Determining Beam Width

Discussion in 'Multihulls' started by mcm, Nov 2, 2016.

  1. mcm
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    mcm Senior Member

    Curious, what factors are taken into consideration when designing the C/L to C/L beam width of a catamaran?

    How are these factors plugged into an equation to arrive at the appropriate beam width?

    If we have to assume usage let's assume racer/cruiser or high performance cruiser.
     
  2. catsketcher
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    catsketcher Senior Member

    Probably a big case of "whatever the other nice cats have". 50-60% seems pretty standard in the boats around me in Australia. The slower the boat, the less the beam (in production boats built here in the 80s to 2000).

    One word of caution is going too wide. I used to have a boat that was more stable sideways than fore and aft and it was not really that clever (although I loved it). This is probably why most cats seem to be around about 50% cl to cl. I really would prefer to lift a hull before I sail her over her nose and pitchpole. So keeping beam down is a bit of a safety valve in some ways.

    Go get the tape measure and sail on some similar sized cats. That's pretty much how cats were designed here in Australia for decades (probably still is)

    cheers

    Phil
     
  3. mydauphin
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    mydauphin Senior Member

    I heard 60/40 rule. 60 being length.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Resistance, structure (weight) and stability.
     
  5. mcm
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    mcm Senior Member

    Thanks Ad Hoc,

    Resistance being drag of wetted surface area?
    Structure weight Not displacement?
    Stability including desired righting moment?

    And how would these look in an equation to derive appropriate CL-CL beam?
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    1) Residuary resistance increases with decreasing separation.
    2) Increase in structure = increase in weight = increase in displacement
    3) Angles to down-flooding and deck edge immersion, to pass rules.
     
  7. Richard Woods
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    Richard Woods Woods Designs

    There are no "rules" as such, and it is too complicated to give generalities, but adhoc is right. A wider boat means more structure and thus more weight, which offsets any wave interference reduction or stability increase. So a wide boat works better as an open deck boat rather than one with a bridgedeck cabin

    A wider boat will need more wind to fly a hull, generally good for cruising, not so good for racing

    And there are practical reasons as well, like shed door width, marina berth, travelhoist, road transport etc. In the same way planes are designed to fit in hangers and ships to go through the Panama Canal

    You are right, it is CL spacing and LWL that are important, not overall beam/length.

    The boat with the widest proportions I have personally built/sailed was the Strider Turbo, 22ft LWL, 17ft BOA, so wider than say a 28ft Telstar trimaran. It sails really well with no fear of nosediving/pitchpoling

    Power cats tend to be narrower, in part for the docking problems, but also because they run at set speeds and of course don't have the sail carrying power issues.

    Talk to Brandon at Turnpoint or Russell Brown as they both sail very narrow, fast catamarans with bridgedeck cabins

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  8. mcm
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    mcm Senior Member

    So designers don't necessarily start with righting moment as the first factor in determining C/L - C/L beam?
     
  9. Petros
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    Petros Senior Member

    there are a lot of factors that affect choice of beam to lenght ratio. with the same hull design, making it wider means it is more likely to drive the bows down and pitch pole rather than raise one hull. flying a hull is much easier to manage while on a reach, usually once a pitch pole starts, the bow dives below the surface, there is no stopping it (I have experienced both). In my limited experience I would tend to want more pitch pole resistance.

    Most designers seem to settle into about 50 percent, though it varies. Rig size and height, the amount of flotation of the bows, weight, and a lot of other factors will affect it. I have read a lot of experienced designers consider 50 percent beam/LOA "ideal". Though in reality there is no such thing, but 50 percent seems to be a recognized acceptable ratio.

    good luck.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    No not at all.

    The successful design of any boat/ship is greater than the sum of its individual parts combined. In other words it is all about compromise. You must 'design' the boat from the SOR and then see what compromises need to be made to ensure your design satisfies the SOR. Every SOR is different and thus every design solution is different. But it is what YOU make it to be.
     
  11. catsketcher
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    catsketcher Senior Member

    As Petros said there is not hard and fast rule. When he wrote about his part in designing Dennis Conners AC cat in the 90s Britton Chance said he wanted HALF as much stability sideways as fore and aft. When I did an article on an Sodebo 10 years ago Irens said that fore and aft stability is a bit of a nebulous concept. Adding more buoyancy can INCREASE the tendency to pitchpole by adding drag. So a boat with a higher static longitudinal stability can be less stable dynamically (when it starts to poke its nose in). The ORMA 60s were tough boats to sail, one reason was the box rule meant they were more stable sideways than fore and aft. There is a good video of Sodebo (the long record breaker) doing everything on its own to stop a capsize by going the low drag bow route.

    https://www.youtube.com/watch?v=QP-67_sPgGc

    On a cruising boat you want loads of longitudinal stability. Because you are at the mercy of the boat and the wave when longitudinal stability is essential you must design in more than sideways stability where easing sheet can help the boat back on its feet. For your boat I think that you are pretty well balanced. You will probably never lift a hull with the boat near 50% in cruising trim. She is pretty heavy and wide.

    If you wanted you could do some quick calcs on beam required not to fly a hull in full sail in 30 knots. Plug in your weight and beam and CE and see what beam gets you safely up to a good breeze without pulling a hull out. I would think that your boat at 50% would be good for well over 30 knots.

    Then calculate the force generated by the main and screecher on a square run in say 30 true. This is the worst case scenario where you get hit by 35 knots and don't furl the screecher quickly. Then you stuff the nose into a wave and the boat slows down from 10 to 5 knots - hence 30. If your computer package will allow it you will be able to calculate the longitudinal stability just as the bows immerse. If not you can rotate the model till the bows are immersed and calculate the distance netween the CG and new CB. This will give you half of the stability. Of course in reality a square run will blanket the reacher so you may try a broad reach where almost all of the buoyancy is generated by the lee bow and the main and reacher are pulling. A full power broad reach is pretty dangerous in big waves.

    If you find that your bows are going to be under a lot then you may want to redesign them with more crown to shed water quicker. If you don't think they will go under then you can have a nice flat deck.

    I like flat bows for walking on with a cruising cat so I would have high bows with flat decks and hopefully a high enough longitudinal stability for the decks never to get very wet. A rig low enough so that it doesn't overpower the bows and enough beam to keep the hulls in the water till the sails threaten to blow the rig off - about 30 knots.

    Actually my cat is slightly under 50% - 5.5 metres wide cl to cl and LOA 11.6m. Seems like heaps to me. I have never lifted a hull and never felt close, I get scared of the loads unreefed above 20 knots true so we go to the staysail at about 15-18 and reef at 22 ish. Never had the bows get close to going under on a square, even when surfing. Kankama has a pretty low rig at 14.1 metres mast length. Enough for me though.

    cheers

    Phil
     
  12. mydauphin
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    mydauphin Senior Member

    Also consider dockage, a wider boat is harder to find slips for.
     
  13. Richard Woods
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    Richard Woods Woods Designs

    mydauphin, see my post above. Over wide multihulls can be very expensive to slip.

    MCM, as Adhoc says, design is a spiral, you cannot ever take one thing in isolation. And when drawing a boat expect to throw everything away at least once after you discover a major flaw where no compromise will work. Thats why (as I have said many times) professional designers draw the lines plan last as until the details have been decided you don't know the real weight, CofG position etc

    Phil - I think you and I are now in the "there are no old, bold sailors" category. Tornados with flat decks can sail with them submerged. I agree re rounded barrel shaped decks and of course a reverse bow can cause major problems when docking a cruising boat - especially short handed.. Newer designs of beach cats and tri outriggers seem to go for the "pitched roof" approach (as I have also done)

    As Adhoc (and others) will know, various navies round the world did lots of research on fast warships in the 1950's/60's. Some of those papers relating to flare/freeboard/wetness are available online

    Richard Woods
     
  14. mcm
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    mcm Senior Member

    From the sound of the majority of replies, longitudinal (fore and aft) stability is a more important factor than transverse stability.

    Concerning beam dimensions what characteristics would improve longitudinal stability and which characteristics undermine longitudinal stability?

    By characteristics I mean CL/CL width or even beam location.
     

  15. tspeer
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    tspeer Senior Member

    John Shuttleworth's stability factor can be a big help in sizing the beam and the rig.
    [​IMG]
    Where :
    D = displacement (lbs).
    CE = height of the center of effort above the center of lateral resistance (CLR) in feet. Use C of E to Waterline for quick calculation.
    SF = windspeed in MPH that the boat has to reduce sail.
    SA = sail area in square feet.
    B = beam between the cenerlines of the outer hulls in feet.

    The beam vs length is determined by how much stability you need when sailing
    off the wind. Here's what Shuttleworth has written about forward and diagonal stability vs sideways stability:
     
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