Repower Inboard Prop Pocket Hull with Outboards?

Discussion in 'Boat Design' started by StandAlone, Feb 5, 2017.

  1. StandAlone
    Joined: Feb 2017
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    StandAlone Junior Member

    Hello all,

    I'm new to this forum but not new to boats. I've had everything from 15' to 42' and the current boat we have is a 1988 Sea Ray 390 Express. It's original configuration is twin Gas 454 Straight Inboards with Prop Pockets.

    We plan to repower her in the next 12 months and are weighing options. Obviously the easiest solution is a new set of drop in engines. However, since we've decided to keep this boat into retirement as I get older I would really rather maintain outboards than engines down in the bilge if I am to continue to do my own maintenance.

    Most would say sell the boat and buy one that is set up with outboards but since the closest to what we'd want I've seen is $1 Million bucks plus that's not going to happen.

    I'm really interested in repowering this boat with twin outboards on a bracket. I realize the transom will need to be reinforced and knees added for strength but my biggest curiosity is how the prop pockets will affect the handling and performance if they are left as is with the outboards mounted directly behind them using the tops of the pockets as the reference for engine height.

    The boat would still be used as a cruiser with desired top speed around 26 knots.

    Looking forward to opinions or hearing from someone that has done this type of repower.

    Thanks, Ken

    [​IMG]
     
  2. Angélique
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    Angélique aka Angel (only by name)

    -
    Hi Ken, welcome to the forum . . :)

    I would think to have the 2 outboards in 2 wells just in front of the transom would be a better idea then to have them on a one or two brackets(s) aft of the transom.

    The wells could be where the prop pockets are now, the outboard props would be on the same place that way . . :idea:

    Many sailboats are repowered that way, I'll post one for info soon, hang on . . :cool:

    For general info ---> Google: - And also try Google Images:

    outboard well - - | - - outboard motor well

    outboard well plans - - | - - outboard motor well plans

    outboard well construction - - | - - outboard motor well construction

    outboard well design - - | - - outboard motor well design

    Good luck !
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Maintaining large modern outboards yourself could be a problem, if you want to stay under warranty, what with the computer programs and electronics that are needed to satisfy the service requirements.
     
  4. ondarvr
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    ondarvr Senior Member

    Interesting project, but will two outboards do it?
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Problem possibly would be getting the full turning arc without fouling the tunnels with the outboard leg structure, 35* each way I think.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    You certainly can get them big enough nowadays But I'd like to know more about the current cruise speed setting, if it is less than 25 knots, I'd say the outboards might not be the way to go.
     
  7. ondarvr
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    ondarvr Senior Member

    HP is available, but prop selection may limit the performance, two props half the size of the current ones.
     
  8. Angélique
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    Angélique aka Angel (only by name)

  9. Angélique
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    Angélique aka Angel (only by name)



     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    It will lose quite a bit of weight with the outboards, but if it is a heavy and fairly slow boat as it stands, I don't like the idea of outboards that much, for the reason you mention, props, those big banger outboards are built for speed, not plugging along. Gearing and prop diameter says 30 knots plus is their go.
     
  11. Angélique
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    Angélique aka Angel (only by name)

    -
    Hi Ken,

    Study High Thrust Outboards. - And also listen to the other opinions and suggestions here, they could be better than mine . . :idea:

    Here's some High Thrus Outboard Info, in a for this thread's application a bit edited repost of...


    Good Luck !
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    You won't find "high thrust" 250 hp + outboards, angelique, the propellor diameters available don't cater to it, These babies are built for speed, heavy, slow boats (admittedly both terms are a bit subjective) are not a good fit. Maybe one day they will have an offering that suits , but they don't seem to exist at the moment as far as I can see.
     
  13. Angélique
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    Angélique aka Angel (only by name)

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    I didn't check for the availability of High Thrust 250+ HP outboards, my thought was if they're available it could be an option here.

    If they don't exist then the idea is out.

    It would need a hell of a strong outboard well though . . :cool:

    And also attached to the transom . . :idea:
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    You are a mine of useful info, Angelique, and an asset to this site !
     

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

    Welcome to the forum.

    Nope, there aren't 350+ HP high thrust options in outboards, which is what this boat requires, along with it's 300+ gallon fuel tankage. She's built relatively light and originally used pretty low grade hardware (replace all the plastic thru hulls). This permitted them to get up and scoot, so twin 375 Cats, or 454 (Mark IV's) were very common. They don't like to be driven hard into a small swell or current. Unfortunately the tunnels cause her to squat badly while coming up and it seems like forever for them to do so (needs tabs). They also have low speed handling issues, worse than usual, even for a full plane hull form. Even with the 750 HP Cats, they're a little under powered, but they can manage 30 MPH in smooth water, but once it chops up, they beat you up pretty good.

    To directly address Ken's questions and concerns, the simple answer is outboards just aren't a good propulsion option for this boat, for several reasons. First is the radical weight distribution changes this will cause. You could trim it out by moving stuff, but this makes the project more involved. Tankage will need to move, stringer issues will likely crop up other then the obvious reinforcements necessary for a bracket mount. Layout reconfiguration might be a welcome thing to think about, as the gen set and other stuff can now be moved to where they will not get soaked in the bilge anymore, but (again) more renovation and redesign stuff. Then you'll have to justify the costs. A set of crate Mark IV's are in the $3,000 range (long block, ready to install). This is pretty easy to swallow, compaired to just the cost of a bracket and a set of used, but good Black Max's, let alone new ones and reconfiguring the boat for this setup.

    These boats had a number of issues. If it was me, I'd focus on making her a much better version of what she is, with some new or reman engines. All the hardware on these was low end stuff (have you fixed all the port and windshield leaks?). Production habits also was on the low end of the spectrum, so have the stringers and other structural elements inspected and repaired. A lot of the 1980's versions used really cheap covering materials which tended to delaminate in time. It's all bolt on stuff, but replace this sort of thing would make a better boat. A boat of this era will have some need for tabbing reinforcement and other repairs and this is where I'd toss my money, after a repowering. For the cost of one new 350 HP outboard, you could make this puppy really nice and solid.

    In short, I'm not trying to insult you or your boat Ken, but these were what we think of as entry level craft (for their size and performance envelop). Because of the way (and era in which) they were built, there's lot of improvements that can (should) be made. You can make her solid and reliable or spend your budget, on a bracket and outboards, with all the other stuff on the back burner.
     
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