Stability/Seakeeping of Deep V catamarans

Discussion in 'Stability' started by waterbear, May 3, 2019.

  1. waterbear
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    waterbear Junior Member

    There's a certain designer of Deep V catamarans who would have you believe the Deep V hull section has the best sea keeping properties, and that other hull shapes are trading safety for performance/amenities/etc. Although I've never seen one of these boats sailing in person, I've been aware of the designs for decades and the seaworthiness claims always sounded plausible to me.

    Fast forward to today, I can watch his designs sailing on youtube, and I've noticed they exhibit a surprising amount of pitching. This has me wondering if the V hull section really has ideal seakeeping properties.

    This designer's boats seem to have a good safety record, but they also have low windage, low centers of gravity, and low center of effort sails.

    All things being equal, are deep V hulled catamarans more seaworthy than catamarans with other hull shapes, eg the Dory? With other shapes the designer has more control over the distribution of volume in the hull, how does this factor in?
     
    Last edited: May 3, 2019
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Seakeeping is a terribly complex subject. It crosses over into many different disciplines of naval architecture.
    Shape is just one attribute to consider and is not the sole reason. Thus claims of such are extremely biased and subjective to say the least.

    If one wished to, and i am not suggesting one should, but if one wished to boil it down into small bites size click bait chunks, it revolves around i) the weights and centres - i.e. distribution of CoG vertically and longitudinally and ii) the waterplane area and the inertia of said waterplane area.

    Again, this is a click bait bite sized dumbing down....thus treat with caution.

    ERGO claims of such superiority are just that - claims only.
     
  3. BlueBell
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    BlueBell Ahhhhh...

    WateryBeer, no, sorry, guilt by association...

    Waterbear, I wish it were so easy.
    Hull shape doesn't really matter as much as the way the whole system works under the circumstances.
    Length, beam, weight, CoG, etc, etc, etc... it's endless, and hard to quantify.
    Best trade-off in design on that particular day.

    So, what length of cat are we talking about here?
    Australian?
     
  4. Dejay
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    Dejay Senior Newbie

    How would one even approach seakeeping?

    I thought one could write maybe a simplified but super fast CFD simulation and then run 1000s of runs with the virtual ship in different sea states and winds from different directions and then record all the motions, slamming loads, capsize and near capsize and pitchpoling events, deck floodings. And then make some sort of statistical comparison. Maybe precision isn't as much needed than comparatively plausible results.

    Intuitively I would speculate that a V section above the waterline leads to harder movements while vertical sections are much softer and gentle on the motions.
     
  5. waterbear
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    waterbear Junior Member

    Bluebell, I'm thinking small ocean cat, maybe 30-40 feet long?

    Obviously there are differences right? I'm not a NA, but I can think of some. Eg the deep V has more volume down low, which means a lower center of buoyancy (?), which could mean less stable (if only slightly).
     
  6. Dejay
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    Dejay Senior Newbie

    Center of buoyancy should have almost no effect at all on stability for a multi hull. Except maybe for a SWASH type vessel. Center of gravity does if you build too high. Look up metacentric height, lots of good info there, the spacing and length define the second moment of area. And a square or round bottom hull would have more volume, a V has almost the lowest volume below.

    Also stability is different from seakeeping / seaworthiness. And then the most difficult and hard to quantify subset of that is how the motion affect life on board, e.g. seasickness and your beer not falling down and getting all watery :)
     
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  7. BlueBell
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    BlueBell Ahhhhh...

    Lol, good one Dejay!
    The watery beer thing was more about USA beer vs Canadian beer.
    Generalization and just kidding around, love beer, but, can't drink it... barley.

    Anyway, Waterbear, deep V hulls have their attributes and their drawbacks.
    Generally, they track well, point well, ride well but can be slow in light airs.
    I have no idea who's design you are referring.
    Can you post a youtube link or give us a riddle to solve in order to figure out what the hell you're talking about?
    I realize you don't want to slam the guy.
    Perhaps we can be more constructive in our diatribe, than critical, of his/her designs.
     
  8. BlueBell
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    BlueBell Ahhhhh...

    This is not correct.
    It is the opposite!
    A "U" shape hull has a lower centre of buoyancy,
    and a square cross-section hull has the lowest centre of buoyancy.
    But it's a catamaran so it's not overly important where the centre of buoyancy is
    in each hull.
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    Indeed, since if two systems are not the same, ipso facto, the outcomes shall be different. How much different, is the matter at hand, not the difference, per se.

    Now you're getting into the realm of religion based science - I think - i feel - etc etc. Based upon nothing other than a subjective thought.

    As I noted seakeeping is terribly complex and cannot be boiled down to a sound bite.
     
  10. waterbear
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    waterbear Junior Member

    Ok, that's not intuitive to me..

    Say we have two hulls with the following dimensions below the waterline:

    1. Equalateral deep V section, 4 units / side

    2. Rectangular section 4 x √3

    Both sections have a waterline beam of 4, and an area of ~6.93

    But the geometric center of the V is √3 below the waterline, vs √3/2 for the rectangular section. Is √3 below the waterline not lower than √3/2?

    Regarding the designer, your right, I'm not interested in bashing anyone. Think Polynesian style catamarans with beam lashings and naked women lounging about.
     
  11. rwatson
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    rwatson Senior Member

    I knew who you were talking about after the first sentence. Mr W makes a big deal of the "seakeeping" of his boats, but as Ad Hoc hinted at, there is a lot more to it.

    The most obvious consideration is that the smaller W cats, have no navigation station. The idea is that its less windage. But, if you have ever been steering a boat for more than a couple of hours in cold and spray laden wind, the boat might survive, but you may die of hypothermia. As a result, there are very few ocean going W Cats that don't have a home designed "dog box" on them. Likewise, their "add on" steering stations are invariably less comfortable and cramped than modern "bridgedeck" designs - and if you are trying to keep navigation charts and electronic gear arranged on a rough night while making landfall, what does that add to the safety factor ?

    The other "safety" thing is low aspect rigs of that design. But, if you take an extra day out on the water than a high aspect rig would take, that's one more day of exposure to the elements. You might miss a storm by being faster. Add to that, engine size or even lack of an engine. Generally speaking, the bigger the engine the safer you are.

    The V hulls have two other profound characteristics. Severe space limitations is one, even with the more modern stressed curve designs. It's like living in a coffin. But, I can verify that the V hulls are very smooth riding in waves, with excellent directional control.
     
  12. Dejay
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    Dejay Senior Newbie

    I think the centroid of the triangle is (y1+y2+y3) / 3 so be 1/3 of the lowest point below the waterline so it's 4 * cos(30) / 3 = ~1.15
    Vs √3 = 1.73

    Sound tactic, strafe and go, then feign innocence.
    I think you'll fit right in here! :p

    But wrong math. You should have instead used a much deeper triangle vs the rectangle instead, then the center of buoyancy would have been indeed deeper!

    I'm just kidding of course and I don't think "He-who-must-not-be-named" really minds us discussing the pros and cons of his beautiful "ticky tacky'ing" boats ;)

    You initially mentioned pitching, maybe that is something that can be more meaningfully answered than general seaworthiness. There is a recent discussion about it here on the forum.
     
  13. BlueBell
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    BlueBell Ahhhhh...

    "Ticky-Tacky'ing" must be a clue...
    "Mr W"... RW?, no, those are stabilized monohulls (aka trimarans).
    Sounds like a "Down-under" dude to me.
    How about RD, no, those are proahs...
    Come-on, somebody post a picture, a sketch, anything.
    I give up.

    Centre of buoyancy of a cross-section is simply the centre of area.
    Simple, no math needed and painfully obvious which has the lower centre, no?
     
  14. waterbear
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    waterbear Junior Member


    You're right, the centroid of the triangle is 1.15 below WL. However center of the 4 x √3 box is √3/2 (not √3), making the centroid of the box ~0.87 below the WL. So in this particular case, the CB of the V is still lower, right?

    Also, thanks for the link on pitching.
     
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  15. waterbear
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    waterbear Junior Member

    No, not a "down under" dude. Edit: Also not RW.

    I'm comparing an equilateral triangle cross section with a rectangular section of the same area and waterline beam. I'm not sure what the optimum rectangular cross section is in terms of wetted surface area, but 4 x √3 is probably in the vicinity.

    Maybe you're thinking of a square with equal cross sectional area? Or a square with the same depth?
     
    Last edited: May 6, 2019
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