The perils of edgy design offshore

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

  1. daiquiri
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    daiquiri Engineering and Design

    The important part here is, apart modelling the correct shape and volume of wings, the estimate of the vertical center of gravity, Vcg. That's a parameter which can give significant differences in both AVS and maximum RM values. One can get any desired AVS by placing the Vcg in this or that place. For a boat of that shape 20 cm of error in the estimate of the Vcg can change the AVS by 10 degrees or even more. So, the question now is - how did you calculate the Vcg of the boat?
     
  2. Paul B

    Paul B Previous Member

    That is as I expected. Including the keel fin and bulb has made your Cp meaningless.

    I don't think you would find any section of the wing (forward, middle, or aft) that would match the profile you used.

    You might take time to scale the hull draft and BWL from the model Jim Teeters presents in the report, to compare to your model. As I said earlier, I would hope Mr. Teeters used the actual lines to build his model.

    If you simply took your previous model and sank it until it measured 1800 kgs then I would expect it to show more stability.

    I don't think you have mentioned yet if you have included anything but the hull, keel/bulb, deck, and wings in your CG calculation.
     
  3. Paul B

    Paul B Previous Member

    The 37kg would be a bit light even for the alloy tube only. The tube is roughly 40 feet long and it was made by Johnson Sails using the Schaeffer/Hall section 135 (if memory serves). That section was about 2.2 lbs/foot. On top of that you would have the spreaders, spreader bars, shrouds, stays, halyards, runners, and fittings.

    The thing I find interesting in reading through the report is the claim of saving 100 lbs by going from an alloy tube to a carbon tube. Since the alloy tube only weighed about 88 lbs I wonder how they could save 100 pounds by changing to a carbon tube?


    There was one Kiwi 35 that removed the wings and replaced them with tubular racks. They claimed they saved 400 pounds by doing this. That means each wing weighed more than 200 lbs. So that should tell you something about the weight of the construction of the Kiwi 35.

    If you think about it, the boat was supposed to weigh 2850 lbs. Remove the 1100 lbs of ballast and you have 1750 lbs. If the wings weighed more than 400 lbs that would leave only about 1300 lbs for everything else. Given the weight of the wings construction it seems unlikely they could have built the rest of the boat to the same standard and met that weight budget.
     
  4. quequen
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    quequen Senior Member

    Daiquiri, PaulB, all data about CG is in the hydro report I attached earlier. No other items were included in the model.
    After reading part of the report, I don't thik I can question anything on it. The stability analisis rests on mr. Teeters professionalism, and I'm nobody to question it. Mr. Stan Dent says wings hit water at 22.5 deg, and this data match exactly with the break on mr. Teeters GZ curve. This are the kind of coincidences that reaffirm an analisis. GZ curve is so different from mine that you can think on different boats. Then I assume my data and/or analisis are very wrong.
    This data match with the report: LOA, LWL, hull Beam, winged hull Beam, total Disp., Ballast Disp., total Draft.
    As I dont find big shape differences on hull and wings, I assume the error must be on VCG. To match mr. Teeter's GZ curve, I must to move VCG to 0.75 m.
    Here is the only hull image I find at the report, so I assume it comes from the model used by mr Teeters and it's a non-perspectived bodyplan image. And here is also a superposition between both images (my roughly model and this much more accurate render). They match more than I expected, considering the lack of data I had to make mine. Wing section is almost the same. But there is a big difference on deck: the render has not cabin, I wonder if that makes some difference on the negative parts of the curves. Also, there is a big difference on bulb's width, so weight relation sword/bulb could put ballast CG higer on my model than mr. Teeters one.
     

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  5. Paul B

    Paul B Previous Member

    You have done significant work on this, but without a result to match the report. My only suggestion is for you to provide your work to Mr. Teeters and ask him to help you understand the differences in the results.

    I don't know if you are a student or not. If you are a student and approach Mr. Teeters from that perspective I would hope he would find the time to help you. On the other hand, maybe your work would end up helping him!
     
  6. Doug Lord
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    Doug Lord Flight Ready

    The Perils of Edgy Weather in Any Boat

    From the report thread on SA by the man at the helm of WingNuts when the storm hit:

    Posted Yesterday, 10:31 AM by "Wingnut"

    Thanks to Mr. Clean for highlighting what the report tends to glaze over, the weather. I can clear up a few points as I was there at the helm. This was one hell of a storm unlike I have ever seen in my 40+ years on the water. We were at the convergence of two intense cells. 67 knots is not the highest wind we saw it was only the last reading we saw on the instruments (we were a little busy holding on at that point). I certainly believe that a microburst could have hit us and few to none of the other boats near by. I have seen microbursts that flattened parts of a forest in a circle or straightline yet left nearby trees standing.

    As for the boats boats stability or lack of that has been exaggerated by the mass media because the used "sound bites" from the report. Many quotes have been used out of context or without explanation. When we have said the boat was on it's side but always came back up was when we were sailing in 20+ knots of wind with a chute up and the boat has rounded up or in some previous squalls (and yes there has been some). I have sailed on many other heavy displacement boats that have rounded up the same way. We have only reefed at 12 knots when we were sailing shorthanded. We often race Wednesday nights with only 3 or 4 people. I am not saying this is the most stable boat and it is tender, but we seldom have to have the crew hiking hard on the wings, you are more likely to see us lounging on them. Our sail plan is typical for any sport boat and not as the media took from the report, too big for the boat. (And what did that have to do with the accident? The main was down and the #3 partially furled). The boat had many hand and foot holds in our current configuration. The reports "scattergraphs" are small scale and don't even have units of measure on them. The boat falls within the group of others except when they take the +/- into account for WingNuts and not the others.

    Just to clear few other questions or misconceptions. The boat was on it's side for a bit and then started to slowly rollover.(emphasis dl) The boom was secured by the main halyard on the end with the main sheet and traveller tight and cleated. (the boom was off when the boat righted itself. And yes it righted itself once the port wing filled with some water to allow some heel. We trailer so the boom was on with a large clevis pin with a split ring. the ring may have been snagged by the tangle of lines allowing the pin to come out???). The other thing the report doesn't address is the difficulty accessing the release shackle on the tether when it is being pulled in a downward direction against the body instead of up and out from the body.

    To echo Clean's earlier comment about doing a Mac on a Kiwi 35, I would still do it again as well. This was a freak intense storm. Yes this area is prone to storms, I have been sailing in northern Michigan since I was a kid and seen many storms. I would treat it more like a multihull in preparing safety equipment. I would wear a harness but not with auto inflate. The great thing about sailing is our experiences add to our knowledge and enjoyment of the sport.
     
  7. sharpii2
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    sharpii2 Senior Member

    I thought about the idea of plain holes. I rejected it because the lee wing would start taking on water as soon as it started immerse. Any righting moment the leeward wing might add would be lost. Its weight would add to capsize moment instead.

    The WINGNUTS design concept is similar to a dory. A dory has wide flaring sides and a narrow bottom that gives it a range of stability that is beyond what a normal un-ballasted boat would have. The price for this is it becomes more likely to flip, once the gunwale goes under. The sides of the dory were probably not flaired for that reason, but they had that effect.

    The trick is to get it to fill only when it's upside down, so you end up with a hydrostatically wide boat, right side up, and a hydrostatically narrow one, upside down.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    That's the beauty of "design". Making a simple concept, like holes, work for you. Being creative to allow both attributes that are required to work. This is what design is all about....creating something from nothing.

    That is why i feel today's graduates are less creative...they rely far too much on a computer program for the answer, ie just numbers! The graduates I've trained up in recent years have become more and more reliant on a computer for the solution these days. Real design, seems to be lost.
     
  9. quequen
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    quequen Senior Member

    Here some speculations (and must be taken just as that):
    Crew from wingnuts says that report puts the emphasis of this disaster on boat's design, not on the extreme weather conditions. This image comes from instruments of the "Fast Tango", 5 miles away from Wingnuts. They could be reached by similar conditions. Registers overpass 100 knots (!) for more than 20 minutes. So I made some calculations (using my spreadsheet posted before, hope someone can check this for accuracy!). Because of the wings, Wingnuts exposes 17 m2 (aprox.) when heeling at 90 deg., if hitted perpendicularly, air pressure at that tremendous speed is more than 150kg/m2. It makes a moment to the CLA of the submerged volume that overpass 4Tm (!). That's enough to easily capsize this boat (and many others, I think), even with a VCG of 0.27m.
    On the other hand, to reach the 0.75m of VCG to match mr. Teeters report, I must to twist weights of my model to an unreal situation, so I start to belive VCG is under that level.
    Also, I'll thank opinions on compatibility between this GZ curve from the report, to this picture also taken from report showing a guy at the top of the mast while two other guys are standing over deck. This boat is too narrow and has almost nothing of Shape stability between 0 and 23 deg, I think (speculation, offcourse!) this picture is possible just by a very low VCG, wich as said, doesn't helped them anyway.
     

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  10. MikeJohns
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    MikeJohns Senior Member

    quequen thanks for your effort. Keep up the good work and try and get your model so it matches that of the consulting NA. But that's still only a start.

    There was talk earlier that a sister ship had blown over and capsized while on a mooring. The very narrow initial waterline beam and the very low initial form stability are a bad combination. But don't forget this is in a seaway and the surface conditions are also affecting the craft, consider for example if it is hit every 3.5 seconds by a steep to 4 to 6 foot breaking wave.

    The wind heels the vessel but the crew can also move around to counter that heel (or make it worse). There is no indication of where the crew weight is and that GZ curve is too simple. The crew should be accounted for and to some extent they should be treated as movable ballast that are not all that well fixed throughout the angles of heel. If they were all nimble and were all hanging from the top wing edge for example as opposed to all in the water and essentially off the boat.

    A combination of deck edge immersion sideways translation and being buoyed up all reduce the righting moment then there is the wind. You don't need anywhere near as much wind if you consider the loss of stability from other factors.

    But whatever way it's approached the conclusion is the same, the boat lacks any sensible level of inherent stability and there were few good options available except maybe using the motor as and when possible to keep the bow into the wind and waves, or using a drogue.

    The chap up the mast is probably protected from the boat rolling to an angle by tying the wing to the post.
     
  11. quequen
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    quequen Senior Member

    Mike, I agree, crew represented 600 kg of "movable ballast" that should be considered. And once the boat reached 90 deg (or less) of heeling, this weight can only worse stability or have no influence on it (if they are on water), but never improve it. Also, waves become a big problem specially if boat is not in movement. If they haven't a drogue, I think that trying to keep moving and run the waves was the right decision (at least, that's what I would do on that situation:().
    About the picture, I don't think the boat is attached to the post. Image has a very good resolution, ropes can be seen clearly. Deck has no points of attachment near the post, the inflatable defense seems to be attached to the post, not to boat's deck (I can see a rope there). Check this other picture.
     

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  12. Doug Lord
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    Doug Lord Flight Ready

    The Perils of Edgy Weather in Any Boat

    From Scuttlebutt tonight:

    CHI-MAC REVISIONS ANNOUNCED

    After two sailors' lives were lost during the 2011 Chicago Yacht Club Race
    to Mackinac, event organizers planning for this year's edition sought the
    advice of industry experts in safety and naval architecture, and worked
    with the offshore office of US Sailing in determining what changes were
    needed.

    The result of this process has seen the committee make some modifications
    that are now reflected in the recently released Notice and Conditions of
    Race (NOR) and the Mackinac Safety Regulations (MSR) for the 104th Running
    of the annual event. "The changes we have instituted compliment the
    approach to organizing a quality and safe race that have always been a part
    of the rich tradition of the Race to Mackinac," said race chairman Lou
    Sandoval.

    Among the changes implemented for 2012 is the issuing of a minimum
    stability for a boat's eligibility to compete in the race. The handicapping
    rule used for the Race to Mackinac (Offshore Racing Rule-ORR) has adopted a
    new version of the Stability Index, as discussed in the US Sailing report
    of the 2011 Race. The committee established a minimum Stability Index of
    103, with additional changes made in personal and boat safety equipment,
    continuing sailor education and the vessel inspection process.


    Full report: http://tinyurl.com/ChiMac-012912

    Well thats it ,I guess.
     
  13. JosephT
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    JosephT Senior Member

    Great news. I'm sure some competitors will fuss, but at the end of the race day at least all competitors will find balancing safety and fun is a good thing.
     
  14. Guillermo
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    Guillermo Ingeniero Naval

    Yeap! :)
     

  15. Silver Raven
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    Silver Raven Senior Member

    G'day Doug. Well guys - I guess that means that the new extended 'top' 4 mtr plus extension to the top of the AC 45's wing is just not going to make the cut in 'The Mac Race' - Great for safety but not for lunitic-fun. Golly - ciao, james However that said - the new extension sure does look the goods.
     
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