Why do boats use water cooled exhaust manifolds?

[chuntington101]

Pretty much as the title says really why do they bother? Is it really just to stop them setting things on fire? Also why aren't moderen heat shields used? Im pretty sure things like Inconel would reduce temps pretty low. I have heard of people with cars that have ceramic coated and then exhaust wrapped the header be able to touch them with bare skin. I guess there isn't ANY airflow around the engine bays within a boat though!

It just seems to me that there is alot of energy being weasted by cooling the exhausts (and thus the gasses inside them). Even more so for anything turbocharged, as this is the energy the turbo(s) need to work.

Thanks,

Chris.

[Brother D]

Couple of things.

Rightly as you state, the heat coming off an exhaust is significant, and especially in an low air-flow environment and this would impact engine bay temps.

But don't forget the water is also used to 'replace' the radiator... This is the primary reason for using water cooling.

[scubadude]

Unlike planes, cars, trucks etc you are sitting in a limitless supply of cooling water, why not use it? :-)

[chuntington101]

Thanks for the replys chaps. I TOTLAY agree for using water to for the cooling systems (both the engine, oil and any intercooling). In fact the boat guys are VERY lucky if you use forced induction! most cars guys would cut of their arm to have a limitless supply of sub ambient temp water to cool the intake air temps! lol

but the question was more around the exhaust manifolds (inc. turbine houseings) and why keep them cool? why not shield or manage the heat and keep the energy in the exhaust gasses (esp on turbocharged apps)?

Thanks,

Chris.

[Brother D]

Ok cool, (sorry not teaching to suck eggs, some people do over look it).

I'm not sure on the offshore power boats and their FI set up, but 99% of boat engines are NA, But yes, primary it is to keep the engine area cool.

Wrapping can have serious issues with cracking standard cast manifolds, and CVD/PVD, doesn't last for ever, and not sure if it could get the reduction in exhaust temps down to the <100 degrees required amibient in an engine bay...

[chuntington101]

Brother D said:

Ok cool, (sorry not teaching to suck eggs, some people do over look it).

I'm not sure on the offshore power boats and their FI set up, but 99% of boat engines are NA, But yes, primary it is to keep the engine area cool.

Wrapping can have serious issues with cracking standard cast manifolds, and CVD/PVD, doesn't last for ever, and not sure if it could get the reduction in exhaust temps down to the <100 degrees required amibient in an engine bay...

Thanks Brother D, no sucking eggs here mate, im a totally new guy to the boating scene so just bouncing ideas/thoguht around. I guess its eaiser to 'manage' engine compartment temps with water cooled manifolds on N/A engines. actually thinking about it on a N/A application there is probably some performance gains to be had in reducing back preusre within the manifold by cooling the air leaving the ports, probably only slight gains though.

Thanks,

Chris.

[Hooli]

I suspect a water cooled manifold is quieter too?

[MOTORVATOR]

Quieter.

Block / oil / fuel etc will need watercooling be it direct or through heat exchanger and then you have to exit the water somewhere anyway so injection into the exhaust is an easy way to get rid of the water.

Engine bay temps kept down which improves performance as intake air is cooler.

Reduced fire hazard.

Not all exhausts are water cooled for the entire length. The injection can take place right at the tip of the exhaust of you want.

Some will use a header and riser that is cooled by the fresh water circulation with salt water injection thereafter.

No reduction in exhaust flow as at least the first part of the exhaust will be jacketed and the integration happens after the riser or even at the tip and these pipes are oversized at that point to allow for it. Typically my big block mercruiser is approx 60 ish mm exhaust riser outlets into a 100 mm combined discharge pipe.

Pleasure boats with outdrives will generally have a through propellor exhaust hub and the water exits there too.

[swanny71]

All explained here

http://www.halyard.eu.com/Knowledge/wet_dry_exhaus...

[tank slapper]

It used to be the case that boats would suck up water and circulate it around the engine before discharging it through the exhaust. This had the downside that salty sea water comes in contact with the engine block and components and can cause corrosion. This wasn't too much of a problem with older engines as they were built properly and there was plenty of material to spare (The volvo in my dad's boat is of this type and is 30 years old and still going strong - it is 13hp and weighs 200kg).

More modern diesels tend to have a closed cooling system like a car, but instead of having a radiator they have a heat exchanger that does the same job. Sea water is still drawn in and ejected through the exhaust, but it never comes in contact with the engine.

Either way, it is somewhat annoying when a blockage or problem occurs with the raw water feed and it stops flowing. The exhaust gases coming from the manifold are no longer being cooled and so rapidly heat up and melt the plastic water trap emptying its foul oily contents into the bilge and filling the boat with fumes. Even more annoying is when you go to replace the water trap and discover that you are being charged £150 for what is a simple moulded plastic box and you have no alternative because you can't continue with your journey without one.

[Simpo Two]

My boat doesn't even have a thermostat - it just sucks up river water, blasts it around the wotsits, then passes into the exhaust and out the back. Runs at 45C, possibly 55C on a hot day!

The exhaust hose is rubber so you wouldn't want it to get too hot!