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  1. #21

    Re: Old iron is out!!!!!!!

    I did rework the cockpit. Mezzanine seating, bigger deck hatches and ER access from the cockpit. This did require moving the rear fuel tank aft. There are some pictures posted.
    The current plan for gears are ZF325A with 1.73:1 ratio with 28' 4 blade props. Still working through the pitch. The M3 rated John Deere's make 500 HP at 2000 RPM. A software change is available to upgrade to an M4 rating making 575 HP at at 2100 RPM. Now for my black magic estimates. I have some test data from Cummins engineering of a 53C with QSM11. She ran 20 knots at 1800 RPM and the QSM is making 320 HP here. Per side of course. This is per Cummins engineering. I have additional data from another 53C owner with QSM's that match that. So if I used that to estimate the JD is making 320 HP prop HP at 1800 RPM and about the same torque as the Cummins. I don't know if my QSM numbers are crank or prop HP but I'm thinking I should be close to 19 or 20 knot cruise with the JD. Maybe just a bit lower cruise since JD wants me cruising at 50% load. That would technically be 64% but the Deere torque curve is really flat through here making tons of torque down to 1400 RPM before falling off. The 500 HP JD makes more torque than the 670 HP QSM by just a bit at 1800 but at 1400 it's over 10% more.. We are back at that displacement thing but torque turns the prop. So that's how I got here, big displacement, low HP and high torque I hope will equal a lightly loaded happy engine. FYI the JD is basically the same displacement as the 12V71N that it's replacing. 13.5 vs 13.96. Make about the same HP at 300 less RPM. Can't find a torque spec so not sure of that but at 3500 lbs lighter there is just not way I won't get a slight bump in performance with all the other benefits of a modern engine. Well that's the plan anyway. We all know how the best laid plans work out sometimes.
    Last edited by harnett; 08-23-2019 at 04:08 PM.

  2. #22

    Re: Old iron is out!!!!!!!

    FWIW I think you are right. I suspect your boat will go a bit faster, use probably 60% of the fuel that it used to use, and generally run cleaner and be easier to maintain.

  3. #23

    Re: Old iron is out!!!!!!!

    So just thinking this through since i've already been down this road with a similar boat.
    If it was me i would go for the higher horse power rating of 575hp, and then run them slow to cruise the 20 knots you're looking for. Is there a big cost increase to the JD 650hp and 750hp models?
    Looks to me the JD is more like a CAT c15/3406E than the QSM as QSM only has 661 CID, CAT 893 CID, JD 824 CID with the CAT pumping out 850 hp compared to the JD's top rating @ 750hp, so JD is already being conservative.
    I'm a big believer in high horsepower run at moderate RPMs for longevity and fuel savings. There's a reason the charter boat fleet gets so many hours on a 2300 RPM engine they cruise at 1600 RPMs, as an example.

    There's 2 projects we should have done, move the aft tank back to be able to get the ER access through the cockpit and not making the bridge wider and also pushing the bridge front forward.
    Extending the aft bridge floor ended up being one of our best improvements.
    What we ended up with is using what was the generator hatch in front of the door for access moving the genset to the other outboard side, worked out pretty well with the Amtico flooring in front of the door area.
    Last edited by Cricket; 08-23-2019 at 05:05 PM.
    CRICKET
    1966 HAT50C101
    Purchased 1985 12v71Ns
    Repowered 1989 with 8v92TI
    Repowered 2001 with 3406E

  4. #24

    Re: Old iron is out!!!!!!!

    I don't disagree. I bought the AF version which means no seawater cooled aftercooler. The SF version has a saltwater cooled after cooler and goes to the higher HP. I am limited to the 575 HP. Its a simple software upgrade they do. Well plus paying them a few more grand to do it. It also changes to engine to an M4 rating. I'm holding it in my back pocket til I see how things shake out. It I need the extra HP it easily available to bump from 500 to 575.

    I think you are right. It is closer to the C15. JD is just very conservative in the ratings.
    Last edited by harnett; 08-23-2019 at 05:15 PM.

  5. #25

    Re: Old iron is out!!!!!!!

    The last 53C I know of with a QSM11 repower was Hookshot, done by Slane Marine. I think her M11s were rated at 700hp. She topped out at >28knots on her sea trisals; this included the weight savings of the Cummins engines, and the bigger bridge fitted by Slane, and some other mods. Very good speed for a relatively small engine in that heavy big hull.

    I don't think you'll have trouble cruising 20 knots. How much faster she'll go is an open question.

  6. #26

    Re: Old iron is out!!!!!!!

    I definitely get going with the freshwater cooled after cooler, just removes a major potential issue.
    Think it's going to be a wash cost wise for the upgrade to 575hp and then have to modify/mess with the props etc. Would that be "more is less" ??????? Looking forward to watching your project.
    CRICKET
    1966 HAT50C101
    Purchased 1985 12v71Ns
    Repowered 1989 with 8v92TI
    Repowered 2001 with 3406E

  7. #27

    Re: Old iron is out!!!!!!!

    That's a good point. I'll have to rethink that a bit. Might just should start out with the 575 hp.

  8. #28

    Re: Old iron is out!!!!!!!

    Are the new motors 3500 lbs lighter on each side? If so, what effect will the removal of that weight have on seaworthiness and stability? When I was considering a 61’ cpmy with the DD’s replaced with Cummins, the consensus here was that I needed an NA to run stability calculations for safety’s sake. Does that not also apply here? If not then why not?
    Michael & Beth
    Hull Number CV312
    63’ Cockpit Motor Yacht
    1986 model launched in August 1987

  9. #29

    Re: Old iron is out!!!!!!!

    The 3500 lbs is total not per side. The JD pair is about 1000 lbs heavier than a pair of Cummins. The 3500 Lbs is about a 5% weight reduction in my boat. The other thing is that by going to a 7 degree gear box the engines will sit lower in the hull than the Detriots did. I don't believe the weight difference will matter especially when coupled with sitting them lower in the hull. I'm certianly hoping that is true. With so many boats out there with Cummins running I think if it made a big difference we'd have heard about it by now. I haven't ran the calculations but I feel certian that the change in weight will have much less effect on stability than say a full tuna tower.
    Last edited by harnett; 08-25-2019 at 09:52 AM.

  10. #30

    Re: Old iron is out!!!!!!!

    I don't think you will have a stability problem at all. You could get a NA to look at it, but frankly the money would be better spent on the install. You might find you have to move some things around. You would think that putting in engines which are the same amount lighter on each side would not cause the boat to list, but I have heard of it happening. I have no idea why.

    The JDs are very good engines. Lugger used to use them in some applications; they were very strong and reliable.

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