Is free 1.5" shaft too big for planing 25'er diesel 150hp 1:1

Discussion in 'Inboards' started by Northeaster, Mar 6, 2014.

  1. Northeaster
    Joined: Sep 2011
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    Northeaster Senior Member

    Hi Folks,

    Boat is a 25' Glen L Double Eagle planing aluminum hull i am building, Cummins 4BT 150hp, 1:1 Velvet Drive
    Although I haven't asked anyone to run any numbers for me, based on feedback from someone with a similar boat- I think I should be looking for a 1 1/8" or 1 1/4" shaft???? But, I have a free 1.5" shaft that came with my engine. I have not found a used or new prop yet - likely buy something used to try. (#s below are what I entered in free prop calc)

    I could have a local machine shop turn down the end to be 1 1/4" if required, so I could use a prop with a smaller hub.
    But, would the extra drag of the large diameter shaft, stern tube. and cutlass be that noticable? Not sure how much machining would cost, but I don't see many used shafts in my area, and if so, they are around $400.

    If it's a bad idea I want to know, and will bit the bullet for a smaller shaft.
    Thanks in advance for any advice!



    150 Engine Horsepower
    2800 Engine R.P.M. (max)
    281 Engine Torque ft/lb

    146 SHP - Shaft Horsepower at gearbox output.
    2 Enter number of bearings between gearbox output and propeller.
    1.00 Enter Gearbox reduction ratio. Eg. 1.5

    3.00% Percentage power loss due to shaft bearings.

    145 Shaft Horsepower at propeller.
    2800 Propeller RPM
    273 Propeller Torque ft/lb



    4,500 Maximum Displacement of vessel in pounds.
    23 Waterline Length of vessel in feet.
    25 Required maximum speed in knots.

    5.213 Speed:Length Ratio.
    Suggested max practical displacement hull speed for LWL input ---> 7.19 Knots

    145 Shaft Horsepower available at propeller from "Torque & Shaft Horsepower" sheet
    31 Pounds per Shaft Horsepower available (power/weight ratio)

    525 Shaft Horsepower required at propeller
    9 Pounds per shaft horsepower required.


    For planing hulls, where C= 150 for runabout, 190 for fast, 210 for race.

    150 Enter "C" for your hull.

    27.0 Approx maximum speed attainable with motor data from "Torque & Shaft HP" sheet.
    125 Shaft HP required for speed required at top of page.


    25 Speed in knots required from Power Reqd page
    2800 Max prop shaft rpm from Torque & SHP page
    2240 80% of max prop shaft rpm

    2533 desired speed expressed as feet per minute.

    1.13 desired speed divided by max prop shaft rpm to give prop feet per minute.

    13.57 Theoretical required prop pitch in inches.

    22.35% Estimated prop slip at required top speed.

    17 Required prop pitch for top speed.

    3 blade prop #s below:

    This is an automatic calculation from shaft horsepower and rpm at prop on Torque sheet.

    15 Propeller Diameter in inches.
    14
    17 Propeller Pitch in inches.
    19

    0.33 mean-width ratio blades. (This means 33% of the "disc" area of prop dia is blades)

    Rules of thumb.
    One inch diameter = 2.5 inches of pitch.
    Two inches extra pitch will cut engine rpm by 450.

    If you can't fit the indicated diameter due to clearance, or have plenty room left, the rules
    of thumb above will be a useful guide.
    The alternatives in light blue squares

    Two blade propeller.

    15 diameter in inches.
    17 pitch in inches.

    Four blade propeller.

    14 diameter in inches.
    16 pitch in inches.
     
  2. Northeaster
    Joined: Sep 2011
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    Northeaster Senior Member

    Is it really that dumb of a question? Please someone tell me if there would be that big of a difference with the 1.5" shaft vs 1 .125" or 1.25"?
     
  3. tom kane
    Joined: Nov 2003
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    Location: Hamilton.New Zealand.

    tom kane Senior Member

    The first thing I would do is check out if the free shaft is straight and true.I would be happy with that diameter if it fitted with thrust brgs and other running gear.
     
  4. Northeaster
    Joined: Sep 2011
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    Northeaster Senior Member

    Thanks for the reply Tom. It is fitted with a thrust bearing. But, I am not sure i have room for it, or need as well. I assume that it was used along with the ajoining u-joint to allow the engine to be mis-aligned or similar purpose. If I align my motor and attached velvet drive properly, I don't believe I would require the thrust bearing. I guess using it and the u joint would allow me to adjust the motor position and not have it straight to the shaft, if desired.
    If I am off on my assumptions, or reasoning, please let me know!

    The 23 x 26 or so prop that came with it is much too large - I need something in the 15" - 16" diameter range. The engine, shaft and prop were used with a 2.91:1 gear in the previous boat, and I am now planning using the engine with a 1:1 gear.
    I will d omy best to check to see if it is straight and true. I could take it to a local machine shop to have it checked.
     
  5. tom kane
    Joined: Nov 2003
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    tom kane Senior Member

    Is free 1.5" shaft too big for planing 25 er diesel 150hp 1:1

    A quick eye check or roll the shaft on a flat surface may save you taking the shaft to a machine shop as any serious wobble should show up.
    Remember the 20 40 Rule. Support bearings no closer than 20 times the shaft diameter or 40 times the shaft diameter apart. Compromise may be necessary.

    A thrust brg that takes forward and reverse thrust I consider essential, plus at least one U/V joint for minor bad alignment with at least 5 degrees off straight alignment for more quiet running and lower U/V wear. Twin U/Vs would be my preferred choice (to mount the motor at 10 degrees) along with soft engine mounts to dampen vibration.
    The prop needs to be chosen to suit the hull.
     
  6. Northeaster
    Joined: Sep 2011
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    Northeaster Senior Member

    Thank you Tom! Had some other priorities so have not checked this yet.
    Don't have a pic right now, but the thrust bearing looks very heavy duty, likely 30cm long or so. It was used on a displacement vessel with a 2.91:1 gear and large prop.
    I was wondering, if I now use it with the same engine, 2800 max rpm and 1:1 area (of course smaller, correctly sized prop) the shaft will be spinning 3 times as fats as before, through this heavy duty thrust bearing. Would that introduce much friction or loss of fuel efficiency, as opposed to juts going with the typical setup of Velvet drive gear straight to shaft coupler and shaft.

    The thrust bearing turns by hand, but I assume is grease filled or similar. Have not had a chance to take it apart yet.
     
  7. tom kane
    Joined: Nov 2003
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    tom kane Senior Member

    Are you sure your velvet drive has forward and reverse thrust brg ? there are plates and other systems.
    If you are going to run a prop at motor speed direct drive do not be afraid to keep as large a diameter as possible and at least 2" prop tip clearance with finer pitch rather than a course pitch and smaller diameter. 2800 RPM would not be excessivly fast to run a prop efficiently.
    Thrust brg friction has to be tolerated. I would be interested to see a picture.
     
  8. Northeaster
    Joined: Sep 2011
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    Northeaster Senior Member

    I will get a pic in a day or two when I am down at the shop.

    I could be wrong, but was under the belief that all Velvet drives, being marine specific transmisisons, had thrust bearings.
    Just looked at a couple of manuels for Velvet drive on line. There is a thrust washer in parts list and the installation section shows direct connection to the prop shaft coupler - no mention of requiring a separate thrust bearing.

    There are (at least) two guys with similar boats to the one I am buiding, running inboards / shaft drive.
    One has about gas 100hp and runs a 1.5:1 gear, and 16" 3 blade prop. He had tried a 17" but didn't see a difference.
    The other guy has the same Cummins diesel as me, but 130hp and also runs 1.5:1 gear with a 16" 4 blade.(and he may be a bit underpropped)
    I think the numbers would have called for my 150hp setup requiring a 1.2:1 gear, but as they are not common, or cheap, I went with a 1:1. It was either that or use the 1.5:1 and I think tah twoudl have been worse.
    The boat plans call for a max 16" prop. But, I have stretched plans by the acceptable 10% and I may need to have the skeg and strut a bit longer to maintain a proper angle, which woudl give me more tip clearance. I think 17" would still be about all I could fit under though, without really changing the design.
     
  9. tom kane
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    tom kane Senior Member

    You can make all sorts of calculations and use many formula but not get the results you expect. Talking with guys with similar boats and set up`s can be a more rewarding. The story is check and double check your information. There are lots of improvements since the days of firmly bolting a motor in a hull with inflexible mountings, stern glands and such.
     
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  10. gonzo
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    gonzo Senior Member

    Propellers will be the same price whether 1 1/8 or 1 1/4 diameter hole. The slight difference in shaft diameter won't make a considerable drag difference at those speeds.
     
  11. Northeaster
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    Northeaster Senior Member

    thanks to all for th replies and advice! Will check for true/ straight this weekend and snap some pics of the large thrust bearing or whatever it is.
     
  12. Northeaster
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    Northeaster Senior Member

    pics of the trust bearing or whatever it is...

    I doubt that I would have room for this type of setup with a u-joint, sgaft and bearing. I was incorrect earlier - there are two u-joints on the short drive shaft.
     

    Attached Files:

    Last edited: Mar 23, 2014
  13. tom kane
    Joined: Nov 2003
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    tom kane Senior Member

    Is free 1.5" shaft too big for planing 25 er diesel 150hp 1:1

    Your " whatever" it is does not appeal to me in a boat could be a "valuable " antique. It looks interesting and exciting to take apart.

    Thrust brg`s take up little room on a shaft, these diagrams may help.
     

    Attached Files:

  14. Northeaster
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    Northeaster Senior Member

    Thanks Tom. I figured it was more suited for a fishing museum.

    The drawing definitely helped!
    Although not used for a thrust bearing, I have used an skf self-aligning bearing, similar to pictures, as the top rudder post bearing when I refit my old 30' sailboat. Worked well, but it just uses set screws to hold against rudder stock. How are the skf type thrust bearings fastened to the prop shaft, when used as thrust bearings? I would not think a set screw would hold the force.
     

  15. tom kane
    Joined: Nov 2003
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    tom kane Senior Member

    Is free 1.5" shaft too big for planing 25er diesel 150hp 1:1

    Have got a picture but can not find it new computer and all software in win 8 is ...... I probably will find it sometime. No thumbnails to guide me. The set screw does not do any thing but lock the thrust brg collet in place when fitted on to the shaft. Collet takes the load.
     
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