Now roll on, Buddy?
Why ya roll so slow?
How fast can I roll
With muh weight so low?
-- Adapting a Folk Song
Metronomics: Understanding Roll
Being an Old Fart, I had the privilege of growing up with mechanical metronomes. Mechanisms - as opposed to algorhythms - that set and keep the beat.
Metronomes keep the beat, adjusting their tempo by sliding a weight higher or lower along a bar.
Higher Weight = Longer / Slower
Lower Weight = Shorter / Faster.
Puts me in mind of the dreaded short and snappy roll that fatigues and throws crew around if not overboard. Longer, slower roll periods are desirable. Up to a point. Beyond that point, they can become self-reinforcing and enter 'death roll' dynamics. We're with Goldilocks... we're looking for a sweet spot that's juuuuuust right.
Being a sailor, I can't help but see roll dynamics in the metronome.
Bar = Mast
Weight = Rig's Center of Gravity (CG)
Axis = Hull's Center of Buoyancy (CB)
Counter-Weight = Ballast CG
Tempo = Roll Period
The connection is easy to spot. We can adjust roll period (by raising and lowering the Rig's CG while all other points remain constant.
A baseline is established by weight distribution along the mast, most likely at construction time. Hollow masts offer lots of options for messing with distribution via placing of internal weights. Choice of hardware (e.g., masthead fittings, blocks, spreaders, etc.) for weight placement all count. Designers aim for one-size-fits-most.
Junk Rig is interesting... as the sail is raised, more weight goes aloft. Typically the yard is heaviest at the top, while battens average out(?) along the mast. The total effect is that more sail slows roll.
This suggests the possibility of elective roll period manipulation via weight that can be raised and lowered to adjust for conditions. The challenge would be a 'doohicky' which would observe these Thou Shalt Nots...
- Not be difficult to deploy
- Not interfere with the rig
- Not swing wildly
- Not be difficult to dowse
- Not be difficult to stow