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Waterflow on Ohno MDKBT 27.5

Discussion in 'Technical Discussion' started by Pascal, Jul 27, 2017.

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  1. Pascal, Jul 27, 2017
    #1
    Pascal

    Pascal Senior Member

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    We have a pair of 27.5 MDKBT ( 7 years old and 2500hrs) which are both a little louder than other genset i ve used, especially starboard one.

    I ve done all the basics... first a new impeller and cleaned all the crap previous captains had left at the heat exchanger inlet. I then replaced the water pump altogether. I checked the thruhull, pulled the hoses between the sea cock and strainer to make sure it was clear, did the same withthe hose between strainer and raw water pump. New basket and seals in the strainer (basket was rotted...)

    I even temporarily replaced th hose between the pump and the heat exchanger with clear hose to make sure there was no air being sucked in.

    It runs just under 180, has never shut down, but is just a bit louder and once in a while may show a bit of steam in the exhaust.

    Not sure what else I can do.. am I chasing a ghost, are these 27.5 ohno just louder than northern lights?
  2. captholli, Jul 27, 2017
    #2
    captholli

    captholli Senior Member

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    I assume your "Ohnos" (I'll have to remember that acronym!) prime movers are Cummins 4-BT 's? when you say that they're louder than what you've been used to with N.L's are you speaking in terms of standing next to them in their sound enclosures or listening to their overboard exhaust dumps? The port unit is louder than the starboard unit or vice a versa? What type of muffler system do you have installed? Vernalift with exhaust and water mixed at the outlet or a separated system? Several different types of noise to be had between radiant, structure, frequency etc. Interesting topic.
  3. genea10gy, Jul 27, 2017
    #3
    genea10gy

    genea10gy Guest

    we have a 13.5kva Onan with sound shield and water separator on the exhaust and is very quite, you could have salt / rubbish build up in the exhaust mixer, we cleaned every 500hrs and there is normaly enough crap to limit a bit of water flow
  4. Pascal, Jul 27, 2017
    #4
    Pascal

    Pascal Senior Member

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    No they re Kubota engines not Cummins

    Talking about exhaust noise outside, basic lift muffler no separator

    Direct noise in ER is fine although you can tell the exhaust is a bit dryer.
  5. Capt J, Jul 27, 2017
    #5
    Capt J

    Capt J Senior Member

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    The 27 Onans don't move as much water through them as other brands in that size and I think that contributes to a slighter louded exhaust note compared to Northern lights that move a ton of raw water out of the exhaust. You could try a different muffler, or it is what it is.
  6. Pascal, Jul 27, 2017
    #6
    Pascal

    Pascal Senior Member

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    Problem is that the exhaust is close to the stern so when on the aft deck you hear the genny. It may be what it is... :)
  7. Sea Gull, Jul 28, 2017
    #7
    Sea Gull

    Sea Gull Member

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    I have the same unit, but 29 kw version. Vernalift muffler. I've been complaining about the exhaust noise since new. It's the loudest generator I've owned and really annoying when sitting in the cockpit. But it's really just in the cockpit. In the engine room it's not too loud, and in the living spaces it is perfectly acceptable.

    It hasn't been a high priority, but I'll be following this thread for ideas.
  8. Pascal, Jul 28, 2017
    #8
    Pascal

    Pascal Senior Member

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    Onan has changed the water pump design at some point Originally the pump used 1 1/4 " inlet and outlet hose with the custom outlet hose reducing to 1" at the HE inlet. The new pump has 1" inlets and outlets so the intake hose from the strainer has an adapter to 1"

    I m no engineer but 1" raw water hoses for a generator that size doesn't seem big enough. It's enough to keep it cool but not quiet
  9. Capt J, Jul 28, 2017
    #9
    Capt J

    Capt J Senior Member

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    That's correct. I'm no engineer, but the only way to quiet it down IMO would be a larger water pump with a bypass running some water from before the HE to the muffler.
  10. Capt Ralph, Jul 28, 2017
    #10
    Capt Ralph

    Capt Ralph Senior Member

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    Anybody looked at the exhaust riser lately? Maybe a water pressure T before/after the H/E?
    Lil less water, lil more noise.
  11. Pascal, Jul 28, 2017
    #11
    Pascal

    Pascal Senior Member

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    No but I can't imagine an issue with risers or exhaust outlet after only 2500 hours when I didn't get any I nealry 10k on NLs!
  12. Capt Ralph, Jul 28, 2017
    #12
    Capt Ralph

    Capt Ralph Senior Member

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    AND you didn't expect other problems on these gen sets either.
    Squeeze the raw water hose after the raw pump and crank it up. If you feel the hose trying to expand in your grip, there is a restriction.
    Compare this between both sets.
    I guess a restriction or air leak before the pump could reduce water flow and louder exhaust but I like my first ideas better.
  13. Pascal, Jul 28, 2017
    #13
    Pascal

    Pascal Senior Member

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    Can't be become the HE because I pulled everything apart, hoses are clear etc... by using a temporary clear hose between the pump and he I could tell there is no air leak either

    Good idea on trying to feel pressure on the hose.