Measuring Roll Behavior
In terms of roll attenuation, there are of course many variables. What works well on one boat, may not be as effective on another boat. For example, paravane size relative to boat size / displacement / righting energy will definitely affect the results.
The percentages quoted above relate in many cases to roll amplitude, which is only one component of rolling behavior... One can isolate several components, as follows:
* Amplitude (measured in degrees)
* Period (measured in seconds)
* Acceleration / deceleration (a result of the above, measured in feet or meters per second squared)
* Rate of Decay (number of cycles to rest or to some other benchmark)
Among the behavior patterns directly observable from the numerous data sources are the following...
The wildest rolling is referred to as synchronous rolling, i.e. rolling in beam seas when the wave period is close to the natural roll period of the boat in question. For example, when a given boat rolls to some extent in harbor, say on receiving the wake of a passing boat, it might not do so given a slightly different wake or wave pattern. Another boat that did not roll so much at a given wake or wave pattern may roll wildly with a different wave pattern or period.
The higher percentages for roll attenuation quoted above for any given roll attenuation method (paravanes, keels, etc.) are from measurements of the
attenuation of synchronous rolling, and appear to have their effect due to putting the boat out of sync with the wave pattern. The lower percentages in the range of effectiveness quoted appear to be an average of the overall effectiveness. In most of the published research, the majority of measurements were of
amplitude. The next most common data quoted were measurements of
acceleration.