What Hull mods are required to use an Inboard motor

Discussion in 'Powerboats' started by Popeye, Apr 15, 2004.

  1. Popeye
    Joined: Apr 2004
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    Popeye Junior Member

    Hi All

    I have a 17' fibreglass hull, no motor, little money and 7 kids.

    I think the way to go to get this boat in the water is with an auto (V6) inboard.

    My thinking is, the motor can be had cheep and will only require hard work and some lateral thinking.

    So far my searches have turned up no information on how the prop is positioned in relation to the boat.

    Probably quite a few other things I have failed to take into account as well.

    All your thought will be appreciated.

  2. yipster
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    yipster designer

  3. Popeye
    Joined: Apr 2004
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    Popeye Junior Member

    Thanks for the quick reply!

    I had a look through that thread before I posted.
    I couldn't find any answers that I could make use of just yet. I'm still on the lower end of the learning curve.
    I need basic info, like does the prop poke out of the transom, or through the bottom x distance forward of it?
    What angle is best?
    How far under the boat?
    and as the hull is designed for an outboard do I need to add a keel (don't know the proper name) or something?

    I really have no idea at the moment. can't even find a picture.

    I only want the boat for a spot of fishing (Sea) and maybe pull the kids around on a tube.

  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Frankly on a boat that length, you're much better off with an outboard. It will cost far more for the straight shaft or I/O conversion then finding a reasonable used outboard and hanging the thing on the transom. With the number of kids you have you'll need the room (both inboard setups take up valuable floor space in small boats) and they will need be smallish if all are aboard.

    There's a great deal of effort required to make both the straight shaft or I/O conversion not to mention cash outlay.

    Maybe you should think about setting up bicycle peddle affairs for the kids to work, tied to a shaft turning a shallow pitched prop. What do you think? Say 3 sets of side by side peddling positions (with a kid in reserve) simple gear reduction and shaft mounted prop.

    To answer your questions, typically the prop sticks out of the bottom, not the transom, though it could. The angle should be as near level to the waterline as possible. Distance away from the bottom of the boat is subject to debate, but half a blade length will do. A skeg will help directional stability, this would be on the centerline and well ahead of the prop (the straight shaft setup only), leave at least a 4' gap to help cut down turbulence. You will also need a rudder if using a shaft and prop.

    This project isn't for the beginner, most yards wouldn't touch it. Find an outboard and use good sense taking yourself and kids into deep water (sea fishing) in an open boat. In fact take a boat handling course and learn how to deal with the issues that can come up farther from shore then you (or the kids) can swim back to.
  5. Popeye
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    Popeye Junior Member

    Thanks PAR

    I re-read my posts, I do sound a bit like a cowboy!
    Now how do I fix that, without irrelevant prattle?

    First the kids.
    Yup 7 at home (2 are mine), love them all. I wouldn't even consider taking them out to sea (too young).
    Besides I go fishing to get some peace. :)
    I also have 4 more kids that have left home 2 of these are mine also, they sometimes go fishing with me.
    Mostly they just want to ski though! (tube).

    I do have a small "going" boat, an old rebuilt (by me) 13'x4' fiberglass "pongrass" with a 1963 Australian 40hp Johnson Super Seahorse.
    I had to make or adapt all the parts for the motor as Johnson no longer support the Ozzi version of this engine.
    Not a very impressive project for those of you that have access to, or work in a boat yard. But considering my nearest boat shop is 2.5 hrs drive each way I'm happy with it.

    I do have my boat license, but that gives NO info on how to build or fix one. (here anyhow)

    PAR, I knew that "keel" was the wrong name, but for the life of me I couldn't think of the proper one.
    I suppose that comes from teaching yourself, the gum trees don't care if you call it a "thingamybob" or a "whatsit", its all the same to them.
    Sorry, I'll try to improve on that now I have found this forum.

    When I picked up the 17' hull I thought I'd just pop the old Johnson on the back and it would do for fishing, But:

    1 The "new" transom is 5" higher.
    2 I think it will be asking to much of the "old girl" to tug around a bigger boat and a tube.

    Then the other day a "boaty person" I bumped into suggested an inboard.
    Having only ever seen one inboard boat of this size I started to investigate the idea. (I have never seen one below the water line).

    Before this exercise I did price a new Yamaha 90hp but I can't justify 10 thou on my toys (kids come first).
    Besides half the fun is in building the thing (I think).

    Thanks for your thoughts
  6. yipster
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    yipster designer

    had to sm:Dle on the bicycle peddle idea, i flashed back to an old strip album where frits the cat discovered 100 pedling chinese as propulsion a chinese rocket. (offcourse in reality chinese discovered the rocket)
    i try to keep up my I/O's, but that (more economic) straight shaft i sure want to learn more about at a later time.
  7. bobber
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    bobber Junior Member

    Brave man thats for sure!
    Umm, think its all been said really, the negative sides anyway. Personally i would go with i/o set up myself. as then you can mount the engine as far back as possible and open up some deck space, but on a boat that size, your going to be tight either way.
    Money been said, work been covered, umm, by auto V6 im not sure what you mean by that. maybe the terminology differences between UK and everywhere else are comming into play here, but do you mean an engine out of a car with automatic gearbox? or a marine engine, and the auto bit is what we call something else?
    If its a car engine, for a start, the auto gearbox wont work on a boat. Due to the way automatics work, and the way a propshaft and prop operate. i couldnt go into all the technical side of things on that, i just know it doesnt work.
    Umm car engines also need marinising, obviously. depending on the engine, some kits are available, but i wouldnt know where to start where you are.
    if you meant something else to that, then ignore all that, i obviously am confused.

    but ok negative sides addressed, lets assume you are still determined to do this. umm, first engine bearers will be needed to mount your engine on. Shuold be at least 2x4", 2x6 ideal is space permits (aus laws might require you to use something else, i dont know) and run the length of the boat, or as far forward as possible. Your existing stringers might already work, if they are big enough, and poisitioned right for engine mounts, more luck than skill though on that.
    Also gotta think about the weight of an inboard. without knowing the dimensions of the boat, etc, i dont know, but might want to check into that. engine, gearbox, drive etc, going to weigh in at a fair bit compared to an outboard, and last thing you want it to go to all that work, put it in the water, and find the waterline as raised so much its dangerous.
    I dont know for sure, but sounds to me like this is more a test of your skills and ambition, rather than practicallity. Which is fine. If you manage it successfully, fair play to you, i would be interested to see how you did it. but if it really is more a personal ambition, then maybe you could consider going a bit smaller than a V6. Maybe a 4cylinder. You all have the Opel Calibra over there i believe. The 2litre turbo lump from one of those pumps out around 200odd horsepower, and are fairly small and light compared to a V6. With some creative fabrication of exhaust manifolds to be water cooled, and some minor modifications to the cooling system, that would work. I say the calibra engine, as i have designed a set up using this engine (mainly because i had one and so could work from it) and it wouldnt be as torquey as a V6 granted, but probably a bit more space saving and weight saving than a V6, and easier to modify. If you use it on fresh water mainly, cooling will be much easier, salt water will cause problems though on any car engine.

    Umm bout all i know to tell you. Not a project i have ever taken on, and cant say i know anyone who ever has. I have only gone as far as designing it. never actually put paper to hammer so to speak on this. good luck to you. but if it is more a practical issue, then i would go outboard, inboard is probably going to end up costing you near on the price of a 90hp outboard anyway, when you account for fabrication, parts, etc.
  8. Popeye
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    Popeye Junior Member

    Thanks Bobber

    Thats just what I was after, an honest appraisal of the idea. Totally unbiased by yuppie (If you cant afford to buy a boat you shouldn't have one) thinking.

    When this "boaty person" suggested the idea it sounded like it might be a "Goer", but now it seems I am better off sticking to my bath tub and saving up for an outboard.

    Oh well! it was fun while it lasted.

    Thanks for your thoughts.

    Another posible option would be to cut down the transom to make the short shaft 40hp fit.
    Any thoughts on this idea?

  9. bobber
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    bobber Junior Member

    that might be a good option. or well not cut it down as such, but maybe cut the middle of the transom down. only enough as needed though.
    I dont know for sure, but i believe you can buy extensions for outboards. that allow you to extend the leg from short shaft to long shaft.
    or you could shop the local ads and see if you can find a long shaft version of your engine going cheap with motor problems or something and swap out the leg. pretty simple to do on a johnson, just 6 bolts i think or 8, cant remember, but anyway, thats an idea.
    Although the engine in particular may be discontinued, it will share a lot of the same stuff as other models in the range. Also dont forget Johnson is OMC, Evinrude and Johnson are the same engines, so widens your search a little better.
  10. gonzo
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    gonzo Senior Member

    To adapt a car engine, even if it free, will cost you more than a good used outboard. A set if manifolds and risers is about U$850.00 for an off-brand. That is assuming your engine has parts available. The rest is even more. You'll need a transom assembly, exhaust collector, outdrive, trim system. Then you have to build stringers and engine mounts.
  11. 202_ski
    Joined: Mar 2008
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    202_ski Junior Member

    I own a 15 foot fiberglass runabout that i converted from outboard to mid mount inboard with a holden 6. It took me about 9 months to complete (still fine tuning) and about A$2000. Most of the parts are second hand or home made. The main problem with my boat is it doesn't get completly get up on a plane coz the hull is designed to have weight hangin off the back not in the centre. It only revs to 2400rpm. But im hoping i can fix that. Anyway, unless your prepared for alot of work and making/searching for parts an outboard maybe the answer although they are expensive for a good,reliable, powerful motor. I assume you have a 3.8 litre commodore motor, they make kits to convert/ marinise these motors for boat use.
  12. RealityBoatCo
    Joined: May 2004
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    RealityBoatCo Junior Member

    Bolt a jack plate to the transom, and mount the outboard at any height that you want.

    You can usually find a nice used 150Hp motor for less than $1,000 plenty of power for a 17.

  13. tom28571
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    tom28571 Senior Member

    For lots of reasons, most already offered, an outboard is the most reasonable alternative for you. The following book is dedicated to guys like you, and me, plus a few thousand others.


    Max offers advice on obtaining, rebuilding, fixing or otherwise just hanging an old Johnson/Evinrude on your boat and keeping it running forever. Very expensive advice to be had very cheaply.
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