Marine Diesel Engines, Part 1 – Overview of the Raw Water System

October 30, 2014

Understanding the raw water cooling system of your boat’s diesel engine will help you keep her running cool.

Keeping your marine diesel engine running cool is explained here:  From the raw water inlet through the seacock, the strainer, and the heat exchanger and then out the the wet exhaust, water from outside the boat circulates to keep the engine coolant cool.

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To see the whole series, you can click "Marine Engines" in the breadcrumb trail above (just below the video).

Equipment and tools are readily available to maintain your diesel engines from marine suppliers like Hamilton Marine and Jamestown Distributors. There you’ll find Jabsco impellers, Groco separators, the correct hose clamps and hoses, and more.

A special note of appreciation to Mack Boring & Parts Company for lending us the Yanmar marine diesel engine seen in the video. It was Mack Boring (named for founder Ed “Mack” McGovern, Sr. and his post-WWI specialty) that, back in 1974, imported the very first Yanmar diesels, which have since spread throughout our boating world.

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– [Narrator] You know diesel engines are pretty intimidating. It’s a big hunk of hot iron down there, with all those systems and fluids and whirring belts. And then there’s that alarm, waiting to go offwhen you least want it to. Even when it’s running you’re thinking there’s a hundred things that could be going wrong here. But actually, diesel engines usually run pretty well. We really don’t have to worry about ’em too much, as long as we treat them right. Jon Bardo, he’s been working on big engines for tug boats and fish boats and the like for 30 years. And he’s been teaching at the wooden boat school for many years and the ins and outs of smaller yacht diesels. After a walk-through with a seasoned pro like Jon, you’ll know what to keep an eye on. You’ll know what to replace. I bet you’re gonna be surprised at how comfortable you’ll feel getting your diesel up and running smoothly.

– In this series, we’re gonna be walking you through a YANMAR engine, which is an extremely reliable engine, and there’s lots of ’em out there. If you don’t have a YANMAR engine, and you have another brand, all these diesel engines have similar components in similar locations, and once you’ve been through this series you’ll be able to identify them on your own engine. So, one thing probably you’re wondering is how come I got such good clothes on. Well, when you’re working around diesel engines you definitely don’t want to wear good clothes ’cause you’re gonna get quite dirty. But, when the wife tells you what you’re gonna wear before you go out the door ’cause you’re shootin a video, you better listen, because happy wife, happy life. That’s just the way it is. So that’s why I got my good clothes on. We wanna talk about the engine cooling system, both the Raw Water system, or the Salt Water system, and the engine coolant system. We’re gonna start with the Raw Water system, so we’ll go over to the bench, where we have all the parts laid out. We’re gonna start outside the boat where your boat may have an outside what’s called an outside sea strainer. This particular one opens, so that you can then, when the boat is hauled out, or if you have to have a diver, access the thru-hull fitting, which may possibly be filled with barnacles or something that blocks the passageway of water going in to the boat. So, this is the seacock, the sea strainer, and this is a ball valve, which you can see it’s open now. That’s in a closed position. That’s open. The handle will be in line with the seacock when it’s open. This’ll be inside the boat. Very important to know where this is and how to shut it off. Next we’ll have a fitting that screws in to the thru-hull fitting. And attached to that fitting will be a very high-grade, marine, flexible hose, with two clamps at each end. We always like to use two clamps on on the Raw Water system. Next in line, you’ll find in your boat, you may have an inboard sea strainer. This is a perfect example, this is a Groco. And it has a removable cover, so you would shut your seacock off. You would open up the Groco sea strainer, and inside is a little basket that’ll catch all the debris. That would either clog the system somewheres down line, or it would cause debris to enter our salt water pump and possible clog that. So, we want to use this device to protect our water pump impeller. If debris gets in to the water pump impeller it could break the veins off and cause more destruction. Threads right in very nicely, doesn’t have to be real tight, just snug. Then you would go back, you would open up your seacock, you’d check for any leaks, ’cause you don’t want the bilge to fill because you got this cross-threaded. Also, on the sea strainer, is a drain plug. This would be for draining the water out of the plastic bowl here, in case you were to be in cold-weather climate. Next in line will be our salt water pump, which is engine-mounted. It may be gear-driven as this one is, or it may be belt-driven. You’ll see the impeller here. Some salt-water pumps will have what’s called a zinc. Zinc is a sacrificial anode, so that, if there’s any electrolysis in the system, it’ll eat the zinc rather the salt-water pump. And this is a maintenance item that must be checked twice a year. Here is the salt-water pump mounted on the engine. You can see the impeller. You can see the nice O-ring seal, which we really like, as opposed to a gasket. Because usually when you take the cover off, the gasket will shrink. Then what do you do? Also, this pump is belt-driven. This is the inlet port of the pump. Our sea strainer would come before this. Water goes through the pump and comes out this tube, and goes through over to the device that we call the engine heat-exchanger, which is in the lower part of this whole housing. The heat-exchanger goes right through here. We have engine coolant flowing through the thermostat when it opens, into the heat exchanger. And then the coolant flows back, into the engine block, and it continues it’s circular rotation. But what’s happening inside this box? Inside this box we have what’s called the heat exchanger core. And the heat exchanger core is, is either a plate bundle or a tube bundle. In this particular case, we have the engine coolant flowing through the tubes and we have the raw water flowing around the tubes, so that the two fluids can transfer heat from one to the other. And then the engine coolant goes back into the engine. The raw water flows out of the end of the heat exchanger. And here is an example of a heat exchanger core that’s removed from the actual housing. Here you can see the plates that I’ve described. In this case there’s six plates. These plates would be surrounded by engine coolant. The other fluid will enter a port here, and exit here, and flow through the plates, thereby transferring the heat from one fluid to the other fluid. Also, you may find a zinc in your heat exchanger. Some engines have them, some engines don’t, but this is an example. This is called a pencil zinc, with a bronze plug, brass plug. So the raw water or salt water will pass through here and exit through this tube and then will go to the exhaust elbow. And the exhaust elbow is where the raw water is introduced in to the hot exhaust gas stream, so when it exits this elbow, which we call a 180 degree elbow, the exhaust gases will be cool and they will not melt our marine exhaust hose that goes to our muffler. So, now we’ll go back over to the table. And we have another example of a marine exhaust elbow. This end being the inlet, where the engine is. This end being where the marine exhaust hose attaches with two clamps. And we like to see this style of clamp, which is called a stainless steel t-bolt clamp. And we like to see two on there, just to be safe. The salt-water enters here on top, goes inside and is mixed with the exhaust gases on the downslope. We go from here to our muffler. And the muffler is, again, plastic, and would melt if there wasn’t any water in it. So this fills up with water, and the exhaust go through the water, and exits at this port right here. Also, for winter lay-up, this muffler has a drain plug that you can take off and drain the water out. Keep it from freezing and busting. So, when you launch your boat next time, in the Spring, the bilge won’t flood. From this port, the exhaust runs out the outlet here through a marine exhaust hose, which may very well, if this muffler’s blow the water line, may very well go in a U-shape, up, over the water line, and back down again, and in that, the highest point, will be a anti-siphon valve, so that the boat can’t flood, should the stern go below the exhaust pipe. So now we come to, and we don’t have an example but, we come to an exhaust thru-hull fitting, which will be fitted into the boat, there’s no valve, it’s just a through-hull fitting. So the rubber hose will hook on to the inside of that, and on the outside we’ll have a flapper valve, and the flapper valve will prevent any wave action, any water from entering the system going the wrong way. Which could, in some instances, flood the engine, which is not a good scenario that we wanna have.


19 Responses So Far to “Marine Diesel Engines, Part 1 – Overview of the Raw Water System”:

  1. Antonio (Tony) Osse Antonio (Tony) Osse says:

    Another wonderful series. We´re building a Black Crown 30 (Sam Devlin´s design) at home as a family project. Already bought the “prescribed” engine, a Volvo Penta D3 200HP with a STERNDRIVE….. Any chance to cover this type of system and, above all, how to install a new engine + sterndrive to a new boat!!! Thanks a LOT to all of you involved with OCH… Fantastic idea and wonderful result! Oh, and talking about wives, mine got the engine above as a 25th aniversary…and was happy with that!!! hahaahahahah

  2. Avatar Captain Nemo says:

    Terrific video sir! I’m looking forward to watching the rest, but I have a few questions for you, if I may.

    1) In your opinion, which type of heat exhaust core configuration is more efficient and/or lasts longer – the Plate type or the Tube type, or does it not matter?

    2) some of the hoses on the engine in the video didn’t have double clamps. Ought there to be a double clamp on all hoses, no matter which type or where they lead?

    3) Why the HECK do they make mufflers out of PLASTIC? I would think the exhaust gases would definitely melt plastic!

    G Mactye

  3. Herve Depow Herve Depow says:

    I wasn’t sure which video to comment on so chose the first although they were all extremely helpful. Having the same Yanmar used in the demos was an added benefit. This year I changed the oil filter, oil including the transmission, replaced both the primary and secondary fuel filters, and winterized the motor myself and felt confident in doing so thanks to these wonderful videos. I had Calder’s book, the operating manual, and my iPad watching the videos during the process. Sure, it took a while, but I did it and now feel more confident by knowing the motor better. During our holiday in New Brunswick my wife and I made a day trip to Brooklin, Maine and had a fantastic day. Thanks for your excellent work.

  4. Avatar Steven Bergmann says:

    I have taken a 4 hour diesel course and this video series taught me more and was worth the membership fee. Extremely clear explanation. I would like to see part 4 and also I would love a similar series on the fuel system from the tank, throught the filters. Wonderful clear explanation. Kudos!

    • Steve Stone Steve Stone says:

      Hey Steven, your holiday wish is coming true. Kevin is editing the diesel fuel system video(s) as we speak. Exactly what you’ve asked for. Hope you’ll enjoy those as much as you did these.

  5. Avatar Al Velieri says:

    I enjoyed the videos but was disappointed that the remainder of the exhaust system was not addressed. Most of my questions arise from the manifold to the exhaust thru hull.

  6. Avatar william kohler says:

    Real good job.

  7. Avatar Dave Paroulek says:

    Glad to hear there are more videos like these in the works. As first-time sailboat owner, this will be my first time winterizing and this type of info is very useful (and timely)! Thanks!

  8. Avatar Stephen Wolfer says:

    I’ve watched videos 1 through 3 so far, and I’m really happy with what I’m learning. Great job! I look forward to the next set you release.

  9. Avatar Thomas Cockrell says:

    Excellent job! I look forward to the upcoming videos in this series. I hope I can keep my clothes clean too. ;>)

    Are the caps for sale?

  10. Avatar Tom Lucas says:

    Wow, thanks, great information. Where did you get the hat?

  11. Avatar Roger Johnson says:

    I appreciate the “Happy wife, happy life” comment! They don’t look like my boat working clothes either, but you gotta do…

    And thanks for this series. Any chance of covering fuel bleeding and such?

    • Steve Stone Steve Stone says:

      You bet, Roger, there might be 8 to 10 videos in this series before we’re done. Kevin is editing them, and we’ll release them over the coming weeks.

      • Avatar Conbert Benneck says:

        Hi Steve, Our daughter’s boyfriend had to have the same boat as we had; a Tripp-Lentsch 29 with an Atomic 4 gasoline engine. He found, and bought one in Rochester, NY. With the exuberance of youth he said, “Connie let’s go up this weekend and start to bring the boat back to Staten Island, NY”. I told him, let’s go to Rochester and first see what sort of problems you bought before we commit to the delivery trip. In Rochester, the first thing I checked on his new “old” boat was what was at the bottom of the fuel tank? I found a lot of red grit and water…. What’s this? After lots of pumping of water and dirt into a Mason jar, my hand pump was finally delivering clean fuel. I then went to the fuel pump and finally cleaned the carburetor. Eventually, while refueling, I found that someone had replaced a brass nipple under the fuel filler plate with a steel nipple that had rusted on the inside surfaces and dropped rust into the fuel tank for years. The moral of the story, is check the contents at the bottom of your fuel tank so that you “know” that you have clean fuel when the chips are down.

    • Avatar Al Velieri says:

      Why is the mixing elbow above the manifold? I have recently installed a yanmar engine in my sailboat and when I started the engine water entered the cylinders. The manifold looks to be in god shape with no cracks. The boat is out of the water so i filled a 5 gallon jug and raised it above the engine to feed the raw water pump. Still have no clue as to how water filled the cylinders. Any advise.

      • Avatar Jon Bardo says:

        The exhaust elbow mixing valve is higher than the exhaust manifold to prevent water from backing up into the manifold and then into the cyl. that has the exhaust valves open.
        The flooding of your engine may have occurred due to the 5 gallon jug being higher then your exhaust lines highest point. Whenever the sea water is higher then the loop ,water can enter the system causing flooding of the engine. This can also happen Whenthe engine fails to start and cranking continues!! Best of luck, hope this helps, JCB

        • Avatar Jon Bardo says:

          Check my comment on Diesel video 2. Another real happening!


        • Avatar Al Velieri says:

          that is what a friend told me but i didnt think that there would be enough pressure to make a difference. i guess it does. thank you for your response i really appreciate it….keep the engine videos coming,there is a great need for them. most of us are continually scratching our heads.

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