Water cooled exhaust- WHY?

[kenimpzoom]

Why is the exhaust water cooled on a I/O boat? I've wondered this for years and I know there is a good answer, I just cant come up with it.<br /><br />Seems to me tons of problems could be eliminated by putting a radiator on the engine and run it like your car.<br /><br />Ken

 

[DJ]

Re: Water cooled exhaust- WHY?

kenimpzoom,<br /><br />The reason for water cooling of exhaust is because of the tremendous heat given off by the combustion process.<br /><br />Marine engines work very hard, thus exhaust temps. can easily reach 1000 degrees F. or better.<br /><br />Boat manufacturers typically put the engine in as small an area as possible to save deck and interior room. <br /><br />Because of this, there would be no way for the exhaust manifolds to disapate heat like an automobile that is sitiing in open air.

 

[kenimpzoom]

Re: Water cooled exhaust- WHY?

Seems like to me with the proper shielding and a little extra forced ventilation, they could get away from it.<br /><br />Most modern car engines are more cramped than any boat I have seen.<br /><br />I guees they are worried about the fiberglass melting, but again, with proper sheilding, that could be eliminated.<br /><br />I always believe in K.I.S.S.<br /><br />Just my 2 cents, Ken

 

[DJ]

Re: Water cooled exhaust- WHY?

Ken,<br /><br />Possibly so (shielding) that is. However, if you shield it, the heat is still there. If you ventilate it, you run into the concerns of noise, complexity and potential water ingestion.<br /><br />If you are thinking of trying this, I would not.<br /><br />There are some inboards and I/O's that do not run water cooled exhaust, however these are typically in boats where the engine is more than half exposed to open air.

 

[kenimpzoom]

Re: Water cooled exhaust- WHY?

Not trying it, just curious.<br /><br />Ken

 

[DJ]

Re: Water cooled exhaust- WHY?

Ken,<br /><br />That was a very good question though.

 

[Capt Ken]

Re: Water cooled exhaust- WHY?

Cause the CG says to, that's why!!!! No really, Coast Guard regs requires water cooled exhaust unless you have proper insulation around the exhaust system and proper spacing around the exhaust area. Commercial boats have always used a dry exhaust system that is wrapped in a fire retardant material. The exhaust then cannot come within (I forget) so many inches of anything flammable. Two years ago I was captaining a friends sportfisherman when I saw a vessel pouring black smoke from the cabin. The commercial fishing vessel had just undergone a refitting and the mechanics had routed the dry exhaust too close to some wood in the cabin wall. We stood by with our pulpit hanging over the back deck while they fought the fire in case they needed to unman the vessel quickly. In case you're wondering, I made sure there wasn't any propane or other gasses on board before doing so. Diesel I wasn't worried about and the Coast Guard was tied up and wasn't able to respond for a while. The mechanics that did the work called later to thank us for the help and offered to perform a free safety check of the vessel I was on. Yeah, right!!

 

[geriksen]

Re: Water cooled exhaust- WHY?

It is just a practical matter. Where you put the radiator on a boat? But you have unlimited water to use. Plus after the water cools the engine, you have to dump it out somewhere. why not cool the exhaust while it is on the way out? I think it is a great design. Now if you could get those impellers to last longer you would really have something.

 

[BobZ]

Re: Water cooled exhaust- WHY?

In regard to the analysis of a boat exhaust to an automobile.<br /><br />Running down the highway at 70 MPH in a Chey with a 454 under the hood would put your RPMs at about 1800 - I would estimate. <br /><br />Pushing my 25 ft. Grady at 25 MPH with a 454 under the box requires approx 3400 RPMs. <br /><br />I would guess that that would mean twice the exhaust gases and twice the heat through the manifolds.<br /><br />Although I leave the bilge fan on all the time while running, my boat engine does not get anywhere near the air flow that comes through the engine compartment of the Chevy. <br /><br />My thoughts.

 

[coupedeville]

Re: Water cooled exhaust- WHY?

You are actually asking two questions. The first is about using an automotive type cooling setup; ie. an air to water radiator. In a car you don't have much choice. You could run an air to water system in a boat - ever seen an airboat? But the packaging would be inefficient. Water can carry off much more heat for a given volume than air can. And, of course, your boat is floating on water, so why not use it? Combine that with the fact that there is an airflow under the hood of any and every car that helps remove heat from not only the radiator, but the whole engine assembly.<br /><br />Your second question deals with a water cooled exhaust. Two reasons for it: Firstly, those exhaust can get very, very hot under load. They would have to be wrapped in some sort of insulation to keep from overheating the rest of the engine compartment and causing a fire. You would also have to increase the size of your engine compartment by a considerable amount. On a small vessel, that's going to really eat into cabin/deck/storage space. Not only that, but you'd need very large air intakes, and probably a fan of some sort. Secondly, there's the matter of noise. There are no mufflers on a boat. The act of spraying cold water into a hot exhaust stream takes some of the energy out of the exhaust. The energy is used to heat the water up or turn some of it to steam. You can buy dry systems, most competition boats run dry type exhaust, and you can hear them for miles. Imagine sitting on the deck of your boat at high speed with straight exhaust coming out of the back. It would be mighty noisy.<br /><br />This is how hot exhaust can get under a load:<br />

<br /> <br />You want that in your engine compartment?

 

[Boatist]

Re: Water cooled exhaust- WHY?

I like the water cooling, It works great in fresh water. Even if have a minor leak never run low on water like in a car.<br />Saltwater is where I have a problem.

 

[kenimpzoom]

Re: Water cooled exhaust- WHY?

I work on offshore oil rigs. Our cementing pumping unit had two great big detroit V8 diesels. They had insulation on the exhaust. You could touch that insulation with you hand while it was running.<br /><br />I am certain that water cooled exhaust was required in the past, but with new technologies, I just think there might be a better way.<br /><br />With all the water ingestion problems mentioned here lately, just thought I would throw it out.<br /><br />As far as a radiator, some boats have a vertical area that encloses the engine. You could rig up a radiator right there and have electric fans do the cooling. Seems simpler to me. Each has its pros and cons.<br /><br />Just wanting to stir up the pot! <br /><br />Ken

 

[DJ]

Re: Water cooled exhaust- WHY?

KZ,<br /><br />Keep stirring! <br /><br />Anyway, there are radiators in some boats. They're called heat exchagers. Some just cool the engine and others cool the engine and manifolds.<br /><br />The issue with heat exchangers is expense and complexity.<br /><br />You can add a heat exchager system if you like. They cost anywhere between $500.00 and $1,000.00 for the average I/O or inboard.

 

[coupedeville]

Re: Water cooled exhaust- WHY?

Again, don't confuse water cooled exhausts with water cooling of the engine. The water ingestion problems you read about here are related to the cooling water being injected into the exhaust stream and having it [under certain conditions] flow back into the engine. You could route the water from the manifolds to be dumped overboard directly, without entering the exhaust stream, but it brings up the noise problem.<br />As far as using water to take away the engines heat, you'd need a mighty large air radiator to take away the heat from a boat engine, maybe 4x6 feet, like on a large truck. I can't think of a single application, no matter how big, that uses an air to liquid cooling system on a boat/ship [outside of the airboat Ipreviously metioned].<br /><br />Those big Detroit's on board the oil rig? Bet you have a team of mechanics to keep an eye on them, and I'll bet they don't sit in a small enclosed box below decks.<br /><br />The system in use on small boats today is efficient and relatively cheap. It will probably change in the next few years though, as catalytic converters are mandated for boats. Catcons don't like wet, cold exhaust.