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Old 11-24-2018, 08:20 PM   #1
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What is it?

So I have this discharge hose coming from my 2008 Yanmar 370hp up to a tee with a gate valve. One side of the tee goes into the muffler inlet, the other side goes thru the gate valve and to a thru hull on the port side. I want to replace the hoses and also replace the (starting to corrode) brass gate valve with a bronze ball valve. Many have told me that brass and gates don’t belong in the marine environment due to zinc content. Before I just change it out I would like to know what it is for and why it was installed.
I’m guessing it is some kind of balancer to control muffler temperature or flow.
Any ideas and is there something I need to do before replacing it like measuring temperature? Hoping the experts have a idea, thanks.
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Old 11-24-2018, 09:39 PM   #2
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If that is the raw water coming from your engine the idea is to only put enough water in your exhaust system to keep it cool, the balance goes to a thru hull and overboard. This helps reduce back pressure in your exhaust. I first read about doing this in a Tony Athens, Seaboard Marine, article.

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Old 11-25-2018, 11:22 AM   #3
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I'd check how much water comes out the bypass and then try to reestablish the same flow after replacement.
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Old 11-25-2018, 11:58 AM   #4
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Here (link below) is some ABYC data pertinent to marine exhausts. They cover suitable materials in Table 1. Perhaps there is newer guidance, but I am not a member and cannot get it on their site. I think using the preferred materials will make things easier in the long run (e.g. future survey, maintenance).

I did not see bronze on the list, so you are on to something.
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Old 11-25-2018, 04:06 PM   #5
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Most big engines are plumbed to send some exhaust water overboard. Many have a line to the shaft seal for positive lubrication of the stern bearing, too.
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Old 11-25-2018, 05:46 PM   #6
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I`m fairly sure a member in Western Australia had a similar issue with the same engine. If I could correctly remember his name, ? Yarrabeen or similar, it would help, but I`m sure this is not virgin territory on TF.
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Old 11-28-2018, 10:13 PM   #7
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On some boats, especially those that have been repowered with a more powerfull engine, that leaves the exhaust system on the small side.


Sometimes the exhaust is simply to small resulting in a too high back pressure which restricts air entry to the engine which limits power or causes dark smoke due to insufficient air.

Changing the exhaust to a larger size can be a serious problem. One fix is as you likely have. Reduce the water flow through the muffler and the rest of the system by sending some of that water volume over the side. That reduces the need for a larger exhaust and reduces back pressure problems.

One advantage is a look over the side tells you the RWP is working.

It takes a bit of fiddling to get the water flow through the muffler right. Too little water can result in a hot or burned exhaust hose or muffler.

But of course some effort must be made to ensure that this will work as expected.

They usually start with too much water, then turn the flow down while measuring the temps to ensure sufficient water is flowing through the muffler to keep things cool even a WOT.

A better explanation can be had by perusing,

The Seaboard Marine site by TOny Athens, which by the way is where I learned about this. His exhaust system articles.

But yes, get rid of that gate valve by going to a bronze ball valve. Just be carefull it is good, real bronze. Buck Algonquin or Groco and some others.

The only not as good part of the ball valve is the adjustment is coarser but some real care should take care of it.
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Old 11-29-2018, 06:42 AM   #8
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Throttling with a gate valve is a NO-NO.

The meek will inherit the earth but, the brave will inherit the seas.
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