Multihulls and 70' waves

Discussion in 'Multihulls' started by ImaginaryNumber, Oct 19, 2012.

  1. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    Armchair planning typically comes before successful sailing.
     
  2. cavalier mk2
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    cavalier mk2 Senior Member

    When you add up every variable safety at sea is only assured by not going near the water.....of course then you have a different set of things to worry about.
     
  3. Corley
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    Corley epoxy coated

    It's very unusual for that time of year and as noted by Frosty wave heights are often exaggerated and as mentioned in the Fastnet report even a much smaller breaking wave could have had the same effect. A 40' breaking wave from the deck of a boat looks enormous. I'm not saying that the sailor in question was necessarily wrong, I wasn't there I'm just noting that a far smaller breaking wave could have had the same effect and it's hard to make an accurate judgement of wave height from the deck of a low freeboard yacht.

    Rogue waves exist and have been confirmed by satellite studies and recorded events on ships and oil rigs however the effect is temporary and generally returns to a state of equilibrium with surrounding waves in a short period of time. Strong wind and opposing currents increase the possibility. A good reason to avoid storms and dangerous areas as much as possible.

    Here is an interesting paper on rogue waves and attempts to predict their formation with a mathematical model

    http://arxiv.org/pdf/1207.2130v1.pdf

    and some further reading

    http://surfspots-gps.com/oceanographers-scientists-study-cause-of-monstrous-rogue-waves
     
  4. brian eiland
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    brian eiland Senior Member

    Don't know that I agree with you there, There may be some small differences with a sailing vessels mast in inertia terms, but not that much difference with a sailing vessel with no sails up.

    I'll take a look there when I get a little more time.
     
  5. MikeJohns
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    MikeJohns Senior Member


    Brian, here's some pictures for you! And you should read that thread.

    Both craft have similar construction both are Crowther 48 foot designs one a power cat and one a sailing cat.

    The power cat has over twice the displacement a lower CG and a narrower beam, it also has a very full bridge-deck that extends right to the bow. The hulls have considerably more reserve buoyancy and have completely different volume distributions ( Cp, Block coeff ) these sorts of power design are very robust performers in a rough sea providing the CG is kept low, the effective ballast of two large engines help (480 HP to the sailboats 32 !)

    The sailing vessel's only similarity is the length, the number of hulls, and the designer. Frankly anyone using the argument that the power cat wave tank RAO applies equally to other multihulls needs to learn some basic naval architecture.
     

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  6. peterchech
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    peterchech Senior Member

    Any na will tell u that it doesnt work if u scale a boat plan up or down by more than 15% or so, due to some rule of physics. Please someone here elaborate. So why would a tiny model in a tank stability test be an accurate way of testing hull shapes?
     
  7. harrygee
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    harrygee Junior Member

    I deliver sailboats on that coast, mostly to Tasmania across Bass Strait.
    There is nearly always a current flowing south, rarely more than 3 knots but I've had 5 knots at times, with a lot of eddies off the headlands.
    Northern NSW gets the worst of it, between Coffs Harbour and Seal Rocks.
    The hardest conditions I've experienced were off Cape Moreton in a hard southerly gale against a strong current. After tearing out fittings and losing my parachute anchor, I lay ahull in a 30' catamaran, surfing sideways in big breaking seas for a hundred miles until conditions eased.
    Thought provoking.
    Conditions were worse than I experienced in a cyclone off Fiji.
    I take my hat off to the bloke who found himself in trouble. He may have made a bad decision to put himself in that situation but he did what he could to get out of it.
    Harry
     
  8. Autodafe
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    Autodafe Senior Member

    Hi Mike,

    I'm curious why you feel lowering VCG and and increasing reserve buoyancy have such a large beneficial effect on catamaran stability?

    In as much as lowering CG increases AVS it must be good, but this is offset in the power boat by reduced beam (reducing AVS for the same VCG).
    A higher CG also provides some small benefit in inducing a righting moment (due to acceleration of the boat by a force below the CG) in addition to the (unchanged) overturning moment acting on CLR.

    Reserve buoyancy forward (in proportion to total displacement!) is beneficial against pitchpole certainly. In other areas it seems to simply increase the area (and hence force) of a wave strike.

    Based on my reading and (very crude) numerical modelling of wave dynamics I would have theorised from the two pictures the sailing cat would by slightly more seaworthy than the power cat, but the two would be fairly similar.

    The benefits of low CG and high displacement for a monohull are clear but I can't get the physics to show a similar relationship for catamarans.

    In as much as the main factors affecting catamaran capsize are beam and AVS, I would have thought power cat and sail cat modelling of roll were fairly representative of each other. Note Southampton U has conducted similar tests of sailing multihulls with similar results, somewhat limited by maximum wave size in the tank.

    You obviously have a different take on it, I'd be interested if you care to expand your reasoning.

    Cheers,
    George
     
  9. Autodafe
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    Autodafe Senior Member

    Peter, the people who do the scale modelling put a lot of time and effort in to making sure that they correctly scale the model parameters they want to study.

    Any given model would not correctly represent all the variables of the full size boat, but it doesn't have to.
     
  10. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    What time of year was it when you had your experience off Cape Moreton? Would you say that one should be particularly cautious sailing that area in October?
     
  11. Autodafe
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    Autodafe Senior Member

    Mike,
    re-reading your post I see you are only referring to RAO, in which case I agree that the two catamarans are different. However in large waves I would take it that RAO of all catamarans is 1, so the differences would have little impact on motion.

    I think Brian was talking about breaking wave strike, which is another subject.

    George
     
  12. Red Dwarf
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    Red Dwarf Senior Member

    I think a power cat could have one big advantage over a sail cat. The power cat does not have a keel or dagger boards. This will increase the likelihood of sliding down the face of a large wave. Of course, if possible, the sail cat could raise its dagger boards.
     

  13. harrygee
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    harrygee Junior Member

    Hi

    I wouldn't think that October would be a bad time to be out there.
    I got beat up in July, about twenty years ago. At least three other boats, that I know of, were damaged or sunk with several fatalities. A cat off Coffs Harbour capsized after half-filling, with one fatality, a man dropped from a jammed helicopter winch when the cable was cut. A 40' mono went missing out from Sydney with several on board.
    A southerly buster can hit any time, mostly in the warmer months.
    I've sat out a lot of rubbish weather in NSW ports but you have to be able to get into port before the weather hits - there are few all-weather entries and some of those are pretty dodgy.
    It's probably no worse than any other area but the current and the challenging entries keep a lot of people from cruising the area. Though it's Australia's population centre, there are only 10% of the cruising boats that would be found on the Qld coast.
    My last-resort tactic of laying ahull worked but I'd already exhausted myself faffing with the parachute anchor and then hand-steering for a hundred miles in ten hours under bare poles. That kind of sustained fear is exhausting, hence my respect for this bloke who's stuck it out for 9 days.

    Harry
     
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