Six Degrees of Freedom...

[Charles Burgess]

A boat has six degrees of freedom of motion, described by naval architects as
Fore and aft movement is termed surge.
Transverse movement is termed sway.
Vertical movement is termed heave.
Movement about a fore and aft axis is termed roll or heel.
Movement about a transverse axis is termed trim or pitch.
Movement about a vertical axis is termed yaw.


As an interesting term for the vertical motion of a boat, when the heaving of a boat gets within the frequency range of 0.15 to 0.2 Hz, the heave of the boat also causes serious heaving in humans - AKA seasickness. Thus, the frequency of retching is in direct proportion to the frequency of the heaving of the boat... 0.15 to 0.2 Hz being the not so sweet spot.

[John B]

Heave eh. how appropriate:D

A friend of mine was describing a very recent delivery to Fiji to me the other day . It was an S and S from the 60's and a miserable trip due to that exact problem. I was perplexed , because a sea kindly hull like that typically makes a fair average and constant speed with a fairly easy motion. He told me he thought it was overloaded though , and thats what caused the nasty motion.

[paladin]

Any sympathetic vibration or low frequency audio signal will trigger the same reaction.....animals can feel vibrations this low better than humans....a low internal rumble of a volcano will be felt by animals normally long before humans, as also similar vibrations created by wave action or tsunamis......a weapon using this technique is also used for riot control....early experiments prior to WWII by the Germans and later developed by the French resulted in design of such devices.

[S B]

This suggests that a seasick proof boat is possible, provided it can be designed not to move in that frequency. We have a hydrofoil ferry,between St.Pierre and Newfoundland, that has earned the name "vomit comet", everyone gets sick.

[Larks]

I reckon they've missed one - I've experienced what should probably be added as a seventh motion of freedom of a yacht that doesn't seem to be covered here. That motion of freedom experienced when you fall off the back of a hollow 40' wave in the middle of Bass Strait, I'm not sure what the proper "naval architecture" term may be, but "ohhhhhhhhhhhhhhhhhhhkahthumpertheregoesthacentrebo ardthroughtheslotwe'reallgoingforaverycoldswim" seems appropriate.

[Charles Burgess]

This suggests that a seasick proof boat is possible, provided it can be designed not to move in that frequency. We have a hydrofoil ferry,between St.Pierre and Newfoundland, that has earned the name "vomit comet", everyone gets sick.

Seakindness and seakeeping are components of seaworthiness.

Design plays a vital part in seakindness. Ship handling also plays a vital part too. Just as when the onside wave frequency resonates with the hull's roll frequency the vessel is in danger of capsizing and is remedied by changing course, speed, or both - the same principle applies to the retch:heave ratio (as MMD put it so well...we're pondering what Greek symbol is suitable for this factor....LOL). The retch/heave resonance is probably directly linked to the biomechanics of the inner ear.

[Charles Burgess]

I reckon they've missed one - I've experienced what should probably be added as a seventh motion of freedom of a yacht that doesn't seem to be covered here. That motion of freedom experienced when you fall off the back of a hollow 40' wave in the middle of Bass Strait, I'm not sure what the proper "naval architecture" term may be, but "ohhhhhhhhhhhhhhhhhhhkahthumpertheregoesthacentrebo ardthroughtheslotwe'reallgoingforaverycoldswim" seems appropriate.

Physiological effects of low frequency vibration - for what you described is properly termed the brown note....from wikipedia and MythBusters: The brown note is a frequency that is said to cause humans to lose control of their bowels due to resonance.

[rbgarr]

Sway is a new one on me. Does it refer to the whole boat moving bodily sideways, like an exaggerated slip?

[Charles Burgess]

Sway is a new one on me. Does it refer to the whole boat moving bodily sideways, like an exaggerated slip?

Sway is the formal term...slip is more descriptive, especially for sailboats. Much of the modern academic NA terminology borrows heavily from aerodynamics.

[Mad Scientist]

I've observed that every hull design will react badly when it hits the wrong combination of wave size and distance between waves. Not just small boats, either - you'd be surprized how a 24,000 ton fully-loaded tanker will pitch under the 'wrong' conditions.

Tom