Deciding factor on outboard or I/O?

Discussion in 'Sterndrives' started by mnmattie, Jan 31, 2013.

  1. mnmattie
    Joined: Jan 2013
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    mnmattie New Member

    Hey guys,

    Tried searching and didn't find what I was looking for (unless it was right under my nose) What are the engineering requirements to use an I/O (Let's say, a 120 Mercruiser-pre alpha) and an outboard. For exampls, 70's glastrons in a 16 foot design came either with an outboard or an I/O setup. Both are the same hulls and same transom (or at least very similar) whereas a 15ft has just an outboard available.

    I have the pdf of the mercruiser sterndrive installation manual (alpha/bravo) and either missed where it was talking about requirements of the hull or it didn't see anything.

    What is your take on what is required, hull wise, to use an I/O setup? Thanks!
  2. gonzo
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    gonzo Senior Member

    The I/O will need a small bulkhead or floor for the front mount of the engine. Also, if the boat has an outboard well, the transom will have to be filled in and the deck modified. Otherwise, the transoms should both be structurally adequate.
  3. mnmattie
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    mnmattie New Member

    Well that was easier that I thought!
  4. tunnels

    tunnels Previous Member

    I/O AND outboards for the simular hp the weight is quite differant . a 90 hp outboard on a 16 foot hull will outperform a 100 hp i/o stern dive every time !
    Plus theres the room lost with a motor sitting inside the boat Sterndrives unless you getting up in size are a pain ! they are heavy ! they take up space !! outboards on even big boats are starting to catch on now check out searay and there 30+ with twin outboards
    We have a sterndrive bow rider and has a 125 hp its a pig . with a 90 hp out board on a bracket its a totally differant boat and will run rings round the sterndrive boat everytime :D:p
  5. Ike
    Joined: Apr 2006
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    Ike Senior Member

    In addition a I/O weighs a lot and takes up a lot of room. As the boat gets shorter the amount of room the engine takes up becomes a much greater percentage of the interior volume of the boat. In addition to having the requirement for a closed engine space, the I/O needs hefty stringers to mount the engine on, that can transfer the thrust to the hull without causing undue stress to the hull.

    Some other factors:

    It is far easier to remove an outboard to work on it or replace it than and I/O.

    An I/O moves the center of gravity farther forward and balances the boat better. (that is a generalization, there are exceptions)

    An outboard moves the CG aft, which on some hulls (say a bass boat with a "pad") improves the lift and planing speed.

    Generally speaking an inboard is more powerful and more economical (4 stroke etc) than an outboard. (but newer outboards are getting much better economically)

    In other words it's a balancing act. There are pros and cons for both.
  6. mnmattie
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    mnmattie New Member

    Well what is making me lean toward this potential project is I have a Forester Rebel 1600 that needs a transom, and gel work, currently setup for an o/b (johnson 50hp that needs work) and I have a 17ft reinell that was in a fire but has a good mercruiser 120 and pre-alpha drive.

    I have plans of adding a sunpad to the rear, but the o/b splashwell would be in the way. Using an I/O would create more room for the sunpad while doubling as the 'doghouse' for the motor.

    I also wanted to put on a smaller swim deck so I can climb in and out of the boat easier.

    Plus it will get use from the mercruiser 120 since I can't find a bare hull to put it in.

  7. IMP-ish
    Joined: Jan 2011
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    IMP-ish powerboater

    Solid stringers the right spacing for front engine mounts and a solid transom.

    Also think about space to access every part that needs to be removed or maintained. Twins are usually tight knuckle breakers. I still like them :p

    A single IO you will like working on and it's easy to access everything unless you have things built-in that are in the way when you go to change a starter or oil cooler or drain plug. To often there is something in the way. Planned right it's easy to work on.
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