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

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

  1. Ad Hoc
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    Ad Hoc Naval Architect

    Only 30years...gess...you're a newbbie. I thought you knew more than that ;)

    When making statements about "hull speed" as if 'fact', you know you're talking to someone with extremely limited knowledge of hydrodynamics, yet uses these meaningless terms, to sound important. Sounds like he doesn't want to learn all the difficult stuff to understand his own questions, then asks them on a forum then doesn't like the replies...go figure!
     
  2. BMcF
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    BMcF Senior Member

    That is why, in a nutshell, why I always defer to my elders and those who have amassed more experience than I. Like..yourself, for example. :p
     
  3. Landlubber
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    Landlubber Senior Member

    well Antonio has left the room, but my views on ocean passagemakers is that a good boat is in the water, a bad boat is on the water...take your pick if you want speed or comfort...but when the going gets tough at sea, the boat in the water is a dream compared to the trip you get with a boat on the water.
     
  4. sinbadwr
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    sinbadwr Junior Member

    seasickness

    This is not a problem just for the elderly, sick or those of us that are just plain wimps. Recent research has reported that of 79 US Space Shuttle missions 94% of astronauts used some medication during flight. 47% of that medication was for the relief of space motion sickness. Hope that makes you feel a whole lot better!

    Ok just pulled this from a preivous post. I really do not think that a space ship is moving in the right megahertz required for sea sickness on a boat. My point is that sea sickness can not be measured accuratly with a formula for every boat in every kind of seas and every kind of waves and every kind of sailors. And not to double decibles.
     
  5. MikeJohns
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    MikeJohns Senior Member

    There are numerous very comprehensive studies and it's well understood. Navies put a lot of money into understanding how debitating the predicted motion will be for human operators.

    I think you are confusing vomiting with sea sickness from your comments. They aren't the same, once a threshold is exceeded everyone's ability goes down no exception, it takes longer to perform both physical and cognitive tasks.

    Also your example boat would be assessed for each loading condition, and it's not a good example. Part of using simple ratios to assess a design is understanding the application. The real NA study is a series of RAO's which much more accurately predict vessel response to sea state headings and vessel speed. RAO's tend to support the simpler ratios you have problems with for the types of boats they are applicable to.
     
  6. sinbadwr
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    sinbadwr Junior Member

    you say that " Everyones ability goes down no exception". Well that is just not true. Can anyone on this forum tell me that everyone on your vessel gets sea sick? With no exceptions? due to the same stimuli on your boat?
    My problems with your formulas are statements like this. You have rated a large number of boats with very specific numbers.
    Lets start with your example of how sea sickness is caused, with the victim siting facing forward with a set up and down motion at a set frequency. No mention is made of sitting facing the beam, where roll could cause the same frequency of motion. Or any number of other variables, like experience, temp, fear factor, oders, or the number of other people sea sick in the same area, ect ect. Now with the up and down motion that is claimed to cause sea sickness at a set frequency, Would that not be more an effect of the wave effects rather than some designe difference from boat to boat. Would that frequency change with the speed of the boat? The angle of attack of the boat upon the wave. Wave comming from the bow vs the stern or beam? Would any of that change the frequency of the motion from the required 10 seconds? Why does heaving to cause these effects to end, on a well rated boat or a poor one?
    Are there boat designes that are cause the passengers to get sea sick less. Yes I would believe that. Can you figure this out with a formula for very different boats to 2 decible points. A formula that dosn't take into effect the state of the seas, wind, sailtrim, size of waves, speed of the boat, ect. No I don't believe that.
    So that leaves us with the question of why you would figure out sea sickness number (to the hundredth place) for so many boats using a formula that does not take into effects so many things. Speed for example. Stop any boat in calm water and no one should get sick. So they would all have the same number, right? To get your correct frequency of up and down motion,would be more a factor of the speed of the boat hitting waves rather than your formula.
    but back to your rating system. You have good boats and bad boats. Would it not be better to start from best and end at worst? Without labeling some boats as bad and some as good? I don't know about you, but I would never have enough time in my life to figure out all those boats with out some specific reason. So what is it? Why this instance on an obviously flawed system rather than an honest discussion on what causes boats to be good or bad, or cause crew to be thrown accross the cockpit. And what should we do about it. Or what we should look for in a prospective vessel.
     
  7. MikeJohns
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    MikeJohns Senior Member

    There is a motion induced fatigue that's just a fact of human physiology. Different people respond differently but once a certain threshold is reached it's detrimental to everyone.
    That doesn't mean you are palid and hanging over the side. It may just mean you take longer to complete tasks.
    Another issue though is the effect of violent motion on sleep, after a few days of heavy weather sleep deprivation also adds significantly to the physiological response termed sea sickness.
     
  8. sinbadwr
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    sinbadwr Junior Member

    Then I guess that all those cruisers sailing across oceans are just dead tired. I now have new respect for people like Lyn and Larry Paraday and their like whom have spent their whole life on the ocean. Or people in the Navy.
    But back to the point. Even if what you say, that if you are exposed to this motion, EVERYONE will get sick or lose effectiveness what does that have to do with putting hundreds of boats into two groups. Good boats and bad boats? I know some people get sick or lose effectiveness in adverse conditions. I just don't define those conditions as narrowly as you do( your definition being sitting facing forward with a up and down motion of a set frequency). I would agree that could cause problems to some, maybe even most people. That is not my problem. I have heard that such conditions are even used to interogate prisioners. My problem is that first, other things cause seasickness, many are prone to it, many are not. Even from what I have read here in this string, the major cause of it in the ferry tests was being around other people who were sick. I have seen this effect in action. But my biggest problem is how you can use this one criteria, not different wave effects, wind effects, speed,(lets take a moment here, The speed of the boat is what actually produces the frequency of the roll, and you say that a certian frequency is required, so speed should be the biggest effect) ect ect and look at measurements on boats, that we agree are not relaible, and come up with a formula to rate boats to the hundreds point. And then spend the time necessary to rate so many boats, and finally put them in good boat or bad boat catagories. Why not spend time analyzing what boats rate poorly and determing what causes that. And seeing what causes boats to score better. That might be something that I would like to know.
     
  9. MikeJohns
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    MikeJohns Senior Member

    Yes in heavy weather. It's more a question of how miserable you are. In a heavy displacement boat you'll be less miserable than a light displacment boat unless the course sailed is downwind and the light boat has sufficient drive to match the wave speed. Vertical acceleration is a significant factor no matter which way you face. The only way to reduce vertical acceleration is to increase D relative to the waterplane area.

    All the aspects you seem to think are novel considerations are not. The models for comfort ( read less misery) in heavy weather are well understood and a lot more emphasis is being placed on human operators and the level of induced disability through motion fatigue. That includes which way you face and where you are on the vessel. I'm afraid there is nothing novel in your spiels.

    Niether is categarisation a black and white affair ( Good or bad boats as you put it ) there is a scale of assessment. And the the main adage of any yacht design is compromise, you can compromise comfort for speed but safety and comfort are usually hand in hand.
     
  10. sinbadwr
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    sinbadwr Junior Member

    I agree. I can agree on a wave length of time on the up and down motion. But that is a result of speed through the waves. Not so much boat design. Yet again my problem is with rating boats down into the hundreds . And then saying which boats are good and bad. With some kind of formula that dosn't have the most important factor being boat speed in waves. Or all the other factors that relate to it.
    I only have to look up my boat. A columbia 26, which is in the bad coloum. quite bad. Yet it is a heavy displacacement boat of the old school. With a full keel. Someone else pointed out that a T52 or something was very bad.
    As it was pointed out earlier in the thread. Does someone have an agenda on making some boats seem bad and some seem good? And dumping all kind of numbers, references and studies, but no real facts about which boat is bad because of X or good because of Y.
     
  11. masalai
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    masalai masalai

    "The roll acceleration: What´s the best for crossing oceans?" - - The headline deserves a cheeky response (NIL) but reality and individuals may accept either end to some level relating to ones ability to remain useful and not rendered useless by "sea-sickness" in the quest for an ever faster passage in the shortest possible time.

    To be somewhat pedantic - boats and boating (SOR?), is usually contained in a series of compromises. - - So "BEST" is a very individualistic thing. - - In essence the OP answered the question in the first post. - - I am 'comfortable' with a certain amount of movement and happily cruise at 7 knots on longer voyages, but I would like to travel 10 to 100 times faster at the same mpg, but that is quite unrealistic and well beyond my financial resources to even contemplate.

    At 7 knots I am quite happy with a 40knot wind and following sea - meaning I will likely be surfing at 19 knots for a goodly portion of the time, quite at ease on autopilot. - - Whereas, with a beam on sea from a short fetch it feels very uncomfortable - so I don't...
     
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  12. dougfrolich
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    dougfrolich Senior Member

    "the effect of violent motion on sleep, after a few days of heavy weather sleep deprivation also adds significantly to the physiological response termed sea sickness."

    The pic below is me on the start of a 10 day ocean race and I will agree totally with MJ in the staement above.--by day 3 without any real REM sleep I was "not myself" but once the bouncing stopped, and 15 to 20 min of REM sleep in every 3-4 hr period rested the mind and body, then reasonable human performance was restored.
     

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