# newbie question, what encounter frequency means?

Discussion in 'Hydrodynamics and Aerodynamics' started by cristianon, Apr 9, 2017.

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### cristianonJunior Member

hello sir,

image hosting gif
is there anyone who can explain what's the meaning of encounter frequency and it's formula on RAO graphic?
thank you

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### jehardimanSenior Member

Encounter frequency is the derived apparent seaway frequency corrected for vessel speed and direction. So a vessel heading into the seaway has an encounter frequency greater than the seaway. Likewise, when running with a seaway, the encounter frequency is lowered. Beam to seas gets kind on tricky as the encounter frequency could be zero, but not really due to the difference between a nominal single frequency calculated RAO response and an irregular seaway response.

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Looking at the graph, the peaks are around 1.5 rad/s...so this means a period of roughly 4.2 seconds. So if the vessel encounters a wave every 4.2 seconds, from whatever speed direction (as noted by JEH), the response is increased. So for a unit wave of say 1.0m the response is almost 4 in heave. In other words, when encountering a wave every 4.2 s with a wave height of 1.0m the vessel shall heave 1 x 4 = 4.0m.

Similarly if the encounter is say 6 rad/s or every 1.0 s the response is almost negligible; since 1.0m x 0 = 0 !!

Conclusion, avoid the peaks, or resonance.

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### cristianonJunior Member

thank you for explanation sir,
Sorry but i wanna know, how do you get the period 4.2 seconds sir?
thank you

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Hmmm...sorry, if you are unable to work this simple one out...then the rest is way way beyond you.

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### hoytedowCarbon Based Life Form

Isn't the period the amount of seconds it takes 2 consecutive wave crests to cross the same geographical coordinate? If the coordinate is constantly changing as in the bow of a ship the period between crests would be lengthened or shortened by the rate of travel of the vessel.

I think I just wrote the same thing jehardiman wrote but in different words.

http://serc.carleton.edu/introgeo/interactive/qotd/waves2.html

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