Outboard conversion, Converting back to I/O

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

  1. FMS
    Joined: Jul 2011
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    FMS Senior Member

    It's a good thing if you have enough space between manifolds to service the engines.
     
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  2. romeomikehotel
    Joined: Feb 2019
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    romeomikehotel Junior Member

    Very true. An older formula I was looking at last year was so tight you couldn’t fit a finger between the exhaust manifolds.

    Thanks for the help.
     
  3. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Converting an engine to marine is its own can of worms. You are in Florida where used marine engines are plentiful and cheap. In fact, get a whole package so everything fits. There is more than a flywheel to change. Many engines are externally balanced, so you can't simply change flywheels and expect it not to vibrate. The transom thickness should be between 1 5/8 and 2". Go online to Mercury Marine and download the installation manual. It has all the dimensions and instructions.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    For every boat switching back to sterndrives from outboards, there'd be dozens going the opposite way, and all because of large four stroke outboards. The heyday of petrol sterndrives was in the era when the only competition from outboards, burnt through fuel like it was going out of fashion, and even though fuel was cheap, people still preferred the sterndrive because it used a lot less fuel per hp. Then podded outboards freed up internal space, and left the sterndrive with a box in the way. Diesel sterndrive, maybe, petrol seems much less attractive than formerly, if there is a reason to keep petrol sterndrives, it is to maintain boat appearance, balance, and have the boat sit on its lines. Something that probably applies to a 20 foot Bertram more than many others
     
  5. fallguy
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    fallguy Senior Member

    Personally, the ob looks better to me and opens up the cockpit for fishing or an aft salon. Perhaps noise is more with ob?
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Sterndrive is visually cleaner, if that is a consideration, but other things are more important to most,
     
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  7. romeomikehotel
    Joined: Feb 2019
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    romeomikehotel Junior Member

    The Bertram 20 I have is most likely going to be put back together as OB, mainly because I already have the engine and bracket and don’t have plans to keep the boat.
    In my estimation, there will be more buyers for an OB boat given the mass market buyers lack of interest in the level of perceived maintenance required by an IO.

    The conversion to IO that I have planned is strictly based on my own personal preferences in regards the engines I want powering my long-term boat. I understand that in most ways, it’s not practical, but practicality is low on my list of priorities for this one and I’m more interested in building something that I’ll have fun building and using.

    I will say, based on the info you folks provided to read and the info from THT, that I have plenty of confidence in this working and I think it’s going to be a lot of fun to build. I’ll be sure to update this once I get started.
     
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  8. Mr Efficiency
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    Mr Efficiency Senior Member

    I recall seeing an early Formula 233 that was powered by two Mercury outboard in-line sixes. That thing rolled around something awful. No doubt the weight of sterndrives would have fixed a lot of that. The wisdom of switching back or forth between outboard and sterndrive is largely a function of the hull design and boat weight.
     
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  9. fallguy
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    fallguy Senior Member

    This plan make a lot more sense to me. There are 20 foot Bertrams available in the US regularily.
     
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  10. Let_Freedom_Ring
    Joined: Apr 2020
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    Location: Edmonds

    Let_Freedom_Ring Junior Member

    Please elaborate if you would. Relevant to me. Too much v? What else should I take into consideration?
    Nose up, COG aft? Ultimately hoping to get your view on my best bet as far as my Cog and moving stuff like gas tanks, pod design. I’m thinking there may be a few things I haven’t considered. Just borrowing thread...
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    OK, well they are a very deeply veed boat for a 20 footer, very few would be more than 22 degrees at that size, this one is at least 25. I don't think I ever saw one with an outboard, when they were a popular boat out here, the weight of the sterndrive sat them down on their lines better. And they did seem to run bow up, from my observations, which an outboard perched out on a bracket, would be unlikely to improve. There is a limit to what you can do with fuel tank position, putting it forward means a big difference in trimming effect between full and low. Any way, you can always use tabs if you don't like the trim angle. My biggest concern would be a boat bobbing higher in the water, at rest, and making it more tender. But the time you do your sums with engine and pod weight, and whatever other additions you build in, may not be that far different to the sterndrive boat's weight. But it is no accident that few originally carried outboards, though I did hear of one with twin V4 Johnsons. That was more a commute boat, not a fishing boat.
     
  12. Let_Freedom_Ring
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    Let_Freedom_Ring Junior Member

    Mr. Efficiency, is it possible I could get you to weigh in my current project. My aim is to do as much as I can now to get the best result. I can’t message you it seems...
     

  13. Mr Efficiency
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    Mr Efficiency Senior Member

    I don't know there is much more I can tell you, I have seen a few Bertram 25's around that were converted from twin sterndrive, to twin podded outboard, and the chines aft were pretty well clear of the water, which is not ideal, but that is a beamy boat and I don't think the deadrise is quite as much as the old 20 footer. You don't want to see those chines standing proud of the water, at the transom.
     
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