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

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

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

    surfbus justin

    Aloha everyone,,
    the wife is warming up to the idea of getting back on a boat!

    so i need some help with my conceptual drawings before handing them over to one of you fellows to engineer.

    i am trying to commission the most efficent power cruiser possible for 2 families.
    i see no problems with going long and super narrow?

    is it correct to assume the narrower and longer the more efficient or does the efficiency of a high l/b ratio at some point diverge? (my thoughts were to make (the hulls only 2' wide at waterline and only flare slightly but be 8 to 10 feet in height.)

    2nd question, beam and forward and aft spacing of amas?( i am loving playing with the layout on the bridge deck , but should do it scientifically, if possible.

    Also to keep the build cost down I was thinking to have the 3 round bilge canoe keels laid , and a hundred 4'x8' finished flat composite panels shipped to s.e. asia to complete?

    all help/advise and sarcasm will be appreciated!

    lets go surfing!
     
  2. Ad Hoc
    Joined: Oct 2008
    Posts: 6,203
    Likes: 415, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Well, your first task is to finish that sentence!

    You need to write down a full SOR. Once you have that, then and only then can you begin to look at:

    Your SOR will dictate to you what kind of arrangements are possible...which you need to review and then and only then can you establish:

    But by then you'll realise it matters not. What matters is, what arrangement/layout/cost satisfies your SOR. Whether you have an L/B ratio of 1 or 100 is irrelevant. The design needs to satisfy your SOR. Then and only then can the process of refining begin if at all possible.
     
  3. Richard Woods
    Joined: Jun 2006
    Posts: 2,194
    Likes: 149, Points: 63, Legacy Rep: 1244
    Location: UK, USA and Canada

    Richard Woods Woods Designs

    Agreed, you need to write down a full Statement of Requirements before you can think about hull shapes.

    2 families, or 2 couples? How big are the families - 10 people in each - children or grandparents?

    what are your cruising grounds/range/speed?

    So many questions, no sensible answers yet

    The best way to start is always "Use your eyes and plagiarise". Look at all similar boats to what you want and write down what you like/dislike about them.

    Richard Woods of Woods Designs

    www.sailingcatamarans.com (plus a few power cats and trimarans)
     
  4. gonzo
    Joined: Aug 2002
    Posts: 13,772
    Likes: 432, Points: 93, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    That's great advice. Freak designs are usually full of quirks and defects. Shapes have developed through thousands of years of experience and trials.
    Also, can you define "most efficient". It means completely different things to different people. For example, in the world of superyachts, owners call them "green" because the multiple A/C units can be programmed for a 15% economy.
     
  5. surfbus
    Joined: Dec 2014
    Posts: 18
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: oahu

    surfbus justin

    well thanks for the advise guys!

    soon as i get time i will put together my estimated weight i will need to displace, housing and feeding 8 on mostly 700 mile trips....

    but i am assuming that the most efficient seakeeping and lowest fuel burn to float us would be on the submerged 2 foot wide diameter round bilge canoe hull with 6 foot of freeboard under the trimarans wing deck.

    but again , my question is does the curve of hull efficiency ALWAYS increase with the higher length to beam ratios?

    THANKS ALL?

    and does this whole plan sound awesome or what?
     
  6. Ad Hoc
    Joined: Oct 2008
    Posts: 6,203
    Likes: 415, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Ignore, ignore, ignore.

    Sort out your SOR first. A realistic one.

    If you can't decide whether to have blue or red chintz curtains or a gas or IH hob, or carry 100kg or 10 tonne of water etc, L/B's are the least of your worries!
     
  7. Leo Lazauskas
    Joined: Jan 2002
    Posts: 2,696
    Likes: 146, Points: 63, Legacy Rep: 2229
    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    You still haven't answered what you mean by "efficiency".

    No. Because there is no "plan".
     
  8. surfbus
    Joined: Dec 2014
    Posts: 18
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: oahu

    surfbus justin

    wow ,, i really love this,,,you guys are hilarious!
    just hope you guys are still with me.

    for me efficiency means a displacement hull that has a lower power requirement for any given speed and the ability to maintain higher speeds in a deteriorating sea state.

    but to be more specific i would like to go over 20 knts at times or slow down and get well over 1000 mile range when i need to.

    so far i remembered a good about 2/3 the weight ill be moving around to 15,000 pounds :

    8 adults 1440pds
    1 dingy 500pds
    2x honda 250s or 1 diesal 1244pds
    1000 gallons fuel 6000pds
    400 gallons water 3360pds
    80 pds gear per person 800pds
    100 4x8' duflex panels 830pds
    aircon, freezer,cooker 900 pds
    gene,tv

    don't worry you guys can come too when we are done!
    what you think now there Leo?
     
  9. surfbus
    Joined: Dec 2014
    Posts: 18
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: oahu

    surfbus justin

    if I had a realistic SOR, I wouldn't be considering a boat.
     
  10. Richard Woods
    Joined: Jun 2006
    Posts: 2,194
    Likes: 149, Points: 63, Legacy Rep: 1244
    Location: UK, USA and Canada

    Richard Woods Woods Designs

    We don't know how "big" a boat you want, but it is unlikely that 100 8x4 sheets will be enough material. And I doubt if each sheet will weigh 8lbs and be strong enough for the boat you want. I would imagine the weight of glass/epoxy needed for bonding all those panels will weigh more than 800lbs. Why not make 40ft panels? I assume you will ship them in a container to your build site

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

    surfbus justin

    yes true brother, big panels mo bettah!
    i started this same conversation under the boat design forum also , and "Groper" said that after 25 to 1 L/b ,because of wetted surface friction longer starts to give "diminished returns".

    he said all the big racing MULTI sailboats HAVE around 25 to 1...

    looks like maybe we will have to go 75 long 3 wide.....

    and now can someone help with some rough ideas about how long and where those sponsons go?

    MAHALO!
     
  12. Richard Woods
    Joined: Jun 2006
    Posts: 2,194
    Likes: 149, Points: 63, Legacy Rep: 1244
    Location: UK, USA and Canada

    Richard Woods Woods Designs

    The best slenderness ratio depends on what speed you want to do, but unless you plan to go very fast all the time (say 30-40 knots on 75ft) and have limited payload then I wouldn't go more than say 15:1.

    I suggest you stop one thread and concentrate all your replies to the other one, if these threads get much longer (which is what you hope) then both you and us will get in a real muddle

    Richard Woods
     
  13. Leo Lazauskas
    Joined: Jan 2002
    Posts: 2,696
    Likes: 146, Points: 63, Legacy Rep: 2229
    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    That depends on many factors. Among others, the sea states you plan to
    operate in and (because you said you were interested in "efficiency") the
    expected speed profile, i.e. what percentage of time do you expect to spend at
    each speed.
    Your weight estimates was a good first cut but you are still on the first turn of
    the design spiral. There are still several levels of hell to descend through :)
     
  14. surfbus
    Joined: Dec 2014
    Posts: 18
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: oahu

    surfbus justin

    thank you master

    i would like to go 20- 25 knots 70% of the time off the west coast of sumatra where usually there is 3-6 ft of 13 to 15 second interval swell and 1/2 the time there is 1-3 foot short interval chop on top of that...pretty nice boating.

    but in the future i would like relocate to the pacific and thats where i might need some range at lower speeds.

    I just did some basic volume math for a 3 ft diameter by 75 ft long half cylinder
    and a 3 ft by 1 foot high by 75 ft long prism. meaning my 75ft canoe with a 2.5 ft draft...28,816 pds of displacement...... I think we can live with that right? (funny , I think thats about what my 60 ft cat was, but both its vee hulls were 3ft wide at waterline...but she's not mine anymore- so i wana get back in the game.)

    sooo,, how bout those yankees,,,I mean sponsons
     

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

    longcours62 Junior Member

    Did you se the power trimaran of this man ?

    http://www.trawlerforum.com/forums/members/boatgm-2907.html

    Good efficiency in term of consumption and speed.

    Personaly when we are thinkins for the eventual next boat, we also come back to the idea of built a power trimaran, but for exemple Nigel Irens ,said to us the efficiency of power trimaran is not necessary of a power catamaran (the drag of 3 hulls vs the drag of 2).
    In any case we just know to built in metal, and for the total weight for a 21 m trimaran may be the volume of our hull form could handle 15 T ...but we are not abble to put the total displacement under ....19 t !!
     
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.