Re-powering a 27' Uniflite with a 6BT

Discussion in 'Powerboats' started by crankshaft, Apr 8, 2011.

  1. crankshaft
    Joined: Apr 2011
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    Location: Big Lake, Alaska

    crankshaft Junior Member

    A little bit of background.... I am a chrysler guy, love the 318 as much as I can love any gas engine, and my 27 Hardtop express happens to have a single 318 and a 71C, I built the 318 out of the (literally) best junk in the garage. (not joking, I can build/mod/port/tune small block chryslers in my sleep, comes from years of hotrodding them when I was younger) The Boat had a 360 when I bought it, guy said it was a rebuild, it had nice paint on it, ran well, but the line bore was only about .060 off on the back main, with mismatched over bearings on the top and the bottom to make the crank spin, and it had a piston beat to snot where a valve dropped at some point after it was *cough cough* rebuilt. It eventually broke a crank at #4 main in those seas where you stand on the deck in the trough and look the crest of the swell in the eye......... 8HP Honda steered by rope ran through the rails saved the day, and night, and day, and night...... Anyway, I have since grown up, and Small block Chryslers no longer really flip my switch, considering I have a 92 Cummins truck with 780,000 miles on it, and would outpull ANY gas power pickup happily during those miles.

    Short version? I want a 6BT in my boat. I happen to have a 180HP P pump 6BT, so I have been thinking, and measuring, and thinking. But a new(ish) to boats guy can get in trouble thinking, right?

    I can get the Cummins in there, using a 71C 1:1 with a half inch to spare between the crank pulley, and bulkhead. The 71C is rated for 183HP in diesel form, and the shaft is 1 1/4"...... The 71C in it now, is a 1.52:1, and I cruise at like 7.5KTS at 1800RPM. Not really a trawler, but it is heavy enough it does OK, following seas really blow at that speed, but I don't get bored.

    From what I gather, the 180HP Diesel in a boat seems to be about the equivalent to a 250 or so HP gas engine, when it comes to pushing the boat through the water. The 318, with the meek 220HP? WILL run at 16KTS at about 3600RPM, right under the 4bbls opening, but I have a hard time asking a gas engine, even a 318, to do that for hours and hours, the Cummins? PSHHH, no issue at all.

    So, my question is, CAN I replace the 220HP 1:52-1 71C gas engine, with the 1 1/4" shaft and 16" 3 blade prop WITH a 180HP 6BT Cummins, 1:1 71C, with the same shaft, and prop? (re-pitched, of course) I have been wading through all kinds of Propulsion math voodoo for months and logically I can, I think, but I am hoping a guru can tell me ya, youbetcha, or, your a blathering idiot....

    I'm just real sick of 7.5KTS, here in AK where the current is quick, and no matter which way I am going, I seem to be in a following sea with the current coming at me.

    Thanks in advance. :)
     
  2. 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 are wanting to get back to a plaining boat , even with the diesel, at speed the fuel burn will be 1mpg.

    IF you are planning on staying in the displacement mode a far smaller engine will do better.

    What are your Desirements?

    FF
     
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  3. crankshaft
    Joined: Apr 2011
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    Location: Big Lake, Alaska

    crankshaft Junior Member

    I am wanting to stay at displacement speed to burn less fuel, BUT, I want to have the HP to run on plane if I have to. Will the diesel really burn more than the 318? The 318 burns 10GPH at 15KTS........ at 7.5 water speed, 1800RPM, it burns about 3.5 and hour.
     
  4. crankshaft
    Joined: Apr 2011
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    Location: Big Lake, Alaska

    crankshaft Junior Member

    Here's what I can gather about the 6BT fuel consumption. IF I just slap it in, with a 1:1 transmission this very second, and take it out, at 7.5 KTS, according to the fuel burn chart, it will be somewhere around 2.2-2.5GPH, and at planing speed 2400RPM, it should burn about 8GPH........... That is asking it to make the same amount of power the 318 is right now, spinning the prop a half turn more more per crank revolution, which it has the torque to do.

    Am I that far off?

    Will my combo with the 1 1/4" prop shaft and 1:1 tranny that is rated for 3HP over what the motor is putting out work? The 318 is actually real efficient, I built it that way, bigger EX valves, porting, distributer timing, carb tuning, I just want the reliability, and the less chance to expolde! I don't think I could go wrong with a Cummins.
     
  5. Easy Rider
    Joined: Oct 2009
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    Location: NW Washington State USA

    Easy Rider Senior Member

    Crank,
    You wrote: "From what I gather, the 180HP Diesel in a boat seems to be about the equivalent to a 250 or so HP gas engine"
    Horsepower dos'nt know diesel from gas. I would think 180hp is 180hp. At lower speed the diesel would make more hp and torque but at their rated rpm they should be the same. And I think you're selling the Chrysler short on engine life. Think of how many years an old gas truck can roar down the highway. I'd just blow the bilge faithfully and run the Chrysler. Also I'd much rather go for a ride w you in your gas boat than a noisy, vibrating and stinking diesel as long as you pay for the fuel. How many hours would you need to accumulate on the diesel to pay for the diesel engine and all the work and expense required to install it? Start lusting after smooth and quiet.

    Easy Rider
     
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  6. crankshaft
    Joined: Apr 2011
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    Location: Big Lake, Alaska

    crankshaft Junior Member

    That old Dodge truck isn't running up a 10% grade pulling a 10K bumper pull trailer though.

    the problem is, the 318 will not last long running as hard as it takes to plane. Also gasoline is explosive, not as reliable, the carb ices up in the colder weather, flooding it out until the whole bilge heats up, I could go on and on. The main reason is, IF I stick with the 318, I am stuck running at 7.5KTS............ It's a big boat for a single 318, almost all of these were twin 318s.

    My other option would be to go 440 or something, then I burn that much more fuel!
     
  7. Yellowjacket
    Joined: May 2009
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    Location: Landlocked...

    Yellowjacket Senior Member

    Easyrider is right, the boat doesn't know what is spinning the shaft and horsepower is horsepower when you are trying to cruise at planing speed. Yes with a diesel you make more torque at a lower rpm, so you have to gear it differently, but if you put less power in the boat you aren't going to cruise as fast.

    If you are stuck on a diesel you need to think about a bigger one than you are currently if you want to cruise as fast as the gas V8.
     
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  8. Three Ts
    Joined: May 2011
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    Location: San Diego

    Three Ts Junior Member

    You should contact this guy who is selling a 27' Express that was converted to diesel on craigslist. He might have helpful feedback. I happened to see this ad the other day, and it might be relevant to your questions. Info copied below:

    "1972 uniflite 27, custom interior, low hour cummins 5.9 220hp diesel with Bosch " P " pump for extra reliabillity and lots of extra power potential with simple adjustments. 100gls fuel, 60 water. windlass with all chain, dickinson diesel heater, kvh electronic compass, furuno depth / fish finder, livewell, ocean or lake ready, a great boat at a bargain price. a veteran of many sea of cortez crossings. 2.5 gph at 8 knots, max speed close to 20 . $14,500 with trailer, $12,500 without, might consider partial trade for older jeep cj5, autopilot, older " R " bmw motorcycle... many more pictures at the link bellow. can be towed with a 3/4 ton truck
    https://picasaweb.google.com/gon2sea/Ereus#

    480 280 0950"
     
  9. copenhagen
    Joined: Apr 2011
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    Location: copenhagen

    copenhagen Junior Member

    my two cents is as follows.
    Hoserpower is a calculated number with two variables, torque and speed of revolution. so a 180 hp engine is able to spin a shaft at a certain speed with a certain amount of tourque, regardless of weather its electric, gas, diesel or turbine power spinning the shaft..

    the expression that diesel hp counts for more has to do with dutycycle and longevity.. if you were to put your 318 in your truck, load up the trailer and drive up a 10% incline with the transmission in second and the engine spinning 5500 rpm it would pull that trailer just fine... but would you ?.. no¨

    so if i were you id install that cummins in your boat, it wont snap a crank if you ask it to do hard work and it has plenty of potential if you should decide to want more power.. i mean with a p-pump you can easily coax 400 reliable horsepower from that engine with an afternoon of adjusting boost and full "throttle" injection pressure

    and diesel is just not as dangerous in a boat
     
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  10. Carteret
    Joined: Jan 2004
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    Location: Eastern NC

    Carteret Senior Member

    Good Points all around. I agree with Copenhagen. For safety and reliability. The 6bt is a good engine.
     
  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

    "but if you put less power in the boat you aren't going to cruise as fast."

    NO gas powered cruiser operates at max power, at least for long.

    The 318 will probably be pulled down to 2800- 3200 rpm for cruise.

    SO the factory peak hp rating is meaningless.

    A smaller diesel can spin the shaft at the same cruise RPM , with a large gain in MPG.

    The match up is not peak hp or torque but the Cruise requirement , if efficiency is the goal.

    Diesels frequently are delighted to operate at 90% of their mfg rating , gas needs to be cruised at only 50% or so.Therefore the confusion of "smaller" diesel .

    If you simply want fast , big power (big fuel bill) always wins , gas or diesel.

    FF
     
  12. crankshaft
    Joined: Apr 2011
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    Location: Big Lake, Alaska

    crankshaft Junior Member

    Thanks guys.....

    I have figured out that I can stuff the 6bt in it with a 1:1 71C, and keep it at 180HP for now. The 318, takes 3800RPM to cruise at 15KTS..... And burns 17GPH according to my Garmin GFS10 fuel flow sensor.

    7.5KT cruise, it runs at 1700 burning about 3.1GPH. I have just installed a new shaft, repacked the stuffing box, new cutlass bearing, and have the prop looked at by Superior machine in Anchorage. They weren't sure, because it was in such horrible shape, but they think it was a 15" pitch. (16x15) However the prop was trashed. I am taking it to the lake right now to break in my packing, and am excited to see what a difference the reconned prop will make. When I first got the GFS10, I was at 4.2GPH at 1700RPM, carb tuning, timing curves, and indexing plugs have saved me about a gal an hour!

    I figure I'll just run it slow until I get all my pieces and parts together for the Cummins.
     
  13. whitepointer23

    whitepointer23 Previous Member

    hey crankshaft, can you post some pics of your boat and the cummins, i am interested in this topic.
     
  14. crankshaft
    Joined: Apr 2011
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    Location: Big Lake, Alaska

    crankshaft Junior Member

    I haven't put it in yet............ I have a 318 Chrysler at the moment.
     

  15. Akgramps
    Joined: Aug 2008
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    Location: Alaska

    Akgramps Junior Member

    Crankshaft,
    What ever became of this project, Cummins or Chrysler..........?
     
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