4.3 to 5.7

Discussion in 'Powerboats' started by easyc, Nov 16, 2002.

  1. gonzo
    Joined: Aug 2002
    Posts: 16,923
    Likes: 1,771, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    There are limits to what you can increase in pitch before it becomes inefficient. What is the pitch now?
     
  2. Eagle1
    Joined: Dec 2022
    Posts: 13
    Likes: 0, Points: 1
    Location: Maine

    Eagle1 Junior Member

  3. gonzo
    Joined: Aug 2002
    Posts: 16,923
    Likes: 1,771, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    To maintain the same ratio, the pitch should be a bit over 23". Extreme pitched propellers are good for very light boats at high speed. However, they are really poor for initial acceleration.




    upload_2022-12-12_17-40-35.png
     
  4. Eagle1
    Joined: Dec 2022
    Posts: 13
    Likes: 0, Points: 1
    Location: Maine

    Eagle1 Junior Member

    So according to that chart, higher horse power engine has a lower gear ratio?
     
  5. Eagle1
    Joined: Dec 2022
    Posts: 13
    Likes: 0, Points: 1
    Location: Maine

    Eagle1 Junior Member

    So I get some of the accessory drive parts in today and moto should be here tomorrow
     
  6. gonzo
    Joined: Aug 2002
    Posts: 16,923
    Likes: 1,771, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    That is correct.
     
  7. Eagle1
    Joined: Dec 2022
    Posts: 13
    Likes: 0, Points: 1
    Location: Maine

    Eagle1 Junior Member

    Those are mercruiser, do it's the same for volvo penta?
     
  8. baeckmo
    Joined: Jun 2009
    Posts: 1,698
    Likes: 706, Points: 113, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    Yes, it is the same principle for all engine/propeller matchings, no matter what label colour. It is because the power absorbed by the propeller is primarily depending on the rpm's it is rotating with. A 15" propeller coupled to a 130 hp engine will have to rotate slower than the same screw coupled to a 300 hp engine. Now, if the engines produce max power at about the same rpms, the gearing has to be different. The prop does not know what is turning the shaft, only that it has to rotate at a certain speed to transmit a given power input.

    You can use the following rule of thumb for a rough preliminary estimation of diameter or shaft speed : D = 630 * P^0.2/N^0.6;
    Here D: diameter in inches, P: power in HP, N: prop shaft speed in rev's/minute.
     
  9. Eagle1
    Joined: Dec 2022
    Posts: 13
    Likes: 0, Points: 1
    Location: Maine

    Eagle1 Junior Member

    Now you give me alot of mathematics to do but I get your point. All of what i'm throwing at this crate moto it's going to have alot of power.. Holley EFI with timing control enabled.. but I don't think will able to get that gear ratio swap anywhere around here in Maine, cause there isn't much here..
     
  10. gonzo
    Joined: Aug 2002
    Posts: 16,923
    Likes: 1,771, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    What model and year OMC do you have?
     
  11. Eagle1
    Joined: Dec 2022
    Posts: 13
    Likes: 0, Points: 1
    Location: Maine

    Eagle1 Junior Member

    1996 volvo penta sx colbra
     
  12. gonzo
    Joined: Aug 2002
    Posts: 16,923
    Likes: 1,771, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    If you have a joint venture era drive, it will be interchangeable with other volvo penta parts. You can change the gear ratios on the top, bottom or both.
     
  13. Eagle1
    Joined: Dec 2022
    Posts: 13
    Likes: 0, Points: 1
    Location: Maine

    Eagle1 Junior Member

    I don't, I wonder how it would cost for a set of gears
     
  14. gonzo
    Joined: Aug 2002
    Posts: 16,923
    Likes: 1,771, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Volvo Penta gears are expensive and you need a complete set of tools to install and set them up properly. You will be better off to try to trade it in for an outdrive with correct gears.
     

  15. Eagle1
    Joined: Dec 2022
    Posts: 13
    Likes: 0, Points: 1
    Location: Maine

    Eagle1 Junior Member

    Oh I see
     
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.