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closed loop vs. raw water cooling


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#1 jmiska

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Posted 02 October 2011 - 09:45 AM

My old Sunesta has an Alpha system with straight raw water cooling. The boat has only 220 hours, but they're all in the salt. I'm told (and the evidence supports) that the previous owner always kept the boat on a trailer, and flushed the engine. It was a well maintained boat, from an expierenced owner. I always flush the motor with muffs every time I'm done using it, unless I'm going back out in 24 hours or less and it's just tied to the dock. In that case it's moored overnight in brackish water. I know that I'm due for the risers to come off, and am planning on pulling them myself in a few weeks to just inspect the insides of the manifolds and see what is due. The engine always runs about 160* and the discharge water it clean, but I worry about internal leaks at this point and know I need to look in there.

Here are my questions.
1. For a boat that is used like mine, is there a real benefit to closed loop cooling? I've been told conflicting information. A couple Mercury mechanics said there isn't a real benefit, as you end up changing manifolds anyway, and engine rust won't be a problem if I always flush.
2. Does the factory Mercury system isolate the manifolds, so that there is not raw water in the manifolds, only the risers? My big worry is an internal leak into an open exhaust valve and a hydraulic-lock on my engine. Can this still happen with the closed system? Or is that risk eliminated?

If I pull my risers and see I need manifolds, I'm inclined to buy the closed loop kit and change over, but only if it's actually protecting my engine. I don't mind changing risers every few years, and would continue flushing the engine. However, if closed loop meant I could rest assured that a bad manifold won't dump water into my engine, and it meant I'd be safer leaving the boat in the brackish water for a few days at a time, it seems worth the investment.

Thanks.

Joe

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#2 951Tom

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Posted 02 October 2011 - 10:44 AM

The manifolds are isolated from the risers if the system is designed that way. The newer dry-joint manifolds are best if you want to prevent water intrusion.
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#3 Toddler

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Posted 02 October 2011 - 11:59 AM

My 496 has a closed system but it only has antifreeze in the block. So the raw water goes threw the heat exchanger and the exhaust manifolds. So IMO one would have the change the heat exchanger and the exhaust manifolds after awhile if it was run in saltwater.


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#4 jeff

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Posted 02 October 2011 - 01:12 PM

Closed loop cooling systems are available as half or full systems. Half systems only cover the block, full systems alos cover the manifolds, but not the risers.

I had a full system on my last boat, and the manifolds were still good after 7 years in salt water4, 24/7.

In your situation, I don't think that a closed loop system would help much, because you boat is usually out of the water and flushed after each use. If your boat sat in the water 24/7/365, and you could not flush the engine and manifolds or use a Neutra-Salt system, then the closed loop system would help.
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#5 jmiska

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Posted 02 October 2011 - 04:41 PM

Jeff,
I believe you just eliminated a lot of my confusion. Now I know why some say that a closed loop system will prevent water entry into the cylinders, and some say it won't. I hadn't realized there were two kinds of systems.

Do you know if both are available from Mercury for a 5.0 Alpha, or just the half system? I have the impression that the full system may not be. The Mercury mechanics I was referring to were at a large dealership. They said that with the closed system I still had raw water in the manifolds, and that the only benefit was keeping the block clean. They also said that as long as I flush, the block will wear out mechanically long before it rusts out. They never mentioned a fully closed system.

So, a half system doesn't make sense. The full system seems like it would be nice, but, like you say, may not be necessary, anyway. Is it more logical to just make an annual routine of pulling the risers, and can I see enough of the manifolds that way to know if they're OK or not? I've never had them off before, and never owned this kind of drive system before.

Thanks again.

Joe

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Boat name: Watermark

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#6 cyclops2

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Posted 02 October 2011 - 07:08 PM

Look at your car cooling system.

Your CLOSED car system cools everything, BUT, the HUGE HEAT load comming off of the exhaust system.

Most boat "CLOSED" systems are for everything. EXCEPT. The exhaust heat of the exhaust manifolds, exhaust piping and the muffler if used. The cost, weight, bulk and SIZE of a "closed" cooling system of the exhaust system is LARGE.

Water getting into the engine from the watercooled exhaust manifold /s is ONLY from VERY POOR design by a so called engineer.

#7 NNV MIKE

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Posted 02 October 2011 - 10:28 PM

jmiska,
Jeff is correct, and you've got it correct. The half-systems only protect your block, the full systems, all the "wet stuff".
Again, think of your car.
In your situation, I don't think I'd spring for the full system (don't bother with half) until I was looking at a major overhaul or repower.
My Chap has a full system, and came originally from the Puget Sound. Clean as a wistle. Well cared for, too. To be sure, but the cooling system yakes away a bunch of work and wear.
Of course, with the cooling system, raw water will still reach the heat exchanger and some associated plumbing. A good flushing after each outing in Salt water will keep your system working correctly for years. Mine is now 15 years old, and passed a pressure check with flying colors.

Hope it helps, and welcome to the forum.
Mike

Jeff,
I believe you just eliminated a lot of my confusion. Now I know why some say that a closed loop system will prevent water entry into the cylinders, and some say it won't. I hadn't realized there were two kinds of systems.

Do you know if both are available from Mercury for a 5.0 Alpha, or just the half system? I have the impression that the full system may not be. The Mercury mechanics I was referring to were at a large dealership. They said that with the closed system I still had raw water in the manifolds, and that the only benefit was keeping the block clean. They also said that as long as I flush, the block will wear out mechanically long before it rusts out. They never mentioned a fully closed system.

So, a half system doesn't make sense. The full system seems like it would be nice, but, like you say, may not be necessary, anyway. Is it more logical to just make an annual routine of pulling the risers, and can I see enough of the manifolds that way to know if they're OK or not? I've never had them off before, and never owned this kind of drive system before.

Thanks again.


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#8 jeff

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Posted 03 October 2011 - 11:58 AM

Jeff,
I believe you just eliminated a lot of my confusion. Now I know why some say that a closed loop system will prevent water entry into the cylinders, and some say it won't. I hadn't realized there were two kinds of systems.

Do you know if both are available from Mercury for a 5.0 Alpha, or just the half system? I have the impression that the full system may not be. The Mercury mechanics I was referring to were at a large dealership. They said that with the closed system I still had raw water in the manifolds, and that the only benefit was keeping the block clean. They also said that as long as I flush, the block will wear out mechanically long before it rusts out. They never mentioned a fully closed system.

So, a half system doesn't make sense. The full system seems like it would be nice, but, like you say, may not be necessary, anyway. Is it more logical to just make an annual routine of pulling the risers, and can I see enough of the manifolds that way to know if they're OK or not? I've never had them off before, and never owned this kind of drive system before.

Thanks again.

As long as you are flushing the engine after each use, you are also flushing the manifolds. I don't think that you will see as much corrosion when boating in salt water when the systems are flushed after each use. It's also pretty rare that a manifold will fail and allow water into the intake or exhaust valves, especially with the system being flushed of salt water like you are doing.

As to full vs half systems, Merc may only have a half system as factory, but there are aftermarket companies that have full closed loop systems for Merc engines. A Google search should find them.
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twin 4.3L Volvo Penta OSi engines
twin XDP composite outdrives now Ocean X drives...finally
Boating out of Long Beach, CA