Max L/B ratio for a fast cat ?

Discussion in 'Boat Design' started by ryanonthebeach, Dec 26, 2017.

  1. ryanonthebeach
    Joined: Mar 2007
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    ryanonthebeach Junior Member

    Hey All

    Seasons greetings.
    Question: How long can you practically go on L/B with a cat.
    I've been searching around quite a bit and haven't found anything concrete on L/B except for general ranges of over 16 or over 20 for a racing cat. Are there tank tests out there? Looks like the AC72's are around 25?
    Background:
    Working on my own design for an ocean going cat.
    Accommodations are very modest, and can be on the bridge deck only, 2 ppl, hulls for storage, engines and tanks, so they can be very narrow. Since length has many advantages out there I'm interested in seeing how much I can stretching it. Yes, I much prefer to anchor out.
    Would an average L/B of 30 be crazy loco? 60X2
    What are the practical limitations,
    More loading on the bridge-deck/beams?
    More windage? (can make the hulls fore and aft of the beams with low free-board as possible)
    Too shallow a draft? (is this even an issue)
    More materials cost?

    Thanks!
    Ryan
     
  2. Richard Woods
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    Richard Woods Woods Designs

    L/B ratio will be determined by your required displacement and maximum length you want. A longer boat tends to have a bigger rig which will certainly cost more than the hull shell

    Too fine a hull increases WSA relative to a fatter one so is less suitable in light wind areas

    As I and others always say, don’t concentrate on just one feature. Consider the boat as a whole and assume you might well throw the whole design away and start again

    Richard Woods of Woods Designs

    Www.sailingcatamarans.com
     
  3. ryanonthebeach
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    ryanonthebeach Junior Member

    Thanks for the reply Richard. I like the look of your Rhea design that's in the range of what I'm thinking just want to stretch it a bit.
    I'm not completely myopic about one feature but when it comes to ocean sailing, all else being equal, size matters, a lot.
    I'll take some time to solve, parasitic drag vs wave resistance
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    With the objective being,....???
     
  5. ryanonthebeach
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    ryanonthebeach Junior Member

    Safety/comfort and speed in big seas
    Most boats today seem to be designed to fit as many people and stuff into for weekend sailing and maybe head into blue water on occasion which is great, for most people. The 40 foot cats are massive inside, way more than I need. I'm looking at it the other way around and would like to start by finding the optimum L/B (catamaran, tri or Proa) for my mission parameters which is ocean sailing with a small crew or single handed, Cross oceans quickly, safely, including in the less forgiving latitudes
    Would prefer something around 60 feet (if financially feasible) with the accommodations of a 30 footer or less
    As it's always a trade-off, willing to trade off :
    Accommodations for cost
    Not staying in a marina, everg
    Motor-sail in light winds

    Assuming as L/B increases, parasitic drag will become more of a factor at lower wind speed, although the top end will increase.
    Busy laying out a hull in prosurf so I can get some hydro calc's on it... will post once I've managed to figure that app out.
     
  6. jorgepease
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    jorgepease Senior Member

    That's my train of thought as well, right now my design is about 68 feet. LB ratio of about 1:17 but my hull also provides for some reserve buoyancy. I am trying to be as accurate as possible with the weight so she sails at her optimum trim most of the time.
     
  7. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Well, you are approaching this as any amateur would do, ...and it is totally incorrect.

    It is your prerogative to do so, but, if you want any advice from forum members that do this professionally, you should take heed of such advice.

    Happy number crunching and wasting time after you figured out why it is a waste of time...since you're already set in your mind about how what and why...with zero validation of said.

    Good luck...
     
  8. ryanonthebeach
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    ryanonthebeach Junior Member

    What a breath of sunshine
    You may win the most helpful post of 2018

     
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  9. Ad Hoc
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    Ad Hoc Naval Architect

    Thank you for the QED :)
     
  10. Richard Woods
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    Richard Woods Woods Designs

    It is certainly worth taking notice of Adhoc. He has been designing catamarans professionally for over 50 years and is a very qualified NA

    Not sure what profession you practice but I suspect you would have similar comments if an amateur tried to tell you how to do your job

    Richard Woods
     
  11. jorgepease
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    jorgepease Senior Member

    ... I don't think he tried to tell anyone how to do his job, he just asked a question. Sucks when a thread that could lead to some helpful insights for the - amateurs - goes south ... Or is this forum strictly for NA's and Engineers.
     
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  12. TANSL
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    TANSL Senior Member

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

    Nope..it is for anyone especially those without an ego or an axe to grind..then...aahh..as is a different perspective for them.

    The difference is...those with an open and enquiring mind and a willingness to recognise one's own limitations, ehcne seeking assistance...and those...with a predetermined fixation of what/where and why and just require someone to say..hey.yeah..that's it...as validation.

    There are no emotions in engineering...just facts. Sadly many do not understand this..no matter how much protestation they sprout as their "gut" feeling on such matters.

    Go figure...
     
  14. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Thanks for the vote of confidence Richard...but as you know...i seek no adulation, im here just to offer advice where ever wanted. If someone wishes to ignore advice it is their prerogative.
    Just begs the question..why do they bother coming on this forum seeking advice from those with more knowledge than themselves in the first place if they ignore it..!!

    Happy New year to you too...
     

  15. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Ryan,

    You're giving up planing so it better be as slippery as possible.
    I'd try 60x2.
    Hard to keep the weight down though, and THAT is the real enemy of speed.

    My current design is 1:16 monohull with a really low centre of gravity.
    I decided on a flat, dove tail stern that's above the water line, static.
    Buoyancy against squat was the main incentive there.
    It's about the same WSA as a canoe stern.

    You have sailed on the open ocean before. right?
     
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