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  1. #1
    Join Date
    Oct 2020
    Location
    Peoria AZ
    Posts
    3

    Default Confused with gear ratio & props

    Hi everyone,
    We just picked up our SA550 and hitting the lake tomorrow.

    Iíve done a lot of reading and searching since last fall but still confused on the 1.5 gear ratio that we have in the SA550 and prop selection.

    We ordered the boat with the 16x13.9 prop so itís going to pull like crazy.

    Why does the 550 have a 1.5 ratio vs the 1.76 in the 400/450?

    Should we have gotten the 16x15 prop or should I get that prop as my back up/spare?

    I understand the impacts of diameter and pitch but for the life of me canít figure the impact from the gear ratio.

    Thanks in advance for some education.

  2. #2
    Join Date
    Oct 2010
    Location
    Lake Wylie NC Area
    Posts
    2,366

    Default

    Why does the 550 have a 1.5 ratio vs the 1.76 in the 400/450?
    I cant answer why Indmar has made this decision, but I can offer some info on why a lower gear ratio is an advantage.

    A lower pitch prop offers better hole shot. The trade off is higher engine RPM. Going up in diameter reduces RPM, but the trade off is poor hole shot. When you put the 2 together, you get a happy medium of improved hole shot but not as much increase in RPM. With the larger diameter prop, you also keep the prop slip to a minimum, get better speed holding and less cavitation.

    So what does this have to do with gear ratio you ask? It takes power to turn the larger wheels. So the lower gear v-drives makes it easier for the engine to run those larger props, like going up hill in first gear on a 10 spd bike v's the higher gears.

  3. #3
    Join Date
    May 2018
    Posts
    3,746

    Default

    The torque on the 550 is already overkill.

    580ft lbs x 1.5 is 870 ft.lbs at the prop shaft

    The 450 and 400 through the 1.76 are 711 and 795 respectively.

    The 550 would be 1020 with a 1.76 and I would think the already larger prop shaft of the 550 would be at risk of snapping.

    Are you sure the 16x13.9 fits the 1.25Ē 550 prop shaft? The 400 and 450ís use a 1.125Ē shaft.


    Sent from my iPhone using Tapatalk
    2021 Supra SA 400
    2018 Supra SA 400 (SOLD)
    Michigan

  4. #4
    Join Date
    Oct 2020
    Location
    Peoria AZ
    Posts
    3

    Default

    Thanks guys. I understand better now and yep itís overkill but never gonna worry about too much weight or elevation.

    Larry, I double checked and it is the 3591 16x13.9 L 1 1/8 prop. Not sure if/when the shaft might have changed.

    1E585B9C-836F-4B2C-8634-71870DD703DB.jpg
    Current: 2021 Supra SA550
    Previous: 2016 MB Sports F22

  5. #5
    Join Date
    May 2018
    Posts
    3,746

    Default Confused with gear ratio & props

    You will love that 550 and prop!!!!! stumpy McStumpersen.

    I would seriously find a stump and try to pull it out.


    Sent from my iPhone using Tapatalk
    2021 Supra SA 400
    2018 Supra SA 400 (SOLD)
    Michigan

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