Dry exhaust?

[PeterSibley]

Does anyone out there have any experience with dry exhausts,something that could be used on a sailing boat?(36'/30-40hp) I've been thinking of ways of having one less hole in the hull!

[Barteld]

The question, I think, is not about the exhaust, but about how you are gonna cool the engine. If you want a dry exhaust with a conventional cooling system, you'll neet an EXTRA hole to get rid of the cooling water.

The only time you'll need a dry exhaust, is in a setup like mine. I have a keel cooling system at the aft end off the long keel. The system contains about 80 litres of water, and conveys engine heat via the hull to the surrounding water. This is on a steel boat, on a wooden boat you'll probably need external pipes or something to convey the heat. The advantage of this system is that you don't have any holes in the hull for your cooling system, the disatvantage is the weight.

The exhaust on my boat is something like a normal car exhaust.

[SchoonerSaraB]

I have a dry exhaust system on my wood sailboat. Basically its a piped loop that runs along the keel(outside).The engine coolant is circulated and the seawater cools it and it also circulates around the muffler helping to cool that. Pluses(No need to worry about a raw water strainer clogging,and when boat is kept year round in the water ,as I do,as long as the anti-freeze is topped off you can run the engine periodically.Drawbacks: 2 holes nedded in hull for keel cooler, Hot exhaust pipe,also muffler tends to be loud. With a wet exhaust all you have to do is look at the exhaust to make sure the water is pumping. When I change the current 51 year old engine it will have a wet exhaust.

[SchoonerSaraB]

Wasn't sure if I was clear. A dry exhaust is a closed system, like an auto, with the keel cooler being the radiator.

[PeterSibley]

Barteld,
thats the kind of system I'm looking for.I was at a boat yard last week and noticed a few boats with keel cooling (wooden hulls) with outside cooling pipes.It seems that an exhaust sysytem that doesn't require cooling water would be a good thing.....one less hole in the hull.Are there any suggestions re taking an dry exhaust line through a wooden hull?

[PugetSound]

Don't overlook the importance of the noise impact with dry exaust. The dry exaust is considerably more difficult to muffle (without screwing up the engine performance). Depending on how big your engine is and how often you run it, a dry exaust can get old fast. I have seen commercial ferry boats switch from dry to wet exaust simply because of the noise factor.

[PeterSibley]

Thank you SaraB.....very good info.The primary reason for my interest is the number of plastic bags floating around in my coastal waters,one of these little horrors in the water intake would probably cause all manner of trouble,either into heat exchanger or exhaust cooling water.

[mmd]

A dry exhaust system will almost definitely be noisier than a wet exhaust. With a dry exhaust you will also have to deal with very high exhaust stack temperatures. The exhaust pipe can be lagged with heat shielding material in the hull interior, but the fitting at the hull will get very hot repending on the distance from the engine. The "fewer holes" argument won't hold water (pun intended) either; water-cooled will have water intake and exhaust outlet, dry exhaust will have coolant out and coolant in fittings plus exhaust out. Finally, an external heat exchanger, unless very cleverly installed, will cause more appendage drag than a mere intake strainer.

[Scott Rosen]

What mmd said. The only reason I could think of for installing a dry exhaust would be if you were operating in frigid temperatures for extended periods of time.

You'll have a lot more holes in your hull for an external heat exchanger and an exhaust. Plus with the external heat exchanger, you will have two holes below the waterline, whereas with a "wet" system you only have one--the intake. Most people with wet systems have a two-stage filter for their raw water: a strainer on the hull, which should take care of the plastic bags, seaweed and such; and an internal strainer, which is much finer and which will catch any particles large enough to foul your impeller.

[Thaddeus J. Van Gilder]

the diesel in my pilot cutter was dry exhauset, AND aircooled.

That kindof worked, other than the engine being 57 years old...

[TomRobb]

Diesel trucks used to be defening as you passed them on the highway. Now you mostly hear the tires singing against the pavement. Somebody's figured out how to silence them w/out killing their power. The intake noise is probably the loudest part these days. And boats don't need jake-brakes :D

[mmd]

TomRobb - check out the size & complexity of the quiet diesel truck exhaust system as opposed to a wet exhaust system. Space on a boat is usually pretty resricted, a problem long-haul trucks rarely have. Also, having the exhaust stack 8 feet above the normal person's head and directing the sound emissions upwards helps with the impression of quiet which is not usually an option on sailboats.

[Noah]

I would also think that the Turbo's on modern engines to much to quite them down. It is amazing how much quiter a turbo car is compared with the same engine sans turbo.

Noah

[Adam C]

All of the Nordhavn yachts we build are dry stack. We build about fifty boats per year with this system. Keel cooling and dry stack are not related, and while often found on the same boat, they are for two different purposes.

Dry stack is not loud if run up past about 20' high as it is on the Nordhavns. There is no perceptible odor. There is also no water or air cooling required. The stack sits inside of an annular enclosure, and ambient air is run in the space between the stack and the enclosure to prevent fires. The stack is wrapped in a heavy exhaust blanket until it penetrates the cabin top.

In my opinion, Dry Stack is barely viable on a powerboat, and I wouldn't do it on a sailboat. In 1989 when we were developing the Nordhavn 46 we had several near fires while refining the design which has been flawless for over a decade. Still, your boat may as well be a 'hull#1' prototype of a dry system, and you might have several 'accidents' along the way. Don't do it.

[winslow]

Some interesting information on the topic on the Metal Boat Society Forum. See the section titled: Exhaust, cooling, shafts, engine mounts...

http://www.metalboatsociety.com/phpBB2/index.php

[ 06-10-2002, 12:39 PM: Message edited by: winslow ]

[Rob Barker]

With respect to the closed cooling system with bilge coolers: Do you have an expansion tank, and does it have a pressure cap?

I'm putting together a small (15') launch with a Palmer PW27 single cylinder gas engine. The boat already had the bilge coolers in place so I thought I'd use them. I rigged up an enpansion tank from a MG Midget. I don't know if it's large enough, and it has a pressure cap so I expect the cooling system will be under pressure as it heats up. Any thoughts on that?

The exhaust runs through a small muffler out the side of the hull. It's wrapped in lagging and gets kind of hot, but it's surprisingly quiet.

[rodcross]

What MMD and Scott said, I agree, with a little more emphasis on the side of urging you to dismiss the idea of keel-cooling as well as dry exhaust.

Injecting water into the exhaust just after the manifold absorbs about 90% of the exhaust noise and eliminates a need for all but the most basic mufflers and eliminates any concerns about hot exhaust and insulation on a 36' sailboat. Sailboats are slow enough without dragging some external plumbing under the boat. Rub the bottom against some rocks and you may not have any cooling.

The whole idea just doesn't make sense to me.

[ahp]

I had a British built auxillary sloop one time with a single cylinder, slow speed, two cycle engine made by Stewart-Turner. It was only 4 hp, a lot less than your interest, but it had a dry exhaust and worked very well and was quiet. Good muffler I guess. The exaust line to the transom was wrapped with asbestos, which is probably not acceptable now.

One thing I don't understand in this thread is the assumption of dry exhaust with a keel cooler, or any other closed cooling system. You don't need to have a closed cooling system with a dry exhaust system. On my boat the cooling water came through a seacock and strainer, was pumped through the engine block and then went overboard through another through-hull fitting above the waterline. You could do the same. The bad thing about once-through seawater cooling is that your engine block is going to rust through sooner or later. Mine was still good after 5 years when I sold it.

[Peter Duck]

G'day Peter, I have both a dry exhaust and keel cooling on my gaff ketch here in Melbourne. I instaslled the dry exhaust because the exhaust outlet is high up under the counter stern, and I was not confident of the power of the exhaust gas to push the water up that far. Also, when I bought the boat, the engine was siezed up due to saltwater vapour from the raw water cooled exhaust rusting one of the pistons through an open valve. The keel cooling was on the boat when I bought it, and I was glad of that. The exhaust pipe which I made up has water jackets where it is straight. They were not easy to make, and the plumbing for them is a rat's nest as the water from them goes to another keel cooler on the other side of the hull. The cooling pipes are placed in the tuck of the hull where they will be most protected. There is a Jarrah wedge at the front of the pipes to deflect anything which may brush against the hull. The exhaust has a Stainless steel truck muffler, which, by itself was not annoying. However, during a recent engine replacement I had a vertical mismatch between the exhaust manifold and the exhaust pipe, so I made up a stainless water trap which also acommodated the two levels, and now my exhaust is silent.
Peter.