DIY Diesel Boat Engine Repair: Raw Seawater Pump Video

Raw Seawater Pump Video

This DIY Boat Engine project shows how we remove and re-install our raw seawater pump on an 85-HP Perkins 4.236 Marine Diesel Engine.  As a follow-up video for our Perkins 4.236 Engine Repair: Raw Water Pump Post, we hope you find the video a nice addition to the steps we used.

Disclaimer: We are not certified diesel mechanics, so please consult a professional before attempting anything in the video yourself.  Since this was the first time we opened our raw seawater pump, we had a diesel mechanic friend direct us through the process.

How the Raw Seawater Pump works

The raw seawater pump is responsible for pulling cooler outside water into the boat.  From there, it is circulated through specific pathways inside the engine to help keep it cool.  The main pathway carries the water through the heat exchanger, the marine equivalent of a car radiator.  Only, instead of using air to cool the coolant, the heat exchanger uses seawater.

Banjo’s raw seawater pump is gear driven and mounts on the port, forward side of the engine.  Hence, the engine turns a gear which is connected by a shaft to the ship’s impeller.  The gear and the impeller are kept apart by a gap exposing the shaft.  In the event of a seal failure, water “should” leak into the bilge, instead of into the engine.  This was one of the first places we checked when trying to solve our water in oil problem.

The Function of the Raw Seawater Pump Impeller 

The seawater is pulled into the boat by the impeller.  This device is basically a flexible “paddlewheel” that creates suction to keep the water flowing.  Water is also pushed into the boat by having the intake hole below the level of the outside water.  This helps reduce strain on the impeller. 


Heat Transfer

As the seawater circulates, heat transfer carries energy from the higher temperature of the engine coolant to the lower temperature of the seawater.  Check out this WikiPedia article if you are interested in learning more about heat transfer.  The raw seawater pump continues to push “new” water into the system.  Since water is not compressible, the water previously brought in is directed by hoses towards the end of the system.  There it is mixed with the hot exhaust gases from the burning diesel fuel.  The injection of the seawater at the end helps to cool the exhaust gasses.  Both are then spit out the back of the boat.

For More

Check out our marine engine playlist videos on our YouTube Channel.  And for more How-To DIY boat project articles, be sure to check out the Boat Projects Section of our blog.  Thanks for watching and we hope you find this video useful.  If you enjoy the video, please subscribe to our YouTube Channel.  

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