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What gear ratio and prop?

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  • What gear ratio and prop?

    Good morning.
    I'm not being lazy. I have been researching this and now my brain is mush.

    The boat is a 1988 260 Predator with a flybridge.
    The engine is a 408 Stroker.
    The Velvet Drive manual says that I need a 72C transmission because of the hp of the engine.
    My goal at this point is to get the correct transmission with the correct ratio so that I can fine tune the prop to get maximum performance.
    The charts show that I need to be able to reach 4200 rpm when everything is correct.
    My goal is to be able to reach 30 mph and troll at 2ish mph.
    The more important number is to get close to the trolling speed however, I can't sacrifice too much top speed to get that.
    The manual says that you might have to change props during certain times of the year for different speeds. That is doable if needed.

    I'll ask about a starting point on the prop when I get to that point.
    Because of the cost and where the boat will be going, I'm hoping that there is a formula somewhere that will pinpoint the answer to my question.

    What transmission do I need to accomplish this?

    Thank you,
    1980 pilot house 351
    1988 260 predator 408

  • #2
    If you buy a ZF or other transmission, you may be able to get a trolling valve. That allows you to size everything for top end performance but then troll at whatever speed you want by bleeding off hydraulic pressure. It's like another throttle. But its costly.

    Last edited by Pedlyr; 05-20-2023, 04:40 PM.
    '78 20ft Cuddy
    Mercruiser 350 FWC preVortec w/Carb


    • #3
      Steve, you have to know three things,. Size of prop you can swing, shaft diameter and torque/HP of engine. Then call zf and borg Werner which I think is owned by another company and ask which transmission they recommend.
      Atlantic City, NJ
      1982 Cuddy, Rebuilt 351 .060 rings, Edelbrock carb


      • #4
        A good amount of strokers here. DaveR has one I know. I have a 71c and no problems. Don't run it hard but none of us do. Not a drag boat,
        1988 26' cuddy
        1975 20' open
        16' Sandpiper
        14' Certified Fiberglass


        • #5
          Thanks Ed and Skip. I always appreciate the help. Jerry, your right. Thanks for keeping me grounded. It makes no sense to go in another direction without trying what I have first. I'm not drag racing. I just want the engine to not have to work as hard. I report my numbers soon.
          1980 pilot house 351
          1988 260 predator 408


          • #6
            Steve,a 71c 1.52:1 is what I ran on my 383 and never missed a beat. Never dragged raced it though. I’ve been running a 3 blade 15x16 with a decent cup maybe a six cup. Performed well under all conditions. Greg had basically the same boat as you with a tower but running a high output carb 351 and the same prop had very similar performance as I did with a wot of 34-36 mph depending on conditions. My Chevy would turn 4800ish and his 351 would turn 4400 ish. Keep in mind that a 72 c is about 7/8” longer than a 71c as well. Going a completely different direction with a ZF or something else seems like you may be better off spending your hard earned cash somewhere else. Especially if you already have a 71c sitting there. I know you will put new clutch’s and seals in it. Should be pretty bulletproof. Heat and water are their biggest enemies.


            • #7
              And for trolling I always liked the door that folded down that I believe Cecil posted specs on. That would get your trolling speeds down to whatever you wanted. I gathered all the parts to make one but never did.


              • #8
                Great information Dave. Thank you. I don't think I've seen the post about the door. I'll have to look it up. Thanks.
                1980 pilot house 351
                1988 260 predator 408