Thoughts on prop load for com-fish/tow boat

Discussion in 'Props' started by NWestward, Apr 23, 2024.

  1. DogCavalry
    Joined: Sep 2019
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    DogCavalry Senior Member

    Yes, that doesn't make sense. Underpropped would over rev, not under. Based on what you observe it's more likely the ratio is 2:1 than 3:1.

    In answer to your other question about ratios, it's not linear. Increasing the diameter of the prop doesn't increase the power the prop can efficiently convert to thrust in proportion to the diameter increase. So doubling the prop diameter wouldn't double the usable power, for example. It would give 4 times the blade area, not 2 times area. But there's more. At the same rpm the working area of the blades would be moving twice as fast, so it's more like 8 times the power converted to thrust.
     
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  2. NWestward
    Joined: Apr 2024
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    Location: Alaska

    NWestward Junior Member

    Well, this answers that. Neither the twin disc model, nor gear ratio, is as was presented in the surveys. Admittedly, the previous owner was very ready to retire. He’d had an amazing run in the industry.

    I’d assumed the gear was replaced with the new 9L engine. But it being original makes a lot more sense. It doesn’t have nearly as good of paint as the JD.
     

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  3. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Good to see my calcs were useful; bad for your efficiency though..... Question is if there is room for a larger prop (requiring a gear change of course), if not a nozzle might be the way to go. But that must be checked carefully, standard shapes probably not suitable. In the mean time I would see if it is possible to downpitch the screw in order to match the engine better.
     
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  4. comfisherman
    Joined: Apr 2009
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    Location: Alaska

    comfisherman Senior Member

    That 5111 was a good gear, probably outlast more than a few engines in front of it.

    Doing the math at 2:1 explains the overloading at the top of the rpm range. Have you got a curb weight from the travel lift when they pulled you light?
     
  5. NWestward
    Joined: Apr 2024
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    Location: Alaska

    NWestward Junior Member

    About 75,000 lbs in the slings
     
  6. NWestward
    Joined: Apr 2024
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    Location: Alaska

    NWestward Junior Member

    As far as larger diameter… not unless I extended the prop shaft aft. As is I have about 2” clearance with hull and keel-shoe. I would gain some clearance in tunnel, though might need to lower keep shoe.

    It would be a bit of a project, as a 40kw generator sits where the moved rudder shaft would go.

    (it would have been a great time to do that when he did the stretch! As he had the deck open to install new fuel tanks and that generator)
     

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  7. comfisherman
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    Location: Alaska

    comfisherman Senior Member

    Yeah with a 2:1 bigger diameter would have you in some odd props.

    75k has you 28x20ish, loaded puts you in 28x16. A bigger diameter has you in some really odd prop dimensions.

    8 inches of pitch is a lot to take out of a 28 inch prop.
     
  8. NWestward
    Joined: Apr 2024
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    Location: Alaska

    NWestward Junior Member

    I think over-propping com-fish boats is really common. I’m pretty sure this for sale posting is Rebel. Doing some rough calcs (I gave him 60,000 lbs) it says he should have half the pitch as is listed, for unloaded conditions.

    Back to real life, I’m going down to 26” pitch (from 28”) on my boat for this year. By my limited notes from last year, this should let me over-rev unloaded. And be a little kinder while loaded. I’ll take better notes this summer.
     
  9. comfisherman
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    comfisherman Senior Member

    Original rebel main was more pumped up 8v92.

    The trend is most definitely over pitched, that's actually common across almost all boats as weight management plagues us all.

    Most folks do a rodeo light, so max pitch for that condition makes the most sense. See less and less of that on the big boats that are perpetually tanked. Keep in mind the perpetual tanked status of many boats was only widespread adopted 15 years ago or so. Considering these boats are 35 to 50 years old that's relatively recent.

    At the end of the day most everybody just accepts some level of "least worst" and call it a day.
     
  10. DogCavalry
    Joined: Sep 2019
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    DogCavalry Senior Member

    A huge market for a proper 2 speed transmission. It's a no-brainer. Unless you're a transmission manufacturer.
     
  11. comfisherman
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    comfisherman Senior Member

    Not really, it's gonna be a while before we have enough money to buy new gears for the sake of minimal gains. The 2 speeds that are out definitely seem to work in the very upper end of the speed range. Market segment is just so small to warrant the engineering cost.

    Gotta love a soft chine overweight tunnel hull boat, but there isn't really anything that will make a huge improvement. After lots of money the best performance upgrade was a really comfy helm chair and a good gps compass driven autopilot. Fished them since I was a kid, probably will till retirement. Clean bottom, keep the prop outta the rocks, and accept they are slow.
     
  12. DogCavalry
    Joined: Sep 2019
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    DogCavalry Senior Member

    Well, actually the market is huge, it's the adoption that's failing. What percentage of comparable work boars in Europe use controllable pitch props? Almost all. It's another solution to the same problem, and unless most commercial fisherman in Europe are wrong, the extra expense and complexity are justified.
     

  13. comfisherman
    Joined: Apr 2009
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    comfisherman Senior Member

    They have deeper drafts to work with, changes the decision making flow chart significantly. Our boats are 35-40 year old boats that are very similar in design to smaller 50-60 year old hyper specialized fishing vessels developed when statehood put an end to fish traps.

    Regulations set in place in the late 50s and relatively unchanged since the early 70s means there has been little change in boat design and construction. The way the fishery is likely to evolve won't make two speed transmissions needed, it will make most the small boats that would benefit from one not needed. The ratios of the current crop of zf gears wouldn't take much to work alright in this setting. But the gains would be minimal compared to the cost.



    Not a slight on the Europeans, they fish different than we do so have different solutions. Even state regions make different boat features more or less attractive. Lobster style hulls have been tried up here and failed fairly significantly to catch on. But I'd hate to lobster fish with a high sided bay boat with no keel and twin jets. Different tools for different tasks.
     
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