prop rpm

Discussion in 'Inboards' started by jeemboNC, Oct 21, 2007.

  1. jeemboNC
    Joined: Dec 2006
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    Location: North Carolina

    jeemboNC Junior Member

    Rick - your post came up just as I hit 'send'. This spring I will get into this in more detail and with mo' money! For now I just want to get her to her home port in NC. Maybe some perpetual motion/otherworldly efficient props will be willed to me by a rich benefactor... JeemboNC
     
  2. Guest625101138

    Guest625101138 Previous Member

    Jeembo
    I still intend to do a model of the hull to check what could be expected if all was good. The numbers you posted for Cemberci will be helpful. As will the photos.

    I would not be too worried about the different props. Angled shafts are more of a compromise and this is seen on almost all small shafted boats.

    Rick W.
     
  3. jeemboNC
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    jeemboNC Junior Member

    BTW - isn't 1 hp per 500# displacement for hull speed a good approximation for properly running/setup trawlers? But apparently with twins you need another 25% due to waste in drag and heat losses for running two engines - yes? JeemboNC
     
  4. Guest625101138

    Guest625101138 Previous Member

    Jeembo
    I did a quick sketch of the hull and did some power estimates based on Michlet data on the hull, prop efficiencies, allowance for mechanical efficiency and extra drag for the rudders and keel.

    The image and curve are attached. It is not often you get a dip in the power v speed curve as you see here. This is showing power to do 8kts is more than previously stated but I previously had no alllowance for the gearbox and other mechanical losses.

    It looks like you may not be getting full engine power as you could expect to do better than 10kts with 320HP but lots of variables like sea conditions, fairness, surface finish under water etc. It is in the ballpark at lower speed. Motors might be in need of good overhaul.

    The curve suggests 7 - 7.5kts will be more economic than 6 - 6.5kts.

    Rick W.
     

    Attached Files:

  5. jeemboNC
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    jeemboNC Junior Member

    WOW! Your analysis is very revealing. I did not expect the power for 7kts to be that high. I will try some runs to get better data. And yes, the bottom is NOT really clean nor smooth at this point, so I am sure that is causing some loos in speed.

    I launch Sunday AM (two days). Wish me luck! JeemboNC
     
  6. Guest625101138

    Guest625101138 Previous Member

    Good luck Jeembo. Hope the trip is uneventful but provides heaps of enjoyment.

    Rick W.
     
  7. 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 best mental description on what a dirty prop does came from a Herrishoff book.
    He thought the prop , with imperfections , should be "blown up" to the size of the boat it is pushing.
    EZ to visualize where the speed went!

    "Therefore, she may be a little overpropped.
    Is that bad as I will usually run slow for economy and that will help load then engine? As long as it doesn't smoke?"

    Most cruising boats are "overproped' as the top speed is seldom if ever called for.
    The advantages of overproping are reduced noise and fuel consumption , and a reduced tendancy to "slobber" .

    The danger of course is overloading.

    If you run at least 300rpm OR MORE below what ever the engine will pull , overloading is seldom possible.
    An Exhaust gas temp gage is not expensive at all and is best insurance if you REALLY try for economy.

    FF
     
  8. dsharp
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    dsharp Junior Member

    I read this thread pretty quickly so if mentioned disregard. If the gears are velvet drives the 1.9 to 1 gear reverses in the gearbox. They are built that way since both engines rotate the same direction. I don't understand the difference in mounts and shaft lengths though since they should be physically the same size. The 2.1 - 1 box turns the same as engine rotation.
     
  9. jeemboNC
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    jeemboNC Junior Member

    Well - I made it!! 8 days, ~600 statute miles, 80 tach hours, ~75 steaming hours, ~300 gallons of diesel fuel. Only real problem was a dripping fuel supply hose which was fixed en route. Ran for 30 minutes or so on one engine at 2000 rpm - made a good 8.0 kts, about the same as on both. She is happy at 8 kts with a flat trim, pulling decent power from engines, and good maneuvering authority. Going faster seems uneconomical (it's a trawler!). But I sure had some headwinds and current - felt like a salmon. And everybody (~300 vessels) looked at me funny as I steamed north in November!

    Therefore burned 4 gph total for 2 mpg with a 10-20% loss due to currents. Seems reasonable.

    Ran mostly at 1500-1600 rpm with an occasional slow down in no wake zones and a burst or two to 2400 to see what would happen (black smoke and a hair raising 10 kts!!). Engines never burped. Racors were changed with 30 microns before I left - gages still show no clogging. Burned ~3 quarts of oil in each engine - I hear that might be considered high. I hear a slight 'bumping' sound in the port prop - the cutlasses were apparently ok - so I don't know what it is. Vibration seemed normal according to the surveyor, but he did not hear the bumping (I heard more in 75 hours than he did in one).

    My engines do not reach the same rpm at WOT. I think I adjusted the tachs accurately, but they seem to run okay. I expected them to burn cleaner now, but they still smoke when cold. My temp gages may not be accurate, but they both ran ~180 degrees even at higher power settings. Isn't that a little too cool for Lehman 160 turbos?

    My bottom and running gear needs a better cleaning/protection. This will happen this spring.

    Re the reverse tranny - I still think it was a cobbed up fix for the failed port tranny. My surveyor poopooed my concern, but he may not be very pure as he was a friend of the selling broker. It is what it is and has been there for years - so I'll live with it. The boat runs fine, I shouldn't complain, and I have a million other things to work on.

    Thanks to all - come see me in New Bern!! JeemboNC
     
  10. Guest625101138

    Guest625101138 Previous Member

    Jeembo
    Well done. Seems a bit thirsty but always hard to estimate the cost of wind, current and waves.

    I did a bit of cruising around the north east coast of Australia and I always remember the words of an old seaman used to the area - Average wind speed is 12kts; nothing at night and 25kts through most of the day.

    Same thing occurs with ocean currents. The Pilot shows 1 to 2 kts southerly set on parts of the east coast at certain times of the year but I have sat beside a headland all day doing 6kts. Had to motor and sail at 7 - 8kts to make an impression. This was before GPS and you think your charting skills are rubbish until it happens a few times.

    Rick W.
     
  11. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    and a burst or two to 2400 to see what would happen (black smoke and a hair raising 10 kts!!

    Black smoke is a sign of an overload , OK if its only during the acceleration and goes away at speed.

    NOT OK for cruising , pull back at least 10% of full throttle rpm , and the smoke "should" go away.

    FF
     

  12. jeemboNC
    Joined: Dec 2006
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    jeemboNC Junior Member

    I will check the fuel burn in the future, but this was probably my best opportunity to get a reading. All the running was in the ICW, so the tides were just simply not with me. And the wind was 10-30 kts in may face the entire trip.
    Re the smoke - it did go away if I pulled back the throttle a pinch. With the bottom so rough, I suspect the engines were overloaded. From what I can ascertain, that is okay as most of my running will be well below WFO. As I said, 1500-1600 seems to be a nice cruise. I am now focussed on boat liveaboard issues, so engines will have to live with being lower priority for a while. I expect to do some local '3 hour tours' with Ginger and Mary Ann, but that's about it. They seem dialed in enough for that...

    Thanks Fred and especially Rick for all your input. Come see me in NC! JeemboNC
     
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