Is a marinized diesel necessary if....

working on a boat now that has SS flex hose off flex mounted engines to dry mufflers then water injected and overboard
Its 17 years old.
Best of both worlds I would say

 

how high does is the exhaust outlet in regards to the water line? If the engine shut down in heavy seas, can water come back into the engine? Sounds interesting.

 

I think what Powerabout describes is common on a lot of medium to large older yachts. The hot piping usually goes up near the overhead in the engine room then comes down and discharges just above the waterline. The water is injected right at the hullside.

This allows passage around the outboard side of the engines (maybe have to duck to miss the pipe) and the water injection at the hullside reduces soot on the hull sides and allows the exhaust to discharge in the engine room instead of making a run all the way aft to the transom.

This setup is fairly common in engine rooms that are midship, saves a lot of room and complexity from running exhaust a long way aft, usually through accomodations.

It would be highly unlikely water from wave action could find its way as far uphill as I have seen is typical in these installations, and there is usually a valve at the hull side.

Another consideration in exhaust installations is allowable backpressure. Dry stack runs add up backpressure fast with every turn and foot in the run, silencers add even more. This means everything has to stay big to allow adequate flows and keep backpressure within limits.

In wet exhaust the water injection adds backpressure, but after that the exhaust gasses are reduced in temp as well as volume allowing longer runs in smaller sizes.

The picture below is similar to what powerabout describes, except this is underwater exhaust.

The black piping is ceramic hard coat lagging coming off the overhead and dropping down into a collecter. The black band just above the silicone connector is a bellows compensator wrapped in traditional lagging. The white piping is water and thats the mixer where its injected just above the blue silicone. The lower pipe going through the hull is the primary exhaust that draws exhaust gas out through the bottom of the vessel with assist from a scoop creating a venturi effect when underway. The upper pipe thru hull is the low speed bypass allowing gas to exit above the waterline at the boot for when the vessel has little or no way on. This keeps the backpressure down and within limits.

Steve

 

Ive never seen a dry exhaust exited at the water line or above, what would be the point . The main advantage of a dry stack is to get clear of any chance of water.

 

." but you would have to give up a LOT of space for dry mufflers,'

Most of the boats I am familiar with install the mufflers outside , plenty of room.

For the quietest setup (but heavy) Google "hospital critical" and see what is installed on generator sets .

Folks with a thick wallet can install wet exhaust manifolds and let a slightly oversized keel cooler dump the heat.

In many New England head boats this is the preferred system as the hot coolant water is fed to the deck hand rail to help the fish killers keep their hands warm.

FF

 

Keysdisease
yep thats normal on a megayacht with the motorised valve
the boat I have injects up high ( below a loop )and then its got flex hose another 15' to the outlets
Long term there is no condensation back up the exhaust like traditional wet with the exhaust elbow injecting on the back of the motor.
Boat builders like wet as cheap quick install and no space issue

 

Now i'm confused. Are we talking about exhaust or water cooling when we say that it is sent to warm the handrails. If it is water traveling as a valved part of the keel cooling unit, i get it. If it somehow has to do with the exhaust, please help an ignorant farm boy out and explain. This thread has gone from exhaust to coolant to exhaust... Both of which are immensely helpful and parts of answering my to be or not to be marinized question.

 

Also, on dry stack, would the fact that the stack went straigh up from the muffler instead of back or down effect the backpressure. Once again, we are talking about an engine of less than 250 hp, or maybe even two of less than 100hp

 

Keel cooling is a captive system. coolant. This is all that is needed to cool an engine.

Saltwater injected into the exhaust...after the manifold.... cools the exhaust and muffles it.
It is possible to go 100 percent dry exhaust with a muffler but the entire system must be insulated and soot, unburned hydrocarbons, accumulate in the dry exhaust. This soot could present a fire hazard. I only know dry exhaust with a muffler on simple open work boats.

 

On a dry stack installation all reasonable lengths of hot pipe run and any turns will contribute to backpressure. I use the term reasonable because if the piping is large enough (but an unreasonably large size) then the length of the run would not contribute significantly to backpressure.

Up, down, sideways, no appreciable difference, turns add the most.

Your engine manufacturer will have recommendations for sizing of exhaust piping as well as allowable backpressure limits. Most engines have very reasonable allowances, so don't be intimidated too much unless you plan some convoluted exhaust run with 720 degrees of total turns of the minimum size pipe.

Steve

 

No odd runs other than one 90 and through muffler and another 90 up the "stack". If i build it the stack, as it exits onto the flybridge deck, will exit into a traditional looking stack house that might also be the base for a mast tower. I have seen one boat where the mast hollow and carried the exhaust up about 5 more feet. This extra length would disconnect when the mast folded and leave the stack shortly above the back of the fly-deck for low bridges and canals. i was pretty handy. As for the boats that have dry exhaust, well they are unlimited in scope from most fishing trawlers to high dollar expedition boats. Most Nordhavn and Seaton's n are dry stack. Most boats where the passengers are more concerned with less through hulls which mean usually means a safer sea boat. Those that have noise or smell concerns that outweigh below water holes in the hulls seem to go for the wet exhaust. Me, i love the smell, sound, and safety of the dry. If i get to build my dream boat, i will even put the generator exhaust up an out rather than down. I will have no through hulls except the drive line. I will go as far as to put external(outside the transom) pipes for the water for air condition,water makers, wash down and fire. I may be oversensitive to the through hull issue, but if i do this, we will be out on the water and making crossings that last a month at a time. rj

 

Your "no holes" concept is the holy grail for many, but often inpractical.

Sounds like you need a Sea Chest.

Steve

 

I'm not a very practical person when it comes to safety. Sometimes obsessive. I am surely not saying that i "will have NO" through hulls. Just that, if i build or refurbish a boat for passagemaking, that i will go out of my way not to. We will see! rj

 

Once you've cut the first hole and sat and wept a bit ,--the second is easier.

 

"i "will have NO" through hulls. Just that, if i build or refurbish a boat for passagemaking, that i will go out of my way not to."

WHY?

Lots of ladies object to tossing a pail of sink water over the side, same for a nightly bucket of poop.

A genuine bronze marine sea cock, with at least 3/8 bronze bolts will take care of most requirements , and can be greased while afloat and re lapped on a haul out.

A seacock that is greased can easily be operated with an outboard shift cable , so knowing valve position and operation with out crawling in the bilge is quite easy.

Beware the SS ball valves with a flange cast on the housing , no repair possible .

FF