Cooling System - Theoretical

I was thinking about typical fresh-water cooled systems used by marine inboards and associated headaches and costs. An idea occurred to me that is probably nonsense, as I am not much of a mechanic. I wrote it up just to think about it. If anyone is curious enough to give it a skim, I'd be interested to hear from knowledgeable engine people if it's ridiculous or has any merit. The engine setup used for this example are the 240 Yanmar's in my Mainship 40T. Here's the idea:

CoolingSystemIdeaForForum.pdf

and here's the pdf referenced in the above document.

marine-applications.pdf

I think it's a pretty interesting idea, but of course it's a bit more complicated than what you show. For example, the expected flow through the various engine coolers is whatever the on-engine pump circulates, and it can be quite a bit of water. That means the new pump that circulates coolant has to be similarly sized, and might end up being pretty large. If I recall correctly, by main engine raw water flow rates is around 50 gpm. And that pump needs to be powered. Electric? AC or DC? PTO driven? Does it require an impeller or can it be centrifugal so maintenance free?

Engine operation is also now dependent on whatever power source runs the pump, so you will have to consider overall reliability, and any additional redundancy required to match the availability of a stand-alone engine.

You show a single engine with a single secondary cooling loop, but it gets a lot more complicated when you throw in another engine, a generator, hydraulic cooling, etc. Presumably you would only want a single secondary cooling look shared among these engines, so you would need to think about how the plumbing works to distribute the coolant through the boat to the various consumer. Also, the agregate flow rate could be quite substancial, and you need to be sure larger engines can't starve smaller engines. Do you need a circulator pump per engine, sized for that engine? Or will a central pump and distribution manifold sufficiently distribute the coolant as it does in many HVAC system. If it's a central pump, then it probably needs to be variable speed to handle when only a generator is running vs multiple main engines.

I'm sure these are all solvable problems, and it will just come down to how large a boat is required before it becomes practical given the extra components and complexity.

Yes. The type of pump, power source and most importantly I think the required flow rate are where it starts getting into the weeds. I'm not sure how to calculate the flow rate. It may need to be similar to what the on-engine raw water pump delivers, but I'm not convinced of that. The boats that I found that use similar systems are like a thousand feet long so someplace between 40 and 1000 is the sweet spot. It would be nice if the secondary loop pump could be engine driven. The other obvious wave of the hands is on the central heat exchanger - How big, what type, etc.

Thanks for your thoughts on it.

All the propulsion engines I have seen specify the heat rejection to sea water, so with that you should be able to calculate what's needed for your central heat exchanger.

Also keep in mind that not all engines have so many separate coolers. Many have oil coolers that reject heat to the coolant loop, not to seawater. And the same is true for some aftercoolers. Some are sea water cooled, and others are coolant cooled. And of course a generator typically only has a main heat exchanger.

I agree it would be nice to leave the engine package unchanged, and just let it suck coolant from a pre-cooled source. But even then you would need to redirect the coolant outflow back to your chiller system rather than the exhaust elbow, and of course you would still need a source of raw water for the exhaust on each engine.

That was something interesting I learned when looking at this. I did not previously know that some existing systems ran the anti-freeze loop through the cooling side of some of the exchangers. I do show the raw water flowing through the wet exhaust and also the dripless valves (and by extension any place else it needs to go). I think the plumbing would be pretty straight forward, but as you point out that's the easier part.

Come to think of it, my last boat was keel cooled and the aftercooler, oil cooler, AND the gear cooler were all coolant cooled. So in essence you are taking a keel cooled engine/gear and moving the "heat exchanger" inboard rather than leaving it outboard.

That is exactly right....that's how I've been thinking of it. The ships use 'box coolers' and sea chests which is a very close analogy I think to bringing the keel cooler aboard.

So the question is, what advantages does it have over keel cooled? The disadvantage is an additional pump that can fail. I don’t see it as a better or worse idea. I see it as different.

tiltrider1 said:

So the question is, what advantages does it have over keel cooled? The disadvantage is an additional pump that can fail. I don’t see it as a better or worse idea. I see it as different.

Click to expand...

I think the advantage is that maintenance can be done from inside the boat rather than diving into the water. That was one of my big complaints about my previous keel cooled boat.

I'm not knowledgeable about keel cooling, but I believe:

1) most boats are not suitable for keel cooling
2) the engine probably has to be 'keel cooling aware' (designed for keel cooling)
- is this true?
2b) the proposed approach could be swapped into existing engines and I believe in case of pump failure could easily be hotswapped for raw water cooling.
3) I would consider it a pro I think that the heat exchanger is in the boat instead of external.

I agree with your assessment that it's not better. I don't propose it as an alternative to keel-cooling. I propose it as an improvement to typical fresh-water cooled systems where I see people have a lot of trouble and big expenses related to maintenance and repair. Keel cooling has it's own set of disadvantages which are easily googled and have a very low percentage of use.

I'm not sure the additional complexity would be worth it vs. doing proper maintenance in a timely manor to the raw water side of an existing setup. It's really not that bad if you service them correctly.
I've pretty much solved my issues by fresh water flushing the system after use and the boat will be sitting a while.

Just a comment that some aftercoolers are intended to get as cold as possible to up the HP, I would think this approach would have a bit of difference between the secondary cooling water temp and sea temp and you could lose some performance.

The cost/benefit tradeoff perception seems to be variable. A small amount of people go through the trouble of fresh water flushing. As twistedtree pointed out, not all engines have as many raw-water coolers as my Yanmars - which is my point of reference. Fewer coolers means, smaller stakes. My friends 480 Cummins I believe just have after-coolers and heat exchangers (coolant coolers). They just had to replace their aftercoolers. I think the cost came to something like 8K and they had their coolers serviced recently which was another few thousand when it was said and done. It's numbers like that that get me thinking. The equation changes drastically if you are inclined and have the time to do your own work. I would love to get the salt water out of my coolers and that secondary closed loop doesn't seem that complex or expensive to me. On the other hand, setting the boat up to easily/conveniently freshwater flush, seems a lot less complex - but it's another task that gets added to the endless list. Also, some people don't return to home port for long periods which is where the fresh water flushing would tend to happen.

Might want to look at this link. Apparently some like Northern use a hybrid system avoiding the problems with classic pure wet or dry exhaust systems. Don’t know enough to offer an opinion but at least seems a logical way to do it. Still need heat exchangers for lubricants but if I read this correctly you get rid of the one for exhaust.

Hippocampus said:

Northern Marine Exhaust Systems – Northern Marine

Might want to look at this link. Apparently some like Northern use a hybrid system avoiding the problems with classic pure wet or dry exhaust systems. Don’t know enough to offer an opinion but at least seems a logical way to do it. Still need heat exchangers for lubricants but if I read this correctly you get rid of the one for exhaust.

Click to expand...

I would disagree with Northern Marine on this, and think in some ways they have the worst of both worlds, not the best.

The arrangement is dry muffler in the ER, followed by mixing elbow transition to wet exhaust. The whole bypass thing is a distraction. That has been around forever to deal with back pressure issues, and is very common in wet exhaust boats.

The muffler is where I think they made the wrong choice. From all I've seen, a wet muffler of comparable size will attenuate noise more than a same-size dry muffler. Look at how big the muffler is in the pictures. Plus, dry mufflers rot out over time. I've seen a number of replacements chronicled, and it's one of the most miserable jobs I've ever witnessed. In contrast, a wet muffler is zero maintenance and unless overheated will last the life of the boat.