Trying to buy, need help!!!

Discussion in 'Diesel Engines' started by Wavewacker, Dec 7, 2012.

  1. Wavewacker
    Joined: Aug 2010
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    Location: Springfield, Mo.

    Wavewacker Senior Member

    There is an older Navy MKII, originally a '66 navy personel carrier that has been converted as a work boat. 56' LOA, 15' beam pushed by a Detroit 8-71, single diesel. Can someone tell me about what the burn rate, gph to expect at a 12k cruise speed, broker didn't know or could be didn't want to say. It's, I think, a full displacement hull design.....I can't really see this plaining like a Sea Ray.

    Info on the engine or any experience with such a vessel really appreciated! The broker says his phone is ringing and I'm close to an offer after some due diligence on something I have not a clue about!
     
  2. WestVanHan
    Joined: Aug 2009
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    WestVanHan Not a Senior Member

    You'd need to know the max HP rating of the 8-71...probably around 300.
    Assuming a 80% load at cruising gives 240 hp.
    DDs give about 15 hp per gallon per hour,so about 16 gph-assuming a lot of things.

    Fast Fred will know better-about DD and I think he knows about these military boats-I suggest PM-ing him.
     
  3. Wavewacker
    Joined: Aug 2010
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    Wavewacker Senior Member

    Thanks, for the estimate, I'm under the impression from what I can find it's 450hp. I would run slow. I read info on a diesel site and while picking up what was good stuff, no burn rates were found. At least now I have an idea of what a bilge diaper is.

    Sounds to be like you need to change oil every 100 hours! It takes 8 gal of oil on a DD 8-71, at $12 a gal. that's $96 plus filter. $1 an hour for oil, doesn't sound bad until you consider it's about $1,200 or more to do the great loop. Break Out Another Thousand!

    I think running at 80% = 1800 rpm, getting 6gph @ $5 a gal or $30 an hour for a 6 hour day cruising 5 days a week is $680 a week! About 400 miles with a decent current or $1.70 a mile?

    Probably talking at least $2.00 a mile for fuel, oil and minor maintenance.
    Throw in an overhaul in 7/8 years, or $1,200 a year for your maintenance fund, another $100 a month.

    It looks to me that it could be about $3.50 a mile. Considering haul outs (I have no idea) gen set usage and other maintenance you'd be at $5 a mile.

    Is any of this close? If so, I see why owners are moored most of the time!
     
  4. WestVanHan
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    WestVanHan Not a Senior Member

    If it's an "N" meaning non turbo-hp is from ~250 to 320 IIRC.
    TA (turbo aftercooled) run from 350 and the 600 hp ones only last a few hundred hours if used hard.

    Next step would be to ask them if it's a turbo or not.

    Maybe it's a 250 hp and "cruise" is really WOT...or is 12 knots the speed at 1700 rpm..who knows...
     
  5. Wavewacker
    Joined: Aug 2010
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    Wavewacker Senior Member

    It's natve urally asperated, so good to hear it's a lower hp, I just as soon have 120 hp! 6 to 8 knts seems fast enough in that!

    Not to change gears, but also found a 39' Willard, same navy heritage, with a 6-71, but has some blister problems, if that could be cured....probably better for my needs.

    Opinions on either or both? Burn rate on the 39', @ 70-80%?
     
  6. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    IF you decide you must go 12K , yes indeed you will only go 1mpg or less.

    These hulls were built for the Navy where fuel is FREE , and the kids are going to run full throttle all the time.

    Most non turbo installed are N series which was a very optimistic 318 HP.

    My suggestion is 7 or 8K, not 12K as cruise speed.

    Our 50 ft Navy Utility with a 6-71 does under 3 GPH at 7 or 8K , so is very easy on the budget.

    IF you purchase the engine you might consider smaller injectors , when its injector time.

    These should run $40 or so each , BUT who ever installs them MUST be experienced in DD , and have all the special required tools.

    While the desire to run at 80% is admirable in theoretical terms of engine life , it would not do much on a DD.

    Engine lives on some have been 20,000 hours and 30,000 hours before rebuild.

    So if under loading , running at 30% power reduces the engine life in half SO WHAT.

    5000 hours worst case for a rec boat that might see 200 hours a year is a 1/4 century!!!

    The 6-71 is very easy to maintain and with "Da Book" and an injector setting pin from DD , you can work on it yourself.

    To me the nicest thing about a 6-71 is the ease with which the engine can be pickeled for storage.

    About a half hour (the second time) and your good to go.

    WE ran the "great loop" a few years ago, be advised 10Klicks is the speed limit for all that is in canals or rivers (lots !)

    We plugged along at 1200 rpm, almost no noise , or wake .

    Sorta like the ICW , the gaggle goes from lock to lock, instead of from low bridge to low bridge, all waiting for 'Tail End Charley"..

    Yes the oil should be changed and ONLY SAE 40 CF II oil used , never multi viscosity , but with loop running the interval can be stretched .

    One cold start , 4-6 hours of operation at light loads , little idieing, (mandatory shut down in locks) 150/change hours is no problem.

    WE pull oil samples with EVERY change , and the results are excellent.

    Oil has gotten much better over the years as has filtration.
     
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  7. Wavewacker
    Joined: Aug 2010
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    Wavewacker Senior Member

    Thanks Fred! You gave me something to look up I guess, LOL, "pickle" have no idea what that is.

    Good info, so at 1200 rpm, what kind of fuel bill should I plan on or better the GPR since fuel prices change.

    I think 3 gph or $15 an hour, plus other expenses would keep me at the dock too much.

    And asking in another way, is 1 or 1 1/2 gph out of the question, what speed (if any, lol) would that provide?

    I'm talking two different boats here, 1 @56' with an 8-71 and the 39' with the 6-71. Either works, in fact the thought of me in a 56'er is kind of scary, I'd feel comfortable at 39'.

    Lastly, I guess you're saying in a pleasure boat running at low output really doesn't hurt since I'll never max out the hours anyway. I'll probably be done in 10 years, maybe 15.
     

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

    FAST FRED Senior Member

    On most boats that look like a boat some simple rules of thumb can help.

    A displacement ton (not the measurement tons ((volume))carved into a bulkhead) is 2240 lbs.

    So take the displacement in pounds and divide 2240 into it and you have the boats displacement in tons.

    Find the boats LWL length on the waterline and figure the sq root of the LWL in feet.

    Say the 50LOA is 49 LWL, rt is 7K ,about your cheapest cruise speed .

    To go this speed will require usually 3hp or a bit less per ton.

    Take the number of HP and figure on a lightly loaded (inefficient ) engine 13 to 15 HP will be produced by a gallon of fuel per hour.

    A 6-71 will use a gallon an hour to idle , more at 1200rpm

    While all this is great fun , the real cost you need is dollars per mile.

    Have fun,

    To get optimum fuel mileage only an engine transplant to a modern 20+HP per gal , where the engine would be much smaller really hard working would help.

    No engine swop I have ever heard of costs less than just feeding and maintaining the std already installed engine.

    Pickeling is what you do for an engine that will be out of service for a while.

    Weather a while is 30 days or all season is the MFG decision.

    Do not bother with pickeling and 30,000 hours to wear out may only be a few seasons .
     
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