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39 ft viking repower transmission and prop questions

Discussion in 'Viking Yacht' started by Jus bills, Sep 19, 2018.

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  1. Jus bills

    Jus bills New Member

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    ive got a 38 ft viking sportfish im repowering from 485 tib detroits to cummins 500hp qsc engine, i want the boat to be capable of 40 mph cruising mid 30s and getting fuelmilage, i have 1.7 zf and i have 1.5 twin disc transmissions, wheels are 25x26 what should i be going to for both, boat lost 2000lbs in engine swap any imput would help im sure some one did the swap
  2. Pascal

    Pascal Senior Member

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    Find a good prop shop, give them all the specs and they will crunch the numbers for you That s why i did when i repowered. They were pretty mich on target
  3. C team

    C team Senior Member

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    For sure what Pascal said. That sounds like a great engine swap!
  4. PacBlue

    PacBlue Senior Member

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    What are your speeds right now with the Detroit's - WOT and cruise at what rpm?

    What gear ratio do your Detroit's have right now?
  5. Trinimax

    Trinimax Senior Member

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    Those speeds sound a little optimistic for that power. I know that there was a 37 bertram on yachtworld a while back that had 600 hp qscs and I think the top speed was around 35 kts, so close to 40 mph. But this is a smaller boat with 200 more hp.

    I wish you the best of luck with your project
  6. Jus bills

    Jus bills New Member

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    detroits 31 mph with 1.5 twin disc, 24x26 wheels , cummins are 2000 pounds lighter which will bring boat up out of water more which will increase speed, they are actually 550hp qsc engines have 200 more rpm then detroits only reason i asked is i was on a sea ray 44 sedan bridge which weighs 30k pounds which is 450 pounds less than my viking and wot was 40mph it had 1.75 zfs but not sure of wheelsid like to see close to 40 gotto make goals,lol
  7. Pascal

    Pascal Senior Member

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    15 extra hp and 2000 less weight isn’t going to get you 9 mph... again check with a prop shop although the Cummins dealer should have done that already
  8. Capt Ralph

    Capt Ralph Senior Member

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    You have a hull design that offers a better ride but way more wet surface. WET FRICTION. You may find better mpg, maybe some speed increase, but I'm not sure of GREAT speed increase.
    I am not a speed fan but reliable cruise speed. 1.5 reduction, good diameter with some pitch may offer the best and most reliable cruise. If your really looking at near WOT speed (8.3s can do it), then lean to a lil less diameter and more pitch. I advise not to go there.
    I bet a cup of coffee your prop shop will advise the same.
  9. PacBlue

    PacBlue Senior Member

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    Here is one sample of a boat test for your Viking:

    upload_2018-9-21_7-43-57.png
    A light factory boat did 36.3mph at WOT and looks to be about 33mph at a max cruise of 2300 rpm. For longevity sake, I would not cruise them much more than the 31 mph at 2100 rpm. Keep in mind that these old magazine test boats where light and had no owner's gear or equipment to speak of.

    Is 40 mph at WOT possible with QSC's rated at 550hp and the weight savings? I would tend to say yes (just barely) based on inspection of these numbers, but the key is how heavy has your boat become? The best thing you can do is do an inventory of all your boat gear, keep all necessary safety and spares, get rid of as much unnecessary inventory as you can and start from there.
  10. PacBlue

    PacBlue Senior Member

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    Looking at the QSC 550:

    upload_2018-9-21_8-33-46.png

    I see that the QSC 550 is a 3000 rpm engine. The Detroit's were 2500 rpm engines with a 1.5:1 gear ration > 2500/1.5 = 1,667 shaft rpm. So for the QSC 550 > 3000/1667 = 1.80 Gear ratio to maintain a similar level of propeller size before accounting for additional horsepower.
  11. PacBlue

    PacBlue Senior Member

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    With regards to props:

    Most manufacturers who select a 3-blade prop do so because they are good for getting the best top speed (less drag because of fewer blades) and do it for a lower cost solution. The higher top speed numbers make the marketing guys happy, but it does not tell the whole story. Cruise speed, cruise efficiency and smoothness (less vibration) are typically sacrificed in a 3-Blade propeller.

    Your prop shop will check certain criteria, such as tip speed and cavitation numbers. When increasing horsepower in an existing set-up, you will not want to sacrifice blade area, and therefore would avoid reducing diameter as a solution unless you offset this by increasing the number of blades. In short, you need sufficient blade are to avoid the critical cavitation zone. Propeller design has improved since your boat was built, and there are some options to look at.

    I would most likely go with a 4-blade propeller in your repower, as you appear to be looking to optimize your cruise speed. It will also be noticeably smoother underway. Propeller brands like Michigan Wheel, ACME and ZF have models that are good choices up to 40 mph (34.7 knots). The costlier Veem's are better for 40 mph + applications, although they are manufactured like a piece of jewelry and are a sight to behold for the mechanically inclined types.

    Personally, my first approach is to select the optimum Gear Ratio by running the Cavitation numbers and checking on Tip Speed first, then I look at the set-up to see if the "optimum" calculated gear ratio and the resulting propeller model can fit the installation. This is a bit difficult in repowers, as you are locked into an existing shaft system, and any changes to shaft angle, shaft diameter will drive up costs.
  12. Capt Ralph

    Capt Ralph Senior Member

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    I like the idea of the lower ratio, but now your leaning on the shafts a lil bit more.
    Any idea of the material and factors in youe existing shafts?
  13. Capt Ralph

    Capt Ralph Senior Member

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    How many blades on your wheels now? Are they fresh props or high milage and thin (used up), near end of life?
  14. PacBlue

    PacBlue Senior Member

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    A 2" Diameter AQ 22 shaft would be good with either ratio.