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12v71 slow cruise modifications...

Discussion in 'Technical Discussion' started by VikHatBer, Dec 6, 2008.

  1. VikHatBer

    VikHatBer New Member

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    What has to be done to cruise a pair of 1271's at 1000 RPM's without harming the engines over the long run? I heard that the turbos have to be modified to prevent carbonm buildup that results from low RPM cruising. Any help would be appreciated.
  2. Capt J

    Capt J Senior Member

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    I run a set of 12v71TI's like that, BUT I run them at 2000-2050rpm's for 30 minutes every six hours to clean everything out (per a Detroit Diesel factory recomendation) turbo's and airboxes. These particular 12v71's are low hours smoh, but tend to run a little on the rich side at 1,000 rpm's or less and sometimes I would get some turbo caking and hear a whine coming from the turbo's. Running them at cruise would clean them out. Also I use a product called "Diesel Kleen" it does a terrific job at eating algae in diesel fuel, and has a little more cetane and stuff and they tend to run better and cleaner with it. These were covingtons, it depends who marinized them as Johnson and Towers tended to be the most "hot rodded" of the detroits and had larger injectors, turbo's etc then the Stewart and Stevens, Covingtons, and I forgot the name of the other company that marinized a lot of them. I see a burn rate of 10-12gph (both engines) at 1,000 rpm's and 90-100gph at 1950rpms. This particular yacht run's 16.5-17knots at 1950rpms and 9.5 knots at 1,000 rpms so it really doesn't pay to run it at cruise.

    However if you want to run at 1,000 almost all of the time. Another yacht I ran was setup to run like that with 8v71TI's, at rebuild Detroit Diesel went to one size smaller injector....... instead of the 110's(lph) they installed 90's(lph) and adjusted the rack accordingly. You'll have to cut a few inches of pitch out of the propellors if you do this and it will effect cruising speed by 5-20%. However, that yacht ran crystal clean at cruise and slow speeds and all of the way around even if you ran at 700rpm's all day. Also fuel consumption at cruise went from 45-50gph (both engines) to 30gph (both engines) at 1950rpm's and cruise speed went from 15.5 knots to 13.5knots at 1950rpm's. This was actually the best running set of detroits (aside from DDEC's) I've ever ran they'd start up and idle perfectly with no visible smoke whatsoever, they ran smooth and felt torquey down low. This was a 58' Hatteras YF.
  3. VikHatBer

    VikHatBer New Member

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    I asked this question referring to the 1271's in either a 61' or 63' CPMY (Hatteras) with the intent of long range cruising in the Bahamas/Yucatan/Cuba. I know 1271's are very efficient at low RPM, and they can be tweaked to be even more efficient.

    What do you think about a noise shroud on a generator? Is containing the generator's heat good?
  4. Capt Bill11

    Capt Bill11 Senior Member

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    I guess you could look into changing the injectors. But in doing so you may lose the ablity to run a higher RPMs/speeds. And perhaps changing thermostats so they will run at the proper temp. at low RPMs.

    I don't see any advantage to containing the genset heat. Some might say it's a bad thing.

    Now noise is of course a different issue. If you're going to use a shroud, I like the ones with a clear panel or two. While they may not deaden the sound as well as the fully insulated ones, it sure is nice to be able to look over the genset with out having to remove the panels duing an engine room check.
  5. VikHatBer

    VikHatBer New Member

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    Would that boat serve well as a trawler w/ modified 1271's?
  6. Capt J

    Capt J Senior Member

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    Why would you want to contain the generators heat? The cooler the intake air, the more power you make from it. Cooler air is denser and also needs less fuel to prevent detonation. If anything put a new Northern Lights or new electronic onan to replace the old onan if you want to cut down on noise.

    The vessel I have run with 12v71TI's is a 65' Hatteras MY that was extended into a 75' CPMY. I average 1.5 g.p.n.m. running it at 1,000 and then 2000 rpms for 30 minutes every 6hrs. If you brought it down to 800rpm's your range would even increase greatly from there. You can run at cruise (1950rpm's) with smaller injectors without an issue because the fuel rack is adjusted to go with them, you just make less HP. They didn't change the turbo's on the 8v71ti's that smaller injectors were put in. The propellors do need less pitch so they make the proper rpm (2300rpm on 12v71TI's) with the smaller injectors. The 8v71TI's I ran were de-rated by DD from 525hp or 550hp I forgot to 462hp with the smaller injectors.

    It's like the Cummins 6B series, they make 270hp in normal trim, 330hp is achieved with larger turbo, and 370hp is achieved with larger turbo and larger injectors. Not much else, if anything is changed on them.
  7. Capt Bill11

    Capt Bill11 Senior Member

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    Maybe. Roomy and I personally like a cockpit boat.

    But they tend to be top heavy and can be a bit of a hand full in a following sea as well as a little rolly at anchor.

    And it should go without saying it and you will benefit greatly from stabilizers for that kind of cruising.
  8. progreseños70

    progreseños70 New Member

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    some help about detroit engine

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