the previous owner, you should be able to source the required information on the Internet.

Your local chandlery or boat yard should be able to provide you with either original manufacturer’s parts, or quality after-market replacements for less than $50 in total. Make sure that you obtain a new impeller, gasket, O-ring (if any) and lubricant (glycerin will do).

Out With The Old
Prior to removing the access plate for the impeller, it is prudent to close the sea cock on your raw water intake, lest water rush into your boat making you look like an idiot…

Remove the screws fastening the cover plate for the raw water pump and put them someplace safe. In fact, on that trip to the chandlery, consider taking one of the screws ahead of time to get a couple more to replace the ones you are going to drop into your bilge, never to be seen again (trust me, it will happen). If the screws have seized you may need to use some PB Blaster (available at Canadian Tire) to loosen them. Don’t spray it on as it will eat through engine seals! Spray a bit into the cap and apply to the screw heads with a cotton swab; this should loosen the seized screws after sitting for 5-10 minutes.

Next, pry off the cover plate taking care not to mar the gasket mating surfaces and peel off the remains of the old gasket and/or O-ring (whichever your engine uses). Check the inside of the cover plate for wear; if it is scored and worn, it should be replaced, meaning another trip to the chandlery for you.

Before re-using the cover plate and application of the new gasket, be sure to clean the back of the plate and mating surfaces on the pump to remove any remaining vestiges of the old gasket material. A 3M ScotchBrite pad works well for this – remember you aren’t trying to sand it smooth, just remove the gunk to make a clean surface to ensure a good seal for the gasket.

Inside the pump, you’ll see the old impeller in its housing. The vanes on the impeller will be “bent” in the direction that it spins; take note of this direction as you’ll need to orient the new vanes in the same way. On most engines you should be able to remove the impeller simply by gripping it with a pair of needle nose pliers and drawing it straight out of the housing. If it’s worn or damaged, you’ll be able to tell straight away.

Now that the impeller is out, be sure to check the interior of the pump housing to ensure it is free of any impeller bits and/or corrosion. Once this has been done, you’ll need to lubricate the inside of the pump housing with the impeller lubricant. Squeeze some onto your finger and make sure you lube the entire insides of the pump, including the back wall since the impeller will spin against it.

In With The New
Lubricate all of the rubber surfaces of the new impeller with the supplied lubricant (usually glycerin); ensure you don’t use any kind of petroleum-based lubricant as it will affect the composition of the rubber, considerably reducing the life of the impeller.
Now orient the vanes of the new impeller in the direction of the old one you just removed and slide the impeller into the pump housing, making sure it’s seated properly.

Seat the gasket on top of the cover plate and slide two screws through holes to ensure proper alignment as you position the cover plate back onto the pump. For pumps that use paper gaskets, make sure they are applied dry (don’t lube the cover plate); those that use rubber O-rings should be lubed with the supplied glycerin lubricant before application. Then simply screw the plate back on using the remaining screws and you’re done.

Two Final Things

  1. Since you’re there anyway, take the extra time to check your intake hose for corrosion or wear and replace as necessary.
  2. Remember to open the raw water intake sea cock!!!