Understanding Lift Muffler Operation

[Gary Marshall]

I have been pouring over Tony’s articles on exhausts, wanting to improve my own. I would like to understand the lift muffler operation a bit more.

Designing a Marine Exhaust System

When the engine is in operation, how full is the lift muffler? Does the pressure of the exhaust gases keep it practically empty, so it is only the water draining down the hose from the mixing elbow which might drain into it when the engine is shut off?
As if it stays relatively full of water when the engine is on, then it would be full when the engine is shut off (so would be pointless), but I am just making sure this isn’t the case.

Would a safety feature then be adding a drain valve in the lift muffler that one could open if the sea was rough, and let any sea water that got into it just drain into the bilge where it would be pumped overboard (rather than filling the lift muffler, and risk getting back into the engine if the seas are rolling heavily)?

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[Gary Marshall]

Thanks Tony. While I read all these articles a few years back, they seem a lot clearer to me now with the benefit of a bit of experience.

I understand better now, so it looks like what you have labelled as the “muffler dip tube” should extend down fairly close to to the bottom of the lift muffler, say 3 or 4 inches off the bottom, and the water level would stay in line with the bottom of the muffler dip tube when engine is on.

Does adding a lift muffler of this nature to an exhaust system generally add a lot of back pressure? (it’s a yanmar 6lpa I have installed)

I don’t have the gauges and fittings to measure back pressure, so I am just hoping to not to increase it by too much. Also was reading your tips about possibly reducing back pressure by diverting some of the exhaust water out of the boat before it enters the mixing elbow (not sure if you have any such inline fittings to add, I suppose about a 3/4″ hose or so).
I suppose reducing the waterflow out of the exhaust might reduce back pressure a bit, but if too much is reduced, the temperature of the exhaust might increase. A lot of variables to study for a person who didn’t have a clue about this stuff a couple of years back!

[Tony Athens]

Water level inside the muffler when running is as shown in the drawing.. When you shut down, the water level may rise 1–3+ inches depending on the outlet design on the system after the muffler.

Your sakety question–Too may varibles to give your a yea or nay, but in general, yes. Drairing then system when shut down could add safety if the the syyem was designed with marginal / proper engineering..It’s all about “gravity” and proper desigh, meaning the size and elevation of all the components within the system in realtion to each other and the LWL of the vessel. .

2 users thanked author for this post.

Brad, Gary Marshall

[Gary Marshall]

Picture on the article I am referring to.

[Author]

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