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

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

  1. Wavewacker
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    Wavewacker Senior Member

    Guillermo,Thank you, so if it is considered it should be with other boats of the same length, I have not found much published by manufactures concerning stability or by any sales advertisement.

    Goodwilltoall, point taken, thanks for your reply, I'll take this with a grain of salt, as I'm not too salty anyway.

    How do you, or what do you look for. as an indicator to compare boats as to thier ability to resist rolling, by looking not so much through the calculus, I assume it's beam width and flare, but doesn't that deminish motoring effeciency?
     
  2. Guillermo
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    Guillermo Ingeniero Naval

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  3. Wavewacker
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    Wavewacker Senior Member

    Thank you! I'm not sure I know enough to understand how to use it, but I'll play with it and see if I can figure it out. It appears it can provide great information! Kinda like handing a 6th grader a financial or engineering calculator...LOL:D

    Wow, nice, just mastered the hp, prop and baot type (LOL) for cruise/hull speed.....very handy, thanks!
     
  4. TeddyDiver
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    TeddyDiver Gollywobbler

    Take it with a lot of Alka-Selzer.. :p
     
  5. Wavewacker
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    Wavewacker Senior Member

    Budwiser, who cares.....? Thanks!
     
  6. El_Guero

    El_Guero Previous Member

    OK, I understand about half of this.

    Would someone break this down so those of us who are not familiar with the terms can understand?

    Thank you!

    wayne
     
  7. BMcF
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    BMcF Senior Member

    Get your hands on a copy of ISO 2631. There you will find the definitions and applications of human whole-body motion criteria, including everything I referred to in my post.

    In really simple terms....low-frequency motions make you sick and higher-frequency motions make you tired. The degree of each and the specific frequency ranges associated with each are all nicely spelled out in ISO 2631.
     
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  8. sinbadwr
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    sinbadwr Junior Member

    calculations

    I believe that the motion of any said boat is dynamic. And with current equipment including wave tanks can not be measured well enough for one boat to be compared with another in different winds, waves, sail set, ect. For example move one crew member from one side of the boat to the other and you have a whole new hull configuration under the water. Or pull in or let out one of the sheets and again you get a whole new underwater hull configuration.
    Yes you can add up numbers punch them into a calculator and come up with a result. But there is just to much variance in real life for them to have enough meaning to compare boats that are not very similar.
    Take overhang. We know that there is such a thing as hull speed, easily figured by the LWL. but we know also that with an overhang, and when the boat heels the water line is increased, so you have a different hull speed. So which hull speed are you going to compare? And that is only taking in to account the waterline on flat water. Waves would change that, and different waves would change it differently. And effect different boats differently
    Then there are the sails. Reef, or change the sheet at all changes the underwater configuraton hence the LWL. And the dampening motiong, beam to beam and Fore and aft. Speed of the boat, and the size of the waves are also variables that are not being taken into account.
    Think about it. Are the same formulas going to apply to a sunfish in 2 inch waves and a sunfish in 10 ft waves? And apply the same way when judging a sunfish vs a 40 ft boat?
    Would it not be better to state what makes a boat more stable than wip out our calculators, pull measurments off the internet on boats we have never sailed and come up with "Facts?" about which one is going to make you seasick? You may know that most weight numbers from the manufacturer are wrong, and even right, don't take into account everything and everyone that is currently aboard.
    Oh, one more thing; most people have some remedy for sea sickness, and somehow it seems to work without changing the boat in any way. So even if you have the correct motion on boat A that will make you sick , person 1 may not get sick, person 2 may not get sick because he took sea sickness meds, and person 3 is going to get sick because he is next to person 4 who is definatly sick today. But not tomorow. If that is the case, why all the measurments and formulas?
    The total lack of consistency of the underwater area of the hull due to wave size and angle, speed of boat, heel, dampening motion of sail configuration ect; preclude you from being able to use what ever formula you have and come out with consistant results. Without that how can you compare one boat to another or come out with the results you have? More so when you are comparing two compleatly different boats.
     
  9. TeddyDiver
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    TeddyDiver Gollywobbler

    Two completely different boats give you actyally much more reliable results. And more so becouse of the varying conditions we can use avarages in our calculations (as you said no consistence in this regard).Thou I admit much of the manufacturers give displacements way of the reality, shame on them, but anyone with something more precise knowledge can allways make the calcs easily.
    BR Teddy
     
  10. Guillermo
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    Guillermo Ingeniero Naval

    Average is the name of the game. Being true that it is almost impossible to take everything into account, crunching some simple numbers give most of the times a useful clue on expected behaviour. Specially when no other information is available.

    :)
     
  11. sinbadwr
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    sinbadwr Junior Member

    avarages

    No it dosen't. You come up with numbers for boats into tenths and hundreds which assumes that you have accuracy to this level. You do not. Consider my boat, a columbia 26 weight listed at 5200 lbs. Add 2 crew and the motor and now you have 10% more weight. 2 more crew and you are now off some 18%. Now there is the water in the tanks. 8 lbs per gallon, 30 gallons=240 lbs. A dingy and all the spare parts and everything else on the boat add even more.
    Then to compare a displacement hull aginst a planing one, or even the same boat under different conditions with it's self and you are way off tenths and hundreds. Think about it, with the boat heeled and half the hull out of the water; it is going to preform differently than when upright. The changing waterline on boats with overhang changes numbers too.
    How about we concentrate on what makes a boat more or less likely to make someone sick. List these traits and let the potential boat buyer aware of it. At the same time noting that one of the biggest factors in making someone seasick is the presence of others around them that are allready sea sick. Isn't that what the tests on the ferry proved? So when you state that boat X has a 22.36, And the ferry tests state that you are much more likely to get sea sick if the guy next to you is getting sick, Many times more likely than the differences in your numbers from one boat to another show, Well what do those numbers actually mean?
    Then sea sickness itself, as also stated is a variable. Again a variable much greater than the differences in your boat results. And as stated by someone here on this string, it isn't really that well understood. You say that a vertical motion with a certian time span is known to make people sick. Well that isn't exactly true, because it makes some people sick and not others. And other factors, such as if you are doing something, or your placement on the boat or your view of the horizon ect modify it more than the differences between most boats. So again your numbers, such as they are, are missleading.
    It's kind of like saying that a 1992 ford mustang gets 22.64mpg today. Well that depends on how much weight is in it. how well it is tuned up, if it is going down hill, how good the driver is ect. There are just to many variables to come up with usefull results into the tenths and hundrededs
     
  12. Guillermo
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    Guillermo Ingeniero Naval

    If you are happy thinking that way, it's OK to me. Kind regards. :)
     
  13. BMcF
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    BMcF Senior Member

    You would know the answer to that question had you spent 30 years in the fast passenger ferry design and build business.;)
     
  14. BMcF
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    BMcF Senior Member

    That is exactly what we have been doing for decades. Route comfort analysis...hull selection and/or modification...motion control system design and optimization...etc. All using proven prediction tools and/or scale model test results, and the vast body of published biodynamic research results, some of which is internationally condified in ISO 2631 as our basis for quantitative conclusions and recommendations.
     

  15. TeddyDiver
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    TeddyDiver Gollywobbler

    So maybe you should test passamgers instead :rolleyes: For the rest of us the formulas and Marchaj will do..
     
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