The roll acceleration: What´s the best for crossing oceans?

Discussion in 'Stability' started by Antonio Alcalá, Dec 18, 2007.

  1. drshaddock
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    drshaddock Design Engineer

    Roll moment of inertia--in my defense!

    Paul B, I'm starting to understand that you're not really interested in straightforward discussion of ideas. But I'll provide this information just to see if you are:

    (You do realize one needs no expertise to be published in sailig periodiacals or on websites and message boards)
    And so? You appear to imply that those who are published do not have expertise. Surely that's not what you truly believe!

    And what would cause it to "snap back"? There are forces that cause the heeling moment that the righting moment is working against. Are you trying to say that it suddenly, completely disappears?
    Lack of roll moment inertia, and high ballast/displacement ratio. As I said before, the boats I'm discussing have very high initial stability. This makes them return or recover very quickly relative to an older, more traditional design--and in rough water they're going to have a very different motion.

    The TP52 has a B/L of 0.28. The B/DWL is the same.
    The STP 65 (big sister to the TP 52) has a B/L of 0.24.
    The Catalina 27 has a B/L of 0.33. The B/DWL is 0.38.
    Finisterre had a B/L of 0.29. The B/DWL was 0.41.

    Obviously, ratios of beam to length don't tell the whole story. You're too focused on just one element of design. Look at Finisterre vs. the TP52--similar B/L, but vastly different B/DWL--because the Finisterre had magnificent overhangs, giving her a long of pitch inertia and plenty of reserve buoyancy (unlike the TP52, which has a plumb bow, I would assume, from the fact that her B/DWL is the same as her B/L). She was also the leader of the movement toward beamy, roomy centerboarders. She displaced 11 tons on a waterline of only 27 1/2 feet. What's the displacement of the TP52?

    How on earth do you think lighter rigs make a boat "quickly heel" or heave more? More BS people like you love to lap up and regurgitate.
    Okay, here's some information from a chapter in Desirable and Undesirable Characteristics of Offshore Yachts, 1987 Edition. This chapter was co-authored by Karl Kirkman, a naval architect and marine engineer with extensive work in 12-Meter and ocean-racing yacht development, and by Richard McCurdy, chairman of the USYRU's Safety at Sea Committee and a former CCA commodore. On page 61, they write about tank tests that found behavior of boats capsized by wave action was controlled by dynamic principles rather than static flotation, and that when properly scaled masts were mounted on tank models, the models wouldn't capsize anymore when struck by waves that readily capsized the mastless models. This seemed counter-intuitive, since you'd think a mastless hull has to float more staunchly upright...
    When they thought about the roll moment of inertia, they noted that the heavier parts of the boat are located close to the roll axis. But the rig, although lighter, was much farther away, and constitutes "over half of the total roll inertia, in boats both big and small."

    So what I'm saying again (I mean, regurgitating) is that a lighter rig makes a boat have a much lower roll inertia, and therefore it's going to be much more reactive in a sea.

    Does this make sense to you yet or not?
     
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  2. Paul B

    Paul B Previous Member

    I've watched your discussions in the past and plainly see you don't learn very well. Keep telling everyone how unsafe things are, maybe you'll sell more of your product. Yep, if it isn't a steel hulled, long keeled boat designed by the greatest ENGINEER in all of sailing, then you are taking your life in your hands.

    You may or may not realize it, but there are some pretty damn good engineers designing boats of the type you don't like.

    If you think the MCR is SCIENCE then there is little hope for you.

    Please enlighten us, when is it likely to find modern designs "snapping" to and fro, pitching helpless sailors across the cockpit?




    I've been on mastless boats, and they do have scary accelerations. I've also sailed on identical class boats with both tin and carbon rigs. You couldn't notice any difference in roll or pitch. I know there was a change, but it is not something you feel. Ditto changing from Dacron to Kevlar sails.

    When out puttering about without sails up boats can roll about a bit. Once you put the main up things quiet down. The drag of the sail is a great dampening device, eh? Maybe that's why powerboaters use steadying sails? If a little steadying sail on a big trawler works, why don't you think the big main and headsails on sailboats act in the same way?
     
  3. Paul B

    Paul B Previous Member


    Let's anchor two boats side by side in an area with the waves coming from the bow. One boat is narrow and heavy, the other light and beamy.

    As the waves come toward the boats the bows lift and the boats eventually crest and drop into the next trough.

    Which of the boats has a greater vertical acceleration?
     
  4. Paul B

    Paul B Previous Member

    There are many people who are published who don't have any idea what they are talking about. I know this for a FACT.

    Again, the MCR does not take any of this into consideration. It does not take stability into account at all.

    I am not focused on any element. I am focused on actual performance. You're the one who made a claim that a TP52 was some overly beamy design with a crap MCR compared to your Cat 27. I was simply educating you.

    Your choice of words, "magnificent overhangs" tells me a lot about your point of view. I've sailed on TP52s. Have you? They may not have the kind of "reserve bouyancy" you like to see, but they don't go down the mine either. I can't say that for the Cat 27. See what happens when you get in large waves with that.

    See my comments to Eng. Johns. Tell me why one might sail a J120 with a carbon rig and another with an aluminum ring and not notice any difference in pitch or roll.

    I'm still waiting to hear some real info about this "snapping" motion of modern boats that pitches poor sailors headlong across the cockpit.
     
  5. drshaddock
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    drshaddock Design Engineer

    Vertical acceleration

    The light and beamy one--less inertia. It's going to lift faster and drop faster. This isn't about gravity--it's a dynamic analysis. That's the whole point.
     
  6. drshaddock
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    drshaddock Design Engineer

    What the MCR considers

    I never typified the TP52 as some overly beamy design. I mentioned several factors, lightness among them.

    I don't understand how you miss the stability element of Ted Brewer's MCR, which he admits was a tongue-in-cheek idea in the first place but it turns out to a be a fair way to judge boats of similar design.

    In Ted's own words: "Given a wave of X height, the speed of the upward motion depends on the displacement of the yacht and the amount of waterline area that is acted upon. Greater displacement, or lesser WL area, gives a slower motion and more comfort for any given sea state.

    Beam does enter into it as as wider beam increases stability, increases WL area, and generates a faster reaction. The formula takes into account the displacement, the WL area, and adds a beam factor. The intention is to provide a means to compare the motion comfort of vessels of similar type and size, not to compare that of a Lightning class sloop with that of a husky 50 foot ketch"

    The MCR accounts for displacement, beam, and length, but if you examine the formula, it is also accounting for the ratio between LOA and LWL, which contributes to pitch inertia.
     
  7. Paul B

    Paul B Previous Member


    Here, I am sure you mention broad Beam:



    Because it is not there.

    Finally someone reads what Brewer said himself. It was a joke to begin with (and remains a joke). A fair way to to judge boats, not many professionals would think so.

    Are you sure about that? The MCR does not specify BWL.

    However, the anti-moderate mob always likes to pull this calculation out as if it is some sort of magic formula, simply because it shows their boats in a better light.

    I think you are reading the wrong thing into the equation. The use of 70% DWL and 30% LOA is a crude way to approximate the actual sailing length of the boat. Similar calcs have been used in many rating formulas over the years. It has nothing to do with calculating any pitch inertia.


    OK, compare two boats. Both have LOA 50 feet. Both have LWL 42 feet. Both have a beam of 12 feet and a DSPL of 25,000 lbs.

    One has a shallow (5 feet draft), full keel with a lot of internal ballast. It has a normal masthead cutter rig of 60 feet "I".

    The other boat has an America's Cup Class style keel, 12 foot deep thin steel fin and 15,000 pounds of bulb at the bottom. Oh, and there is no mast.

    These two boats HAVE THE SAME MCR.

    R-u-b-b-i-s-h.
     
  8. Paul B

    Paul B Previous Member

    Hmm,

    The two boats are anchored next to one another. The wave it the same height for both and passes under both at the same velocity.

    They both rise to the top of the wave together and drop to the trough together.

    The only way for the lighter boat to rise faster is if it is moving forward faster than the heavy boat. But they are both going the same speed (the speed of the wave passing beneath), so how can one rise faster than the other?

    Magic?
     
  9. drshaddock
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    drshaddock Design Engineer

    Here, I am sure you mention broad Beam:
    Yes, I mentioned broad beam. I also mentioned light weight. You somehow keep missing that.



    Originally Posted by drshaddock
    I don't understand how you miss the stability element of Ted Brewer's MCR, .


    Because it is not there.
    The only reason I can imagine why you don't see stability as part of the equation is that you still don't grasp some of the factors that contribute to stability--both dynamic and static.


    Originally Posted by drshaddock
    In Ted's own words: "Given a wave of X height, the speed of the upward motion depends on the displacement of the yacht and the amount of waterline area that is acted upon. Greater displacement, or lesser WL area, gives a slower motion and more comfort for any given sea state.

    Beam does enter into it as as wider beam increases stability,.


    Are you sure about that? The MCR does not specify BWL.
    Those are Ted's words, not mine. You can ask him if he's sure. But don't bother commenting that the MCR does not specify BWL--you're right. But there was no mention of BWL. The MCR just factors in Beam.

    Originally Posted by drshaddock
    increases WL area, and generates a faster reaction. The formula takes into account the displacement, the WL area, and adds a beam factor. The intention is to provide a means to compare the motion comfort of vessels of similar type and size, not to compare that of a Lightning class sloop with that of a husky 50 foot ketch".

    However, the anti-moderate mob always likes to pull this calculation out as if it is some sort of magic formula, simply because it shows their boats in a better light.
    Hmm... never thought of myself as being part of a mob, and I'm not sure what anti-moderate is. But I don't pull out MCR because it shows my boat in a better light; heck, the C-27 doesn't really come out all that high!

    The other boat has an America's Cup Class style keel, 12 foot deep thin steel fin and 15,000 pounds of bulb at the bottom. Oh, and there is no mast.
    She'll sure be slow. Or does she use a sailkite?
     
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  10. drshaddock
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    drshaddock Design Engineer

    No--you're completely missing the inertia part of the concept. You have taken a physics class somewhere along the way, have you not? Newton figured it out centuries ago--his Law of Momentum Conservation.
     
  11. Guillermo
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    Guillermo Ingeniero Naval

    I think you should read some basic books before coming into this kind of discussions. May I suggest Marchaj's venerable "Seaworthiness..."? I'm afraid you are bringing to light a certain lack of understanding of some elementary physical phenomena.
    Cheers.
     
  12. TeddyDiver
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    TeddyDiver Gollywobbler

    Sorry Patric, I was only commenting Paul B(S), just forget the "quote"
    It sure looks like he irriteted others too..
    Maybe better to ignore or something..

    Salu! Teddy
     
  13. TeddyDiver
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    TeddyDiver Gollywobbler

    After all there's something we all agree?:D
     
  14. MikeJohns
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    MikeJohns Senior Member

    The acceleration is variable with time as the wave passes since the vessels do not leave the water it is a complex solution. Firstly you need to define the wave shape, period, and amplitude then you need to know the rate of change of the volume of the vessel as it intersects that wave (which alters along with the vessels response), then you need to calculate acceleration from the resulting change in centre of buoyancy which requires a knowledge of the mass distribution (Inertia), a hydrodynamic damping coefficient (zeta) then you end up with a characteristic higher order differential equation which is not trivial in it's solution, there will also need to be some assumptions refined through observation.


    You want me to learn your ‘Truth’ without any discourse? It seems to me that the boot is on the other foot, but I would rather discuss naval architecture than discuss your personal slurs. Perhaps in the future you could actually contribute something rather than ‘watching’.

    Why this imagined duality? If you have ever designed any offshore boats surely you would be aware of the tradeoffs and compromises in vessel design?

    What don’t you agree with that we have so far been discussed? Relevant to your interests would be:

    Inversion times after capsize related to low AVS
    Keel strength issues
    Design life
    Collision and grounding damage resistance
    The likelihood of certain types of vessels to be endangered in heavy weather.

    When debating these issues you as ‘PaulB’ have not contributed anything significant at all except for the occasional jibe, and pretending a superior position.
    You can always open a thread and discuss your views and I will be happy to contribute.

    Who are they and what don’t I like about their designs?


    In Australia there are chiefly two prominent professional engineers who design lightweight performance vessels under their own banner. Both of these engineers have recently been voicing concerns over lightweight design trends. One of their frustrations is that they are bound to produce designs for heavier (stronger) vessels than designers without proper engineering knowledge.


    Flinging satellites into orbit? This is your interpretation but it is not what I asked you to consider .

    It’s called damping not dampening (to make wet).
    Sails can also excite rolling. This depends on the course sailed relative to the conditions (wind and sea) levels of parametric coupling etc
     

  15. Pericles
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    Pericles Senior Member

    Gentlemen,

    There are those whose opinions, up with which, I will not put!

    (Never end a sentence with a preposition, Sir Winston Churchill).

    In such situations I go to the recalcitrant offenders' profiles and add them to my ignore list. Thus, I am informed of a "ignored" contribution to a thread, but my sense of wellbeing remains intact, as my eyes are spared the task of informing my brain about the contents of the post.

    There are some good and some not so good consequences to such an action. :p :p :p :p

    Hello, hello, is anyone out there. I am not getting posts nor am I getting replies to mine. Why am I so unloved, why, why? It's not my fault nobody wants to communicate with me . I did nothing wrong, d'you hear! Please!:D

    Best wishes ,

    Pericles
     
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