The perils of edgy design offshore

Discussion in 'Sailboats' started by CutOnce, Jul 18, 2011.

  1. Doug Lord
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    Doug Lord Flight Ready

    The Perils of Edgy Weather in Any Boat

    ===================
    You may have missed the bottom of post 38. There is also more on this on SA under "Sportboats": http://forums.sailinganarchy.com/index.php?showtopic=124386
     
  2. magwas
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    magwas Senior Member

    This U-20 story signifies that handheld GPS and cellphone (near coast) are safety gear, and good to have some at hand, waterproof.
    Which waterproof phone models have a built-in GPS? (I have a B2100, which is able to make picture at 2.5m depth, but have no GPS, and hangs with any GPS software when I try to use a bluetooth receiver.)
     
  3. Doug Lord
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    Doug Lord Flight Ready

  4. Doug Lord
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    Doug Lord Flight Ready

    From SA today-Wingnuts recovered:

    It was recovered yesterday and is sitting at Irish Boat Shop in Charlevoix.

    We were doing our Wednesday night race last night, and saw Towboat US coming in with the Wing Nuts in tow.

    Looks like the engine either was taken, or fell off.... none of the tethers were on board, and there was crime scene tape still attached to the boat. My guess is much of the safety equipment has been removed for evidentiary purposes.

    I was advised by the guy who brought her to Charlevoix, that the boat was located floating on Gray's Reef, then taken/towed to Naubinway (Upper Peninsula - Michigan). The boat was then towed yesterday afternoon to Charlevoix and was sitting in the hoist well at Irish. Insurance adjuster was supposed to show within the next day.

    That's the story.

    DG
     

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  5. Paul J. Nolan
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    Paul J. Nolan Junior Member

    We've certainly come a long way from the Smeetons and John Guzwell.

    Paul
     
  6. Steve W
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    Steve W Senior Member

    Thanks Doug, i had missed that.
    Steve.
     
  7. Doug Lord
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    Doug Lord Flight Ready

    The Perils of Edgy Weather in Any Boat

    For those that are still thinking this may have some relevance for future consideration. From the front page of SA:

    Meanwhile, the crew of the SeaCart 30 Trimaran ‘Sundog’ also had an incredible run from Chicago to the land of ******** and public drunkenness arrests, crossing the line third, ahead of every monohull except the MaxZ86 Windquest and STP65 Equation…with four crew and 30 feet of waterline.

    For those that don't know the Seacart 30 is a very high powered racing trimaran.......
    18 min of vid is avail. on the front page: http://www.sailinganarchy.com/index_page1.php
     
  8. Doug Lord
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    Doug Lord Flight Ready

    The Perils of Edgy Weather in Any Boat

    From Alan "Mr. Clean" Block on SA yesterday:

    " I learned today, from Meade Gougeon (during his speech at the memorial ceremony) that the Gougeons and Morleys increased the keel's weight from 1200# to over 1500#. Combined wth switching the rig from alloy to carbon, the increase in stability was substantial. Morley* told me that the gust that knocked them down and held them down was peculiar in both its strength and more importantly, its duration. That being said, it was a mellow knockdown, and he has no idea what caused the head injuries that most likely killed his brother and Mark's girlfriend, but it was most definitely not some kind of ultra-quick violent knockdown."

    *brother of the skipper and part of the crew of Wingnuts during the incident

    Block quote from here: http://forums.sailinganarchy.com/index.php?showtopic=124750&pid=3349498&st=25&#entry3349498 post 27
     
    Last edited: Aug 2, 2011
  9. sharpii2
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    sharpii2 Senior Member

    Thanks, Doug. for sharing this.
     
  10. bntii
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    bntii Senior Member

    Is it know yet if the boat rolled fully inverted during the initial knockdown, or was just pressed down and held over by the force of the wind?
     
  11. Cheesy
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    Cheesy Senior Member

    It is still just an arbitrary line in the sand, how about you sketch a line on that plot of what you think the K35 would look like and as well as a curve that you would consider sea worthy. Then as most of us here are not NAs perhaps you could explain what makes one seaworthy and the other not?
     
  12. Doug Lord
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    Doug Lord Flight Ready

    The Perils of Edgy Weather in Any Boat

    ========================
    Not known yet. One thing that is interesting: Block points out that ballast was increased to 1500lb. In post #2 (I think) the specs showed 1100lb-the increase was fairly substantial any way you look at it. A lot of wind could blow on the hull at 90 degrees and the boat still have +RM.
     
  13. TeddyDiver
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    TeddyDiver Gollywobbler

    Look Ad Hoc's post #71 the first drawing. It's quite close what you request IMHO. Wings of K35 makes things only bit worse compared to dinghy.
    The arbitrary line is quite well defined in the ISO standard considering the AVS and the area of the positive RM.
    BR Teddy
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    I asked GB if he is a qualified naval architect to “draw a line” in the sand, simply to ascertain whether “we” are debating with someone with just opinions and “thoughts” or someone with real technical knowledge on stability; obviously the former. We can all opine, but understanding the mechanisms behind such opines clarifies what is what and if one is obfuscating and subverting opine as fact.

    Teddy has already given you the answer. But without more data on the Kiwi 35, I could only guess. Once that line is drawn in the sand, the naysayers will pop up criticising without providing any proof of their own (just as in the endless Brent Swain debates).

    Seaworthiness is terribly complex and cannot be reduced to simply the Righting Moment of a statical stability curve. It is a start but there is far more to it as already noted by MikeJohns and Daiquiri, and I am certainly no expert in seaworthiness, as it is not even defined legally, owing to its complexity. But some simple facts can provide very clear indications even before going setting sail on whether a boat is suitable for certain conditions or not.

    So, lets first address the righting moment. The righting moment is to counter the heeling moment Mh. The stability curve of any hull shall be like that below:

    Y-stab-1.jpg

    If, for now, ignore the ballast curve line, you can see that there is a maximum and then falls away to a minimum back to zero and then below this line. The maximum is very important. If you have a hull that has a low freeboard, as in the case of a wide shallow draft boat, the angle at which the gunwales/deck edge becomes immersed is very low. This defines the where the maximum angle shall be.

    Please bear in mind we are looking at stability only for now, nothing else.

    So, with a wide flat bottom boat the GZ curve will have a maximum angle that is much lower than one with a high freeboard and/or deeper draft. The beam provides additional stability, so it is advantages to have a wide beam. But, if, as I noted earlier, if this is at the expense of draft, the increase in stability is only at small angles, at larger angles it is seriously compromised.

    beam-draft.jpg

    So what does this mean? Once the boat heels past the max angle, the ability to self right, becomes more difficult. It is the angle of limiting stability. Beyond this angle you must reduce the moments acting on the boat. How do you do that?...you can reduce the heeling moment by reducing the amount of sail area.

    So, if we add deep keel and ballast, what does this do?...again looking at the curves it aids stability. It affects the shape of the curve. It does not alter the angle of deck immersion! Also note that it is a curve with a keel that has volume and hence also adds to the restoring moment, it is not a “dagger board” with a blob of weight, which is subtly different.

    Next we have the dynamic stability of the boat in relation to the wind and its stability on performance. The resistance of any boat is a function of its weight. And if we know that the ability to sail fast must be the ability of the hull to counter a high heeling force (Fh) at the same time as a low angle of heel. This is can be easily described by the ability to carry sail area as a function of its weight. This where it starts to get interesting and terribly complex too.

    Most ballasted boats have a high percentage of ballast to total displacement. But the right arm (RA) of any ballasted boat is zero at zero angle of heel. The increase in RA is slow and gradual with increasing heel. To counter a large Fh, a ballasted boat needs the RA. But at low angles there is insufficient RA, so either reduce the Fh (sail) or increase the RA, heel of the boat. The paradox is that for a ballasted keel boat the maximum RA is at 90degrees (see below). But does this produce fast sailing??..clearly not!

    For a multihull it is different, since the RA is a function of the hull spacing, ie the beam..which is considerably greater than a monohull, again as noted here:

    Y-stab-2.jpg

    So, how does a ballasted boat increase the RA…you have a the crew become “live ballast”. So if you can move the crew further outboard, this applies another moment to restore the boat, and can increase the ability to carry sail at lower angles and maintain speed.

    And then we have the boat itself and it natural periods of rolling. This is all a function of the hull shape (roll stability) and the moment of inertia of the boat. The moment of inertia is simply the mass of an item times the distance from the centre of rotation of motion. How close was the period of free rolling (natural period) of the hull to that of the waves? The angle of heel can become very large and can exceed the boats limiting angle of stability and cause a capsize.

    You can change the moment of inertia of a boat by standing up or getting the crew to sit out on the sides, well, this coincides with getting more RA. You can also increase dampening by use of rudders/dagger board. ..if a heavy dagger board this also pushes the mass further away to increase the moment of inertia. But this can also make the motion too stiff, and will thus have a period of motion faster (lower period) than when unchanged by moving crew etc. This could in turn, in short sharp steeps seas move the period of roll to that of the encountering waves, which is dangerous, this is called resonance.

    If the crew experiences excessive rolling due to resonance, they can try 3 options:

    1...change the period motion of the aerodynamic heeling forces., in other words reduce the amount of sail.
    2..change the period of rolling..ie moving crew about
    3..oppose the motion, add dampening and move crew from side to side to oppose to reduce the amplitude.

    The above is just a summary of many contributing and sometimes conflicting aspects that goes into a boats ability to sail and be seaworthy.

    So, going back to the K35.

    The shallow draft and low freeboard would produce a low max angle in the stability curve. The ‘wings’ would provide a temporary reprieve and increase in angle, but then a massive drop off in RA once beyond this angle. To aid and counter this ballast is used, in the lead bulb. But the ability to sail fast needs additional live ballast to increase the RA at lower angles of heel. The increase in RA using ‘live ballast’ would increase moment of inertia and alter the period of motion. The weight of the crew accounts for some 35% of the weight..which is high, ie easy to alter/change m0tions compared to a heavy boat.

    To maintain speed means to carry more sail….oh, it is a race!...to reduce the sail area means the speed would lose a significant amount of speed as all the elements that are finely tuned to increase the RA are lost.

    If the sail area was not reduced sufficiently it is not inconceivable to see how the crew at the sides coupled with a high Fh (gust of wind) pushed the boats wing into the water to submerge it which in turn cascades into a sudden and massive loss of RA. (Traditional ballasted “slow” boats have a more gradual deterioration of stability by comparison and hence time to do something about it). With crew either suddenly moving to the centreline (noting something serious was about to occur) or indeed if one or 2 crew fell overboard this compounds the loss of RA. The hull shape at increasing angles has little reserve of buoyancy, which would be reflected in a poor stability curve after max angle is reached. The AVS may be fine but the right moment, RA x buoyancy would be poor (compared to before the limiting angle)…which is the dynamic stability. Then you have as already noted, the now expose wing on the weather side being pushed by the wind and rolling the boat further still and the submerge wing digging in and driving the hull down from hydrodynamic forces (which are greater) making a recovery next to impossible, from within the boat. The result...capsize..at an angle lower than one wish for in such conditions. The amount of "recovery" must be related to the sea conditions as this influences the dynamic stability and hence the safety and ergo seaworthiness.

    Seaworthiness is terribly complex it brings together many aspects of the design and behaviour under many and various conditions, all are different too.

    Just by casual inspection and knowing some “simple naval architecture” when looking at race rules such as here: http://www.rorc.org/special-regulations/safety-and-stability-indices-ssi.html
    It is clear that K35 would be, at very best a category 3, which would only allow the boat to race at or near populated coast. In other words, the seaworthiness is not considered suitable for "offshore" racing.
     
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  15. CutOnce

    CutOnce Previous Member

    Thank you to AdHoc for the concise and clear explanation to Cheesy's question. It is refreshing to have content on topic from an expert.

    I started this thread to try to facilitate responses such as this - as I well knew there would be people working hard to justify the inclusion of edgy designs without the support of science and established naval architecture practices. Edgy designs have their place, and the racing world will demand their presence to keep pushing the records and performance envelope.

    I'm not convinced that edgy designs belong offshore and unsupported. I'm certainly not certain a boat that needs/may be safer with close support can be considered a valid racing sailboat in offshore events. Consciously choosing to reduce safety margins for crew for higher speed potential is a serious topic needing serious debate.

    There has been a terrible amount of misdirection trying to emphasize the edgy weather, not the boat design. The presentation of catamarans as justification for edgy lightweight winged boats is another attempt at misdirection away from my original question. I was hoping for an apples to apples discussion on what exactly constitutes an offshore racing boat, and where the line should be drawn for offshore overnight events.

    Forums like this are an appropriate place for this kind of discussion - perhaps better than enthusiast sites. Naval Architects are the people who are liable for the failure of their designs (and frequently unremembered for their successes). Liability is the difference between amateurs and professionals, and liability is a great catalyst for discussion of expectations, standards and safety.

    --
    CutOnce
     
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