Semi displacement conversion to outboards

Discussion in 'Powerboats' started by wildsideboats, Mar 3, 2019.

  1. wildsideboats
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    wildsideboats Junior Member

    Hello.. Considering converting a single engine 30 x 10 full molded keel semi displacement hull to tvvin outboards. I'll be extending the hull aprox 3' to create the engine bracket adding full length planning rails . My greatest concern is adverse effects of the keel . Curious if anybody has experience in such a project ? Thanks in advance ... DSCN0640.JPG DSCN0614.JPG DSCN0635.JPG
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Compared to a single diesel, twin outboards will use a lot more fuel, even if you select engines carefully, by which I mean motors with a good gear reduction spinning larger diameter props, and you have the possible issue of that keel creating turbulence in turns.
     
  3. DCockey
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    DCockey Senior Member

    Current 4-stroke outboards are much more fuel efficient than old 2-stroke outboards. The cost difference between a Diesel with reduction/reverse gear, shaft and propeller installed and two outboards will buy a lot of fuel.

    Judge Yachts in Maryland builds boats with warped bottom / variable deadrise hulls and they now offer an outboard version of their 36 foot boat. Chesapeake 36 OUTBOARD http://www.judgeyachts.com/chesapeake-36-outboard.html
    Comparison of Judge 36's with two 300 HP outboards, Cummins 480 HP Diesel, and two 350 HP outboards. Fuel consumption with the outboards at cruising speeds is about 20% higher than the Diesel. The Diesel has a much larger percentage difference at slow speeds. I'd be interested to know the fuel consumption of the outboards at slow speeds with only one outboard running. The boat with the two 300 HP outboards is $50,000 less than the Diesel, the boat with the two 350 HP outboards is $50,000 less.
    Owner review of Judge 36 with twin 300 HP outboards: Judge 36 Chesapeake with Twin Outboard Review Part 1 Overview - Page 3 - The Hull Truth - Boating and Fishing Forum https://www.thehulltruth.com/mid-atlantic-chesapeake-bay/903047-judge-36-chesapeake-twin-outboard-review-part-1-overview-3.html
     
  4. DCockey
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    DCockey Senior Member

    If the original poster is ambitious and comfortable with structural fiberglass work he could take several inches off the bottom of the keel or even most of the keel for the conversion to outboard power.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Depends on what sort of speed his boat is comfortable running at, if its less than 20 mph, the outboards will fall away a bit. Will cutting away the keel cause tracking problems ? There are a few things to consider, including weight changes, and weight distribution, but obviously outboards today have infiltrated the markets that once were the preserve of inboards, and because of the lower fuel burn of fuel injected 4 strokes, and direct injection 2 strokes.
     
  6. wildsideboats
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    wildsideboats Junior Member

    Thanks for the input..All good things to consider. VVith this shape hull , leaving the keel in place has advantages , tracking , roll stability , grounding protection being the most obvious . By theory, if the outboards are mounted far enough apart they should have plenty of clean vvater . The comment about cavitation in a turn is a good point. The Judge hull is very different no large keel and appears to have hard chines so I dont think that is a good comparison . Atlantic / Duffy 27 has a more similar hull shape and is being offered vvith OBs. My original intentions vvere to go back Inboard but considering the pros vs. cons , OB may be an option. I hate to give up the open transom space the OBs require VS additional speed , ease of service , quiet , smooth ,cost ,opens up additional interior space ect... I am also proficient at installing , rigging , servicing , running OBs but have little experience vvith IBs . This post is in hope somebody vvho has done a similar conversion can state it operates great or is a flop ? Thanks again...
     
  7. fallguy
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    fallguy Senior Member

    Let's be clear here, I am no an expert.

    But!

    I don't understand how a semi-displacement hull becomes a planing hull with a bracket.

    I submit it does not AND the results you will achieve are spurious or unknown and somewhat unpredictable.

    I will let others coin in, but absent an NA reviewing the hull; it would be w crapshoot, imo.

    My guess is you won't plane easily and have a horrible hump and terrible fuel economy unless you are wide open and then the gas goes as fast as the boat.
     
  8. fallguy
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    fallguy Senior Member

    The deep, deep forward vee might make for an interesting time to plane.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Would have no adverse effects, I'd say, but without knowing what make it is, and seeing a lines drawing, hard to say much, but cutting away the keel could lead to treacherous handling in following seas. And there would need to be a weight study if moving from an inboard diesel to outboards on a pod, there would appear to be shift aft, of the COG, which is not desirable. Is the pod ine that adds buoyancy ? Many things to consider, but if the OP wants to know if someone else has done similar, mentioning the make and model of the boat might help.
     
  10. DCockey
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    DCockey Senior Member

    Boats with similar "semi-displacement" hull shapes going fast:


     
  11. fallguy
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    fallguy Senior Member

    All well and good, how are they on fuel?
     
  12. DCockey
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    DCockey Senior Member

    Generally less fuel than a "planing" type hull of similar size and displacement. Low resistance as long as they are not loaded so that the stern is deeply submerged. The lobster boats have very little deadrise at the transom and moderate deadrise amidships so tend to low resistance unless heavily loaded. Very little if any "hump" when accelerating. Not the boat to use for jumping waves though. Outboard versions typically have a shallower and smaller skeg/keel than needed on inboard versions to accommodate the shaft and propeller.
     
  13. wildsideboats
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    wildsideboats Junior Member

    The hull is a 1986 Hunter Alura.
     
  14. DCockey
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    DCockey Senior Member

    Do you have any photos of the aft portion of the bottom?
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    You would need to retain some of that keel, question is how and where you truncate it. "Planing rails" would be unproductive, I'd say. I think I would stick with inboard, unless you can find someone who has already done similar with the hull, to what you are contemplating, and it has worked out well, and I guess that is why this thread exists.
     
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