Not a Design but a Use Case change: Prop changes as needed?

Discussion in 'Boat Design' started by Swamplizard, Mar 13, 2024.

  1. Swamplizard
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    Swamplizard Senior Member

    As I get older I find I rarely go fast in my go-fast cruiser. Some will say "sell it and buy a trawler" which is possible but I like my boat.

    Instead I tend do turn on the Autopilot and "voyage" at hull speed, fish, or simply enjoy the views as I go from anchorage to anchorage for overnights. (young folks - I know - BORING!).

    Candidly, 1,000 rpm gets me 6mph and a 6 gallon (496 gasser w MPIs)/hour burn (3 gals/motor) or 1 mile/gallon. She is not an efficient girl displacement-wise. Can I do better?

    Since I can change my propellers in 5 minutes and I am unlimited in propeller diameter, why not change the props and putts along more efficiently when that is the agenda? DOWN SIDES?

    If current vessel (deep Vee, no steps, 9.5'x42', 15,000 lbs sport cruiser) which has arneson drives and surface piercing propellers can do 40 mph at 5000 rpm with 3-blade 20x20 props when on plane, what would happen if I put big bigger props with higher pitch for when I am in "Trawler mode"? As required, prop Pitch is limited to what the motors can handle load-wise when on plane and must e able to pull WOT (5,000 rpm) to not over-burden the engines. Transmissions are 1.5:1 by the way.


    What would happen if I used more traditional propellers - 5-7 blade 30x35s or 20-35s????

    Curious what the engineers would say on this topic!
     
  2. BlueBell
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    BlueBell . . . _ _ _ . . . _ _ _

    Larger diameter
    Fewer blades
    Lower RPM
    Skinnier cord (higher aspect ratio) blades

    6 gallons/hr is terrible.
    What do you get at speed/cruising?

    Have you tried running only one engine at a time while below "hull speed"?
     
  3. fallguy
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    fallguy Senior Member

    He is in a planing hull, below the hump.
     
  4. bajansailor
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    bajansailor Marine Surveyor

    I don't think that surface piercing propellers are going to be terribly efficient when running at slow speed, even if you install propellers with more pitch.
    For props that are fully submerged the formula for developed power P is
    P = 2 x 3.142 x Revs x Torque

    If you increase the pitch, you will get more torque, so the revs will decrease for the same amount of power.
    But does this apply to surface piercing propellers as well?
     
  5. baeckmo
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    baeckmo Hydrodynamics

    What can be achieved by a change depends on what you have right now in terms of blade shape and how deep you operate the "sting pins". So if you can provide close-up image showing the prop blade sectional shape and perhaps also a picture of the wake of the boat, when loitering at the preferred "lazy speed", it would help.

    And, btw, what is the power setting of the engines? Do you have a power curve available?
     
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  6. Swamplizard
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    Swamplizard Senior Member

    Great comments - thanks!
    Motors are Mercury 496 EFI inboards - they are supposed to consume 1.3 gph at 800 rpm (each) or 2.6 gph@1,000 rm each - hence the 6 gallons per hour. Torque is close to 400 as low as 1500 rom and peaks at 4,000 rpm at 450.

    I have not tried running with single engine - Power steering is driven off starboard motor only but port has a pump and reservoir (could be linked)

    I have two pairs of props - same diameter and pitch 19 D x 17 P for both (my 20x20 above was incorrect.

    Picture below was to show prop shop damage to my spares I need repaired so not great over-all prop pictures but should help.

    I may have access to some 4-blade 17.5x26 cleavered props - be interesting to see if my speed at idle and 1000 would increase or if the motors would be overloaded.
     

    Attached Files:

  7. baeckmo
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    baeckmo Hydrodynamics

    You didn't respond to the question "how deep do you operate......". What I want to know is if you can trim down the shaft units so the props are working in green water or if they are whipping close to the surface? This is vital info. There have been heavy barges using Arnesons but with screws in submerged mode.
     
  8. Swamplizard
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    Swamplizard Senior Member

    The drives have about a 20-degree adjustment up and down. Sitting, the props can be as close as a foot from the serface.....so fully emerged. On plane the props can be true surface piercing (half out of water). Hope that helps
     
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  9. fallguy
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    fallguy Senior Member

    The question is out of my ability to answer. I'm rather curious though; here in Minnesota; someone with a 150-300 hp engine on their boat almost always uses a 15-25 hp kicker for low speed trolling (10%). Perhaps a swim platform makes it impractical, but it seems like the more ideal situation would be a smaller engine like a 70hp ob to run at low speeds and to keep the setup correct for typical use. But we almost never see it.
     
  10. baeckmo
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    baeckmo Hydrodynamics

    ....so, when putt-putting along @6 mph you run them completely submerged then?
     
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  11. baeckmo
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    baeckmo Hydrodynamics

    Checking some numbers, I can't make the ends meet here so I've got to keep pestering you with questions; The consumption figures you quote, are they measured and if so with what precision? Or are they cited from somewhere, and if so do they refer to propeller loading curves or brake specific fuel consumption at wot?

    Asking, because the props should not pull the power corresponding to the fuel flow you refer to, even if fully submeged.
     
  12. Swamplizard
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    Swamplizard Senior Member

    Yes
     
  13. Swamplizard
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    Swamplizard Senior Member

    YES
    The motors are Mercury 496 EFIs and I have Vesselview which measures real-time fuel consumption in Gallons/hour. At 1,000 rpm with both motors running. I am consuming 2.6Gal/hr per motor or 5.2 g/hr total and my speed is what I believe is hull speed (not much wake and not laboring, 6 mph per GPS. At quick cruise which is between 3,800 and 4,000 rpm it consumes 21.5G/hr EACH.
     
  14. fallguy
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    fallguy Senior Member

    The boat is not 15,000 pounds loaded. I apologize for lurking and comments a bit. I'm always fascinated by these big cruisers and how poorly they do on fuel and why there has never been a push for better economy. I have a friend who used to spend a grand a weekend on gas.
     

  15. Swamplizard
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    Swamplizard Senior Member

    Actually it is. My friend has the same boat with but CAT Diesels (ie much heavier than my gassers) and his weighs in at 17,000. His has been wieghed and he pulls it with a gasser pickup truck (not a diesel dually) and a trailer ball....not a 5th wheel. The boats are kevlar reinforced glass over a light closed cell core - lighter than you would think.

    Today, I took her out again with 300 gal fuel, 100 gal water, 4 adults and couple coolers. Here were some of the numbers I saw (no fast runs):
    800 rpm
     
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