Turbo 400 trans or other Multi speed transmission with Surface drive.

Discussion in 'Inboards' started by MRover, Apr 27, 2021.

  1. MRover
    Joined: May 2020
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    Location: Philadelphia, Pa

    MRover Junior Member

    I want to modify a th400 to use in my 30ft boat. By modifying I want to machine an input shaft so it can drive the pump and forward drum as in the Velvet drive. Use a flywheel mated to a drive plate with a shortened bellhousing. Reverse manual valve body, removal of the parking pawl and governor adding a trans brake and eliminating the intermediate band. Finally machine/ shorten the output shaft and machine a a tail
    shaft housing to accommodate thrust bearings and an output flange.
    I know transmissions and other than space is there really any reason this won't be practical.
    1st Gear 2.48
    2nd Gear 1.48
    3rd Gear 1.00
    Reverse 2.00

    670 Horsepower
    18 x 30 surface prop
     
    Last edited: Apr 27, 2021
  2. gonzo
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    gonzo Senior Member

    What is the purpose of an automatic transmission in a boat? The propeller does a similar job as a torque converter. It would be only wasting power.
     
  3. MRover
    Joined: May 2020
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    MRover Junior Member

    There would be no need for a torque converter, hence the need to machine an input shaft that could run the pump drive and forward drum. Three speeds yes but no longer automatic.
     
  4. gonzo
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    gonzo Senior Member

    OK. What is the purpose of it? Also, are you replacing the parking pawl with a transmission brake? As far as I remember there are only two bands on a TH400: front and rear. Are you removing the intermediate clutch?
     
  5. MRover
    Joined: May 2020
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    MRover Junior Member

    I am having a hard time understanding what you're asking? What is the purpose of the Torque Convertor? It's name implies it's actions. The parking pawl would be discarded and a trans brake would perform a similar function to a parking pawl while the engine is running only. My 72c doesn't have a park function so I think the gearbox can live without. The intermediate band exists to provide engine braking while in manual second gear only.
    What I am trying to find out is if anyone in the forum has tried this and do they have any practical experience with a similar setup.
     
  6. missinginaction
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    missinginaction Senior Member

    MRover, we've been down this road at least 100 times on these forums. Boats are a whole different animal than cars. I believe that you are thinking of the propeller like a rear end gear in a car. A fixed ratio. So you're thinking that the fixed pitch propeller is not an efficient set up and want the ability to shift or change the shaft rotation speed. It's not so simple with a boat.

    The name of the game with a boat is to maximize your prop efficiency at or slightly under your engines maximum rated power. This will be very close to it's maximum RPM. I could write all night trying to explain how proper props are selected. You can have a professional do the work for you or you can dig into it yourself. You'll need a scientific calculator a good head for math and this book:

    The Propeller Handbook: The Complete... book by Dave Gerr https://www.thriftbooks.com/w/the-propeller-handbook-the-complete-reference-for-choosing-installing-and-understanding-boat-propellers_dave-gerr/647484/item/10181377/?gclid=Cj0KCQjwsqmEBhDiARIsANV8H3avvx21GKSWIHsGHG6es0Pph4Eap3HnAAv1b1L8_JToH-xYG0DWCTIaAqHKEALw_wcB#idiq=10181377&edition=3066070

    What you will discover if you spend the time and energy to really get an understanding prop selection, is this. Yes propeller shaft speed is a crucial part of proper drive train selection. But trying to shift is futile. Why? Because you're not rolling around like a car or pushing air like a plane. You're moving water, a vastly denser substance. If you were to run your engine to it's rated horsepower in first gear and then shift to second, the power needed to spin the correct propeller would bog even the most powerful engine down. It just won't work. It's like chasing the holy grail.

    Propeller selection is a compromise. It involves diameter and pitch or course but also blade loading (read the book) shaft speed, engine speed, gearing and much more. But not multi speed automotive transmissions. If it worked others (me too) would be doing it right now.

    BTW, welcome to the forums.

    MIA
     
    Last edited: Apr 29, 2021
  7. MRover
    Joined: May 2020
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    MRover Junior Member

    Thanks for the reply, My thoughts were based on the availability of planetary gears in the TH400 that can absorb massive amounts horsepower and torque. Perhaps using first gear ratio of 2.48 for docking and slow maneuvering and the second gear 1.5 for normal use. I never assumed that I would set this up for some sort of shifting automatic. I am concerned about building a transmission that can absorb my 700 hp and the torque that the motor is currently rated. Currently the boat has a BW72c with a 1.5 ratio and I am very satisfied with its performance right to the point where the transmission breaks. This usually happens half way thru the season. then I can get back to dock with no reverse or neutral. The engine won't change nor will the prop that I am currently using.

    I keep coming back to this 6 speed from Weissman
     
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  8. MRover
    Joined: May 2020
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    Location: Philadelphia, Pa

    MRover Junior Member

    These boats also come to mind , The first setup is a Chrysler 727 that shifts automatically.
    179043840_10159593911923756_7753497275774927089_n.jpg

    178533779_10159593911558756_7347829439734626282_n.jpg
    This second boat has a reverse manual valve body with the transbrake and the parking is intact.
    180770032_4212198642135687_6769065514454648451_n.jpg 180681567_4212197498802468_3074999751634157303_n.jpg
    So it has been done before.
     
  9. comfisherman
    Joined: Apr 2009
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    comfisherman Senior Member

    Shifting transmissions seem to work for the really high speed applications, but the gear ratios are usually much closer together. Mercury has a big outboard in that power range and it has only a 2 speed with I think only a .5 ratio change.

    Guessing unless the boat is made of helium, boats propped for 2.48 and 1.48 have little chance of seeing 3rd...


    Water is a cruel thing to push through, 400s are beefy but I'd wager not tremendously long lived in a water application.
     
  10. MRover
    Joined: May 2020
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    MRover Junior Member

    I tend to agree with you as I have no interest in a shifting transmission. I do however like the idea of having two ratios to select from. I don't see a use for a third gear at all and it's easily eliminated. What I am interested in is the strength of OEM and some aftermarket components of the Th400, In my opinion I can build a transmission for my boat that will last for more than half a season.
    FYI the Blue boat has 1400 hp through a Chrysler 727 auto trans and a V-drive with three speeds active. It's been in use for 15 years with the same trans and 4 different motors.
     
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  11. comfisherman
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    comfisherman Senior Member

    Huh, well give it a shot.

    I don't play much with gas and boats. The difference between a nv4500 behind a 6bt and a twin disc 509 in a work boat is fairly substantial.

    The gas an light weight boats utilize much less torque so that may be the saving grace.
     
  12. DogCavalry
    Joined: Sep 2019
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    DogCavalry I aim to misbehave.

    You say that every time. Drag boats all use 6 speed transmissions. The guys in the Meet the Twins article used a 3 speed, and won every single race, so they changed the rules to ban the guys. I have a torque converter in my chev. Still has 4 other gears. He will use a multispeed transmission to turn a much bigger prop than the engine could manage at low speed, using the available power for once, then shift up. The engine stays close to its optimum rpm range at every boat speed. Allows efficient operation at many different load states. That's why.

    Weismann Surface drive with transmission https://www.boatdesign.net/posts/821460/
     
    Last edited: May 1, 2021
  13. DogCavalry
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    DogCavalry I aim to misbehave.

    We've been down this road a hundred times, wrong a hundred times.
     
  14. gonzo
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    gonzo Senior Member

    OK, so you are building a drag boat.
     

  15. MRover
    Joined: May 2020
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    MRover Junior Member

    No I am not.
     
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