Wet exhaust

When I installed GM diesels in Florida Keys type commercial lobster, crab and bottom fishing boats we used to split part of the raw water overboard instead of letting it all go out the exhaust to regulate the back pressure on the engine. The water could go straight overboard but some would bring it on deck and let it run out a hose to use as a washdown when the engine was running. When it wasn't in use it was long enough to reach a scupper where the water would just run overboard.

You don't need all that water to cool the exhaust gas and the excess can be pumped overboard. This does two things, first it increases available horsepower by reducing exhaust backpressure and it gives you a visual that your raw water pump is pumping. The first part being the most important however only a few boats take advantage of this free horsepower.

Also.
Boats with underwater exhaust have a secondary bypass to relieve backpressure from the exhaust when the the boat is not on plane. On plane there is a venturi effect that sucks the gases out.
Others use it as a telltale so you can see if you are pumping water.
Backpressure and or warning related.

Hi,

There is also the issue of a Salt Water Pump whose output is non linear.

There is a danger with this system in some designs of the exhaust OB filling right up when running at higher engine rpm's hence the bypass.

So this unused "free" HP is needed at low RPM and is available then when there is less exhaust gas?

So you are saying that at high RPM there is no back pressure because the venturi effect?

These three quotes do not jibe with each other. One says there is free HP to be gained at high rpm due to back pressure- the next says at high rpm the gasses are sucked out meaning little to no back pressure, the next says the exhaust may fill up meaning high back pressure due to non linear raw water output. How is a pump which works off a engine driven gear non linear? The three answers seem to contradict each other.

Hi,

Its pretty simple, a pump may produce 450 gal a minute at 300 rpm shaft speed (1.5 gal a revoultion)and the same pump might produce 2500 GPM at 500 rpm.( 5 Gal a revolution)

Welcome to the world of the Centrifugal Pump.

They can easily cavitate at low speed and cavitate at high speed but they do have the advantage of a lot more variables then a fixed impeller type like a Jabsco.

How many of these are found on the boats/engines being discussed here would you guess?

Hi,

It depends how many have Centrifugal Pumps rather than Flexible Impeller Pumps.

Of course it does- and I'd guess very few have those as opposed to the impeller pumps. Do you think they are common on the boats being discussed here? ... because I don't.

I think you are correct but this is one site where everything is discussed from a row boat up to ships. Also around the world. In my experience cooling water being diverted is generally for a tell-tale so the operator can confirm that their is water pressure. Occasionally you will see water separators used for sound reduction or to reduce the size of the exhaust system. Once the exhaust is cooled the water is no longer needed but represents a significant volume of the exhaust. On the other hand some of the workboats in my area will use a car radiator and a dry stack. They're not doing this for horsepower or noise reduction. It's just cheap. Often times they'll keep the car transmission too.

Bamboo quoted; "So this unused "free" HP is needed at low RPM and is available then when there is less exhaust gas?"

My comments were for an above water exhaust discharge and a flexible rubber impeller. With this type of exhaust system, the back pressure increases with HP produced and a separate water discharge does reduce backpressure most notable at high RPM/HP.

Hi,

I think that with any diesel engine regardless of where the cooling water goes the BP will increase as the HP increases, this is as a direct of the gas volume flowing through the system.

More RPM means more HP and this can only be achieved by burning more fuel and producing more gas.

Remember it is the Exhaust Gas that drives the Turbo Charger in most engines found on yachts today.

You are talking the most common type of marine cooling system/wet exhaust. The great majority of small marine engines (100-2600 HP) have flexible impeller raw water pumps that cool the engine and then return the water via the "wet" exhaust. I've not seen a centrifugal cooling pump(s) system combined with a marine diesel/wet exhaust system on any type of wide scale with a split or non split water path.

K1W1 do you have an example of a major builder of boats- not ships or large yachts- that uses centrifugal cooling pumps exiting into the exhaust?

Diverting Exhaust Cooling Water

Most likley I can give you some insight into your question the diverting of exhaust cooling water came about due to the new requirements for more power and the engine company requireing lower back pressure standards.
Manufacturers of traditional mufflers need to divert the water in order to avoid excessive backpressure on the engines. You see the baffles and restrictions utilized by these mufflers are used to breakup the sound pulsations and thus reduce the noise created by the exhaust.

The best example of this swing in exhaust cooling water rates is most apparent with DDC/MTU Series 2000 and 4000 engines. When you remove exhaust cooling water you get a very hot exhaust and the truth is it's a very dangers practice. By removing the water you get steam the more water you have in your exhaust the cooler it is also much safer.

Von'Widmann

You are correct in that MTU engines need to have water diverted to keep the backpressure within limits. However, this happens well before you reach the mufflers. An exhaust system needs to be properly designed so that the mixer can still cool the exhaust while having the water diverted as needed. This also goes for the design of the muffler so that its contribution to the backpressure numbers are kept low while still meeting the attenuation levels required. There are other factors that also come into play. Is the system sized correctly, how many bends are there that can be eliminated, etc. To say that removing raw water will cause high temps, is incorrect and not an acceptable option. No one would think of having to choose between having elevated exhaust temperatures and meeting backpressure requirements.