Trailing Exhaust extensions

Discussion in 'Boat Design' started by sdowney717, Jun 7, 2015.

  1. sdowney717
    Joined: Nov 2010
    Posts: 852
    Likes: 32, Points: 28, Legacy Rep: 274
    Location: Newport News VA

    sdowney717 Senior Member

    I had some hard rubber underwater exhaust extensions that screwed onto the back of the transom. I decided to remove them as they caused a 'rumble' feeling when I sat on the flybridge seat, I could notice it. They did quiet the exhaust a lot, and helped a lot with exhaust fumes.

    SO I was thinking of another way to do this, get a 4 inch PVC connector, the schedule 20 size since it is cheaper, glue in the black pipe adapter to join to corrugated 4 inch drain pipe. Glue the 4 inch PVC to the copper exhaust outlet, then snap in a section of drain tube to the adapter. All parts came from Home Depot.

    This will trail out behind the boat 3.5 feet or so. The drain pipe is very flexible bendy, but wont collapse, made of polyethylene. I have a 2 foot wide Swim platform, so these will extend beyond this, but being so bendy, will easily move out of the way if the boat backs into something.

    My thinking is it will exit underwater as it hangs down. I may run a SS screw into the adapter to help it secure the pipe. The pipe snaps into the adapter. I will tie a small line to this so I don't lose it if it decides to let loose.

    [​IMG]

    [​IMG]
    I cut off 3/4 inch from the end of the black adapter, leaving the second ridge intact so it fits more tightly the inside circumference of the white PVC.
    White PVC glued to Black polyethylene by hand sanding with coarse the parts. Mixed sawdust and PL Premium polyurethane construction adhesive.
    The black adapter has ridges that will physically secure the pipes.

    So 10 foot pipe is $7, 2 black adapters $6, 2 PVC $3, and I had the glue. Under $20 total. When hauled out, simply pull off the tubing from the adapter.

    I will let you know how it works and sounds.
     
  2. sdowney717
    Joined: Nov 2010
    Posts: 852
    Likes: 32, Points: 28, Legacy Rep: 274
    Location: Newport News VA

    sdowney717 Senior Member

    Here is what I took off. They are nice idea. Did that with boat in the slip. Nice to have a swim platform.
    [​IMG]
     
  3. Russ Kaiser
    Joined: Jul 2009
    Posts: 119
    Likes: 2, Points: 0, Legacy Rep: 35
    Location: Winston-Salem, NC

    Russ Kaiser Exuberant Amateur

    Backpressure change?

    Don't you have to worry about significantly changing the backpressure on your exhaust system? Changes in pipe design and backpressure significantly change the performance and economy of car engines. Also, sometimes too little backpressure is just as bad as too much.
     
  4. sdowney717
    Joined: Nov 2010
    Posts: 852
    Likes: 32, Points: 28, Legacy Rep: 274
    Location: Newport News VA

    sdowney717 Senior Member

    I was reading about back pressure too. These extensions may help suck fumes out from the exhaust. Take a look here.

    http://www.sbmar.com/articles/some_thoughts_on_underwater_exhaust_systems/

    At idle, I imagine the ends will hang down a little in the water, dispersing the fumes underwater. As the boat speeds up, I imagine they will start skimming the water surface, so I think they will have less back pressure than the ones I removed and less back pressure when the boat is moving faster. I can easily take them off and on to experiment. I prefer less noise and don't like smelling exhaust fumes. So will see what happens.

    I was also wondering about the extensions I removed, maybe they restricted the exhaust? I added those, they came from a boat being broke up at the marina and were given to me for free, they are a design from the 1960's. My engines are 392 cubic inches.

    My existing setup is twin inboards, exhaust is high risers with 3 inch outlets. 4 foot rubber hoses run to a bronze mixer with a 6 inch outlet and two 3 inch inlets and a water injection of 1.25? inch coming from the heat exchangers.
    From the mixer runs through a bulkhead and onto very large plastic mufflers. Mufflers I think have 6 inch inlets and I suppose 4 inch outlets. From mufflers, runs through a black plastic pipe to a 4 inch copper pipe that exits the transom. All of this steadily runs downhill. Water does collect in the muffler and it has a drain, but I have never drained them for the winter. I am not sure about the muffler size.
     
  5. sdowney717
    Joined: Nov 2010
    Posts: 852
    Likes: 32, Points: 28, Legacy Rep: 274
    Location: Newport News VA

    sdowney717 Senior Member

    Got them on and like them.
    Exhaust is quieter. They sink a little on the end when off.
    When engine starts up they float upwards and are half in half out of the water.

    I smelled no exhaust. Slip neighbor said it was quieter, and we both agreed looked goofy, but we both like them.
    [​IMG]

    [​IMG]
    There is no gurgle sound with engines running, unless I push the ends under the water. They might gather marine growth and get heavier. I could tie them up out of the water, need to think on it.

    I like how they spread off at an angle. means the exhaust gas more likely to flow around the hull instead of hitting the stern and flowing upwards. Although underway, they likely will straighten out some. they are very flexible, more so than a thick rubber hose.
     
  6. rwatson
    Joined: Aug 2007
    Posts: 5,839
    Likes: 277, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    Stay in calm water so the outlet pipes don't get raised above the inlet level on the boat - thereby flooding your engine with salt water. The exhaust pressure wont be a protection.

    The last sentence of the article you quoted "So leave this part of your vessel design or repower to a company or builder well versed in this type of work and has a proven long term record in building underwater exhaust systems."

    Other info I found -
    "Any boat that has a wet exhaust with a discharge opening of about three inches or larger, is at risk. A Muskrat, Water Rat, Otter, or other vermin, can and will, swim up the exhaust to chew through a coupling hose to the inside of the boat or make a den. This also includes boats with underwater exhausts, as these rodents & vermin usually look for underwater entrances, leading upwards to hollowed out chambers. This is how many boats sink, even while tied to the dock. "


    Oh - PS "It was also observed some significant tracks of sea water found its way into the exhaust passages from condensation of the salt water vapour and also from entering some sea water into the exhaust system. "
    O. A. Ozsoysal: Siphoning Sea Water Back Into the Engine in Fast Boats
    http://jmst.ntou.edu.tw/marine/18-4/496-503.pdf
     
    Last edited: Jun 15, 2015
  7. sdowney717
    Joined: Nov 2010
    Posts: 852
    Likes: 32, Points: 28, Legacy Rep: 274
    Location: Newport News VA

    sdowney717 Senior Member

    I will let you all know how it does in the Chesapeake Bay.
    The pipes are self draining where they snap into the pipe adaptors.
    If I lift up a pipe full of water, it comes out at a decent rate at the joint.
    The mufflers are very large ones, and the riser is very high. So there is a lot of volume capacity in the existing exhaust piping.

    An animal could climb into any stern exhaust and setup home. We do have otters here, sometimes they climb up on the swim platform and eat fish. I have seen it one time, other time it left a fish carcass behind.
     
  8. sdowney717
    Joined: Nov 2010
    Posts: 852
    Likes: 32, Points: 28, Legacy Rep: 274
    Location: Newport News VA

    sdowney717 Senior Member

    Well they worked great! Nice and quiet all the way to the middle of the bay, then as I slowed down and shut down the engines to fish, off they came as the boat was rocking in the waves.

    I managed to grab one as I saw it floating away.
    So given enough bending motion, the snap together joint will loose the pipe. There is a lot more weight of water in the pipe than you might think and when the engines turn off the extensions flood with water.

    It will need a SS screw driven in to hold it together more securely. I should have tied them to the boat with some small line.
    Since I cut the original pipe in thirds, I still have 2 pieces of pipe.

    Was interesting, I go slow about 6 to 7 knots, and these tubes curled in, not going out straight. Still they floated on the surface, and I smelled no fumes.

    They do modify the tone of the exhaust.
     
  9. SamSam
    Joined: Feb 2005
    Posts: 3,818
    Likes: 156, Points: 63, Legacy Rep: 971
    Location: Coastal Georgia

    SamSam Senior Member

    In reverse i can imagine the flex pipe folding over with a kink that would pretty much block your exhaust.
     
  10. tom kane
    Joined: Nov 2003
    Posts: 1,766
    Likes: 45, Points: 58, Legacy Rep: 389
    Location: Hamilton.New Zealand.

    tom kane Senior Member

    Trailiing Exhaust extensions

    I tried exhaust extensions like the ones under the cowling in this image but the fumes still were sucked into the boat when moving.
    An exhaust straight up works better.
     

    Attached Files:

  11. kapnD
    Joined: Jan 2003
    Posts: 539
    Likes: 47, Points: 28, Legacy Rep: 40
    Location: hawaii, usa

    kapnD Senior Member

    I was on a workboat that had extensions made of blue discharge hose, a soft hose just clamped over the outlets.
    It was explained that the soft hose would act as a duckbill valve to prevent water intrusion when the engine was off.
    The length was just short enough not to contact the props.
    The exhaust pressure quickly straightened them out when revved, though the huge exhaust leaks in the ER probably prevented overpressure damage!
     
  12. sdowney717
    Joined: Nov 2010
    Posts: 852
    Likes: 32, Points: 28, Legacy Rep: 274
    Location: Newport News VA

    sdowney717 Senior Member

    They are too stiff to collapse. Goto the store and see what they are like.
     
  13. sdowney717
    Joined: Nov 2010
    Posts: 852
    Likes: 32, Points: 28, Legacy Rep: 274
    Location: Newport News VA

    sdowney717 Senior Member

    I have seen boats that exit exhaust out the side. Idea is the exhaust flows out around the hull and wont come in the boat.
    With a stern exit, if the wind blows against the stern, the fumes can be carried up and over the transom.

    I like the exhaust tone with these black extensions and did think it improved not smelling exhaust fumes. It is not like I always smell fumes with it as OEM, just sometimes do.

    At last haul out, I glued on a rubber hose to the Onan MCCK exhaust outlet, and that made a big difference regarding fumes. That hose always stays slightly underwater. My MCCK is all fresh water cooled, and it has a water lift muffler, so there is no chance water would backflow siphon, all the exhaust is above the waterline.
    [​IMG]

    This was an old hose that the steel coil rusted out from, but the rubber hose was still ok. Happened to be an exact fit for the task. I glued to the copper through hull outlet (basically a copper pipe) with some PL Premium Poly construction adhesive.
    It has grown algae but no barnacles.

    Something like this creates a little gurgling noise when running. It can be tied up to the swim platform to keep it quieter.
     
  14. SamSam
    Joined: Feb 2005
    Posts: 3,818
    Likes: 156, Points: 63, Legacy Rep: 971
    Location: Coastal Georgia

    SamSam Senior Member

    i know what it's like. it's cheap flimsy water drainage pipe. maybe it works if you're idling, but give it a bunch of vthrottle while in reverse and then see if that 3' length doesn't fold under the boat, crease and pretty much close off the exhaust.
     

  15. SamSam
    Joined: Feb 2005
    Posts: 3,818
    Likes: 156, Points: 63, Legacy Rep: 971
    Location: Coastal Georgia

    SamSam Senior Member

    If your exhaust outlet is underwater, you cannot say all your exhaust is above ther water line. If you don't have an anti-siphon valve in th exhaust, one that works and isn't plugged or stuck, you have a good chance of backflow siphoning with that set-up.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.