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Thread: 671 DD transmission ratio

  1. #1
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    Default 671 DD transmission ratio

    Hi,

    I have a 49ft, converted to pleasure use shrimp boat with a single 671, 14.5 beam, 4ft draft. The transmission is a 2.5/1 what would the result be of using a 1/1 transmission? On the surface it would seem to be more economical. Any thoughts.


    Thanks, SP

  2. #2

    Default Re: 671 DD transmission ratio

    It isn't that simple. The 'economy' of your drive system is based on a lot of things, including having the right size engine for the job so it isn't working too hard. It's also important to have the right reduction gear on the proper engine so that the engine and transmission are not under too much load, resulting in failure.

    More likely you can benefit from playing with the propeller size and pitch. Pitch in theoretical terms is the distance the boat travels with one revolution of the propeller. From the sounds of it, you have a likely set up, but without knowing the boat, it's weight, and the pitch and diameter of the propeller, it's hard to say what could change. The 6-71 is a great old engine, but it's not known for economy, but for longevity. If the propeller is original to the shrimper, it probably could be increased in pitch, since it was intended to drive the boat with much heavier loads aboard. That's just a guess. A converted wood shrimper in that size is never going to be fantastic on fuel, but at hull speed it should be acceptable.

  3. #3
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    Default Re: 671 DD transmission ratio

    Thanks. It's more complacated than it appears.

    Sea Pearl now moves along at a stately 8.5 knots between 3.5 and 4 gallons per hour, I don't complain, the old 6-71 turning 1000rpms. The longest I've had it running was 16 hours, nice steady song coming from the engine room.

    I was concerned about inertia on startup, what level of strain a 1/1 would put on the shaft and transmission.

    She will be coming out of the water for her bottom paint soon, I'll do some measurements of the prop. It is large, as you say she is very light now, all the shrimping gear is gone. A new prop may be a good idea. She can churn some water at low rpms and reverse!

    Thanks Again, SP

  4. #4
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    Default Re: 671 DD transmission ratio

    A marine transmission does two things. It tries to keep the motor running in it's most efficient RPM band and it tries to keep the propeller running simultaniously at it's most efficient RPM band. You can vary those, marginally, but you can't make big jumps like you envision.
    Last edited by Cuyahoga Chuck; 10-14-2011 at 12:32 PM.

  5. #5
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    Default Re: 671 DD transmission ratio

    If you change from 2.5:1 to 1:1 you are also going to have to buy a new prop. Replacing a large prop will cost a couple of seasons recreational fuel usage. Lowering the gear ratio will increase prop speed. For example at your 1000 rpm cruise speed your current prop speed is 400 rpms. If you go to a 1:1 gear, then your prop speed will increase to 1,000 rpms. You would need to decrease your prop pitch by more than half to accomodate that increase in prop speed. Furthermore, you may find that you don't have enough torque to turn the prop at the faster speed.

    A general rule of thumb is that a slow turning large prop is more efficient than a small fast rurning prop. Furthermore, the boat takes a certain amount of power to push it at a given speed. Fuel burn generally equates to power produced by the engine. 3.75 gph for a diesel corresponds to about 60-65 hp. Basically, what your current fuel burn is telling you is that you need about 60-65 hp to push the boat at 8.5 knots. If you change the gear ratio and prop pitch, you may change that number a few percent, but you won't change it much. Basically, your boat is going to consume 3.5-4 gph at 8.5 knots pretty much no matter what you do to the gear/prop.

  6. #6
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    Default Re: 671 DD transmission ratio

    Todd D and Chuck,

    Like most boat owners I want to tinker. But this doesn't seem to be the correct area.
    I'll check the prop when she is hauled and might fine tune that .......... but if it works leave it along.

    Thanks, SP

  7. #7

    Default Re: 671 DD transmission ratio

    Quote Originally Posted by Sea Pearl View Post
    Thanks. It's more complacated than it appears.

    Sea Pearl now moves along at a stately 8.5 knots between 3.5 and 4 gallons per hour, I don't complain, the old 6-71 turning 1000rpms. The longest I've had it running was 16 hours, nice steady song coming from the engine room.

    I was concerned about inertia on startup, what level of strain a 1/1 would put on the shaft and transmission.

    She will be coming out of the water for her bottom paint soon, I'll do some measurements of the prop. It is large, as you say she is very light now, all the shrimping gear is gone. A new prop may be a good idea. She can churn some water at low rpms and reverse!

    Thanks Again, SP
    I would be surprised as hell to see you get a lot better than that kind of fuel efficiency..... it's already bordering on excellent.

  8. #8
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    Default Re: 671 DD transmission ratio

    I would wonder which injectors your engine has.

    Switching from workboat usage, ie towing heavy shrimp nets, to a pleasure vessel should be a huge difference.
    1000 rpm is a minimum speed for the fuel curve charts. That is barely off an idle. The slower you can run your engine the less fuel you will use, but the engine must be set-up for the power band you are operating in to avoid trouble.

    Here is a link to some fuel curve charts for the different injector sizes on a 6-71 *note that none are rated below 1000 rpm

    At a glance, you could perhaps get it down to 3 gallons per hour with a competent "tune-up"
    (this may include injectors, timing and propellor work which could cost a lot more than the subsequent fuel savings unless you are running many hours per season or many seasons to come).
    Most diesel injector shops practice a bit of legerdemain or VooDoo at the counter, mostly to collect more money. They will rarely give you advice or tune the components to your engine useage specifically, but it can be done.


    A lot of engine adjustments can be made in the field by a good mechanic, but you should find one that specializes in 71 series Detroits.

  9. #9
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    Default Re: 671 DD transmission ratio

    When I see people list 8GPH at 8 knots, I smile ............. Sea Pearl at 4 or less at 8 knots seems very good to me. If I can get it down to three GPH I'd be happy but it's ok as it is. SP

  10. #10
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    Default Re: 671 DD transmission ratio

    As a comparison, P.E. Pruitt the Chesapeake Bay Buy boat we run, is 60 ft LOA, 16 ft beam amd displaces about 25 tons. She has a single 671 in her and burns about 4.5 gph ar 1300 prm doing 8.5 knots

  11. #11
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    Default Re: 671 DD transmission ratio

    Dale, Do you have a photo? SP

  12. #12
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    Default Re: 671 DD transmission ratio

    It would probably simpler to repitch the wheel. More pitch, lower shaft RPM, remember a DD has marginal low end torque and work best at elevated speed. I'm considering an ex fisherman myself with a 4 71 and my idea is to replace it with a comparable 4 stroke allowing more torque at lower RPMs. The other issue I have with this is how fast can your stern bearings take, I prefer to run a cutless bearing at the lowest speed practical as you could have problems with cooling and lubrication. I intend to add a raw water pump to feed water into the stern gland aft of the packing, this has worked in large commercial vessels since there were wooden staves rather than the modern cutless.

    Hope this further "muddies up" the water for you.

    Cheers, Dumah,
    Halifax, NS
    Sanity is optional, but by no means necessary

  13. #13
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    Default Re: 671 DD transmission ratio

    I have a 3-71 in my 37' converted salmon troller. She weighs in at 28,000 lbs due mainly to her poured concrete ballast. The previous owner added some more concrete when he removed the fishing gear. She runs at 7.5 knots at 1250 rpm and burns 1.35 gallons per hour (I keep a very accurate spread sheet on fuel use ). The gear box is what is affectionately called a crash box, its the old mechanical Twin Disc which I had to have rebuilt a couple of years ago. I think it is a 1.5:1, not really sure. I had the prop repitched just a small amount because the engine wasn't working hard enough, ie running too cool. It helped the engine run at a better temp, but seemed to cause it to use a bit more fuel.

    Everyone says Detroit 71's leak oil, or "slobber". Mine doesn't for some reason but it is loud inside the pilot house, of course it is hard bolted to the floors with no rubber mounts.

    Not sure any of this helps you, just anecdotal.

  14. #14
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    Default Re: 671 DD transmission ratio

    Here is a photo of P.E.Pruitt

  15. #15
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    Default Re: 671 DD transmission ratio

    Quote Originally Posted by Dale Genther View Post
    Here is a photo of P.E.Pruitt
    Wow!

  16. #16
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    Default Re: 671 DD transmission ratio

    Quote Originally Posted by Canoeyawl View Post
    Switching from workboat usage, ie towing heavy shrimp nets, to a pleasure vessel should be a huge difference.
    1000 rpm is a minimum speed for the fuel curve charts. That is barely off an idle. The slower you can run your engine the less fuel you will use, but the engine must be set-up for the power band you are operating in to avoid trouble.
    I wondered about this as well. It seems like a very low rate of turns for a 671 at normal cruise. I'd wonder if you have access to the exhaust temperature through a pyrometer or what temperature is normal for your coolant's rig under load.

  17. #17
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    Default Re: 671 DD transmission ratio

    What a beautiful boat. Thanks for the photo. Sea Pearl

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