Water Ingestion Into I/o Merc Engine

Discussion in 'Powerboats' started by Boatwiser, Sep 22, 2015.

  1. PAR
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    PAR Yacht Designer/Builder

    Tom let's face it, you're a little odd in regard to the imagined safety of gas systems on boats. Nothing wrong with this, but a bit unnecessary for the average boat.

    There's no "ram" effect from your insulated or cooled header, though there are some scavenging benefits that can be realized. For these benefits to actually work, you can't cool the exhaust gases as they leave the port. In fact, what is desirable is to keep the exhaust gases as hot and centered in the tubing as practical. This is the way a header works, in terms of flow efficiency. Simply put and anti "reversion cone" at the port, a relatively straight run directly out of the port, equal length tubes of the appropriate diameter for the engine's breathing ability and a well designed collector, to take advantage of the exhaust pulses scavenging effects.

    If you cool the flow, it slows down, creating back pressure and it tends to cling to the tubing walls, further slowing it down, creating more pressure. You can compensate for this to a degree with larger tubing diameters, which is precisely what those big, massive marine manifolds do, but the increased density of the flow still is restrictive, comparatively.

    The only time this might be desirable is on a high output engine, which in most cases is dry anyway, so they can run automotive headers for best effect. The drawbacks of course, are noise and fumes. On high speed craft the fumes aren't an issue, but noise can only be handled one way. The application of noise reduction can be approached two ways, reflective or absorption. With buried exhaust systems in most pleasure craft, you don't have a lot of good options. Absorption, with wrapping, insulation and water jackets are the typical methods. The iron manifolds themselves do a lot, even before water gets introduced, but additional wrapping can help to a degree. Wet exhaust tubing also is effective, but can't take much heat, so you introduce water to solve this issue.

    I once bent up a set of headers, from stainless and used a box to encase it with water injection to cool the pipes, so they could run under deck out a transom fitting. A lot of difficult welding and I got less power than a straight automotive dry header, though this couldn't run under deck or through the transom. My idea initially was to enclose the engine and it's exhaust, to streamline the back half of the boat, which at the speeds it was traveling, could benefit from it. What I got was about 15 - 20 HP lose and less noise. I could live with the HP lose, as I had plenty to spare, but this isn't the case for most pleasure boats. For example, if you have a 220 HP V8 and coke it down to 175 HP (a 20% reduction) you'll decidedly notice the difference.

    In the end it's a trade and you have to decide how much it "costs" for the perceived benefits. If the exhaust is to run below decks, you don't have good options in regard to flow efficiency, though you can have a relatively cool and muffled result, which are the usual desires.
     
  2. Boatwiser
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    Boatwiser New Member

    Water Ingestion Into Engine

    Thank you to everyone who took the time to respond to my questions.

    I very much appreciate it.
     
  3. tom kane
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    tom kane Senior Member

    We have a poster telling us he has issues with water ingestion but our experts tell us it is not an issue,strange, and he is not alone with this problem.
    as some suggest, and danger from fuels can be conveniently ignored.

    Just about any dry exhaust on a boat must be better than the conventional wet exhaust incorporating risers and such like.Yes even dry exhausts need a condensation drain.
    As whitepointer says a home built dry exhaust works OK but perhaps is not attractive to look at.
    The thread poster points out that the issues of water ingestion is partly explained by the manufacturer but no solutions there.
    So where to next,perhaps the thread poster is justifiably getting impatient.
     
  4. PAR
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    PAR Yacht Designer/Builder

    The 5.7/Alpha combo is about the most common I/O install out there and this service bulletin was designed to address some of these installation, particularly in regard to some engines with fat timing overlaps and big block installs. Sea Ray was one of the companies that got caught with a bunch of warranty issues, but there were others. The usual solution was spacers, but each has been treated individually, such as manifold, riser and muffler combinations.

    Since this issue is application specific, it would be helpful if we knew the make, model and year of the boat and the exact repower that was performed.

    As to dry exhausts solving the problem, well they're fine for a work boat, but try to stuff that style of exhaust under an engine box, in a bow rider and watch what happens. Dry exhausts have an extremely small portion of the pleasure boat market (a few percent), which suggests the much more costly and complex wet exhaust systems, must be offering considerably more, otherwise the manufacture all would save money and use the dry setup.
     
  5. tom kane
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    tom kane Senior Member

    It looks like boatwiser is going to be stuck with his water problems like everyone else with wet exhaust systems.
    Looking at the boat design is not going to solve the problem of deliberately injecting water into an exhaust system and expecting to not have problems with that water reaching into the motor and it only takes small amounts of moisture to foul the plugs and set up rust and corrosion on hot pistons and rings,which is good for Manufacturers and repairers.
    All wet exhausts including outboards are subject to the same problems.
     

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    Last edited: Sep 24, 2015
  6. powerabout
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    powerabout Senior Member

    I guess there is a few million sterndrive owners that have never had a problem
     
  7. PAR
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    PAR Yacht Designer/Builder

    Yeah, that's the thing that gets me too. Literally millions of examples and some do have application issues, but this isn't the fault of the engineering approach, just the subtle changes made from one install to the next.

    Again, some installs like the big block Chevy in certain hulls are known for this and there are ways to address it (some recognized in a service bulletin or two), while others are more difficult to understand, because of cam duration and valve timing overlap. This was one of the issues associated with the MkIV, it's fat overlap could actually suck water up at and just off idle.

    Actually Tom, it take quite a bit of moisture, not few drops to foul a plug or to cause rust. In fact, the conditions within the chamber and surrounding runners is such that any small amounts of water can be beneficial to air/fuel charge. Water injection is a common way to get a wee bit more power out of a gas engine and it's been used since prior to WWII, for this reason.

    As to the nonsense about causing rust, well, if you spray moisture into a just shut down chamber or it's runners, the water will flash off in an instant. It takes a lot of water vapor and a lot of time for rust to form, so given all the oil and partly spent hydrocarbon encrusted parts in the related assemblies (rings, cylinders, valves, etc.), you'll have flood the chamber and leave it that way for serious damage to occur. Unless of course something is wrong with the setup.

    So, instead of turning this into a debate on the merits for the concept, if the OP provides the boat make, model and year and the repower install information. I'll bet we can figure out what HIS problem is and recommend the appropriate changes necessary to eliminate it. As I mentioned, some Mercruiser installs on the carbureted "1 piece" 5.7's, in some Sea Ray models are know for this issue and it's an easy fix, but we need to know what part combinations he has.
     
  8. powerabout
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    powerabout Senior Member

    only 2 things kill marine engines
    bad operators and bad install, the combination of which is guaranteed success
     
  9. BMcF
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    BMcF Senior Member

    Sure are...and I'm one of them.

    In all the boats I've owned (many) with wet exhaust inboard or I/o setups, not one has ever had a problem with water ingestion via the exhaust. I was aware of the few examples discussed earlier where the exhaust geometry was not correct to preclude water ingestion but fortunately none of those were ones I owned. If the engines with the problem have center-riser manifolds, adding the spacer block is not a huge undertaking.

    Many..even most..of the high-speed ferries and military vessels that I've worked on used dry exhaust or "mostly" dry exhaust excepting only the last section downstream near the outlet. The final installed diameter that includes the lagging becomes quite large.
     
  10. powerabout
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    powerabout Senior Member

    I am pretty sure the USCG wont allow packaged pleasure engine with non jacketed exhaust as its all up to the installer to make it safe and design it.
    Boat ramp is a classic place to drown an engine even if its all ok.
    Same like having a party on the transom and sinking the risers down
     
  11. PAR
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    PAR Yacht Designer/Builder

    Agreed, there's nothing inherently wrong with this arrangement, though maintenance, buttheaded owners and some installations can be questionable. These rise to the top pretty quickly and in the case of a manufacturer or engine vendor installs, service bulletins will pop up.
     
  12. tom kane
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    tom kane Senior Member

    You can guarantee all long tail boat operators never have such problems too.
    I hope Boatwiser gets his worry fixed before he spends to much.
     
  13. PAR
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    PAR Yacht Designer/Builder

    Why do you do this, Tom (bring up unrelated issues and contention to a subject)? Long tails are very specialized, use very small engines and are a very small segment of the market, having absolutely nothing to do with wet exhaust systems. So, how is the OP going to rig up a 200 HP long tail, on his bow rider? How about some relevance to the thread . . .
     
  14. tom kane
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    tom kane Senior Member

    Just replying to previous post.
    All boats are very specialized and some people want something more practical and less troublesome than others and Boatwiser wants no problems with water up his exhaust.
    There are many alternatives to wet exhausts and one model does not suit all.
    Image shows one of many water cooled exhaust manifolds suitable for any boat design.

    Perhaps I should have pointed out that long tail boats are usually dry exhausts.
    I don`t know how the other things I supposedly said come about.
     

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    Last edited: Sep 25, 2015

  15. gonzo
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    gonzo Senior Member

    There is nothing practical about a dry exhaust in an enclosed engineroom. It is very hard to make it compliant too. The rise in temperature will drop the engine power considerably and will require positive ventilation. It also creates a serious fire hazard. It is complete nonsense to advice someone to install one in a bowrider.
     
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