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
Brilliant Dave. I’ve never connected the comparison of a metronome with a boats roll and stability. It makes visualizing it and its effects so much easier now. Looking forward to seeing what you will do with this knowledge!
ReplyDeleteHi,
DeleteMostly thinking about slowing roll at anchor for MUSTELID and eventually LUTRA. They're narrow enough that it doesn't take much to get 'em rolling in a side swell. Flopper stoppers are also a consideration.
Still, it's usually pretty easy to avoid side swell situations. Mainly, it happens a lot in one of our favorite anchorages close to our home port.
Wouldn'tchaknow?
Dave Z
I used to cruise a neptune 24. Shoal draft sailboat that felt like it was designed to roll while all other boat where comfortable at anchor. The best solution ive found was to run a spring line from the anchor rode to a stern cleat. You can then adjust tension to rotate the boat around until its comfortable. Sometimes ive had the boat 90 degrees to the wind to face wrap-around swell and make the boat comfortable.
DeleteThe spring line should be tied with a rolling hitch half a boat length down the anchor rode, which gives you ability to turn the boat 90 degrees to the wind.
That's a really elegant solution! We'll get a chance to try it out in the next weeks (we hope).
DeleteOur little cove is open to a larger body of water with swell wrapping in... usually calm at night with a light catabatic breeze setting us at odds to the swell. Your solution would be perfect.
Thanks!
Dave Z
Some designs were known for being "sea kindly" with an easy roll in most seas. Snappy rolls are very tiring.
ReplyDeleteSpeaking of such, how has your designs done in that situation? I've never heard you complain about snappy rolls and such.
Hi Michael,
DeleteI'm happy to say that at the 4:1 Length to Beam ratio we prefer for the larger barges, the motion has been very easy and 'shippy'. This is a mercy, as one of the main straights up here is known for its short, sharp chop.
I've not tried it, but I'm guessing that barge hulls that are wider for their length will tend toward snappy in a chop.
Shemaya Laurel (Sailing AUKLET) has more experience than anyone I know in that regard, and I'll try to remember to ask her next time we talk.
She did mention once that her (wider) TBoat angled alarmingly over on the face of large waves (yeah, I know!). But when I told her that's what theory predicts - that such a hull stays relatively flat to the water surface. She said she relaxed after that, and no longer felt any bit out of control. She's never complained about 'snappy'.
Sooo... my guess is that the wide-spread buoyancy is snap damping, though I'd think that a beam on chop would be uncomfortable, and likely more so than in a deeper, possibly ballasted hull.
Dave Z