Installing remote drains in mufflers...thoughts?

Thanks, yes I meant CO detectors. I just put my arm inside as far as I could get it, the muffler is just open inside, air will easily enter from the 6" exhaust exit.

I have plenty of Garolite and similar materials at the shop I consult at as an engineer. I'd helicoil into Garolite threads if I went that way, or use some kind of thread insert. Garolite is tough, but not like steel for threading. More likely, I'd just make a stainless plate and tap it NPT for the valve.

I can see the structural advantage of bonding in a larger threaded block. I'm concerned about making a much larger joint area with another block of material, that has to be sealed around, and could trap water. The finished extra structure around the new bung should be solid, with the pipe thread reducer acting as a thread insert, but bonded much more solidly to the glass than an insert tapped and turned in. That should ensure the 1/2" thread reducer distributes the stresses more widely than simply tapping a 1/4"NPT hole for the valve.

Right, I could simply pump in anti-freeze from the exhaust. The side-exhaust is only a couple feet long, easy to pump anti-freeze into the mufflers. For winterizing, the raw water will have to be run with anti-freeze anyway, possibly enough to protect these mufflers. It spent 12 winters north of Toronto, and my fiberglass guy seems to think this was a manufacturing error, not cracking from expanding ice. So it survived winters as-is, and my fancy valve mod is probably not as valuable as I thought. But I still prefer being able to drain the mufflers, seems proper. I use heated storage, so this will be for the next owner.

The existing drain plugs were put in with some kind of adhesive, not threaded in, and survived an unknown number of years. I'm overthinking and overkilling this, but learning a bunch from you guys in the process, thanks.

Bakelite? Like old phones were made from? I don't remember seeing any fiber in bakelite when I've broken it, just granular surfaces at the break. Didn't know that was the same stuff.

"Are you not the most wicked and ungrateful of reptiles? But for me you must either have given up your journey, or have been drowned in that stream, and what is my reward? If it had not been for the armour which God has given me, I should have been stung to death." "Blame me not," said the scorpion, in a supplicatory tone, "it is not my fault; it is that of my nature; it is a constitutional habit I have of stinging."

I assume a professional chef spends way too much effort making perfect toast, as a habit, like I spend too much effort solving every little engineering challenge. Time and money are part of the equation at work, but at home I get to indulge a bit more. In either case, simple is always the better solution.

Until talking to the fiberglass guy last night, I assumed there was a serious freezing issue, without a simple solution. Also, I'm uncomfortable with large amounts of water sitting anywhere in my boat while stored if it can be avoided. I drain and blow out everything even in heated storage. And water mufflers simply have to have useable drains, these things hold a lot of water. Pumping anti-freeze into my mufflers with a hose would work but is not simple. Vacuuming the water out with a hose maybe, but hard to know how much water has been vacuumed, and a wet-vac full of anti-freeze is again not simple.

Hopefully a surveyor would not find fault with a valve instead of a plug. Worst case, he would recommend replacing the valve with a plug, to remove the remote possibility of the stainless valve failing.

No, not a Carver. An amazingly well preserved Cruisers Yacht, gas. Something simple, my gateway drug back into boating, 30 years after leaving the marina, boat, and charter business. Kind on an experiment on my family, to see if they will truly participate in boating, or get too busy. I'm enjoying the heck out of this little boat, but will sell it and buy a small plane if they don't show some enthusiasm for it. If they love it, I'll look at something bigger. So far, after over a year, there is enthusiasm, but competition with cellphones and social media is fierce. It is a different world than what I grew up boating in. Here we are entering the marina, headed for the slip on a beautiful evening. At least they have confidence in my docking skills.

Mod edit: Oversize image removed.

I use heated storage. I still drain the raw water and everything I can, to prevent things getting slimy, zebra mussels, and just in case the building loses heat. I might store it outdoors in the future, save a few thousand $, but I'm still enjoying going out and tinkering on it during the winter.

They were plugged, each had a different screw held in the holes with sealant, obviously not stock. That is odd, because the rest of the boat is so un-molested and original. The original owner did nothing with the boat but start it, and drive it out to his summer home on an island. He had it professionally winterized and summerized, fluids and filters changed, batteries topped, not much else was touched that I can find. I guess whoever did the winterizing put those plugs in the mufflers? There are no threads in the holes in the mufflers, so I wonder if someone drilled those holes, or if the holes are original and someone lost the original plugs.

Normally I'd expect the engines would be run with anti-freeze, then drained. Draining the mufflers and any other accumulation of water would be normal. There is faint pink stain under the engine drains, and under the mufflers at these holes. Someone at some time squeezed back there and drained these mufflers after running anti-freeze through them. ​

Yes, I use thread inserts often when building industrial machinery. This is basically a thread insert, epoxied into/under the existing hole in the muffler. I can't think of a simpler solution that fixes the need for a threaded drain in these mufflers. Threading a valve into it instead of just a plug, that is adding a potential failure mode, I'm aware of that. But a pretty unlikely failure, and not a catastrophic failure.

A 1/4" stainless ball valve is small and sturdy, reliable, well suited for this application. If it ever plugs, it will be a straight hole I can easily poke through, or unscrew the valve to make a bigger hole. If it leaks, I can plug it with a lot of things to get home.

Putting a threaded plug into this thread insert will require two tools with both hands, and there just isn't access. I should be able to get one hand on this valve, or make a tool to reach in and turn the handle. I bought valves with longer handles for that reason.
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That can't hurt. I described what I'm doing in an email to Centek, will see what they think.