Removing the deck of a 46' express cruiser Ultimate Dive Boat

Discussion in 'Boat Design' started by cgoodwin, Jan 1, 2018.

  1. cgoodwin
    Joined: Mar 2011
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    cgoodwin Junior Member

    I am considering buying a 46" Sea Ray Express Cruiser and removing the deck from the windshield forward to make it into a very large bowrider. The boat would be used to get 20 divers about 20 miles to several dive sites. The issue with current boats is that they carry few divers and have a harsh ride. The deep V hull and size of a 46' cruiser would solve those issues and be more comfortable and more fuel efficient.

    So on to the boat. I am looking at several older 80's Sea Ray Express Cruisers. The deck would be removed from the base of the windscreen forward and the area at the base reinforced side to side to create a transverse beam holding the sides together. The interior would be gutted and a new floor installed higher than the original. Passengers would enter on the rear deck level and walk down through the passageway into the area that contains the head and galley, then up a few steps to the new floor level where seated passengers could see over the gunwale from seats arranged like a horse shoe around a central table and camera washing area. Tanks would be stored behind this table and dive gear under the seats.

    Structurally I think the new floor would replace any structure that had been provided by the original deck. The transom would also be modified and a larger swim step with folding ladders installed.

    Any thoughts?
     

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  2. TANSL
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    TANSL Senior Member

    It seems a bit dangerous what you are proposing. You can always modify the structure of a boat but to give an adequate response you should take into account many things. For example, the longitudinal strength of the booat as a beam can be greatly diminished because, as the new bottom will be closer to the neutral axis than the deck was, the contribution to the longitudinal strength will be less. And this is just one of the many things that must be taken into account. Anyway, without seeing a scheme of what you want to do one can not give a valid answer.
     
  3. gonzo
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    gonzo Senior Member

    It will be hard to get that boat to comply with the regulations for a vessel carrying 20 passengers for hire. A six pasenger (6 pack) may be possible. Also, the amount of gear for 20 divers will probably overload that boat.
     
  4. cgoodwin
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    cgoodwin Junior Member

    This could be overcome with ribs glassed into the sides of the hull. Frankly I'm not sure that the existing deck provides that much strength, it has windows nearly the entire length of both sides...
     
  5. cgoodwin
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    cgoodwin Junior Member

    The boat will not be being used in the US and will be replacing a 36' Panga which carries 20 pax and gear...
     
  6. TANSL
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    TANSL Senior Member

    Naturally, I agree with you. You should not be sure of anything if you do not check it.
     
  7. gonzo
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    gonzo Senior Member

    Where do you dive?
     
  8. cgoodwin
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    cgoodwin Junior Member

    The Sea of Cortez, Mexico.

    TANSL you are clearly making a statement, so how do you propose I check this. This is what I am asking the the point of my post. I didn't just post and say "Hey I am thinking of drilling holes in the bottom of my boat" I posted asking for thought son the matter because obviously I am concerned with this plan. I get it that you think it is possible that the integrity of the hull could be compromised, so do I, that is why I am here asking....
     
  9. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I agree with TANSL, you need to reinforce longitudinal stiffness if you're removing this portion of the deck. Given the local requirements where you'll likely operate, I don't see any issues, as there's just no over site there, so you can do whatever you'd like. This said, if you make these modifications and someone gets hurt, it will be come a huge liability for you and your insurance company.
     
  10. cgoodwin
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    cgoodwin Junior Member

    Yes, that is correct, we have all established that the hull will need to be reinforced, from the first post to the most recent, that has been the point of the post.

    So far I have mentioned reinforcing the section of the deck directly in front of the windscreen making it into a beam, and adding ribs to the sides of the hull.

    So we have all agreed that strengthening will be required, the question is and has been, what type and design of reinforcement?
     
  11. TANSL
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    TANSL Senior Member

    Could you show us a picture of what you want to do?. Without that, it is not easy to give concrete answers.
     
  12. cgoodwin
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    cgoodwin Junior Member

    This is a diagram I did based on a 34' Sea Ray Sundancer. There are no bulkheads being removed only the deck which has 7" high windows running from just under the windscreen to about 3' short of the bow, so I can not imagine that it provides much longitudinal structure to begin with. I would think that strengthening the area forward of the windscreen and the anchor locker would make up for the missing deck... sea-ray-340-sundancer_pic5.jpg sea-ray-340-sundancer_pic5a.jpg
     
  13. AusShipwright
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    AusShipwright Duck

    The foredeck itself does not contribute greatly to the longitudinal strength of the hull girder, but the stiffeners and longitudinal bulkheads that support it can, depending on their overall length. Cutting these out or trimming down will reduce the stiffness in an area usually subjecting to the largest internal bending moments.

    An idealised midship section of your proposed structural layout is required to assess the stiffness of the new hull girder and more detail on the current bottom framing.

    Im unsure about the standards a vessel such as this must comply in that part of the world so others with more expertise in this area may be able to help.
     
  14. PAR
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    PAR Yacht Designer/Builder

    Yes, the foredeck does contribute greatly to the longitudinal stiffness of the hull, as it's a homogeneous structure. Using the beam descriptor; the deck would be the upper flange, with seat risers, seat backs, furniture partitions and the like serving as the web, while the sole and hull shell form the lower flange of the beam.

    This can be done, though it's not nearly as easy as one may think on first blush, particularity on a vessel of that size.
     

  15. baeckmo
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    baeckmo Hydrodynamics

    Placing 20 divers plus diving gear (~over 2,5 tonnes) where you propose will create serious trim problems with this hull shape.
     
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