Underwater exhaust Pros and Cons

[Leighb]

We are entering the world of motor boats and are starting to look at various models to see if we can find one we like.

One feature I have noticed is that some seem to have underwater exhausts. What are the advantages? Less noise? However I wonder how you can then tell if the cooling water is coming through OK. Easy enough if you can see - and hear - the exhaust outlet but when it is underwater??

Perhaps a silly question and the answer may be obvious but I would quite like to know.

 

[matthewriches]

I feel the exhaust pipes in the engine room. You should feel the water going though them.

 

[jfm]

On a big mobo you wont use a visual check on the exhaust outlet as a means of checking you are getting cooling water. There will already be so much going on near the exhausts that the gurgle of cooling water would get lost in the noise. You need flow or temperature alarms to tell you when you've lost cooling water flow

 

[MapisM]

I feel the exhaust pipes in the engine room.

LOL, are you staying in the e/r while cruising?

 

[kashurst]

I have had both, surface exhausts can be noisy on some boats depending on speed, and you can see running water and exhaust gases, so easy to check. Current boat has underwater exhaust, very quiet. You can´t see exhaust gases but you can check water flow by looking in your water strainers. Some people put a ping pong ball inside but I find its usually easy to to see bits of sea grass etc whizzing around inside.

 

[matthewriches]

LOL, are you staying in the e/r while cruising?

Yes shovelling coal lol

You'll soon realise from the temp gauges as to whether there is a problem, so as long as they are reliable. We keep a note of them in our trip log.

 

[rafiki_]

Generally, there will be an overheat alarm, but I monitor engine temps very closely on my mobo.

 

[Deleted User YDKXO]

However I wonder how you can then tell if the cooling water is coming through OK. Easy enough if you can see - and hear - the exhaust outlet but when it is underwater??
.

Depends on the boat. On my Ferretti there is take off pipe from the main exhausts which allows a tell tale trickle of water to exit from 2 ports just above the waterline on the transom so its easy to see whether cooling water is exiting the u/w exhausts. I think a few other boats with u/w exhausts have this arrangement

As for the advantages the obvious one is exhaust noise. I've owned boats previously with above waterline exhausts and I found the drone gets very wearing after a few hours at sea especially if you're driving from the outside helm. The other less obvious advantage is that you tend to get less exhaust soot deposits on the transom but that has as much to do with whether the engine is a modern electronically fuelled type rather than an older mechanically fuelled one but certainly u/w exhausts reduce that too

 

[jfm]

Depends on the boat. On my Ferretti there is take off pipe from the main exhausts which allows a tell tale trickle of water to exit from 2 ports just above the waterline on the transom so its easy to see whether cooling water is exiting the u/w exhausts. I think a few other boats with u/w exhausts have this arrangement

As for the advantages the obvious one is exhaust noise. I've owned boats previously with above waterline exhausts and I found the drone gets very wearing after a few hours at sea especially if you're driving from the outside helm. The other less obvious advantage is that you tend to get less exhaust soot deposits on the transom but that has as much to do with whether the engine is a modern electronically fuelled type rather than an older mechanically fuelled one but certainly u/w exhausts reduce that too

Yep all agreed. Normally engine manufacturers insist on a small bypass exhaust outlet above the w/line, if u/w exhausts are fitted. My boat has these too. But at speed mike, surely you'd agree there is so much white foam and stuff around the waterline that you are not going to look at these outlets as a means to check you have cooling water flow, are you? I never do. You need some other kit to tell you that you have lost sea water flow, imho

 

[Leighb]

Thanks for replies, I will have a look to see if can find the bypass outlets. I would not expect to be able to tell when underway, it is just what I do when I first start the engine on our current sailboat, I forgot once and it was the one time I had forgotten to open the engine seacock. Fortunately no harm done and I was alerted by the temp alarm screaming after a few minutes.

 

[MapisM]

As for the advantages the obvious one is exhaust noise.

Yep, that's definitely the obvious pro of u/w exhausts, but it has its drawbacks.
Firstly, the exhaust backpressure can change depending on speed and sea conditions - though that's not so critical when it matters more, i.e. when going fast and with high engine load.
Secondly, if the exhaust pipe goes straight to the u/w outlet, relying on it being below the w/line as the only mean to silence the exhaust, the result is a cyclic noise (arguably even more annoying), whenever in rough sea the sea motion exposes that part of the hull.
In fact, it's not like there aren't other ways of silencing exhaust.
For instance, SL boats (also their smaller ones) are considered by many among the most quiet boats, but I suppose you have seen how much space inside the e/r is taken by their silencers...!

 

[MapisM]

Normally engine manufacturers insist on a small bypass exhaust outlet above the w/line, if u/w exhausts are fitted.

I didn't know that it was an engine manufacturer requirement. Can't think of any sensible reason why...?
And fwiw, I fully agree that it's practically useless, anyway.

 

[rafiki_]

I didn't know that it was an engine manufacturer requirement. Can't think of any sensible reason why...?
And fwiw, I fully agree that it's practically useless, anyway.

My Azi has these. The handbook says that this is for idling. I guess with the boat stationary, there is more back pressure underwater? However, if you want a quick check on system working properly, then the bypass shows this.

 

[MapisM]

I guess with the boat stationary, there is more back pressure underwater?

That would be a good point, if it weren't that the telltale exit is for water only, AFAIK.
And if so, I don't think backpressure can be affected/reduced by the telltale, at any rpm/speed.

I agree that it can be visually reassuring though, at least upon startup, but hardly meaningful while underway...

 

[baylabayla3288]

Having had both systems the underwater system is the clear winner provided that it is done right. A good feature is to have a low speed bypass. This is needed for two reasons. It helps the engine to start by lowering the back pressure and it allows you to see that water is exiting at start up which gives sufficient peace of mind that the impellers are working and taps are open. The exhaust pipes in the engine room, attached to the bottom, need to be moulded fiber glass pipes that extends high enough above waterline to eliminate that a failed clip causes a major leak. The exhaust outlet should exit deep enough under water at speed, to have good silencing effect. There need to be a "spoiler" in front of the underwater opening that pulls out the gases and reduce back pressure at speed. Many builders fit large silencer chambers together with above water outlets but my experience is that while these are silent at idle and low speeds they can be as noisy as straight pipes at planning speeds when the water trap is blown empty. Also these makes the boat pull a steam-soot mixture behind that tend to stick on the back of the boat

 

[Deleted User YDKXO]

But at speed mike, surely you'd agree there is so much white foam and stuff around the waterline that you are not going to look at these outlets as a means to check you have cooling water flow, are you? I never do. You need some other kit to tell you that you have lost sea water flow, imho

Crikey, no! In my case it would need me to hang off the edge of the bathing platform at 20kts and look underneath to see the outlets. Not even I am stupid enough to do that! No, those outlets are really just for engine start up so that you can check that there is water flow before leaving the marina. At speed I'm entirely reliant on the coolant temp and exhaust temp sensors

 

[Deleted User YDKXO]

For instance, SL boats (also their smaller ones) are considered by many among the most quiet boats, but I suppose you have seen how much space inside the e/r is taken by their silencers...!

Bad news on that front, M. We took a look at a 2011 SL72 in La Spezia which had been repossessed by a bank and was for sale. I'm sorry to say that SWMBO was very unimpressed. Apparently she can't see out of the saloon windows when seated, she thinks its old fashioned anyway and the galley is in the wrong place. I've had my bubble well and truly pricked on this one

Yup agree on the cyclic noise. In a big beam or following sea as the exhaust outlet gets fully or partially exposed the exhaust note does tend to rise and fall but I'd take that any day over the constant blare of transom exhausts. And yup I suppose the ideal situation is u/w exhausts in combination with silencers but as you say silencers especially for big engines take up a huge amount of space which could be used for other purposes

 

[jfm]

For instance, SL boats (also their smaller ones) are considered by many among the most quiet boats, but I suppose you have seen how much space inside the e/r is taken by their silencers...!

Agree - best combo is dustbin sized silencers in the e/room then u/w exhaust. But these days with big cabins important to sell boats at boat shows (yawn!) engine room space is critical and many builders don't use big silencers any more

 

[jfm]

That would be a good point, if it weren't that the telltale exit is for water only, AFAIK.
And if so, I don't think backpressure can be affected/reduced by the telltale, at any rpm/speed.

I agree that it can be visually reassuring though, at least upon startup, but hardly meaningful while underway...

At least in my experience MapisM, the bypass outlets above w/line are for exhaust gas, and a little bit of water splutters out inevitably, but they are not cooling water tell tales (like in an outboard motor, conceptually). Manufacturers insist on them to reduce back pressure at idle, when the u/w exhaust can have perhaps 75cm of water head "blocking" it, typically.
Fact remains that they are not meaningful/useful underway as a method to check coolant flow

 

[Danfarry]

At least in my experience MapisM, the bypass outlets above w/line are for exhaust gas, and a little bit of water splutters out inevitably, but they are not cooling water tell tales (like in an outboard motor, conceptually). Manufacturers insist on them to reduce back pressure at idle, when the u/w exhaust can have perhaps 75cm of water head "blocking" it, typically.
Fact remains that they are not meaningful/useful underway as a method to check coolant flow

That's reassuring, I was concerned that my above the waterline exhausts only had a trickle of water from them, even though I could plainly see loads of water going through the filters.
My kids love how the boat 'farts' when we are on tickover in the marina from the UW exhausts.