If the engine doesn't start, it drowns?

Discussion in 'Inboards' started by CaptainAB, May 8, 2010.

  1. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    Dry exhaust up a stack/funnel brings up another issue in that when you leave them sit for a few months they will quiet often draw air in the inlet and through a cylinder with any valves overlapped and up the chimney so you need to close the inlet and or let the valve gear go.

    A wet exhaust if it has a water lift muffler is sealed from the air so it doesnt happen....
     
  2. murdomack
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    murdomack New Member

    Is it possible to have both inlet and exhaust valves open at the same time? I'm afraid my knowledge does not run to that.
     
  3. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Yes, that's called valve overlap. There is always at least one cylinder with both valves open ( two in a V-8).
     
  4. murdomack
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    murdomack New Member

    Right, I've been on Google so I now have to contradict you,;) . If the overlap is 33 Deg on a 4 cylinder, 4 stroke, then there will be a cylinder with two valves open for only 4X33/720 or 18.33% of the time. There will be a far greater chance of them not being open. Is that right?
     
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  5. CDK
    Joined: Aug 2007
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    CDK retired engineer

    No it's not.

    First, valve overlap depends on engine design, a high revving engine will have more than 70 degrees overlap, performance engines even more.
    Second, the end position of an engine is never random. One cylinder is always at its compression stroke but cannot reach tdc, so the crankshaft turns back. Another cylinder has valve overlap then.
     
  6. murdomack
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    murdomack New Member

    Google does not explain that bit. What you say makes sense when I think about it, Thanks.
     
  7. mark775

    mark775 Guest

    The thing about dry exhaust is, indeed, nonsence.
    A dry exhaust runs hot, and if poorly maintained is a fire hazard. Yacht people are bad at maintenance relative to commercial operators and would have many more fires with dry exhaust.
    The condensation rules CDK pointed out are how it happens. I have put in over 7,000 engine days on wet exhausts. I inflate a childs soccer ball in the pipe if I am leaving for more than a month or two and have never, in I don't care to calculate how many engine hours, had a "ruined engine due to condensation on the valves from wet exhaust." ...nor a problem, at all, that I know. Poor wet exhaust design is what ruins these engines - not wet exhaust, per se. Poor wet exhaust design? - now that is common!
    Dry exhaust is harder to keep quiet.
    Dry exhaust tends to get black specks on everything.
    Dry exhaust allows more flexibility in design of compartments/fish holds. Not wanting to run an exhaust thru a hold is the single biggest reason that boats (of the class you are talking) use dry exhaust.
    Even with the power loss of running an extra pump and the weight gain of the water one is packing, wet exhaust works better for high speed craft because of the large cooling needs, the drag of coolers and the weight of coolers and cooling water. Skin cooling is an alternative conducive to speed if it is designed in.
    Dry exhaust takes up valuable main deck-level space.
    The mechanic in Pt. Townsend was speaking tongue-in-cheek or really doesn't have a clue.
    Wait, there's more to this - on small boats, if it were not for the problem of running an exhaust thru the hold and water intrusion because of the wet exhaust not being designed to accomodate large loads of fish, commercial boats, too, wud mostly have wet exhausts. In short, when designing wet exhaust, use all available engine room height for a (blanketed) dry riser, think of how the water will flow, how today's wind holds your stern to yesterday's swell, and how (the single biggest reason for water-ingestion in wet exhausted engines) when you chop your throttle to avoid a log or the "energised-to-run" solonoid wire comes undone and you have an immediate loss of power, how your own stern wake washes up and into your engine. This is why I always build very high dry risers for my engines, I have a waterlift or surge tube, and I avoid scenarios where I have to chop the throttle.
    A final note; Those little rubber flappers on the exterior of an exhaust? - I have watched boats anchored stern-to a swell and the swell simply raises the flap and proceeds right up the exhaust. Don't trust this method.
    CDK's drawing is good if there is, at least, maybe 13 to 15 inches between waterline and turbo, and I am sure there is.
     
  8. Brent Swain
    Joined: Mar 2002
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    Brent Swain Member

    Your estimate of 98% of all marine engines having wet exhausts is way out of wack. They are extremely rare on any commercial boats, for good reason . Experience has taught them better. Most would never do anything as foolish as pump water into their exhausts.
    The best preventative was on my old one cylindre diesel. I couold crank it until the injector squeaked then drop the decompresion lever. The piston was then top dead centre , all valves closed ,and soaked with diesel.
    60% of diesel engine failure is from water cooling , either wet exhausts or heat exchangers.
     
  9. tunnels

    tunnels Previous Member

    :idea: For people that have petrol engines in there boats it a good idea to mix in and run some 2 stroke oil with your fuel, it helps to keep the inside of the motor clean and gets a light coat of oil over everything . The oil i use if the 2 stroke for the injected motors , it lubricates with little to no smoke .
    Also helps to get shot of carbon and crap that builds up in the intake manifold . I even use the same oil and petrol premixed in my lawn mower , line trimmer and during the winter a little in my car as well . Its the same oil that i use in my 115 yamaha oil injected 2 stroke outboard . :D
    Remember not all 2 stroke oils are equal !! so do your home work , Its oil used in injected outboards you need not just std 2 stroke oil !!
     
  10. mark775

    mark775 Guest

    "Most would never do anything as foolish as pump water into their exhausts."

    "60% of diesel engine failure is from water cooling , either wet exhausts or heat exchangers."


    - So much talking out of your ***, I come to wonder if you even have a mouth. There is NOTHING wrong with water cooling exhaust or heat-exchanging - It simply needs to be designed by someone (not you) that knows what they are doing. 60% huh? Is that truck engine failure or marine? Is that jacket water with corrosion inhibitors or salt water? No engine, EVER, failed because of a properly done wet exhaust. NONE. EVER. Why, pray tell, wud an heat exchanger cause engine failure? Are there no alarms, no safeguards, no maintenance? What are the mechanics of a heat exchanger ruining an engine? - I'll address the fault for you.
     
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  11. tunnels

    tunnels Previous Member

    I know what you are saying ! Over the years have seen some terrible things during my travels here and there . People forget the basics of how the system works or is surposed to work . some also just plain dont know and there are others that just dont care as well . simple is best with just about anything . Keel pipes are just a form of heat exchanger the design of a water lock muffler is all important some work really good and others are a accident in the making . Maintance is all important with any and all thing mechanical . Nothing works better than something mechanical thats looked after regularly . :D
     
  12. CDK
    Joined: Aug 2007
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    CDK retired engineer

    No it is not.
    Your angle of vision is somewhat limited.
    The overwhelming majority of all marine engines are outboards and have wet exhausts. So do all stern drives, nearly all cabin cruisers and most small commercial craft.

    So a dry stack is as rare as a white elephant.
     
  13. powerabout
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    powerabout Senior Member

    Mercruiser had made their one millionth stern drive in 1976
    ( add the inboards and Crusader make most of those, pcm most of the ski boat engines)

    Think of any city on the world and its pleasure craft versus commercial craft
    So maybe its not 98% but its probably more then 97%
     
  14. Brent Swain
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    Brent Swain Member

    Sorry for ther omission. Forgot what level I was talking down to. That is 60% of marine engine failure.
    Heat exchangers involve sucking water into the boat thru a thru hull, then a strainer, then a pump, then thru a complex mass of pipes, then an outlet. You say NONE of the above has EVER plugged in an ocean becomming ever more filled with plastic? It has NEVER happened? ********! It happens all the time.
    My keel cooler involves seawater flowing past my skeg , No intake, no strainer,no salt water pump, no piping, no outlet. That has never plugged. Friends with water cooled exhausts are constantly fighting with salt water pumps that fail etc.
    My father, a steam engineer all his life said if you plug the intake on a rubber impeller pump you destroy the impeller quickly.
    Only a circulating pump on my boat. Those who convert to dry exhaust and keel cooling live happily ever after.
    Practical Boat Owner had an arrticle on the subject of wet exhaust. They concluded that the riser should rise at least 16 inches minimum before any water gets injected. Most commercial built water cooled exhaust sytems , designed by "Experts" have a rise of only a few inches sometime less than 4 inches.
     

  15. apex1

    apex1 Guest

    This debate will never end, as I stated already above.

    But claiming that only one is perfect and the other system wrong, is biased or uninformed.
    Both have their disadvantages, severe ones, and there is no solution in sight at present.

    Regards
    Richard
     
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