Calculating ballast for a motorsailer

Discussion in 'Motorsailers' started by Annode, Sep 22, 2019.

  1. Annode
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    Annode Junior Member

    Well yes... but I am assuming the non sailboat hull will capsize before the heel reaches the point where the water reaches the helm cabin!
    If the object is to prevent the boat capsizing at 90deg like a serious blue water sailboat, then thats going to require a serious keel.
    I was hoping that someone with sailboat design experience would explain the angle at which wind starts to spill out of the top of the sails and the keel counterbalance prevents further heel. I am guessing that is a lot less than 90deg for the majority of sail boat designs.
     
  2. gonzo
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    gonzo Senior Member

    Flooding angle considers all openings, including engine-room ventilation and non-watertight doors/windows.
     
  3. Annode
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    Annode Junior Member

    yes understood. That is still a substantial angle. My guess is it would capsize before that point, hence the need for a ballast calc :)
     
  4. DogCavalry
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    DogCavalry Junior Member

    As soon as it starts to heel, aerodynamic forces lessen. 7 tons craned aboard? That is a pretty respectable load from a sailplan.
    Get CA Marchaj's Aerohydrodynamics of Sailing out of the library. It will tell exactly what the forces from your prospective sail plan would be. But honestly it sounds like you will need very little ballast. Side force from an external keel is the greater lack.
     
  5. Annode
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    Annode Junior Member

    Last edited: Oct 9, 2019
  6. bajansailor
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    bajansailor Marine Surveyor

    This article explains the basic principles of stability very nicely.
    Understanding monohull sailboat stability curves http://marine.marsh-design.com/content/understanding-monohull-sailboat-stability-curves

    And in this paper there is a typical graph plotting righting moment and heeling moment against heel angle - but you have to scroll a long way down to Section 4.6.3.
    IS-Code Code on Intact Stability by IMO instruments - Netherlands Regulatory Framework (NeRF) – Maritime https://puc.overheid.nl/nsi/doc/PUC_3005_14/3/

    Where the wind heeling moment first intercepts the righting moment will be the steady angle of heel for a given wind strength.
     
  7. TANSL
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    TANSL Senior Member

    Be aware, Section 4.6.3 is for "Mobile offshore drilling units", not for motorsailers.
     
  8. bajansailor
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    bajansailor Marine Surveyor

    Ooops, sorry about that.
    I think though that the general shape of the curves shown would be fairly typical of a motor sailer as well?
     
  9. TANSL
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    TANSL Senior Member

  10. bajansailor
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    bajansailor Marine Surveyor

    I have just cottoned on - my apologies, you are correct Tansl - the curve of the wind heeling moment should be decreasing as the angle of heel increases, not increasing.
    I have been trying to find a plot online for a monohull comparing the two moments - the best I can find at the moment is this one
    https://www.sciencedirect.com/topics/engineering/heeling-arm
    Scroll down to section 8.3, and Figure 8.5.
    Although the vanishing angle of stability for a converted fishing vessel / motor sailer will probably be less than the approx 85 degrees shown in the graph (but this is for a 2,000+ tonne ship).
     
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  11. Ad Hoc
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    Ad Hoc Naval Architect

    This summary may help.
     
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  12. bajansailor
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    bajansailor Marine Surveyor

    Excellent, thank you!
     
  13. Annode
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    Annode Junior Member

    Nice explanation Ad Hoc.
    Did I understand correctly that a keel can effectively "trip up" a hull in wave knock over event:


    Perhaps a keel that can rotate into the hull would be a better idea. Trawlers have stout enough bulkheads to support a JCB style joint about which a few tons of keel could be rotated down.

    In reading your explanation, it seems that broad beam coupled with significant draft (2.5m) is a good stable combinbation, and looking at the graphs, 60deg of heel would roughly be about the tipover point given the original design was reasonable.

    The twin keels shown on SVSeeker would act immediately for a given heel angle unlike a single vertical keel, but would reach 90deg sooner and thus contribute to tipover for a lower heel angle.

    With a large steel hull, with large moments of inertia and long period of roll, it is scary to contemplate having to understand by the seat of your pants when a gust has imparted enough energy to the sails to knock over the boat and release the sails fast enough.

    Then there is the Rogue wave consideration...
     
    Last edited: Oct 10, 2019
  14. bajansailor
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    bajansailor Marine Surveyor

    Annode, could you post a photo of the boat that you are planning to convert, or at least a photo of something similar?
    I think that you are worrying too much re your last post above - a trawler hull such as what you have described above is going to be inherently pretty seaworthy so long as it has not been chopped and changed too much. Trawlers fishing in the North Sea in winter will experience much worse conditions than what you are likely to encounter.
    I do not know of any trawler conversions that have a lifting keel or daggerboard - which perhaps suggests that it would be very complicated and expensive for you to carry out such a conversion on your project.
    Twin bilge keels would be useful for supporting the vessel when she 'dries out'.
    I think that you are looking at a vessel around 80' long?
    If you have a relatively modest motor sailer rig on her, then a 'gust' of wind is very unlikely to knock her down easily.
    I crewed once on a 100' ketch rigged motor sailer (a lovely Philip Rhodes design) on a passage from the Caribbean to the Eastern Seaboard of the USA. We stopped in Bermuda on the way - we motored all the way up to Bermuda because we had no wind. But when we left we motored out into a F7 gusting F8. Skipper said that we would put up full working sail - yankee jib, staysail, mainsail and mizzen. We did, and she took off with the wind on the beam - a steady angle of heel, not excessive, and a steady 10 knots on the log, tromping along very steadily and happily. Big heavy boats (yes, big and heavy are relative here) do generally stand up well to their canvas, when compared to smaller craft.
     
    Last edited: Oct 10, 2019

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Agreed.
    Must be based on facts not emotions.
     
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