Optimum position of LCG, VCG, LCB, LCF

Discussion in 'Multihulls' started by manon, Aug 26, 2012.

  1. manon
    Joined: Dec 2007
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    manon Junior Member

    Dear Members,
    Good day. Hope you are fine and in good health.

    Currently I am looking for some reading materials or professional help regarding the optimum positions of LCG, VCG, LCB, LCF of a steel body twin hull passenger vessel. The optimum trim in full load conditions (Departure and arrival) is an important concern. Due to lack of knowledge regarding catamarans, I would be very happy if you can give me some links of reading materials or suggest here in this thread.

    For your info:
    Loa= 30m
    Boa= 10m
    B_hull= 2.4m
    Center line distance between hulls: 7.6m
    Depth_main deck= 2.8m
    draft= 1.6m
    cruising speed= 14 knots.
    Passenger capacity= 200 persons (127 persons on Main deck, and 73 persons on upper deck).
    Main engine power= 2x450 BHP.
    Route= Inland rivers (No sea or bay)

    Please let me know if you need in more information. I will be grateful to you for any kind of assistance.

    /Manon
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Well your Fn is roughly 0.4 -ish...which is approaching the prismatic hump, not so good. But what affects this is the length displacement ratio and its implications on your hull form. Thus without knowing the displacement, difficult to provide much more advice.

    There are plenty of technical papers on catamarans posted by many on this site. Just using the search shall yield many.
     
  3. manon
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    manon Junior Member

    Thanks for your reply Ad Hoc.

    Fn in our case is near 0.4 (0.432 actually, since Lwl is 28.23m) which I could not avoid so far due to the fact that, higher speed than 14 knots will be operable in the inland proposed route. I can play with the length now a bit.
    The displacement is 140 tonnes.

    I will search as you have suggested. Your suggestion and help is well appreciated.
     
  4. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Just type in Molland or Catamaran Design, and many should pop up.

    That means your L/D ratio is 5.5, which is low for a catamaran approaching the main hump. Thus your resistance shall be high and the trim also. You need to design for these features I'm afraid. Unless you can change the design parameters.
     
  5. manon
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    manon Junior Member

    Hello,
    What would be the preferable L/D ratio?
     
  6. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    You really wnat something in the 7.0 range or so.

    Which means either your 28.23m vessel to weigh around 66 tonne
    or
    be a length of 36.4m long at 140 tonne.
     
  7. manon
    Joined: Dec 2007
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    manon Junior Member

    Thanks for your very quick response.

    Its a very big problem than! We cant consider the building material accept Mild steel, for several reasons. Its light weight becomes around 100 tonnes and with passengers, stores and others, total required displacement becomes 140 tonnes.

    So, I can increase the length to 36.4. The question now arise, how much frictional resistance will be increased at this speed limit. I am considering now to reduce the speed to 13knots, increasing the Lwl a bit (29.2m), Fn being 0.395. The Fn still approaching to the hump, is not it?
     

  8. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Indeed it is. That is what naval architecture is all about, coming up with a design to ensure the SOR can be achieved; how ever you resolve the conflicting issues is up to your skill and experience of NA. If it were as simple as pressing a button...every one would be doing it :D

    Good luck :)
     
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