l/b ratios: the longer the better? trimaran float placement?

Discussion in 'Hydrodynamics and Aerodynamics' started by surfbus, Dec 4, 2014.

  1. longcours62
    Joined: Jul 2011
    Posts: 92
    Likes: 4, Points: 0, Legacy Rep: 47
    Location: France

    longcours62 Junior Member

    We already contact Nigel Irens ...


    But 14 years ago and his message was : " not sure than the drag of 3 hulls was better than the drag of 2 "
    Clealy this man with great experience think catamaran could be à better choice and he send to us a drawing of a catamaran nice silhouette of ...30 m and this comment : "sorry I don't know drawing "small" boat" :):eek:

    Actually we are in the process to sold our boat (if the potential customer don't change this decision !!)
    But as wrote before , we just know metal work and not rich (enough ?) for order a hull in "exotics materials" to a boat yard.:mad:
     
  2. longcours62
    Joined: Jul 2011
    Posts: 92
    Likes: 4, Points: 0, Legacy Rep: 47
    Location: France

    longcours62 Junior Member

    A project made by a collegue

    You can find below some view of a power trimaran
     

    Attached Files:

    • 10.jpg
      10.jpg
      File size:
      42.6 KB
      Views:
      750
    • 14.jpg
      14.jpg
      File size:
      47.8 KB
      Views:
      653
  3. groper
    Joined: Jun 2011
    Posts: 2,467
    Likes: 123, Points: 73, Legacy Rep: 693
    Location: australia

    groper Senior Member

    The design parameter that makes any of these types of slender hulls work efficiently, is the displacement to length ratio. It is THE most important design ratio with regards to hull resistance. It doesn't matter if it is a trimaran or a stabilized monohull or a catamaran, if it is very light, and very long it will be extremely efficient in the semi displacement speed regime, for our purposes in the hull lengths already discussed, say between 10 and 20kts.

    Building in aluminium should be fine, and allow a reasonably lightweight structure. Sandwich core Composites would also be a good choice. Don't be fooled into thinking just because it looks fast, and that it's a trimaran, means it will be fast and or efficient automatically. There are fundamental reasons for it.

    What I am trying to say is, if you need this boat to carry a large displacement, then perhaps a stabilized monohull does not fit your purposes ideally. If you attempt to build a design like this, and it ends up very heavy, you will have a very poor performing vessel that you wish you never wasted your years on. The displacement to length ratio is key in these types of designs - which allow higher cruising speeds than classical displacement vessels, with still minimal fuel consumption.
     
  4. longcours62
    Joined: Jul 2011
    Posts: 92
    Likes: 4, Points: 0, Legacy Rep: 47
    Location: France

    longcours62 Junior Member

    You are right

    but we are leave a board guys (and woman !!), it means when we make a list of weight:
    - alloy
    - engine, filter, shaft ,propeller, accumulator, sound insulation etc
    - insulation
    - wood, plywood or cored material
    - books,
    - solar panel
    - hatches, plexiglass
    - anchor , chain, ropes
    - windlass, capstans
    - tools
    - heater
    - watermaker
    - water tanks
    - diesel tanks
    - electrical wire
    - food (and food for our dog !!)
    etc etc
    We can't go under 20 t full loaded, yes the L/D ratio is not big : around 55
    But for a trimaran or stabilised monohull it is a lot compared whit for an "extreme" example "ilan voyager".
    But in any case it is not our "target", but as you wrote before able to cruise somewhere between 10 to 20 knts, say 15 knts with a "low" consumption of around 2,4lt per nm, and for exemple 10 knts @ 1lt per nm.
    May be it is "voie sans issue" , may it will be better to keept the same drawing than our actual boat (we can "decrease the weight under 30 t) with the same volume (even little less) but for 22m loa :confused::?:
     
  5. groper
    Joined: Jun 2011
    Posts: 2,467
    Likes: 123, Points: 73, Legacy Rep: 693
    Location: australia

    groper Senior Member

    You have your answer- simply make the boat longer if you wish to carry more displacement... You can compare benchmarks of the similar vessels outlined earlier. Plot the displacement to length ratios of them on a graph, then once you see the range in which successful designs have been implemented, you will have an idea of how long you should make your boat in order to carry an amount of displacement you want.

    Once you have a rough idea of that, you should then consider where you will build it, where will you keep it, and what slipways can handle it for maintenance etc. and of course, can you afford it.
     
  6. Richard Woods
    Joined: Jun 2006
    Posts: 2,197
    Likes: 149, Points: 63, Legacy Rep: 1244
    Location: UK, USA and Canada

    Richard Woods Woods Designs

  7. WestVanHan
    Joined: Aug 2009
    Posts: 1,374
    Likes: 56, Points: 0, Legacy Rep: 746
    Location: Vancouver

    WestVanHan Not a Senior Member

    I'm still a little curious as to how much you think it will cost....
     
  8. Ad Hoc
    Joined: Oct 2008
    Posts: 6,203
    Likes: 416, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    I don't think he has still really grasped the concept of what is the SOR and what it really means either!
     
  9. Leo Lazauskas
    Joined: Jan 2002
    Posts: 2,696
    Likes: 146, Points: 63, Legacy Rep: 2229
    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    Agreed. It sounds a lot like wishful thinking, and not a serious project with money behind it to make it happen.
     
  10. longcours62
    Joined: Jul 2011
    Posts: 92
    Likes: 4, Points: 0, Legacy Rep: 47
    Location: France

    longcours62 Junior Member

    The cost for one hull is (depending the country ) from 26€ per kgs in France , 15€ per kgs in Lituanie, 10€ per kgs in Vietnam

    For the another cost after building 3 boats ( not in reality 2,5 the big alloy hull was sold before the arrengement was builts but already move by herself at sea )
    - Steel ketch (round bilged) 18,26 m loa 17,40 m lod, 5,25 mbeam
    Displacement 34T (et yes !) lifting keel, twin 90 hp Perkins
    - alloy hul 26,86 m loa 24,99 lod, 7,10 m beam, twin Perkins 185 hp
    - alloy passagemaker 19,46 m loa, 19,165 m lod, 5,03 m beam, displacement 33,1 T, twin Perkins 215 hp

    Own 14 boat during the last 38 years and rebuilt 2 of them .

    Sure I am not a specialist like you guys , but I thinks I know what is a boat and how much it will cost for built...;)

    Depending your choice if (for exemple) you buy a Dashew 64' at nearly 3 millions usd , you are thinking all boat of this type will cost 3 millions to built...sorry it is not "exactly " the reality.
    If for exemple on the cost of our actual boat we put ... say 1 million USD more for nicest electronique , naiad, nice varnish we will stay far , far bellow the 3 millions:eek:
     
  11. WestVanHan
    Joined: Aug 2009
    Posts: 1,374
    Likes: 56, Points: 0, Legacy Rep: 746
    Location: Vancouver

    WestVanHan Not a Senior Member

  12. longcours62
    Joined: Jul 2011
    Posts: 92
    Likes: 4, Points: 0, Legacy Rep: 47
    Location: France

    longcours62 Junior Member

    @WestVanHan

    Thanks for the link we had , may be 30 years ago, friends who built near similar drawing , say "tri-motor sailor" but around 15/16 m length and use her for chartering in Caribeen during all this years.
    May be you are right it could be a more reasonable choice, but this drawing look little "dated".
    Looks like the French trimaran Architeuthis who participate at the Ostar in 1972 and after cruise long time in Carribeen and Amazonia
     
    Last edited: Dec 10, 2014
  13. longcours62
    Joined: Jul 2011
    Posts: 92
    Likes: 4, Points: 0, Legacy Rep: 47
    Location: France

    longcours62 Junior Member

    Trimaran means tris hulls :)

    But for the two smaller one, I saw lot of choice, each designer put them at different place middle, middle aft , aft ...And the length of the amas change a lot depending of the designer : very short one, 1/3, 1/2 or even 1/1 of the main hull...
    Where is the true ? Because each designer claims his choice is the best and the only one ....

    I found on the net some test but for a "little" bigger boat;)

    And some photos with different choice:?::!::(
     

    Attached Files:

  14. surfbus
    Joined: Dec 2014
    Posts: 18
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: oahu

    surfbus justin

    floats

    so glad to see this thread moving again.....i am designing my boat 80-100 foot, but with only accommodation of a 40 foot boat.....I will have navitec here in hawaii do the engineering
     

  15. surfbus
    Joined: Dec 2014
    Posts: 18
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: oahu

    surfbus justin

    ama phenom

    wow! did that paper say that with optimum placement of ama , drag of the ama is negated????
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.