Exhaust question, conversion to thru hull

Discussion in 'Gas Engines' started by Skua, Jul 6, 2013.

  1. Skua
    Joined: Apr 2013
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    Location: Hunt's Pier WW NJ

    Skua Senior Member

    With the boat out of the water ,and pretty much gutted for gas tank removal, and interior remod, Ive been thinking. Engine is probably coming out now, with transom and stringers added to the list , so while it's apart why not convert to 4 inch mufflers and thru hulls? And why are all of the exhaust thru hulls out of the water? I intend to put mine under the surface for further sound control, unless there is some reason the flappers need to be above the surface.

    Boat is a 30ft crusier equipped with a SBC 350 and Volvo Penta Aq280 with exhuast through the cavitation plate currently
     
  2. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    There are several issues connected to subsurface exhausts, like the increased back pressure, danger of leaks and the risk of water being scooped up when anchored. Volvo puts the exhaust in the anti cavitation plate that is only submerged when the boat is at rest or moving slowly, Mercury puts it in the prop hub where pressure is low.

    You could install knees and downward directed pipes on the thru hulls. That would eliminate the danger of a leak in the exhaust hose sinking your boat. At speed the exhausts are no longer submerged so there is little or no back pressure.

    I had such a construction for one year but removed it because the stainless is an ideal playground for marine creatures and proved very hard to clean.
     
  3. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    The constant immersion of the exhaust will not allow the cylinders to ever be dry , rust on cylinders eventually caused oil burning and loss of compression.

    Mufflers do work and are less dangerous.
     
  4. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    sterndrives have them inside the pipes as well are just there so you cant get a surge of water back up the exhaust like if you ran fast then turned the key off or boat stationary in rough sea, going in reverse etc
    If you put in underwater you will need a relief above to close to the water surface otherwise very hard to start
     
  5. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    The question is not why not, but why? It will take a lot of space in the engineroom making service difficult and expensive. It will add holes to the boat which increase the possibility of leaks. It will take considerable time an money. If done wrongly it will damage the engine. What is the advantage?
     
  6. iceboater
    Joined: Jan 2011
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    Location: Iceland

    iceboater Junior Member

    leak is a reason for not to take the exhaust trough the transom under the water line like CDK said. But if you use fiberglass piping and laminate it using the right materials, you will reduce the the risk of leaks.
    Another thing to consider if you go under the waterline is that the thrust might slowly the push the boat while not in gear if the exhaust is pointed that way.

    I´m doing a thru hull exhaust on 33ft commercial fishing boat with Volvo Penta D6-330hp sterndrive.
    The reason I do that is to reduce the risk of water getting to the engine if it shuts down while the boat is fully loaded with the water line above the 35cm recommended minimum to the lower bend
    on the exhaust riser or elbow.

    I take the exhaust trough the transom 20cm above waterline with stainless steel tube and then the exhaust will be extended at an angle with flexible exhaust tube to go below the waterline.

    Here are photos of how I´m doing it.

    Axel
     

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  7. Skua
    Joined: Apr 2013
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    Location: Hunt's Pier WW NJ

    Skua Senior Member

    Space is not an issue, I can throw a party in the engine compartment, but the OEM exhaust outlet is very small in comparison to the exjaust at the manifolds. I'm looking to quiet the exhaust thats the reason for mufflers

    Very interesting setup Axel
     

  8. FAST FRED
    Joined: Oct 2002
    Posts: 4,519
    Likes: 109, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    The quietest wet exhaust are from water lift mufflers of a proper size.

    But the proper size does take a good deal of room , and added work if the boat is winterized , as they must be drained.
     
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