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Thread: New Builder Trying to Pick Reduction

  1. #1

    New Builder Trying to Pick Reduction

    Hi Guys -

    I've built a few boats but this is my first airboat. Hull is wood/fiberglass and is around 11' X 5'. The only real issues I've had is figuring out the motor and reduction drive. Please let me know if I'm on the right track with this...

    The motor is 22hp (max 4000 RPM) that generates a max of 33.19 ft/lbs of torque at 2500 RPM. I'm looking at a 50'' 2-blade wood prop that has max power when tips travel at 890 ft/sec which occurs at ~4000 RPM. So if I'm reading this right, I'll have max power from the prop at 4000 RPM, which means a direct drive in my case. However I'd be generating max torque at half that speed. What is more important in an airboat, max RPM or torque? Since the motor could not turn the prop over its recommended max is a reduction drive needed? Thank you for any help with my newbie questions. From what I've read the RPM/torque/DD/RD solution is part science part magic.

  2. #2
    Administrator Papee's Avatar
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    Re: New Builder Trying to Pick Reduction

    Is it a 5' bottom or Beam? It sounds like you've done your homework and figured out the right prop for your engine and RPM. I'm saying this without knowing the pitch. A 50" prop for that engine would probably be around 18 but I'm just ballparking it.
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  3. #3
    Member yamahaulerG1's Avatar
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    Re: New Builder Trying to Pick Reduction

    Finding the right prop can be the hardest part of building a mini. Me and a friend built almost the same boats same motors different hulls and setups the props that worked good for me wouldn't get his boat on plane and his wouldn't work on my boat it can be mind boggling. He was using an ivo at 2000rpm I was using an ultra prop at 2800rpm. What make of prop did you find that pushes the best at 4000 rpm that seems pretty high for a 50"wood prop. For a 22hp 50" the pitch would have to be really low to get that kind of rpm and that makes for low speeds I would run more pitch, less rpms

  4. #4

    Re: New Builder Trying to Pick Reduction

    Hah! you are right. After reading your post I went and looked, its max RPM rating is 4000, max performance is at 3600 RPM. (its a from Tennessee Propeller, haven't ordered yet). How would you recommend figuring out the correct pitch for this if I ran a direct drive? If I go the reduction route what is best way to figure reduction ratio? Hahahaha the more I think of this the less it makes sense. Help a dummy out please!

  5. #5
    Member yamahaulerG1's Avatar
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    Re: New Builder Trying to Pick Reduction

    I have had a few wood props come apart on me and it's not fun. I thought 860ft/sec sounded to fast so I looked it up 850ft/sec is the maximum speed recommended by the eaa experimental aircraft assoc for a wood prop. For safety mostly but also at that speed the prop losses efficiency. the warp drive and the ultra prop losses it's efficiency after 2900rpm way short of 850ft/sec. That's like saying if your motor will come apart at 5000rpm then run it at 4900 to be safe, NO run it much lower then the max same with the prop don't push it to the breaking point. Keep the prop under 3000 to be safe

  6. #6

    Re: New Builder Trying to Pick Reduction

    OK that is the technical advice I've been looking for! Thank you. So I'll definitely be needing a reduction that keeps the prop under 3000 rpm at full throttle. Higher pitch prop means more thrust but more resistance so that will also help keep RPMs down. So now I know that I need to figure out gear reduction ratio and pitch that keeps me ~2900 RPM. Thank you for the help getting me started in the right direction.

  7. #7
    Member yamahaulerG1's Avatar
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    Re: New Builder Trying to Pick Reduction

    Sorry I didn't see the reply. with each prop manufacturer the pitch needed can vary. call and talk to the person making it they usually have the best info on what fits what motor all I can say is don't give up after the first prop. Wood props haven't done well for me. changing props can make it feel like a different boat

  8. #8

    Re: New Builder Trying to Pick Reduction

    OK, I've read a lot of people that say the same thing. I may buy adjustable pitch prop at first just to see what works best.

  9. #9
    Member yamahaulerG1's Avatar
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    Re: New Builder Trying to Pick Reduction

    If you go with a reduction 2:1 is a bit much anywhere from 1.5:1 to 1.8:1 should work good for you

  10. #10

    Re: New Builder Trying to Pick Reduction

    OK great I'll start looking around. Part of me is leaning toward a little bit larger build since I'll need reduction either way hahahaha.

  11. #11
    Member yamahaulerG1's Avatar
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    Re: New Builder Trying to Pick Reduction

    With the right hull 22hp direct drive will work ok. I had a14'jon boat with a 16hp direct and it worked great. I actually prefer direct drive because of less parts to break and less wear on the engine. Not sure where you live but if your in Florida look on Craigslist for a 32hp generac I see them all the time for cheap

  12. #12

    Re: New Builder Trying to Pick Reduction

    I'm in south Texas. All the reading I've done on this forum about the 4A084 makes me reconsider. They seem hard to find though.

  13. #13
    Super Moderator Corky's Avatar
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    Re: New Builder Trying to Pick Reduction

    There's an NOS 084 engine for sale on ebay...Location is Oklohoma city....New for $350.00...Can't beat the price

    Aluminum .120 14' X 76" hull
    Teledyne 4a084-4 engine
    Circle S 1.69:1 reduction
    67" Whirlwind "Razor X" prop



  14. #14
    Administrator Papee's Avatar
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    Re: New Builder Trying to Pick Reduction

    It's all in what you want the boat to do. If you want a single person boat and rarely carry a second person direct drive is fine. Two people requires a reduction drive. Bottom surface is key, you should have at least a five foot bottom. Weight is another key factor. Degrees of pitch is not the same as prop pitch as related to a wood prop. You can only expect a certain amount of performance out of any given setup under perfect conditions. Tennessee Props makes a good prop, I think they are using the old Culver props templates.
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  15. #15

    Re: New Builder Trying to Pick Reduction

    I cant find it on ebay. There is a 4ao32 for 350 in Deleware but that is all I found. Can you post link? Thanks.
    Quote Originally Posted by Corky View Post
    There's an NOS 084 engine for sale on ebay...Location is Oklohoma city....New for $350.00...Can't beat the price

  16. #16
    Super Moderator Corky's Avatar
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    Re: New Builder Trying to Pick Reduction

    Here's the link..It's listed as used{because it's out of the crate} but read the description...This guy has been selling one every two weeks now for a while...The engine still has the tag,plugs, and the exhaust manifolds are still painted which would burn immediately on the initial fire-up so it's a safe bet it's new as described...http://www.ebay.com/itm/4a084-milita...8AAOSwcUBYPLMv Too bad it's so far away from me I'd have that engine in the shop for some experimenting...
    Last edited by Corky; 12-01-2016 at 09:11 PM.

    Aluminum .120 14' X 76" hull
    Teledyne 4a084-4 engine
    Circle S 1.69:1 reduction
    67" Whirlwind "Razor X" prop



  17. #17

    Re: New Builder Trying to Pick Reduction

    Thanks Corky. Sent him a message to see if he'd ship. I just posted back on you build thread, btw. Honestly from what I've seen, you're build is almost exactly what I'm going for so this 4a084 may be a good thing to grab.

  18. #18

    Re: New Builder Trying to Pick Reduction

    Hey Corky -

    Guy in OK said he'd ship that motor but does not know the weight. Saturn Supply has it at 400lbs... does that sound about right? I need to know to determine shipping costs. Thanks for any help

  19. #19
    Super Moderator Corky's Avatar
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    Re: New Builder Trying to Pick Reduction

    Yes that's the shipping weight used with the engine in the crate..Does he have the rest of the original crate ??? You'd want that for shipping if so....If he's going to ship as it is in the pics it might be lighter but have him at least shrinkwrap it...

    Aluminum .120 14' X 76" hull
    Teledyne 4a084-4 engine
    Circle S 1.69:1 reduction
    67" Whirlwind "Razor X" prop



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