Stability etc.

[Roger Long]

If anyone here is interested in what floats their boat and keeps the sunny side up, you might enjoy this plain language, minimal math, section of my web site:

http://www.rogerlongboats.com/Stability.htm

http://www.rogerlongboats.com/images/Metacenter1.gif

The last chapter is a brief introduction to some of the Coast Guard issues that cause such problems for the large traditional sailing vessels that carry passengers and students.

[Hwyl]

Book marked for later thanks.

[dingbat]

Found it the other day via your yawl dory thread. Great intro for a novice. Thanks very much!:)

[Woxbox]

Good stuff. Thanks.

[matoi]

Thank you very much!

[PeterSibley]

Oh yes ! Lots of reading !

[TomF]

Looks very interesting, Roger.

Within your theoretical model, what's the relationship between stability, and comfort in a seaway? I could imagine that these are largely but not entirely overlapping characteristics ... and would love a rational way to try to make choices between them.

[Roger Long]

what's the relationship between stability

Whew! That's a huge and complicated subject that is largely responsible for what success I've had in my career.

We'll have to come back to that. Quick teaser: The more initial stability a boat has, the faster it rolls. Counter intuitively, you can produce a more comfortable boat by designing for fast roll if you do it right.

[TomF]

Whew! That's a huge and complicated subject that is largely responsible for what success I've had in my career.

We'll have to come back to that. Quick teaser: The more initial stability a boat has, the faster it rolls. Counter intuitively, you can produce a more comfortable boat by designing for fast roll if you do it right.I can see how high initial stability makes for a fast roll - whether induced by form or by hanging a honking piece of lead down low.

I haven't got my head 'round your counter intuitive bit yet ... but I'll think about it over lunch. Thanks Roger - this is great fun. Please feel free to define the boundaries, so we don't overstep them.

t

[Ian McColgin]

I like the way you clarify the basic mechanics of modeling.

For TomF, there's not a single answer because no boat can do everything and trade-off's are the art.

Example, Granuaile, a 55'LOD 10'B 6'D c. 20T LFH Marco Polo as little form stability initially though with those big slab sides she'd pick up stability with heel. So she'd heel very easily to 30 degrees, could be pushed to about 45 degrees, and was nearly impossible to heel her further. Many found this uncomfortable, but she had very little wave or sea induced rolling, which was rather nice.

Another example, two identicle hull fat cats - Marmalade and another Chappequiddick 25. Both are very comfortably seaworthy (within their limits) so long as one's not trying to slam a head sea. Marmalade has a very heavy solid wood mast that deepens the roll a little - there's only so far you can put that massive beam down - and really dampens the return, so she's not only more comfortable than the other Chappy25 but she sails better in a seaway as she doesn't shake the air out of the sails.

And it's not just roll, though that's the usual issue of stability. The shape of the bow and stern and the weight distribution, not just the center of mass, make huge differences in how well the boat can handle various angles of the seas.

A boat is a bit like a symphoney: Your orchestra can have the greatest trumpeter in the world and the composer could make something as grand as the "Voluntary" but if the violin and other parts are not rightly composed, arranged and played, the piece will suck. You may for a given boat and purpose emphasize a part or parts - sail power, sail ease of handling, head sea punch, buttocks power or whatever - but if the whole does not harmonize, she's a dog.

[Roger Long]

I haven't got my head 'round your counter intuitive bit yet ... but I'll think about it over lunch.

This from the UNOLS (University National Oceanographic Laboratory System) site might help.

http://www.unols.org/publications/manuals/SBCompendium/motion.pdf

It's about power vessels but similar principles apply to sail. They are just harder to implement. Catboats have very similar in behavior to the type of research vessels discussed and it's probably no accident that I started cruising in a catboat.

[rbgarr]

I apologoze for not reading (yet) your link, but the animation makes me a tiny bit uncomfortable.

When the boat is at maximum heel it seems that the starboard portion of the section submerged is larger than the port section that had been submerged when level and is now out of water. Wouldn't that result in the center of buoyancy (axis of rotation?) moving down and to starboard some amount, at least for that section of the boat?

I'm not trying to make a big deal of it. Just curious.

[George Roberts]

If anyone here is interested in what floats their boat and keeps the sunny side up, you might enjoy this plain language, minimal math, section of my web site:

Not to offend. While some people have the skill to give a reasonably accurate (in the scientific sense) explanation of the issues involved for laymen, you seem to have fallen short.

[TomF]

This from the UNOLS (University National Oceanographic Laboratory System) site might help.

http://www.unols.org/publications/manuals/SBCompendium/motion.pdf

It's about power vessels but similar principles apply to sail. They are just harder to implement. Catboats have very similar in behavior to the type of research vessels discussed and it's probably no accident that I started cruising in a catboat.Thank you Roger, that was fascinating.

My takeaway summary is:

Comfort reflects an interplay of area at the waterline, where within the vessel you want to provide "comfort" (objects rotate around their centre of gravity), and the moment of the waves for which you'll be designing. Bigger usually feels better - and usually is more cost-effective (and delivers more capacity) than an all-the-bells-and-whistles-expensive but smaller motion-reducing hull.



Increase stability via greater waterline beam, and you may produce more comfort on deck through:

increased damping,
a faster motion than other hull forms, which if designed well will not set up harmonic increases in roll in relation to the anticipated wave period of the conditions for which you're designing, and
a centre of gravity raised to where people are likely to be (on deck).
knowing your passengers - newbies tend to prefer this motion.
as a bonus, you'll get more usable deck space.
Increase stability via greater depth (and/or I suppose ballast), and you may produce more comfort below through:

a centre of gravity closer to where people are likely to be - if the vessel's designed to carry passengers inside;
a slower, moment of roll - though this can more easily get amplified if damping isn't designed well enough;
knowing your passengers - experienced folks tend to prefer this motion;
Crew will prolly find the increase in depth (comparable to the increase in breadth posited above) overall less useful.
Have I done too much violence to your ideas, Roger?

[Paul Pless]

Roger, I just want to add that this non engineer, non naval architect finds the topic and your presentantion of quite readable and interesting. Please continue.

[Hwyl]

I apologoze for not reading (yet) your link, but the animation makes me a tiny bit uncomfortable.

When the boat is at maximum heel it seems that the starboard portion of the section submerged is larger than the port section that had been submerged when level and is now out of water. Wouldn't that result in the center of buoyancy (axis of rotation?) moving down and to starboard some amount, at least for that section of the boat?

I'm not trying to make a big deal of it. Just curious.

I thought the same thing, but I'm trying to keep below 10K posts, I think it's a case of CAD not equalling reality

If all boats behaved like the animation, you'd never see a keel root on an extremely heeled boat, capsized dinghies would have their centreboards at water level.

[Roger Long]

Have I done too much violence to your ideas, Roger?

That's a pretty good summary.

A good example of the principle is the craft which I designed with VP of another company:

http://www.sml.cornell.edu/sml_welcomekingsbury.html

Her seakeeping and comfort are legendary. In a light wind she seem quite lively and you might think, wow, when the wind comes up this is going to be a hard ride. When the waves get larger however, she settles right down. On the delivery trip, we crossed Buzzards Bay during a typical snotty southwester beam to the seas walking around the decks with our hands in our pockets. The trick is to tune the boat to the period of the small waves that don't produce severe rolling and then have the hull out of sync in the 20 - 30 knot conditions.

It was the, frankly, unexpected comfort of this boat that got me thinking about the whole subject.

[TomF]

That's very interesting.

I can see why boats like Redningskoites or Pinkies put so much emphasis on longitudinal balance. That would go a long ways towards eliminating the corkscrewing you'd get from an asymmetrical waterline plane, where one end would pitch more than the other while rolling.

I'm trying to sort out how windage factors in. Seems to me that in a power craft with a "steadying sail" you'd get direct damping of motion via air resistance. Essentially, friction.

You'd get that in a sailing craft too, but in conditions of decent wind and moderate seas, heeling motion would also "load the spring" both regarding initial stability derived from beam, and ultimate stability derived from the ballast arm. Depending on the hull form (i.e. chines or not, soft or hard bilges, keel and rudder size) rolling motion would be constrained by a variety of forces acting on the hull to damp it ... or not.

[Paul Kotzebue]

When the boat is at maximum heel it seems that the starboard portion of the section submerged is larger than the port section that had been submerged when level and is now out of water. Wouldn't that result in the center of buoyancy (axis of rotation?) moving down and to starboard some amount, at least for that section of the boat?


You are correct. In reality the "axis of rotation" changes with heel angle. Most sailboat hulls will "roll out" and trim down by the bow as they heel.

[Roger Long]

I'm not trying to make a big deal of it. Just curious.

The animation was only intended to show the principle of the CB remaining under the metacenter and not be a complete representation of everything that is going on.

[Roger Long]

you seem to have fallen short.

Well, thank you for that:)

[novapamela]

This novice thanks you for posting, Roger.

Cheers!

[Roger Long]

You are correct. In reality the "axis of rotation" changes with heel angle. Most sailboat hulls will "roll out" and trim down by the bow as they heel.

It's not as much as you might think though (at least for hulls similar to my animation) because the effect is not as pronounced in the ends.

The one set of computer model results I can lay my hands on quickly is for a 98 foot waterline schooner with 10.25 foot draft. The draft difference at 30 degrees, from a computer model that finds the exact waterline both in trim and heel, is .45 feet, 4.4% of the draft.

Dinghy like hulls roll out more but, for a 30 foot boat of traditional keelboat shape, it's only going to be in the neighborhood of a couple of inches.

[mcdenny]

Roger, Great explanation, thanks. I've fooled around in Freeship testing the effects of changing ballast but never really understood the GZ and GM terms.

PS: Guess you are not getting too much consulting work from the Coast Guard anymore...

[johnw]

Thank you, Roger, that's the best explanation of metacentric height I've ever read.

[johnw]

After all, the slowest roll of all is one where the boat just goes over and doesn't come back.

After that, you can go to sleep and not have to get up.

[ShagRock]

Thanks Roger from another novice! I'm not ready for the 'metaphysical' yet, as the physics itself is challenging enough. Your visual aids were most helpful in that regard. A few simple questions:

1) When sailors hike to the windward gunnel, do CG and CB both shift by the same or differing amounts? And is one of the purposes here to maintain an appropriate amount of tension on the righting arm?

2) For a small sailing trimaran, is it correct to say that the leeward ama is a 'buoyant' righting arm while the windward ama is a 'ballast' righting arm? Can the use of amas be compared to increasing beam on a monohull, or are these like comparing apples and oranges?

[Roger Long]

A few simple questions:

You should be able to answer these questions now yourself. You'll learn a lot more reading it again and thinking it through. If that doesn't work, get back to me.

[ShagRock]

Okay Roger..upon further reading at tea break, I'll have another go. If a sailboat heels to leeward, the CB will shift to leeward, but the CG will stay put. If there is risk of heeling too far, as with increased wind force in the sails, the crew can hike to the windward gunnel to offset this. Their weight is shifted to windward, so CG will follow suit. If they hold the leeward gunnel at the same level in the water as before, the CB will not shift. Thus the righting arm (distance between CG and CB) will increase thereby increasing stability through the force exerted by the 'righting moment'. Since all factors are in dynamic flux, the sailors' skill is required to keep the whole shambang in balance.

I guess that amas on a multihull will act like increasing beam in a monohull, but this does have it's downside as when flipped by big seas, a vessel with excessive beam would be happy to remained inverted.

While I'm starting to appreciate the horizontal shifts in CG and CB, I am struggling to grasp some aspects of 'vertical shifts in CG'. On one hand, it seems to make sense that one way to increase stability is to throw everything in the hold around the centerline to lower the CG. However, I recall reading somewhere that in the days of schooner trade, weights would be hauled up into the topmasts of schooners on windless days in heavy swell to offset some type of unstable rolling. I'm not sure how the dynamics work here?

[Roger Long]

However, I recall reading somewhere that in the days of schooner trade, weights would be hauled up into the topmasts of schooners on windless days in heavy swell to offset some type of unstable rolling. I'm not sure how the dynamics work here?

They were simply raising the center of gravity to make the vessel less stable which slows the roll period. For reasons too complex to get in here, the GM for any vessel is sort of like the length of a pendulum. There will be a specific rolling period for each GM length. The greater the GM, the faster the roll period.

A pendulum's period is dependent only on it's length. A boat's rolling period is also effected by the distribution of mass away from the center of rolling. Moving weights outboard slows roll. Raising weights to the top of the mast would therefore further contribute to slowing the fast roll of a vessel with its cargo stowed too low when there was no wind in the sails to damp the roll motions.

[Paul Kotzebue]

A boat's rolling period is also effected by the distribution of mass away from the center of rolling. Moving weights outboard slows roll.

Excellent point Roger, and well put. There seems to be a general misconception that the only way to change roll period is to change GM.

[Ian McColgin]

No confusion about that with trawlers. Running out and back they have the outriggers down - paravanes only in when the nets are shot.

A high great weight, like in the masts, will slow the roll but also can make it permanent. Widely spread weights on each side can't.

[JimD]

... I am struggling to grasp some aspects of 'vertical shifts in CG'...

The illustrations in the appendix of Tall Ships Down show the effect of this quite well.