Small liftable stern drive

Discussion in 'Boat Design' started by jakeeeef, Oct 22, 2021.

  1. jakeeeef
    Joined: Sep 2009
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    jakeeeef Senior Member

    I'm looking to make or source/ get built, or at least research a small, hydrodynamically efficient lightweight liftable stern drive.

    Initially a single prototype, but potentially more later.
    I'm researching what is/ has been on the market.

    It will need to handle up to approx 5 hp.

    For my purposes, it would need to be:
    Transom mounted
    Driven by a shaft coming in horizontally at the top
    Lift by rotating 180 degrees about that shaft on a simple pair of lubricated plates.
    So when in lifted position the prop will be out of the water and the leg will be vertically upwards with the prop in the air.
    It will be locked in either the up or down position by simple sprung pins through the plates.
    The pins and the lifting (canting) could all be done remotely by simple lines, cleats and turning blocks.

    Power will come in horizontally at the top, bevel gear down to the vertical leg shaft, bevel gear again to the prop.
    Where the power comes in at the top the shaft might need a small flexible section, so that the trim can be altered, but such trim adjustment would not need to be conducted on the move or on the water.

    The lower bevel gear will need to be as compact as possible for hydrodynamic efficiency as it's a low power vessel hoping for good performance.

    It will drive a relatively slow moving, very high aspect ratio two bladed prop.

    Rudder will be integrated into a section of the (teardrop profile) leg, and actuated by simple hydraulics, so that it is only connected to the hull by flexible pipes, and therefore does not need to be integrated mechanically into the lifting/ lowering mechanism.

    I'm guessing nothing like this is or has been in production, as it's a new concept that I need it for, but has anyone seen anything LIKE it?

    Many thanks

    Jake.
     
  2. jakeeeef
    Joined: Sep 2009
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    Location: Hamble

    jakeeeef Senior Member

    It's so small, and so light, I'm actually wondering if such a drive would fall within the scope of one of the far East manufacturers of larger, faster remote control powerboats. The ones that are a metre and a half long, do 100 mph plus, have quarter inch driveshafts and a couple of horsepower.

    My proposal would only be a bit bigger than some of these.
     
  3. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    What is the rationale behind this, that it offers some utility that a small outboard motor does not ?
     
    duluthboats likes this.
  4. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    Search this forum for weed eater powered drive.
     
  5. jakeeeef
    Joined: Sep 2009
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    jakeeeef Senior Member

    I'm afraid I'm not in a position to divulge the nature of the vessel design or purpose as it's novel and unprotected.

    But, it's a small craft and an outboard, even a lightened electric one, would be too heavy back there on the transom. It also has its source of power located up to a metre forward of the transom for reasons I can't go into, hence the need for an outdrive type approach.

    I'm always amazed that nobody makes ( professionally) a small electric motor that uses a shaft to bevel drive system in a small lower unit. It's easier of course to chuck the motor at the bottom of the leg, and it solves cooling. Hell of a hub size though.

    But I think if someone just made a hydrodynamically efficient thin leg, with a small bevel drive lower unit, it would help people like me out, but also be of much wider market interest:

    Kayak manufacturers could attatch a pedal system above it, and they'd have a market- leadingly efficient drop in pedal drive.

    Electric motor manufacturers could undercut the likes of Torqeedo by 50% or more with a much better product. (Because it could use e-scooter motor, battery, ESC. Which are PENCE compared with the marine stuff). They might need to have an impeller to a water jacket round the motor due to higher constant loading than when in a scooter, but it's doable.

    It will arrive in the next few years I'm sure. Electric hydrofoils will want it most of all, because they still run their chunky motors underwater with a big efficiency loss.
     
  6. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    I saw an old drive once that was a drive leg with the power coming in through a flexible drive, like a hose that attached to it.
     
  7. jakeeeef
    Joined: Sep 2009
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    jakeeeef Senior Member

    Yes, that's still around I think, or was a couple of years ago when I checked.

    Flexi drives rob power though, so isn't my first choice as power and weight are both low and planing is hoped for!
     
  8. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    There have been several custom catamaran sail drives that pivot about the upper z-drive shaft. They normally only pivot about 90 degrees though. They are about 45 degrees down when in the water, and about 45 degrees up when tucked up under the bridge deck. This lets you use any z-drive arrangement such as a ob leg with a swing arm 90 degree gearbox bolted on where the powerhead should be. There are some issues with bottom end lubrication and cooling water pickup that have to be addressed, but nothing too drastic.

    To be clear, the leg rotates about the hull's centerline axis, not about a transverse axis like an ob that kicks up. Unfortunately, you won't do this with simple sliding plates. To get the structure to work, you need some pretty stout bushings and the leg needs to land on a load-bearing pad on the transom. Gearbox and bearings will set you back about the price of a new 5hp outboard, even if you get the old leg and foot for free. and you still need the powerhead.

    What I haven't seen, and what might be cheaper, is to adapt the smallest standard 3-point tractor hitch components and extend the range of motion. It won't go 180, but it should go more than 90. I reckon I could prototype one from the small engine shop's junkyard for $50. Still need an ob leg and a powerhead.

    Category 1 3-point hitch http://salesmanual.deere.com/sales/salesmanual/en_NA/lawn_equipment/attachments/bm23882_category_1_3-point_hitch_x400-500-700.html

    http://salesmanual.deere.com/sales/...ment/x700/457256_x700_3pt_hitch_cat1_275s.jpg

    [​IMG]
     
    Last edited: Oct 24, 2021
  9. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    You can buy a saildrive off the shelf.
     

  10. jakeeeef
    Joined: Sep 2009
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    Location: Hamble

    jakeeeef Senior Member

    Sail drive is too heavy.
    I'm ordering a kayak pedal drive off Ali Baba to start with. And seeing if I can re-engineer it a bit or get something stronger made up. It clearly won't handle the power as standard.
     
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