Motion Sickness Incidence Index

Discussion in 'Boat Design' started by Rounak Saha Niloy, Sep 18, 2022.

  1. Rounak Saha Niloy
    Joined: Mar 2022
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    Location: Sydney, NSW

    Rounak Saha Niloy Junior Member

    I am in need of estimating the MSI of monohull, catamaran and trimarans. Using Maxsurf Motion, I am capable of estimating the MSI of monohull and catamaran only since Maxsurf Motion cannot analyze trimaran hull forms. That's why I have opted for the empirical way of estimating the MSI. How do I do that?
    In the article "The prediction of the Motion Sickness Incidence index at the initial design stage" by "Tomasz Cepowski", I have got the following formula-
    How do I calculate the a_v (mean value of vertical acceleration) at midship?
    And this equation 1 does not contain any term involving the principal particulars of the vessel. Does that mean, MSI is not influenced by hull shape?
  2. Robert Biegler
    Joined: Jun 2017
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    Robert Biegler Junior Member

    Is the equation empirically derived from how motion sick people get? It looks like it should apply to any movement, in planes, cars, trains, lifts, or on bicycles or pogo sticks. In that case, hull shape is important only in so far as it has an influence on acceleration and roll (I assume that the omega parameter is roll), but for the same motion, it doesn't matter what combination of hull shape, course, and sea state produces that motion.

    The mean acceleration would depend on sea state, speed, and course relative to the waves. If you move with the waves, you experience a longer period, and therefore less vertical acceleration, than if you move against them. If you assume wave lengths long enough that the vessel will follow the water contour, and not go so fast that you have to take slamming into account, then you should be able to approximate the mean acceleration from wave height and from the wave period experienced at your speed and course. If you either go fast enough to worry about slamming, or if you have shorter waves, then the larger the waterplane loading, the slower the boat will be to respond to waves, and the smaller your mean acceleration. So I expect that the importance of hull shape for vertical acceleration very much depends on the conditions.
  3. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    The hull shape/size will matter. If you are sitting in a kayak; the forces on the boat are basically equal to the sea state.

    If you are in a massive catamaran; the hulls will have a dampening effect and a single person in the catamaran will not be experiencing the same accelerations as a kayaker in the same sea state. Consider the extreme of a cruise ship, the cruise vessel is rarely rising and falling in perfect rhythm, amplitude and frequency as a small kayak. I dare say never unless flat calm perhaps not even then.

    More precisely, consider a simple case where a catamaran is perfectly on top of a wave on one hull and in the bottom on the other. Midship, the person has experienced a (rise) fall of 50% of that of a person in a kayak. The ship (falls) rises after the crests and so the person in the cat is experiencing less vertical distance. So the amplitude for the person is less. The same waves have less effect on a larger vessel.

    I apologize I have only a real world experience sense on this issue.

    Here is a great article that should help you a bit.

    ABS - Vertical acceleration
  4. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    You need a series of RAOs of the vessel, in various headings and sea states.

    And why is that?

    I suspect.... it is because for multihull craft has different motion characteristics (which begs the Q how did they do it for the catamaran?), in that the lateral accelerations should be parallel to the deck; as it is a result of the of the pitch and roll accelerations as well as the competent of gravity proportional to the induced deck angle.
    Whereas monohulls consider the roll induced lateral accelerations and competent of gravity proportional to the roll angle.

    So, in short, you need to read the how's and why's the programme can do mono's and cat's but NOT trimarans?!
    BMcF and fallguy like this.

  5. mitchgrunes
    Joined: Jul 2020
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    Location: Maryland

    mitchgrunes Senior Member

    I don't know the technical aspects, nor can I give you appropriate references. I do get motion sick easily.

    I can tell you that there are a lot more factors than acceleration.

    From both personal experience, and others' experience, a huge factor is how well you can see the horizon, how much you can control the motion of the craft (because interaction helps you know what motions are about to occur, making motion sickness somewhat less likely - e.g. , driving a car is generally known to be much less likely to cause motion sickness than being a car passenger), and how much attention you are able to pay to the motions.

    I used to get carsick as a child.

    I once got sick in an old (4 meter, probably 1970's era) slalom racking kayak that I tried to use as a sea kayak. (Not optimal, BTW - they aren't as efficient to paddle under those conditions as good sea kayaks.) It occurred in roughly 2' beam wave conditions (i.e., where waves from the side tended to roll the boat from side to side). I never got sick in whitewater, even in the same boat, where the accelerations were a lot greater. I think a big factor was that the sea conditions and heading persisted over a long period, and that the motions were mostly in the roll direction, whereas there is more variety of motion in whitewater, but can't completely explain why that was a factor. But also that the kayak rolled with the waves very easily, and that the motions of those particular sea conditions were approximately in resonance to the natural roll motions of the boat.

    I used to have a job where I had to fly in an aircraft with no windows (in my area), while operating a computer (which made it a lot worse), under conditions where the accelerations (mostly but not exclusively vertical; there was also significant horizontal acceleration, as well as roll, pitch and yaw) exceeded 1 G. I threw up every flight. The aircrew and others told me that if I could have closed my eyes, and lied down (instead of being strapped securely into a seat for safety), it would have been better. It would probably have been better had I taken appropriate medications, but I didn't. I did learn (and was coached) to eat a small amount of salty foods (lick Saltine crackers), and water, but not to fill my stomach too much. I was told that it would have been better if I could see out of a window to the horizon - which seems inconsistent with the idea that closing my eyes made it better, but both are true. I can't explain that.

    There have been studies which indicated that some motion sickness is sometimes caused by a confusion between what your brain figures out from the sensors in the ear canals, and what your eyes see. For example, many astronauts got "space sick" under null gravity conditions, with NO net acceleration, presumably because people are not used to being in zero-gravity conditions, and their brains haven't figured it out yet. It was found that athletes - who are presumably most tuned into their bodies and the balance interactions from those ear canals - have a greater tendency to get space sick. Many also get sick while training in the "vomit comet", an aircraft which flies in short trajectories to simulate zero g. OTOH, others do not, and love zero g.

    I think discomfort conditions (like the loud engine noises in that aircraft - my ears are shaped in such a way I couldn't make earplugs stay in, though I eventually used earmuffs designed to quiet gun use) may also be a factor.

    I've also gotten motion sick (without throwing up though I feared I might) in small passenger planes, even when I can see a window. I haven't figured out the factors that affect that. But it specifically occurs under turbulent conditions. I have learned to try to avoid the front and back of the plane, where pitch accelerations are greatest.

    Anyway, be aware that acceleration isn't all that matters, and that other forms of acceleration (e.g., horizontal, roll, pitch and yaw) matter too.

    E.g., if your crew and passengers can easily see the horizon, with as large a window as possible, and which isn't too dark at night, and adjust their seat positions to be more comfortable, it might possibly help. And there are significant dietary factors that might be paid attention to. Plus, passengers with the greatest problem probably shouldn't be placed near the front or back of the boat, with the exception that being able to see ahead to the horizon might help.

    Hope that helps, though it wasn't what you asked for.
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