Boat stretch help

Discussion in 'Boat Design' started by Orlando Melendez, Feb 11, 2021.

  1. Orlando Melendez
    Joined: Jul 2018
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    Location: Orlando Florida

    Orlando Melendez Junior Member

    Hi there, I'm a boat builder and have a customer that wants me to stretch a 1969 28 feet Cary, just like to get some guidance and opinions if it will be ok to stretch this hull 6 feet. Here are the specs 27'10 long 9'10 beam 9 feet wide at transom. I already told him to buy a boat of the size he wants but he has sentimental attachment to this design and wanted stretch. Some help will be appreciated. The pic on the post is a pic I found on the net I couldn't upload my pics.[​IMG]
     
  2. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    If the deadrise is constant, and the chines parallel in the aft 6-8’, it should be doable.
    That dropping break line below the gunnel will be interesting, but that’s just cosmetics.
    Maybe that’s just the camera?
    Six feet is a BIG jump on a boat of this size!
    I’ve done even more radical stretch jobs, but on commercial fish boats to increase capacity, not on go fast boats.
    Maybe some of the more mathematically inclined here will offer some wisdom?
     
  3. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    It's not uncommon, but as mentioned, the pic makes it look like it would be a bit of a challenge to make it look appealing after that much of a stretch.
     
  4. Orlando Melendez
    Joined: Jul 2018
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    Location: Orlando Florida

    Orlando Melendez Junior Member

    My designer and I caind it followed the factory lines and add a little hight to the cap and it doesn't look that bad what we came up with, I just need to know if the stretch is not going cause any problems or sacrifices the factory ride quality. (Talking to other boat builders) some said that it might even get better than before the stretch. But that's the reason I end up here because I'm not sure about that. In the design pics is 34'11 including the bracket.
     

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  5. Orlando Melendez
    Joined: Jul 2018
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    Location: Orlando Florida

    Orlando Melendez Junior Member

    Latest design were we achieved the 6 foot stretch on the design without a bracket and with the upper chine, the customer loved but right now is just a fantasy, I will not build it if is not going to ride acceptable. Please help
     

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  6. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Those renderings don't appear to be accurate to actual appearance, it could become an ugly beast. I think you need accurate lines to start with, and proceed from that, should not be too hard to get accurate offsets by measuring at the sheer line, the centreline, and the chine.
     
  7. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    +1 re Mr E's thoughts above - in your computer renderings I do not see any sign of a stretch at all.
    How about trying an old fashioned and easy approach initially?
    Print a copy of the side profile photo above. Measure the length of the boat on the photo, and then divide the LOA of the boat (28' ?) by this length to get the scale.
    Now that you know the scale, draw lines on the photo extending 6' aft, to lengthen the sheerline, chine and keel, and see what it looks like then.
    It will look very different to the hull shown in the computer renderings.

    You could also try drawing a plan view on paper, by taking some rough dimensions off the boat, unless you have a drone that can take an aerial photo of the hull from above.
    Assuming that the maximum beam is amidships, then the beam on the present hull decreases by 10" over approx 14'.
    And you want to 'stretch' the boat by 6'.
    You will need to keep this rate of change of beam reduction, otherwise you will end up with a slight 'kink' in the hull at the joint.
    So the width at the transom will be less than 9' - probably about 8'8".
    And the new LOA will be 34'.

    What is the reason for stretching the boat? Does your customer simply want more room for fishing?
     
  8. Ike
    Joined: Apr 2006
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    Location: Washington

    Ike Senior Member

    The real problem here is one of strength. I have seen quite a few boats that were stretched and not done properly, and the stress, usually at the point where the old stern was, eventually causes cracking and can also result in twisting of the hull. In addition to designing it so it doesn't look ugly, you need to do an engineering analysis of the stresses. Hire an engineer who does Finite Element Analysis to determine if it is going to break apart. You are also now moving the weight of the engines 6 feet farther aft which creates a huge lever (moment) wanting to tear the back end off the boat. It also changes the longitudinal center of gravity, and moves the center of buoyancy and pressure farther aft, changing both the static stability and dynamic stability of the boat.

    I agree, he should just buy a bigger boat, it would probably be cheaper in the long run.
     
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  9. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Something of an essay whichever way you look at it, structurally or aesthetically, but unless I am cross-eyed (possible) it does appear to have some reverse sheer, if it was some slab-sided old thing the only problem would be the structural.
     
  10. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    This is done often enough that the structural aspect isn't much of a problem. Tying it all together and making it look acceptable will be the challenge. At least from what can be seen in the pics.
     
  11. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    Assuming that it’s powered by an I/O, you might want to consider leaving the motor in its original location and adding driveshafts to the stern drives, especially if it has twins.
    Ride will almost certainly improve, added waterline is always good that way, but it’s all dependent on the configuration of the existing hull, and how well you can get it balanced after modifying.
    Also I’d consider gutting the entire hull and adding beefier new stringers and bulkheads, making them come clear up to the deck to form a box grid belowdecks.
     
  12. Kayakmarathon
    Joined: Sep 2014
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    Location: NewEngland

    Kayakmarathon Junior Member

    Adding all that volume will lower the center of buoyancy / raise the center of mass. There will be a decrease in righting moment, less static stability. I wouldn't modify a boat designed for speed. Too many tradeoffs that could compromise safety.
     

  13. Orlando Melendez
    Joined: Jul 2018
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    Location: Orlando Florida

    Orlando Melendez Junior Member

    Thank you gentleman's for all of the feedback, this is enough information to abandon this job. I have done several stretch on boats but never this many of feets.
     
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