General HydroStatics
Ship Stability Software
Command of the Week
(New or interesting aspects of GHS that you may not know about)

(Requires GHS version 16.28a or later with SK)

Passenger vessels are frequented by...well, passengers. It comes as no surprise that many, if not most, of these passengers spend very little time at sea. This means motion sickness is a reasonable concern when the general public goes for a boat ride. So how can the naval architect use GHS to evaluate their designs for seasickness? Motion Sickness Incidence, or MSI, is one easy-to-use option, and it is now built into GHS.

MSI is not a new concept. This "standard" approach for quantifying motion sickness was originally developed by a group of researchers in the 1970s. They built a motion chamber (a platform suspended by hydraulic cylinders) and paid college students $5 to take a ride. They counted the number of students who vomited. This is perhaps not a very glamorous research experience, but the data set allowed the researchers to develop a statistical model of motion sickness incidence for unhabituated passengers, and the model fit the data quite well. The model returns the expected percentage of passengers that will experience seasickness when exposed to certain vertical motions.

The /MSI parameter in GHS is attached to the SEA command and it is very simple to use. All you need to do is give the location(s) where MSI should be computed, and SEA will return the standard 2-hour exposure MSI for the specified seaway.

Fishermen, tanker crew, and OSV operators aren't really the general public, so to realistically demonstrate /MSI, we'll have to use a different model. We specify the motion sensitive locations on our vessel using Critical Points, as shown:

The five motion sensitive locations on the vessel are then passed to SEA in the example code below. The /MSI parameter is applied to all critical points using the CRT identifier.

 wave (br) 11,13.12
 sea /speed:20 /heading:135 /samp:30 /crt:1,2,3,4,5 /msi:crt
For the specified Bretschneider spectrum, speed, and heading, the MSI output looks like the following:

The 2-hour Motion Sickness Incidence "MSI(2)" is given for each critical point. The absolute RMS vertical velocity and acceleration, two quantities that are primary inputs into the MSI model, are also given for verification.

It is also possible to change the length of exposure time by using the /TIME parameter, but the default MSI exposure time is 2 hours, as shown in this example. Note that MSI is computed for "unhabituated passengers", meaning long time periods may over-predict the actual incidence. This is because people tend to become less prone to motion sickness once they're exposed to the motion for a while, but that can take many hours. For those poor chaps looking forward to enjoying the foredeck during their passage, things might get a little woozy.

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