creating a dry riser from NPT pipe

Discussion in 'Powerboats' started by sdowney717, Mar 29, 2016.

  1. sdowney717
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    sdowney717 Senior Member

    I have limited my choice of metal pipe to bronze or SS.

    Pipe diameter would be 3 inch or 2.5 inch, standard thickness metal pipe.

    Water injector would be a bronze pipe attached to end of riser with a short section of 1/2" SS small pipe drilled at a sharp angle and brazed to the bronze pipe. That I already have brazed onto a cast iron riser and has been fine for 16 years in salt water.

    All the pipe pieces would screw together and sealed with Permatex Ultra Copper.
    Then wrap the riser with heat resistant pipe wrap.

    Someone said bronze pipe would get soft from exhaust heat?
    Bronze pipe would be much easier for me to work with. I have not priced any of the pipe yet.
    Any thoughts?

    My current riser is ok, it is a cast iron riser all FWC, but the inside surface exposed to the exhaust is corroding. So I am thinking ahead.

    2 elbows, 2 pieces of pipe (6 inch, 12 inch, I would work with what they have), one flange to attach to the existing exhaust adapter on the exhaust manifold is about what I need.
     
  2. gonzo
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    gonzo Senior Member

    What alloy of bronze can you get pipe on?
     
  3. sdowney717
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    sdowney717 Senior Member

    I can easily get bronze pipe from Hampton Rubber in Hampton Virginia.
    I do not know exactly the alloy.
    What I bought from them in 2000 has been in the salt water exhaust circuit without any corrosion. I brazed it on the end of the cast iron riser.

    Here is picture of my modded riser, to left is the bronze pipe with the ss raw water injector. Scroll down to see,

    https://photos.google.com/share/AF1...?key=NVZlQTNmblNxOXJUd1JHT3dMUDRCR0EtQ2ZmeGxR

    The riser outlet is a 2.5 inch pipe. Inlet measures 2.25 inches, so making a whole new riser would have to be 2.5 inch pipe.
     
  4. gonzo
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    gonzo Senior Member

    Dry risers produce a huge amount of heat in the engine compartment. Have you considered fabricating them out of bronze sheet? By the brazing you show on the photos, your workmanship is really good.
     
  5. sdowney717
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    sdowney717 Senior Member

    Thanks, brazing and welding I find fairly easy but I do have some small experience.
    I am thinking threaded pipe would be the simplest way to go.
    I have been reading about dry wrapped exhaust pipe, apparently can work ok, dont know the cost or what kind of heat suppression.
    I had read that bronze gets soft with heat, so was wondering if wrapped bronze pipe would maybe even sag with exhaust gas flow?

    The exhaust adapter goes between manifold and riser. That was fabbed up from steel, the OEM was cast iron and rust destroyed the part. Rust was also eating the steel part, so I cleaned it and brazed over the steel. The exhaust manifold just on the end was showing some small rust erosion, so I filled with braze. There was no flakey rust there, those manifolds are from 1970 FWC and not available, but they are still ok. I coated with bronze rod just to stop any more rust potential.

    There is another photo of the other adapter from the other cylinder bank.
    I found that coating the steel with Permaflex Ultra Copper sealed the steel from hot salt water and no more rust!
    If you see, each log manifold has 2 adapters bolts to their ends. One photo shows an adapter that was on the boat for 2 years, the ultra copper was intact under the baked on soot. I carefully cleaned some soot to reveal it. I thought that was amazing, it withstood the exhaust heat and prevented rust, the steel was smooth like new.
    So I cleaned off the adapter and gave it a good coat inside and outside. I also coated the end of the other exhaust manifold instead of brazing to fill up the small erosion from the last 50 years of use.

    I could use SS pipe, but do not have the welding equipment t work with SS, although I could learn.
    A new riser really would need not 90* elbows, more like 60* elbows. I know I could cut bronze pipe and rework to get the angles I would use.
     
  6. gonzo
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    gonzo Senior Member

    The outside water jacket will have much lower temperatures (about 140F) so could be fabricated with bronze sheet metal and the edges brazed to the pipes. As for the bronze softening, each alloy has different characteristics. You should be able to get that information from the vendor.
     
  7. sdowney717
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    sdowney717 Senior Member

  8. gonzo
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    gonzo Senior Member

    The exhaust jackets look nicer but are more expensive. The wraps work fine as long as you put enough layers. I usually wrap thin wire over to make sure it doesn't come loose.
     
  9. sdowney717
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    sdowney717 Senior Member

    I was told the bronze pipes and fittings I can get are are 'Naval Brass' It has a more reddish color to the pipes, not bright yellow like brass hardware store fittings..
    All I know is what I used has held up great in the salt water since year 2000.
    I do see it is speced for marine use.
    http://www.nbmmetals.com/collections/naval-brass

    Maybe If I press them, they can tell me more alloy number.
     
  10. FAST FRED
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    FAST FRED Senior Member

    "I am thinking threaded pipe would be the simplest way to go."

    The thru hull folks have nuts that screw on to pipe thread.

    Why bother with welding or brazing?

    The water supply down tube can simply drilled in and be extended in, say 6 -8 inches and a couple of nuts used.

    I doubt that using plain old brass pipe would de-zincify enough to ever cause a problem with most engines lasting only 10,000 hours or so.
     
  11. sdowney717
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    sdowney717 Senior Member

    Problem is the heat issue affecting bronze pipe integrity.

    other issue is angles, you can only get 45 and 90, the pipe near the manifold needs more like 60, upper one can be 90.
    I was thinking, slice pipe take out a wedge, bend down to get an extra 15 degrees, then weld it up. So from manifold to a 45* pipe coupler, then the 12 inch middle do the wedge cut and weld.
    Runs up to a 90 elbow coupler to which my existing bronze water injector can be screwed on.

    The flange at base of riser that attaches to the current exhaust adaptor can be plain steel 2.5 inch threaded flange. I would need to weld up the holes in the flange, drill new ones and tap them.
    Anyway, still thinking no immediate need yet.

    Those 2.5 inch threaded flanges are 1/2 inch thick (good), 7 inches wide (way to wide, would need to be cut down to 5 or 6 inches)

    And for sealing joints, use the white ceramic muffler paste in a tube, good for 2000* F
    Would also give the SS threads a cushion against galling, if that might happen.
    Or stick with the Permaflex ultra copper, I dont know, I have sense in an insulated pipe, the Permaflex would burn, but still it would seal and prevent galling.
    I have galled SS on SS in fine thread bolts, NPT pipe thread is very coarse, so less likely to gall. Other idea is just use anti seize paste.

    Antiseize might be best idea, would aid in assembly, then the oil in the paste would burn out leaving the metal dust and the joint would lock. Not that it would need to ever come apart, but I think if you heated the threads it would unloosen.
     
  12. Chuck Losness
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    Chuck Losness Senior Member


  13. sdowney717
    Joined: Nov 2010
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    sdowney717 Senior Member

    I learned I can stick weld SS with my AC welder.
    308-16 0r 308-17 for 304 series, 316-16 or 316-17 for 316 series.
    The '-17' sticks are for horizontal welds.

    I have been thinking galvanized steel pipe would work, Would be cheaper.

    But of course eventually rust. I have been finding SS pipe is pricey and I would not need much maybe a total of 4 to 5 feet at most.. I called Old Dominion metals, they may have some used SS pipe to sell.
     
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