Twin 250hp on 24ft I/O Project

Discussion in 'Boat Design' started by ermaclob, Apr 13, 2015.

  1. ermaclob
    Joined: Sep 2013
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    ermaclob Junior Member

    So i been working on converting a 24 ft cuddy cabin into a twin outboard on a Armstrong flotation bracket. im pretty much almost to the end of it though i cant help thinking this is a really bad idea :rolleyes: So i was wondering if you guys could give me your input on the matter.

    this is the boat im converting
    [​IMG]

    it has 24 ft in length, 8'6" beam, and about a 25 in deadrise

    i plan on putting twin 1992 yamaha 250 ox66 2 strokes on it.
    [​IMG]

    i did not rebuild the transom. instead i plugged the i/o hole with by putting a glassed wood backstop and a plug that fit into the I/o hole and then glassed that on the out side till the plug was even with the plug.

    this i know is not the right way to convert I/O to outboard because the transom normally need to be reinforced. so what i did is make a knee brace that mounts to the outboard bracket and then strait the stringers of the boat where the inboard engine used to mount as well as deliver all the force.

    like so.
    [​IMG]

    the thing is made out of 3x3 1/4in galvanized steel angles and will be coated for extra protection.


    i feel confident that the transom is solid and wont give me any problems... then again i could be wrong. My real consern however, is the weight distribution of the boat. im told i need to move weight up front to balance the boat out now that theres about 1000 lbs hanging of the back of the boat. whats the best way to do this?

    so with all that said, how fast do you guys think its going to sink?
     
  2. BMcF
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    BMcF Senior Member

    For whatever its worth, I converted a 25' Rampone Deep-V from twin Volvo inboards to twin outboard V-6 Yamaha's on a transom bracket, and the result was an excellent performing vessel. I don't see anything "fundamentally wrong" with the approach you are taking there.

    Top speed with the twin 140HP inboards was 28-29 knots. Top speed with twin 150HP outboards was 40 knots.

    An older pic of mine after the conversion.
     

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  3. Mr Efficiency
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    Mr Efficiency Senior Member

    You might be able to use those old engines as a test bed to see if the bracket set-up works with your boat, but the fuel burn will kill you ! I doubt many boats are converted from I/O to outboards and use old non -DFI 2 strokes. Because there will be a loss in mpg, even though speeds may increase.
     
  4. ermaclob
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    ermaclob Junior Member

    out of curiosity how much did your hull weight without the motors ?
     
  5. BMcF
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    BMcF Senior Member

    I really don't know. The original Rampone's (later Sea Vee) were very heavily built and so the hulls were definitely heavier than the typical look-fast boats in same size range...
     
  6. BMcF
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    BMcF Senior Member

    Fair point. The fuel burn on my boat increased rather dramatically going from inboard Volvos to carbed 2-stroke outboards, even though I usually operated at the "sweet spot" for that boat of around 26 knots.
     
  7. ermaclob
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    ermaclob Junior Member

    The boat I have is 3400 lb with the I/O so its probrably 2700 with out the inboard. I think thats a little on the light side considering im going to plop 1200 lb on the back
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Did you need those trim tabs on the boat as it was ? Is your outboard pod a continuation of the hull bottom, or elevated ?
     
  9. ermaclob
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    ermaclob Junior Member

    i do not know about the trim tabs, the boat was my grandfathers a while back but he hadnt used it for some time. he kinda just gave it to me. Motor was shot beyond repair. there is actually no hydraulic pump in there so the tabs dont work. i will need to get a pump and switches to work em i guess. defiantly important for what im doing.

    the outboard bracket is elevated. 4ft x 16in foot print on transom. 27 inch of extension. doing a little math its about 12 cuft of air so it should lift about 768lbs if it was fully submerged.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    I would be a little concerned if it was needing the tabs as a sterndrive boat, you have quite a bit of weight perched out there with those outboards, and getting no support underway from the bracket/pod.
     
  11. BMcF
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    BMcF Senior Member

    ?....like every other transom bracket mounted O/B setup...including the conversion I put a pic up of. Transom brackets are not an extension of the hull bottom.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    In Australia, some "pods" are tailored or customized to be an effective extension of the hull, wide enough for one or two motors. Many others are as you describe. depending on the hull, wider bottoms are more likely to work with these. The former type of installation is better able to resist excessive squat, and/or porpoising. I realise you get no advantage from having a motor set back, in terms of operating in water less affected by hull drag, if it is an extension of the bottom of the hull proper. What started as mainly an idea to gain boat speed with set-back, has morphed into a way to gain more internal boat space, as the I/O and box, or the outboard well goes, extending the cockpit length appreciably.
     
  13. ermaclob
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    ermaclob Junior Member

    are you referring to this? the V bottom on the bracket? [​IMG]
     
  14. rasorinc
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    rasorinc Senior Member

    Does your state or the US-CC have rules regarding overpowering small boats? You did not list the age of your boat and some builders list a max HP for their boats. You might just check on that. PS you might check out insurance also as they have lots of exclusions.
     

  15. BMcF
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    BMcF Senior Member

    Where I live and boat..the conventional brackets are easily 90% of all transom OB mounting structures. The only thing that has changed over the last decade or so is the increasing number of boats that include the transom OB brackets as an integral part of the molded hull instead being a bolt-on.
     
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