Outboard to Sterndrive Conversion

Discussion in 'Boat Design' started by Benny theAnimal, Mar 1, 2012.

  1. Benny theAnimal
    Joined: Mar 2012
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    Benny theAnimal New Member

    I'm considering buying something like a Flightcraft 5.4 and am considering removing the outboard and replacing it with a V8 inboard, probably sterndrive.

    Firstly, is this possible, and what is involved?

    For wait distribution would I be better off moving the motor further forward and making it either just a direct shaft drive or V drive?

    For reverse, I'm thinking of fitting a 2 speed powerglide auto, as this will give me reverse and 2 forward going gears, one for trawling and one for skiing

    I have a little knowledge of building boats, but my uncle used to work for Flightcraft and I have friends that currently build Stephens boats, so if I get in over my head I do have a fair bit of back up knowledge.

    Any constructive advice anyone can give me would be greatly appreciated
     
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    To start with, the structure of the boat will have to be modified. A sterndrive will need stringers to attach the engine mounts. The deck will need to be cut to make space for the engine. The fuel tank may or may not need to be moved. The outboard well will have to be removed and a new deck built. The cut-off in the transom needs to be filled with attention because it is structural. Sterndrives already have gears so there is no need for a transmission. A powerglide will give you only headaches anyway. If the boat was also set with sterndrive it should be OK. Otherwise, most likely it will be bow heavy and handle poorly.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    This is not a good plan, IMO. Boat is certainly too small for a V8 sterndrive, fitted in the conventional way. Lots of hacking and buggering around for little worthwhile end result. If you want a V8 ski boat, better to sell the 5.4 and buy one.
     
  4. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Stern drives normally have fwd/reverse gearshift, so there is little need for another gear box. And what do you hope to gain? Certainly no extra space, reliability or fuel efficiency!
     
  5. PAR
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    PAR Yacht Designer/Builder

    I disagree in that a nearly 18' boat isn't capable of hosting a 5 liter V8 and outdrive. It'll be a bit cramped in the cockpit, but not so bad really. Mount your tanks forward to correct trim issues and go for it.

    The real problems that exist are: do you have under sole room for the new tanks, bottom loading and transfer issues (more stringers) and of course trim.

    A small block Chevy and drive will add 900 pounds to the butt of your boat. This is a lot of weight, so a transom width specification is needed to further this conversation, but if it's a reasonably modern boat, than I'll bet you have enough bearing aft to carry this much, assuming you move the batteries and tanks forward to help trim her out properly.

    This isn't an easy undertaking, but certainly doable, given you have some skills. Getting the balance right will be the big issue, but playing around with some concrete blocks or sand bags will usually find the sweet spot.
     
  6. Jim_Hbar
    Joined: Sep 2006
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    Jim_Hbar Junior Member

    In this size of boat, a 4 cyl or V6 would be the more normal selection in front of a stern drive, than a V8 - especially an iron block engine.

    With a V-drive, a powerglide can work, but requires some internal modifications and the elimination of the torque converter to be truly viable.

    I once had an 18ft. boat with a 5L V8 stern drive, and a friend had a 4.3L V6 in a 17ft. ... I burned 50% more fuel, but was a bunch more comfortable in choppy conditions!
    Boats....the ultimate compromise!

    BTW, you will be shocked how much useful room you loose in the cockpit with a stern drive vs. an outboard.. I'd mock up the engine box and sit in the boat, before you go past the point of no-return on this conversion..

    HTH's

    Jim
     
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  7. Mr Efficiency
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    Mr Efficiency Senior Member

    I assume this is the same hull :

    http://www.boatsales.com.au/Tig/UI/...&f_mid=&c_mid=5220866&pid=140921&tabID=303696

    Not a boat to be dropping a V8 sterndrive into, IMO.
     
  8. PAR
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    PAR Yacht Designer/Builder

    Agreed, it'll be a cramped cockpit as suggested, but certainly possible. I'd also agree in that a 4 cylinder would be a better choice, say a 160 HP 181 CID with an Alpha, which would come in a few hundred pounds less. A V8 will have about the same length engine box, as a 4 cylinder, though it will be wider and considerably heavier.
     
  9. CDK
    Joined: Aug 2007
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    CDK retired engineer

    The mock up is an excellent suggestion. We should remember it for future plans concerning major modifications, wood gas generators etc.
     
  10. Benny theAnimal
    Joined: Mar 2012
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    Benny theAnimal New Member

    Thanks everyone for your feed back and opinions.

    Mr Efficiency, yes that is exactly the boat I'm considering.

    It sounds like a V6 might be the way to go. Even though I love the sound of a nice V8

    The main reason I'm considering a conversion is that a boat of this size really needs at least 115hp to get up and go well enough if i want to ski behind it. IMO

    And I was told the older, mid 80's and earlier, big outboards aren't real good on fuel, and since these were built in the early to mid 80's that's what they usually have on them.

    If I was misinformed then I'll just leave it outboard, but what would be the best motor to look for?

    Thanks for eveyones input, I'm learning heaps, so keep it up.
     
  11. gonzo
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    gonzo Senior Member

    If you consider that those outboards use about 30% more fuel than a modern engine, then do the numbers. You will spend 4-5 thousand dollars on a conversion, that may or may not work. To break even you have to spend at least $18,000.00 on fuel.
     
  12. whitepointer23

    whitepointer23 Previous Member

    why do you have to buy a flightcraft, there is nothing special about them. you would be better off looking at caribean, haines, cruisecraft , they all had sterndrive models but most under 21 ft were 4 or 6 cyl. there are lots of cheap american imports , most fitted with 3 liter mercruisers. i have done the sort of conversions you are thinking about and if i did them again i would just leave them as they came from the factory. fitting engine beds , modifying transoms , adding fuel tanks in a production boat is a bit like pissing into the wind just more expensive.
     

  13. Willallison
    Joined: Oct 2001
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    Willallison Senior Member

    I agree with Brendan... there are so many 2nd boats out there for sale that if you can't find one that suits your needs without such drastic modification, then you simply haven't looked hard enough.
    Personally, I reckon that sticking a sterndrive into a boat like that will turn a perfectly practical boat into an absolute pig. Quite apart from the real risk that the thing won't perform as expected (for reason's expalined already by others) you will lose 1/2 of the boats useable cockpit space.
    If you are really set on the Flightcraft, then spend the money that you were going blow on the conversion on a newer outboard. At least that way you'll have some hope of recouping some of it when it comes time to sell again, and you'll enjoy a much better boat in the meantime.
     
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