Wide mounted twin outboards on monohull

Discussion in 'Powerboats' started by fpjeepy05, Aug 11, 2022.

  1. fpjeepy05
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    fpjeepy05 Senior Member

    Anyone have any examples of this? Lots of examples of cats, but I could only find one example for a monohull. I know closer mounted is faster/more efficient, but wider would be better maneuverability. Also would allow for different transom layout.

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  2. bajansailor
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    bajansailor Marine Surveyor

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  3. DCockey
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    DCockey Senior Member

    A potential disadvantage of wide mounted twin outboards on a monohull is the powerheads might be partially submerged if the boat rolls a lot.

    Would be closer mounting be "faster/more efficient"?
     
  4. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Re the glass bottom party boats mentioned in my previous post, here are a few photos of them.
    One could argue that they are not really monohulls as such - more like catamaran barges, but with the bridgedeck significantly immersed.
    They were built here by A & T Marine in 2016 and 2017, and have been working well ever since (apart from the Covid lockdowns).
    There are some more photos on the Builder's Facebook page -
    Log into Facebook https://www.facebook.com/atmarineltd

    The engines for both vessels were supplied and fitted by Marine Power Solutions here -
    MERCURY http://www.mps.bb/mercury.html

    DSCN8369.JPG

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    DSCF2138.JPG

    DSCF2938.JPG
     
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  5. BlueBell
    Joined: May 2017
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    BlueBell . . . . .

    With a pair of 150 Merc's on it.
    Fantastic!
     
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  6. comfisherman
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    Location: Alaska

    comfisherman Senior Member

    I'll dig out the way back machine, doubtful any pictures exist but we had a fun one years back. About 15-20 years ago my friends dad was a salver for the area, mostly salvaging old fishing boats that had found a rock due to a sleepy crewman. After the season he would hire us young guys to come do the physical stuff. One was a beautiful aluminum pilot house pleasure boat that had one big fault.... tight corner on hawse chaffed its moring in an onshore gale and took a decent rock through the hull below the main and promptly filled with gravel and sand. Usually when this happens the boat above ground looks brand new, the boat below ground looks like a popcan that's been rum over.

    Was a decent wait on the tide so we started poking around with shovels. The engine had torn loose and rotated 90 deg and the whole drive train was mangled almost beyond recognition but the hull looked almost passable. Escavator eventually dug it out and we drug it up on deck of the landing craft. Usually these recovery are just a d exercise in aluminum scrap and maybe a salvaged door. Usually we'd cut them up on deck to 2'x4' size chunks and recycle the alloy or landfill the fiberglass ones. At any rate the hull looked like fixable barring a big dent on the cabin from the escavator and the whole in the hull where the main had been.

    Somehow we talked him into letting us revive it, we gutted all the interior that was ruined and found the hull would never be an inboard again, but a few plates of alloy and some new stringers it could probably be an outboard driven boat. Ironically they had just salvaged a boat that had twin 250s that had come off a trailer on the highway at a pretty good clip.... problem was the high sided inboard boards with a deep vee are poor choices for short shaft 250s. Only way to get them clean water was fairly wide on the corners of a 12 foot wide hull on brackets. Could have left them center but the powerheads would have been near submerged.

    Between the extreme weight loss of having most the interior stripped and the extra hundred hp the outboards gave over the cummins inboard... she was a bit of a rocket on a shoelace budget. They drove most items at two speeds... off and full throttle. I remember doing a banking turn on it coming up a remote lagoon dodging some driftwood and the racket it made when the 250 yamaha Aerated at 5000 rpm. It would suck air running abreast in a decent chop and if you got too sporty in a turn. But it was a fun way to kill a few weeks and provided the platform for a few fun stories.

    Probably wouldn't build one like that on purpose, but it worked allright.
     
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  7. DogCavalry
    Joined: Sep 2019
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    @bajansailor , tell us about the stairs that are integrated with the bows of these boats, please.
     
  8. bajansailor
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    bajansailor Marine Surveyor

    Here is another photo of the steps in the bow ramp on the blue boat, and a couple of photos of the ramp on the white boat.
    I was responsible for the design of both ramps, and for talking the Builder and Client into going for a semi-catamaran hull form with twin engines - initially they were just going to have a simple barge hull form, with a single O/B engine.
    I will send you some copies of my sketches of the ramps by email, so that they don't hijack fpjeepy's thread too much.
    The white boat has a single line block and tackle system that hoists both sides at the same time, while the blue boat has a block and tackle on each side.

    Re the second photo, notice how easy it is to bend a fluke on a Danforth anchor - although I suspect that it probably got hooked under a rock, and then they tried to drag it out using 300 hp of power..... :)

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    Tranquility - view on bow ramp .JPG

    Tranquility - view on bow ramp 2.JPG
     
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  9. fpjeepy05
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    fpjeepy05 Senior Member

    One bump
     
  10. BlueBell
    Joined: May 2017
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    BlueBell . . . . .

    No I don't have any examples but I do think it's going to have advantages and disadvantages on different monohulls.
    Usually, your best bet is for a single engine anyway.

    Why do you ask fpjeepy?
    Is there something specific you're trying to ascertain, evaluate or otherwise decide upon?
     
  11. fpjeepy05
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    fpjeepy05 Senior Member

    I am thinking about design. I like narrow hulls and typically a single-mounted engine makes it difficult to have a transom door that a human can fit through. I was curious if having two smaller outboards mounted wide would allow for a center-mounted transom door.
     
  12. BlueBell
    Joined: May 2017
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    BlueBell . . . . .

    Okay, thanks for that.
    Makes sense but what a cost and weight penalty to pay for a transom door.
    I guess if it's a priority then it may be worth it.
    Two other options would be an inboard engine or a new Rotax Ghost:
    BRP Unveils Rotax Outboard Engine https://www.boatingmag.com/boats/brp-unveils-rotax-outboard-engine/
     
  13. fpjeepy05
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    fpjeepy05 Senior Member

    I'm looking in the 40-60hp range (BRP is out at 150hp), so twin 20-30hps. Depending on make and model it will be about 100lbs more or 40% more weight. Definitely a consideration, but not a deal breaker. I don't know the prices without calling.

    [​IMG]
     
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  14. BlueBell
    Joined: May 2017
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    BlueBell . . . . .

    If you've narrowed it down to 40-60hp (x2) then you must have a hull design in mind.
    Let's have a look because a narrow hull combined with two o/b's and a transom door is mighty counterintuitive, no?
    What's the requirement of the transom door? Scuba?
     

  15. fpjeepy05
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    fpjeepy05 Senior Member

    I don't have it modeled yet. I like the lines of the old runabouts. Something like that. A little flare forward a little tumblehome aft. Near plumb bow. High gunnels. Self-bailing.
    Below is a boat I modeled for an inboard. Some similarities, but different.

    I freedive. No tanks. The below boat has a beam of 7'4" and the transom door seems to work well, but I don't think much skinnier is possible. Definitely not the 5'-6' I would like.

    Has anyone seen a single outboard boat with an off-center engine? It would "counteract prop walk" but my intention would be to fit a transom door.
     
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