Why don't we mount lower units to the bottoms of boats?

Discussion in 'Inboards' started by jimmy34, Aug 3, 2016.

  1. jimmy34
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    jimmy34 New Member

    One of the things that I've noticed that you never see is a lower unit mounted directly to the bottom of a boat. Instead we have sterndrives, outboards, and inboards.

    If you were to mount a lower unit to the bottom of the boat, towards the back (farther back than a typical inboard), then rotating the thing on a large bearing for steering would have a number of advantages.

    1) it would keep the prop under the boat, which is good for safety and the like.

    2) Better steering at low speeds (compared to an inboard that is).

    3) Eliminates the universal engine/drive joint found on sterndrives, which is a common (ish) failure)


    I don't really see a whole lot of downsides to this compared to an inboard drive. It doesn't have a way to tilt the prop for shallow water, but then again neither does an inboard. It is more prone to getting damaged compared to an inboard, but if you don't hit rocks that's not an issue, and steering should be better.

    What do people know about this? I've never seen it done, so clearly there must be some reason why nobody uses this design.
     
  2. Barry
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    Barry Senior Member

    Volvo already has such a drive
    IPS drive
    And many ocean going yachts use the same pod principal
     
  3. PAR
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    PAR Yacht Designer/Builder

    A sail drive is a fixed lower unit bolted to the bottom of a boat.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    There are a host of reasons, mainly involving grounding, trailing, ability to clear fouled props, ability to adjust the drive angle etc.
     
  5. Barry
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    Barry Senior Member

    Forgot to mention that the Volvo IPS has the props ahead of the drive, forward facing
    There are a number of boat manufacturers using this drive in boats from 35 to say 45
    feet
     
  6. PAR
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    PAR Yacht Designer/Builder

    Have you priced the Volvo IPS systems? Wow . . .
     
  7. tom kane
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    tom kane Senior Member

    why don`t we

    These are the reasons why a Pivotal Drive, or a steerable, retractable drive or a jack able are such a pleasure to use in a family boat.
     
  8. keith_2500hd
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    keith_2500hd Junior Member

    mercury also has these kind of drives. biggest problem as stated is grounding. bow thrusters used to come so they could be lowered, maneuvering with wind or water currents would lead to grounding. a lot of the time the leg was not only component involved, the turn-table and interface to hull would be damaged. if I wanted a setup of this kind the pod drives they mount on barge deck would be my choice, but large and expensive.
     
  9. Squidly-Diddly
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    Squidly-Diddly Senior Member

    Seems crazy to me to run a prop or pod without a nice burly skeg to protect it from grounding and fend off logs, fishing lines, stray ropes and seaweed.....unless you are going for some record breaking hyper efficient solar powered contest or something.

    Even besides being in the water, when my buddy was trying to get his trailered twin engine flybridge up an apartment drive-way the lowest part that required rejacking on the trailer were the props.

    I'm thinking a skeg would be worth it just for out of water boat handling.

    Wouldn't a skeg and double hung rudder be worth it, just for peace of mind, for any non-competition recreational boat with fixed or pod prop?

    Especially since a skeg could also provide directional stability and anti-roll, the rest of the hull could be more optimized with less need to address those issues.


    I posted about an idea to re-shaped traditional outboards into something like a clamp-on stern-drive. The engine and full tank would be like a swim step across the transom and provide some buoyancy of its own, and greatly lower the center of gravity and stay out of the way. Extra fuel tanks could sit on top if needed and provide gravity feed. It would most likely also be a lot quieter. Sure, it would make tinkering with the engine on the water a lot harder, and require a few extra engineering features, but stuff is more reliable and CAD/CAM makes it cheaper to include fancy engineering.
     
  10. Barry
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    Barry Senior Member

    The benefits of Volvo Penta IPS
    The unique design of Volvo Penta IPS makes a huge difference when it comes to performance, emissions and onboard comfort:
    • 30% reduced fuel consumption
    • 30% less CO2 emissions
    • 50% lower percieved noise level
    • 40% longer cruising range
    • 20% higher top-speed
    • Safe and predictable handling
    • Joystick docking
    The above information is from a Volvo brochure and the comparison is to a standard inboard

    A few boat builders offering or using IPS as a drive
    Hinckley
    Riviera
    Beneteau
    Formula
    Sabre
    Eastbay
    Cranchi
    Four Winns
     
  11. PAR
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    PAR Yacht Designer/Builder

    Ask any owner about shaft seals on their IPS and how they learned about them. Of course do this after calculating their cost compaired to other drives.
     
  12. WestVanHan
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    WestVanHan Not a Senior Member

    I've heard of a few issues with IPS on the BC coast-two log hits and an unmarked drying rock in a remote bay.
    One of the vessels almost sunk before the captain ran it aground.
    And apparently sail drives have a fairly short life span.

    I'll stick to regular props,thanks
     
  13. Barry
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    Barry Senior Member

    I am not an advocate of IPS. But owned Volvo D6 Duoprops
    Perhaps the only parts more expensive than Volvo are Yanmar then perhaps parts for a
    Boeing jet
    Recently had to pay about 900 bucks for starter motor, ouch
     
  14. tom kane
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    tom kane Senior Member


  15. PAR
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    PAR Yacht Designer/Builder

    I do think the Volvo unit is basically bullet proof, but maintenance, parts and the potential for 5 digit repair bills are very likely with this technology. IPS unit it designed to shear away on impact saving the boat, but this is a very costly piece to use as a depth indicator.
     
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