Outboard conversion, Converting back to I/O

Discussion in 'Powerboats' started by romeomikehotel, Jan 13, 2020.

  1. romeomikehotel
    Joined: Feb 2019
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    Location: Ft Lauderdale

    romeomikehotel Junior Member

    Hi guys,

    I’m working through my current restoration project (Bertram 20) and I’m at a point where this question is very relevant moving forward as well as for my next project.

    My current B20 was converted to outboard years ago and I’m considering going back to I/O and my next project will likely be a true inboard that I want to convert to I/O.

    I’d like to hear from guys with real knowledge about the best way to go about converting a boat to I/O and anything that may not be obvious.

    I’m considering:
    -what’s best way to recreate the engine mounts as those were removed when my B20 was converted.
    -how thick should a transom be for a Mercruiser Bravo drive to fit properly?
    -if dual I/O, how to determine correct spacing between the drives when cutting holes in the transom?
    -Best way to mate an engine to a Mercruiser drive that isn’t a standard marine engine (sources for someone who can fabricate a flywheel/parts that can match up)

    I’d like to hear from folks who have real experience in similar projects and avoid the whole I/O vs outboard vs. inboard opinions if possible.
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Just about all that info should be obtainable from the engine and drive makers.
     
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  3. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Y wood u?

    An outboard on a bracket basically makes that boat capable of becoming an unsinkable design done right.

    Twice as sexy to boot.

    The only reason I can fathom is a poor bracket which would be best fixed with a good one.

    Please do tell.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Dunno, a 20 foot Bertram never seemed a good candidate to me for an outboard bracket. Too much vee, and already nose-up without moving the COG rearward.
     
    Last edited: Jan 13, 2020
  5. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Chine barely in the water. If anything, her remodel was light to dwl. Lead works.
    CD532156-C8C6-4E7D-8496-3F1DCBC90DBF.jpeg
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Tippy, I would think.
     
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  7. romeomikehotel
    Joined: Feb 2019
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    Location: Ft Lauderdale

    romeomikehotel Junior Member

    Thanks guys,

    The boat came to me with a stainless Armstrong bracket. I like the extra room it has now but honestly, I’m planning on getting rid of it asap as soon as the resto is done in order to build something twin engine. I may leave this one with the outboard or I may use it as practice before converting my next boat to twin IO.

    My main concerns are in regards to making a boat an IO that doesn’t have any IO components currently.
    And regarding checking with the engine manufacturer for info, that’s not an option as the motors I’m going to be using are high hp automotive engines that are rarely used in marine applications (please refrain from trying to dissuade me, this is what I’m doing. Just looking for help).

    So my questions are based more in principal than in regards to a specific engine as I know nobody will have that specific knowledge. So basically, what’s the best method for adding IO engine mounts? Or do you have good reading material to share?

    Will the Bravo 1 manual tell me how thick the transom should be?

    And can anyone recommend any resources or reading material on mating a non-standard engine to an outdrive?
     
    Last edited: Jan 13, 2020
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    the drawings ought to.
     
  9. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    The manual will tell you just about everything you need to know about installing the outdrive and motor.

    A flotation bracket will balance everything out and save you about a year in getting back on the water.
     
    Last edited: Jan 14, 2020
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  10. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    When you start working “outside the box”, you’re committing to your own engineering skills.
    I would do full size layouts in plan and profile on the garage floor first.
     
  11. romeomikehotel
    Joined: Feb 2019
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    romeomikehotel Junior Member

    Thanks! Just got a hold of the full installation manual. Those 240 pages should keep me busy.

    You’re right about the bracket. I’ll most likely just put the bracket and engine that I have back on and get this B20 back on the water asap and then sold.
    Then it’s on to the next and converting to IO.
     
  12. romeomikehotel
    Joined: Feb 2019
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    romeomikehotel Junior Member

    Amazed to have gotten more direct information from TheHullTruth before this forum but regardless, someone on THT provided some very useful info and I want to cross reference it here in case anyone else comes along with the same questions:

    Quote:
    Originally Posted by mpwitte
    As above, transom needs to be 2 1/4" thick. two layers of 3/4 ply or high density composite plus glass is standard. The drive height is called the X dimension. You will need to do some research to find the recommended measurements but the info is available. Generally with a smaller, lighter, faster boat you can get away with at least 1/2" higher than Merc's official recommendation. Offshore only will be a good source but there are many others as well. Transom assembly should come with a template for the cut out but I am sure you can scrounge one off the net too. The jig is nice if you can beg or borrow one but not necessary if you are careful. There are common or convenient dimensions for stringer width. 28" comes to mind but it has been a long time. First step is to mount transom assembly. There is a jig to locate mounts from there but not very common to find one. Without the jig, lift the engine into the boat and set on the rear mounts. Make sure you use the proper isolation bushings or springs and torque to spec. The front of the engine will be supported with blocking and a jack or better yet an overhead winch. Use the correct alignment tool and set motor height. I would bump the height up 1/4" or so to allow for mount settling during final assembly. The actual mounts will be 6" x 1/2" aluminum angle stock about 10" long. Pre drill 4 1/2" holes for mount bolts on one web of each mount. With the engine in the desired location, position the aluminum mounts under the engine pads and mark the bolt hole location on the side of the stringers. Also mark the engine pad to aluminum angle bolt holes and label the pieces left and right. Pull the engine, drill your holes, install your mounts using plenty of caulk and reinstall the engine. It's all a bit of a pain in the neck but nothing hard or complicated about it. I would be surprised if there weren't numerous youtube videos showing the procedure.

    You will have to do your own research on engine connections but I would strongly urge you to use a power package that closely relates to one that has been available from the factory so you can source off the shelf parts. Trying to have custom one off couplers fabricated will be the death of your project. At least do some research on the old TRS drives. They used an external transmission and something like that may help with driveline connections.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Are you saying those installation instructions are not directly available from the engine/drive makers ? Hardly !
     
  14. FMS
    Joined: Jul 2011
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    FMS Senior Member

    Google Mercruiser Bravo Installation Manual
    http://download.brunswick-marine.co...n/mercruiser/drives/865612t00/865612t00_2.pdf

    ...
    Transom Thickness and Surface IMPORTANT: Transom thickness and surface plane (flatness) must be controlled where the sterndrive mounts. Transom thickness and surface must conform to the following: Transom specifications

    Thickness Between 51 ‑ 57 mm (2 ‑ 2‑1/4 in.)

    Parallelism Inner and outer surfaces must be parallel within 3 mm (1/8 in.)

    Flatness Transom surfaces in area where transom assembly will be mounted (includes vertical as well as horizontal dimensions) :Inner Surface ‑ Flat within 3 mm (1/8 in.) Outer Surface ‑ Flat within 2 mm (1/16 in.)

    Angle 10 ‑ 16 Degrees
    ...

    Drawings and Manuals http://download.brunswick-marine.com/download/dosearch/DESCRIPTION/lang=EN&categories=&mod=7
     
    Last edited: Jan 14, 2020
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  15. FMS
    Joined: Jul 2011
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    FMS Senior Member

    Got to hand it to him, mpwitte's is a good thorough answer too.
     
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