Adding a second story to a displacement cruiser trawler

Discussion in 'Boat Design' started by Mobcat, May 21, 2020.

  1. bajansailor
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    bajansailor Marine Surveyor

    Mobcat, there is no magic formula as such - I think it was mentioned previously that you will need to carry out an inclining test to determine what the metacentric height GM of the vessel is.
    Here is quite a good introduction of the procedure -
    Inclining Experiment- Determining Metacentric height of the ship https://www.marineinsight.com/naval-architecture/inclining-experiment-determining-metacentric-height-of-the-ship/

    Here is a much more detailed explanation - take note of what they say of page 27 about adding extra ballast.
    https://www.dco.uscg.mil/Portals/9/DCO Documents/5p/CG-5PC/CG-CVC/CVC3/references/Stability_Reference_Guide.pdf

    Have a look also at the first calculation on this sheet -
    http://www.shipwonders.com/download/i/mark_dl/u/4013086932/4629493632/Chapter 19 (Calculation of BM and KM; KM curves).pdf

    You will want to find the height of the metacentre above the keel - the ship wonders chapter above makes some basic assumptions re height of the centre of buoyancy KB, and the distance BM between the centre of buoyancy and the metacentre. You should be able to do a similar exercise for your vessel for an initial rough estimate.

    The aim of the exercise is to find the KG of the vessel.
    Once you have this, and you have a rough displacement, then you can take moments about the keel to calculate how much the KG will increase by adding weight on the top deck.

    (new KG x new displacement) = (old KG x original displacement) + (added weight x distance of it's centre of gravity above the keel)

    Where the new KG relates to the vessel with the 2nd story added, and the new displacement include this added weight on the top deck (which you estimate to be about 1 tonne).

    Note that if you reduce your GM, you will reduce the righting lever GZ, making the vessel less stable.
    For small angles of heel, GZ = GM sin (angle of heel).
    I hope that the above makes sense?
     
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  2. Mr Efficiency
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    Mr Efficiency Senior Member

    If you add several tons of weight to this boat, what with added structure and the proposed ballast, you will start to immerse that transom, and notice a discernible increase in resistance.
     
  3. JSL
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    JSL Senior Member

    Don't forget that adding weight (depending on where it goes) can also reduce stability
    BM = I/V
    Previous documents noted by bajansailor will address this.
    Also
    Your deck scuppers look totally inadequate for drainage, what lack of stability does not do, the Free Surface Moment can make things worse.
    So....
    some items to check: GM + FSM + Windage
    & if you are in higher latitudes in winter, allow for snow/ice accretion.
    you have a lot of work to do.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    He isn't going to encounter icing, but there is also the matter of how a new level of accommodation would affect the comfort level, with a different rolling amplitude.
     
  5. Mobcat
    Joined: May 2020
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    Mobcat Junior Member

    Thanks guys bit of reading to do and some planning appreciate it all, mr efficiency I can’t see a problem with the transom I put 4000 litres on board once and the boat sat perfectly just at the right level and swim platform was above waterline just at a nice level
    Happy days.
    Mobcat.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    What would you estimate the value of your boat is, as it is ?
     
  7. kapnD
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    kapnD Senior Member

    What’s that got to do with the OPs question?
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    M'lord, do you consider my question an impertinence ? :rolleyes:
     
  9. Mobcat
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    Mobcat Junior Member

    I suppose it’s worth what someone will pay for it, it’s a amazing boat with its internal fitout it’s been over five years of work this boat and she’s a beauty and to recoup what’s been put into her would be impossible like most boats, we have no intention of selling her she’s been built as a family holiday home with all the comforts of home, and due to this Covid crisis she has been our home for the past few months, we have been able to sit these strange time’s out in some beautiful places the past few months and the kids have had a ball onboard full time, but of course with the time spent aboard of late we have decided it’s a bit tight with all of us, thus the idea to move up the plan is to mirror the nordahavn 62 in its second story lines I think it will look ok and give us everything we desire

    62.jpg

    it will have this style of second story with the open fly bridge forward and acces stairs from the salon and forward deck to the fly bridge.
    I do like the idea of the second story only being as wide as the original wheelhouse giving us a nice walkway from the fly bridge around the second story to the rear deck and tender davit. I will photoshop it when I have time for us all to critique, from early calculations it certainly seems very possible to achieve what we want and if needed a six inch extension to the keel incased in lead could be a easy option, the fullness of time will decide the best road forward I suppose.
    Thanks again everyone for your input it’s much appreciated.
    Happy days
    Mobcat
     
  10. fallguy
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    fallguy Senior Member

    John Wayne's former boat the Wild Goose had just a roof added to the top deck and it is NOT allowed to take passengers put beyond the harbor.

    This add a second story is an abysmally bad idea.

    Sorry. Don't do it. The boat will be destroyed at great cost.
     
  11. fallguy
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    fallguy Senior Member

    How much space do you have forward of the cabin? Hard to get an idea of the width there.

    It might actually be easier and better to add a small space forward of the existing cabin.
     
  12. fallguy
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    fallguy Senior Member

  13. fallguy
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    fallguy Senior Member

    He is using ballast somewhere. Thought he said for'd.
     
  14. fallguy
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    fallguy Senior Member

    ...with my proposal; you move the windows forward

    If you want the flybridge; it is also unrestricted by any cabin for operational views..

    ...windage impacts also minimized
     

  15. kapnD
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    kapnD Senior Member

    M’lady, it just seemed like a pretty severe thread shift.
    I didn’t see your question as relevant, and the owner is obviously not concerned with value or aesthetics.
    Maybe you’re interested in purchasing the vessel?
     
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