Roll Control – We look at systems and techniques for reducing a motoryacht’s roll motion and making the ride more comfortable

Text and illustralions by Christopher D. Barry

)elft Umversity of Technology

Ship Hydromechanics Laboratory

Library

Mekelweg 2, 2628 CD Deift.

The Netherlands

one: +31 15 2786873- Fax: +31 15 2781836

22 PROFESSIONAJ. BOATBUILDER

We. look at systems and techniques for

reducing a motoryacht's roll motion and

making the ride more comfortable.

One

lems associated with motoryachtsof the most notorious

prob-is that they roll in waves, both under

way andoften more importantat

anchor, when the owner is aboard entertaining guests. Superyathts have long, relatively narrow, round-bilge hulls for maximum efficiency, and these characteristics exacerbate roll. Roll can actually be greater on small vessels because their small size cre-ates higher metacentric heights com-pared to their beam than most large ships (more on. metacentric height, below). That is, they are more stable for their size. The smaller the boat,

the shorter its natural roll period.

Since short-period seas are the most

common type, smaller boats are more likely to encounter sea states that

pro-duce large motions. Trawler yachts

and other ocean passagemakers

spend long periods in unfavorable

wave conditions, so roll control is key. to comfortable voyaging. Fortunately,

there are a number of techniques and

systems available to reduce roll

motions. Each has its advantages and disadvantages, but one or a combina-tion can be used to reduce roll and

make a boat more comfortable.

Sefore we look at these in detail, let's

review the physics of rolling.

Roll Fundamentals

A boat rolling in waves can be

modeled as a damped spring-mass

system, as can buildings in

earth-quakes, musical instruments, radio receivers, and many other

phenom-ena. Similar physical principles

govern all such systems, which have

a restoring force (the spring) and

some sort of inertia (the mass). We

can learn a lot from a few simple

experiments with a readily available

damped spring-mass system: the

pen-dulum. A. plastic-rod coat hanger

makes a good pendulum and is

actu-ally an instructive model of rolling. Hang the hook on your finger. Now

pull the coat hanger off vertical; this is

storing energy in the system in the form of a lifted weight. Then let go.

The spring of the system is the

"coup1e"two equal but opposite

forcesproduced by the weight of

the coat hanger acting down and the

support of your finger acting up when

they are out of line. A sideways

restoring force is produced by the couple to bring the coat hanger back to vertical. However, the mass of the

coat hanger acquires some velocity as it swings toward the vertical position,

so when it reaches vertical, it over-shoots. This is storing energy lxi the

system in the form of a moving

weight. By the time the hanger stops,

it is well over on the other side, so it goes back down., The action repeats until the damping effect of friction

against your finger and motion

thro'.igh the air take away all the

ini-tial energy. Energy is cyclically

chang-ing between the two forms of

energythe lifted weight and the

moving weightuntil it is dissipated.

A boat acts in the same way as a pendulum, with its weight supported

at

the "metacenterthe point

through which the buoyant forces

appear to act (your finger, in the case

of the coat hanger). Metacentric

height (GM) is the distance between the metacenter (M) and the center of gravity (G). Under the influence of

wave action, the boat rolls like a

pen-dulum suspended at the metacenter with the bob at the center of gravity.

I

GM

The couple between buoyancy and

weight acts to restore the boat to

vertical, but inertia of the rotating mass of the boat causes it to

over-shoot, the, upright position. The larger the restoring force, the more energy it

can store, and the faster the natural

frequency.

Rolling the boat immerses a wedge

on the low Side of the hull, while a

similar wedge on the other side

emerges. These wedges cause the

center of the buoyant forces to shift to

the low side. If we plot the line of

buoyant force straight upward, it

intersects the center plane at M

(Figure 1). Metacentric height is the

principal measure of stability, at least

for small angles, and it also controls

the roll period. So, the roll period is a

good practical measure of stability. Measure the roll period in various load condjrjons in calm water. If the roll period increases, it's a wa±ning

that stabifity may be compromised by

flooding or increased topside weight from icing or 'improper loading. On

boats such as fishing vessels to which

lots of load or ballast can be added, reducing GM may make a boat "feel better," but reducing it that much might cause potential stability

prob-lems. The operator of a boat on

which the load can vary substantially needs dear loading instriictions so he won't inadvertendy hazard his boat.

Now, back to our coat hanger.

Make slight oscifiating movements of

your finger at different frequencies.

5, 'a a 0 0 to 0 0 0 0 'a C to E 0 0 a 0 at 2.0 Figure 3 Phase angle

Notice that you can get the coat

hanger swinging, quite well if you move your anger at a certain rhythm Any springmass system has a

well-defined natural frequency (also called

eigenvalue). It 'is' said to be in

reso-nance with forces that excite it at this

frequency, and will actually magnify the exciting condition, often by a

magnifi cation ratio of 10 or more.

The magnification ratio is also

dependent on the damping. Figure 2

shows the ratio of boat roll to the maximum wãre slope. The damping

ratio is the percent of damping divided

by'the damping needed to have the

motion die down by the time it

reaches' center, without oscillating at all. Perform the oscillating experiment with the coat hanger again, but watch careftilly the relative motion between

your finger and the hanger. At slow frequencles, the hanger will move in

the same direction as your finger, but

Roll Response Magnification

Phase lag equivalent to angle

Figure 1The metacenter, M, is the point through which buoyant fOrces appear to act; metacentric height, GM, is the distance between M and the center of gravity, G. In waves, the boat rolls like a pendulum suspended at, M, with the bob at G.

Figure 2The ratio of boat roll to the maximum wave slope is amplified at the natural roll frequency, especially in lightly damped hulls.

at resonance, 'it will move in the

opposite direction. It is said to be out of phasethe response lags behind the condition producing it. Because

an oscillatory motion is cyclic, the lag

can be expressed as an angle, called the phase angle (Figure 3). The shift

in phase angle at resonance, shown in

Figure _{4, is a critical element of roll}

and an important factor in some

motion-control systems. You may

hear the term response amplitude

operator or £40. This is the combi-nation of the phase-shift plot and the magnification ratio, which together allow the motions of the boat to be

calculated from the waves.

Unlike itt _{a pendulum (but just as}

with the coat hanger), the mass of a boat is distributed th.toughobt the boat's volumeabove, below, and to either side of the center of rotation, which is close to the waterline on

center. If we divide the boat into

Figure 3The phase angle represents the lag between a wave and a vessel's response to the wave.

FEBRUARY/MARCH 2005 23 slender

-

motoryacht w/bilge keels damping: damping 10% damping 20% damping Resonant waves-Is more than on is moat important ressnance. wave rail damping. 7:1 6 1 I boat ro p slOpe depending . Damping near roli - Yacht Cntica 100% 51

_______

Short waves

boat rolls less than wave

Long

waves-same as wave slope

1 _{slope boat roils}

:i

11

1.8 L6 1.4 1.2 10 0.8 0.6 0.4 0.2

Ratio of natural roll penod to wave period

Figure 4

-

Phase @10% damping

- Phase @20% damping - Phase @100% damping

relatively small masses and assume each is so small it acts as if it were concentrated at a point, each mass will be a characteristic distance from the center of rotation. The distance

from the center of rotation to

the mass is called the gyradius of each mass. The larger the gyradius, the far-ther the mass moves for a given

rota-tion angle, and the longer the

moment arm it acts through. The

square of the distance times the mass

is the moment of inertia of the mass, and the total moment of inertia is the

sum of the moments of all the pieces.

Dividing this by the boat weight and

taking the square root gives the

aver-age gyradius of the whole boat

(Figure 5), which is a convenient measure of moment of inertia. The

larger the moment of inertia, the more

energy it can store, and the slower the natural frequency. This point is

important for sailing yachts in

break-ing beam seas: A yacht with a large roll moment of inertia tends to rotate slowly and resist getting knocked

180" means roll is exactly opposite wave slope

'

Near resonance, maximum boat roll falls increasingly behind wave slope

180

0.. Ratio of natural roll period to wave period

2.00 1.80 1.60 1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00 Phase shift 150 120

-

go

-

60 - 30

Figure 4.A shift in phase angle at resonant frequency is a critical element in roll, and an important factor in

0 some motiOn-control systems. Figure 5The gyradius

is a measure of a vessel's moment of inertia.

down. The mast, though light, has a large gyradius, which when squared

contributes much of the total moment

of inertia. So, keeping the mast up greatly boosts the chance of survival in storms. Bulb keels don't have as large a gyradius, but they do have a lot of mass, so they too will be more effective than a fin keel, even with

the same center of gravity.

Hydrodynamic forces act as if some

water is stuck to the hull and rolls with it, increasing the moment of inertia. This is called added mass.

Specialists in ship motions sometimes

call these added-mass forces imagi-naiy forces (also known as masslike

forces), because they are actually

sep-arate forces in the equations of

motions that are in phase with accel-eration. ("Imaginary" in this context does not mean "unreal"; the term is based on a mathematical method for

dealing with phase angles.)

In

long-period wavesmuch

longer than the roll periodthe boat

basically follows the waves, remaining

level with respect to the wave surface. Short-period waves are too fast to get

much response at all from the boat,

though any response tends to lag

behind the waves. At and near the roll period, however, the boat rolls a

lot more than the wave slope and

generally opposite ita thoroughly

uncomfortable situation, as shown in

Figure 6.

Stability and moment of inertia determine the roll period. The larger

the stability moment (GM times

weight), the faster the roll; the larger the moinent of inertia, the slower the

roll. Stability moment and moment of

inertia both increase with weight, though, so weight cancels out. We can compare the average gyradius of the whole boat with the metacentric

height to get the period:

T = 2rtk/ (g GM)4 or T5 = C B / (GMft)'4

In the first formula, k is gyradius, and g the acceleration of gravity in

Figure 6When the wave period is at or near the boat's roll period, rolling is not only amplified but lags behind the wave slope, making for an uncomfortable ride.

Figure 7A hard chine imitates the action of a bilge keel, but is less effective. The graph at right compares the performance of round bilges, double chines, and hard chines on typical offshore supply vessels.

units consistent with T, k, and GM.

The second formula uses the fact that the gyradius is usually proportional to

beam, B, and inc1ude the various

constants in C, which typically ranges from 0.34 to 0.46. In the second equa-tion, the period is in seconds, and the other units are in feet. The period of a

pendulum also depends on the

square root of its length, so these

equations should be no surprise.

Resonance and damping explain a lot about designing boats for good roll motions. We want to have a hull that damps as well as possible, and has a roll period away from typical wave periods encountered during operation. The hull must not have a

roll period short enough to cause

excessive accelerations, or that occurs

at frequencies that tend to bother

humans. Human beings have evolved

to a point where oscillations at a

period of about one cycle per second are not just acceptable but pleasant,

which is why the motion of a rocking

chair is comfortable.. This is roughly

the frequency of walking. Oscillations

in a range of three to 12 seconds are

unpleasant; with about seven seconds

the worst. (ISO 2631-1:1997 gives standards for discomfort boundaries for motion sickness.) It's also impor-tant to make sure that roll and pitch periods aren't close together. If they are, they'll tend to produce a

cork-screw, or so-called Dutch, roll in

quartering seas that is exceedingly

disturbing. Calculating and estimating

ship motions is a potentially difficult topic, but good software and tech-niques are available to help, as are

ship-motion specialists. A very

approachable practical text on the, subject of ship motions is Bhatta-charyya's Dynamic

of Marine

Vehicles. [Unfortunately this is cur-rently out ofprinl though it may soon be revised or reissued on CD. See References at the end of the article:

26 PRossIoNi BOATBUILDER Figure 7 0. 3 0.2 E 0.1 0.0.. 3

The standard text is Principles of

Naval Architecture, readily available from the Society of Naval Architects

and Marine Engineers (Jersey City,

New Jeiey)Edj

Roll-control Systems'

Once we understand the basics of roll motion, we can start thinking about controlling it, mainly through increased damping and shift of

reso-nant period.

Sails. Let's return to our coat

hanger example. Fasten a stiff paper card to the hanger so that it catches

the air as the hanger swings. The

hanger will slow down quite quickly

and will swing much less- for a given

amount of force applied. (How big a card is required to get 100%

damp-ing?) From our simple pendulum, we

can see that adding damping is one

way of minimizing roll, and the oldest method is similar to the card:

steady-ing sails. A small sail can add a great

deal of air drag, which will signifi-cantly cut down on roll. Fishermen have long used steadying sails. In

order to prevent flogging, a sail

should be cut dead flat with a hollow leech (which also nlakes it inexpen-sive) and be rigged boomless. Full-length battens may be useful as well, to reduce noise and increase the life

of the sail. One ieason sails work well

is that fluid dynamic forces go up

with the square of speed. The portion

of the sail near the top of the mast is

traveling through the air very fast due

to the long distance from the roll

center (roughly the waterline), so the forces generated by it are large. These forces also produce a large

gound

moment, again because the distance

to the roll center-is large.

'Bilge keels. Sails can .be

incon-venient and can cause problems,

especially with steering. Water is 840

times denser than air, so underwater damping appendages can be much smaller than sails. Most s-hips- use

bilge keelsshallow, long fins

welded to the hull at the turn of the bilge. Typically, they stick out a foot or so from the hull of a- large ship

over about half the length of the

ship. Bilge keels can reduce roll by 50% or more. They are a good first

step toward controlling roll, especially

on round hulls. Though their effec-tiveness is limited because they can't

stick out too much or be very fai

from the roll center, they are easy to fit, inexpensive, require no mante-riance, and add only a very small amount of drag. A hard chine will

also act as a bilge keel, though it may

not be quite as effective. Figure 7 shows the effect of round bilges and

double and hard chines frir fairly

typi-cal offshore supply vessels. Bilge

keels- also act as lifting surfaces when

the boat has some speed on, so they are more effective when the boat is

moving than when standing still.

Anotherthough rarely usedoption is retractable passive appendages, which could be a good deal larger than fixed bilge keels. A centerboard or a pair of thggerboards at the bilge would work well. E'en easier (but

odder looking) is a simple pair of lee-boards fastened to pivots at the

sheer-strake, port and starboard. Any of these appendages can extend -deeper than the bottom of the boat when

z

bilge

U

8

down and will damp roll

consider-ably. In shallow water, they can either

pivot up or, in the case of leeboards,

be removed altogether.

Catamarans. Another means of increasing hydrodynarnic damping is

multiple hulls, because when a

multi-hull rolls, the separated multi-hulls bounce vertically. Rolling a log doesn't pro-duce much damping, but lifting it quickly in and out of the water pro-duces a lot. Catarnarans also tend to have high metacentric heights, and therefore fast periods. Waves with short periods don't have very much

energy, so cats are rarely in resonance

with strong waves. The slender hulls reduce pitch damping, however, and

combinations of pitch and fast rolling

can produce high vertical accelera-tions in some sea states; thus, cats especially big onescan get very ani-mated in the right conditions. Plus, the pitch-and-roll natural frequencies get closer together, so Dutch rolling,

mentioned above, is more likely.

Active fins. Many yachts and

ships, especially fast military vessels, use active fins for roll control. Fin sys-tems from Naiad, Wesinar, and others

have been available for years and are fairly common on yachts over about 60' (18m). These fins extend farther outboard than bilge keels, but they have two big advantages: they rotate to oppose roll by providing lift, so

they can produce large forces at small

angles of roll; and they are

automati-cally controlled by gyros that activate

the fins with very small roll angles. Passive-damping devices must allow

a fair amount of roll before they

produce damping, but active devices

can actually eliminate all but the very

small amount of roll required to

activate the control system. (In fact,

you can usually run the fins manually to induce roll in flat water.) The chief

disadvantages of these devices are:

their cost and maintenance; the amount of interior space they require; the fact

that they work only when the boat is going fast enough for the fins to

develop lift; and the small drag

penalty they produce. Another

disad-vantage is that the fins must be

approximately amidships, so the actu-ators may have to be located in public

spaces or cabins, and the passengers might hear the actuators running all night. On some hulls the fins must extend outboard of the deck for

ade-quate damping. Since these fins usually 28 PRoFEssIoNp,i BOATBIJILDER

AboveThe U.S. Coast Guard's

110' (33.5m) Island-class offshore patrol boats are fitted with nonretractable active fins for roll stabilization. Patrol boats such as these often have easily driven hulls with relatively little roll damping, and so are commonly fitted with fins. Shown here is theWrangell,

being loaded aboard a heavy lift ship for transport.

RightA computer rendering

of the Zero-Speed stabilizing system from Quantum Hydraulics. While active fins generally work only when a boat is going fast enough to develop lift, this system can control roll on a boat that's at anchor by rotating the fine vertically and actively flapping them.

retract, they take up more

interior space. And, they

create the possibility of loud,

expen-sive noises if the operator forgets to

retract them when coming alongside a dock or quay.

A variation on the active-fin theme sometimes seen on naval craft is

rud-der roll control. In this system, the steering gear is operated by special

control systems and gyros so the

rud-ders act to suppress roll. This can be

'. -.--- _-;,,

L

-7 0 0 0 0 0

very effective at high enough speeds and eliminates the need for separate

fins. It works because roll frequencies

are much higher than turning fre-quencies, so the two control systems are essentially independent of each

other. Another variation actively flaps the fins to generate forces. It can

pro-duce a force at anchor or low speeds and may provide some roll control

Built by Park isle Marine (Victoria, British Columbia), this 65' (20m) full-displacement trawler yacht is equipped with paravane stabilizers, shown here in the deployed position. The vessel also carries a steadying sail.

in these conditions. The advantage

of such systems is that one set of

hydraulic actuators and fins controls

the boat for its entire speed range.

Paravanes andflopper-stoppers. Paravanes and flopper-stoppers are

other removable damping appendages.

Since a long arm from the damping device to the roll center substantially increases the damping effect, setting relatively small fin or plate dampers well outboard, hanging off an out-rigger, can be quite effective. Para-vanes are usually steel plates shaped

roughly like delta-winged jets. (These

triangular marine stabilizers are

patented by Kolstrand Marine Supply in Seattle, Washington.) They are

bal-anced and rigged so that when the supporting wire goes slack they dive

rapidly. When pulled up by the

30 PROFESSIONAl BOATBUILDER

wire they assume an angle to the

water that produces a large down-ward force on the rigging,

suppress-ing roll. Like active fins, they require

ahead speed to work well, and are sized for a particular range of speed. Flopper stoppers are generally some

sort of disc, plate, or sometimes even

a leaky bag. They produce pure drag

-s ç .sCt,_sa a

?-(or in the case of the bag, drag plus the weight of the entrained water). They are meant to be deployed when

the boat is stopped.

Paravanes and flopper-stoppers are very effective at roll control,

inexpen-sive, and often used by research

ves-sels and fishermen as well as by

yachts. The forces on these devices N

0

Figure 8Ant/roll tanks are designed

to be nearly resonant, but out of phase, with a boat's roll. These tanks also reduce GM slightly, and shift the resonant period.

are substantial, and the outriggers and other rigging must be strong. Disadvantages of paravanes include the difficulty of handling them, the drag they produce, and the narrow

range of speed at which they're

effec-tive (unless they are changed to suit the speed). Most paravane installa-tions are fitted by the owner after a

vessel's construction, rather than

being designed in, and this often

con-tributes to the difficulty of dealing with them. Designers may want to

anticipate the insr2lltion of paravanes

and develop arrangements that

pro-vide for them from the start.

D.W. Bass of Memorial University in Newfoundland recently discovered one more important problem with

paravanes. In 1990, the Canadian

-.

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Shpr

where the 00oot& was

the Fëadthip en

crtin

}it

can adst an aicht.ted ite'ssel

blt),.=-nd tbe Dt Vo..oØr

a'val thy tiId tedtrce the vesseLs o1llk

h.ea41gg aWay fronben s

an,thitectiw finn ..Ud an in-.dth

motion, by .61)% to 75 V1ta 'we1l' edixc& ie fold

Figure 8

fishing vesselStraitsPride II capsized,

with the loss of several lives. Bass determined that one paravane was carried away in bad weather. Due to severe conditions, the crew couldn't

bring in the other paravane on the lee

side, and it acted to increase, rather than reduce, the roll of the vessel. Other fishing vessel losses are sus-pected to have occurred in the same way, although no survivors are

avail-able to confirm this. What happens is that as the vessel rises and rolls away

from a crest, the paravane on the lee side is pulled upward, producing a

strong downwave moment. There are other effects involved as well, but the

key point iS: If one paravane is lost, the other must be either recovered or

cut free immediately. Anyone rigging

look t the.con,apany srad its h,tory, deslgn.d antriI tänlc a subse-

i son arlk

btFge ets.,.Thnd

see- 'O ecs4 at a TLfne

B quent 20Q

stufy Fe1aip con-

actrv stab17fs ar

rh're f th

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ts.ts.to eoeipa.e 1e

1QSt effectve rnen of cntzofl

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tith he Dutdi Maitii1e

peifoiinanoe of the antsroll rank wrth rofl ,r anchor smce th

dotIe1y

Research Titb

(MAIiN) to per-

crive stbzer fin systearis

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on bdtsped fOr thew

eratipn

Tfo eOrputer sthuiaton and tafik peed: these s _{teftis lnove up and}

as &, say' c

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rstr'rig to evajuate varroüs hull -4orn

iucb hife owin,g oars tie

.ñns os paravanes (The pae

s.hzpe and rflQtioa-control Istem rnp

_{roil moion Th}

re.siilts resulting frori Feadsh.ip s

tesCjro'-The designers focused on, etacen'

showel thgs zñ 0 Snt [i6 waviss

grm, tiU,ed Tasir (o)E)n

c he1at as a key factor

roll the ilizer fins refe those e.ffec- tloar !4otofyad iras 1ished

.ffi.btton at anchor arid on of tjae

tive tbr th antirfl tin1

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posiuth n Yabt Degn. Ya

na'tn,ralroll peiiod lower snetaceri,-

systems we about ea In 2m [tJ

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-ttid hegha resuitis m lohger natural

'vates theantiroll ritik was more

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.-32 PROFESSI0NAJ. BOATBUILDER

Roll motion

A

Antiroll moment

paravanes should make sure that it's possible to release the paravanes no

matter what.

Antiroll tanks. To provide roll control at a range of speeds, and to eliminate the drag and potential dan-ger of paravanes, Bass decided to

experiment with a common

roll-control device for large ships: anriroll

tanks. Antiroll tanks depend on the effect of water sloshing in a tank to

partially counteract the forces of roll.

The tank is designed so that it

is

nearly resonant, but out of phase, with the boat's roll (which occurs at

resonance). As the boat rolls one way,

the water starts to slosh to the low

side, except it is a bit late. By the time

it reaches the low side, the low side has become the high side, and the

Figure 9At resonant frequency, an

antiroll tank can add the equivalent of

4O-6O% dampingprovided the

tank's natural frequency, damping, and the amount by which it reduces GM are calculated to optimize the effect.

weight of the water acts to counteract

the roll, as does the inertial force of the rushing water (Figure 8). The

tank also reduces the GM slightly, and

shifts the resonant period. Bass

installed tanks on several trawlers out

of Newfoundland and found that the tanks were actually more effective

than paravane.s.

We can use our coat hanger to

demonstrate an antiroll tank. Hang another coat hanger off the top one. Notice that when you move your finger, the top hanger moves much

less, and the lower one mdves wildly.

Energy is being transferred to the lower hanger. In the same way, roll energy is transferred to the water

sloshing in the tank. Adding baffles or nozzles damps the energy in the tank,

34 PRozssIoNi BOATBUILDER Figure 9

00

_0

2.0 10:1 9:1 6.1 3:1 2:1 1:1 :1 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.

Ratio of natural roll period to wave period

adding damping to the boat moridns.

At resonance, tanks can add the

equiv-alent of 4O%-6O% damping (Figure 9). Nevertheless, the parameters of

the tank, its natural frequency, damp-ing, and the amount of GM reduction it produces must be selected carefully

to optimize the effect. Note that the roll response shown in Figure 8 has a

peak on either side of the natural

Antiroll Tank Effect

0

frequency. If the thmping of the tank were less, it would have more effect at exact resonance, but less effect off the natural frequency (and the two peaks would be higher), so an odd,

jittery motion could result.

Antiroll tanks can be U-shaped

tubes; or a pair of tanks connected by

a narrow, full-depth trough (Figure

10); or a series of tanks connected by

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tanks can take a variety of shapes. Here, a pair of tanks is connected by a

narrow, full-depth trough. Or, they can

be a series of tanks

connected by

baf-fles, as in Figure 11

(facing page). The

flUid may be fresh

water, seawater, or even fuel. Their main disadvantage is the

space they consume.

baffles (Figure 11). The period can also be tuned in some tanks by

set-ting the depth of water. Antiroll tanks

would contain about 1.5%-2.5% of the weight of the boat and reduce metacentric height about 20%. This also changes the roll period, which

may be beneficial. The fluid could be

36 PRossroN BOATBIJILDER

seawater, fresh water, or even fuel. The main disadvantage of antiroll tanks is the space they take up. The wider they are, the better they

per-form. Tanks also work best if they are

above the waterline, because the

force of the water moving in the duct

then opposes roll, too. Unfortunately,

-this means that they take up premium

space. Still, a designer who under-stands the space requirements for the tanks and designs them in from the start should be able to find a clever way to minimize their impact. One

option might be putting them below a bridge that's raised half a deck for vis-ibility, and offsetting the crossing duct forward. A stair could use the remain-ing half-height space. Sloshremain-ing sounds

can be disturbing, so if the tanks are near passenger spaces, they probably

have to be insulated.

A boat's rolling characteristics must

be known so antiroll tanks can be designed with a period close to that

of the vessel. The designer may have to

perform specialized analysis both

to calculate the rolling characteristics

of the boat and the period of the

tank. Roll characteristics of a boat are

usually determined by a computer program, often the Standard Ship Motions Program developed at the

Naval Surface Warfare Center in

Bethesda, Maryland. Tanks can be

designed either with special software

or by standard manual calculations.

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a

Once a tank is designed, it is often

bench-tested to verify its period,

damping, and other characteristics, such as the conditions at which it saturates, due to water hitting the tank top, and loses effectiveness. A

scale model of the tank is made, typi-cally of Plexiglas. The model is filled

with water and rolled at various

fre-quencies in a frame, and the forces it produces are measured to ensure they match the boat.

The most common reason for a bad

antiroll tank installation is that the weight or GM of the boat has been incorrectly estimated: the boat has a different range of periods than that

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for which the tank was designed. As in so many other areas; good weight

estimates and ongoing weight control

during construction are the most

important factors in a good antiroll-tank installation. Also critical are

allowing for some tank modifications,

if needed, after launching, and plan-ning for future weight changes. The

design of antiroll tanks is usually done

by naval architects versed in ship motions, Antiroll tanks are in wide

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service in ships, in particular research

vessels, large fishing ,essels, and

many types of passenger vessels. I first encountered antiroll tanks on Navy combat-support auxiliary ships

just a few days into my first job. Such

ships transfer fuel, ammunition, and supplies to combatants by passing

them across a taut wire stretched

between the two ships, and

suppress-ing rollsuppress-ing is critical to success in this operation. Feadship in the Netherlands

is the only major builder of yachts with antiroll tanks, but there is no reason tanks can't gain much wider acceptance, even on moderate-sized yachts. The fishing vessels on which

D.W. Bass installed tanks were in the

65' (20rn) range. [For a closer look at

Fead.shi:p 's research into antiroli

tech-nology, see the sidebar on page

30

Ed]

Another variant on antiroll tanks is

controlled passive tanks. These are U-shaped. tube tanks, but the tanks

are airtight. The air vents at the top of

the tank are connected to each other

through a valve controlled by sensors

and a computer. The valve controls

the motion of vatr in the tank to

move as required. Such tanks can be very effective, but are relatively rate, partly due to the complexity of the control systems, and to the fact that the period of the tank has to be faster

than the boat's roll period in order for

roll to be controlled effectively at

the required period. In other words, the

tank period has to be faster than

the fastest needed, because the valves

can only delay the water movement, not speed it up. Controlled passive

tanks therefore tend to be larger

than passive tanks.

Antiroll tanks, whether passive or active, are very effective at resonant roll, but can be less effective in con-fused seas with a range of wave peri-ods. Because they saturate in heavy seas, they can't decrease roll in the worst possible conditions. for this

reason, tanks are almost always

installed in combination with bilge

keels, which tend to be most effective: in severe seas.

Gyros. A concept that has been

around for several decades is now

becoming more feasible. It's based on the precession effect of a gyro, or

fly-wheel. Precession turns the force

applied to a spinning object 90°.

Imagine one particle of mass in a

fly-wheel, as if it were a ball on a chain being spun around the center. If we

push it down as

it comes by,

it

deflects on to a new path that has its low point not where we pushed but a quarter of the way around the circle.

Rolling a vertical-axis flywheel in this

way produces roll resistance and a pitch moment, and applying a pitch moment produces rolling moment

(and pitch resistance). A flywheel can

produce large momentsprovided

the gyro is big enough and spins fast

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enough. Early gyros were of the

passive roll-control type, until

design-ers realized that actively applying a

moment to precess the gyro opposite

the roll (by tipping the axis in the fore-and-aft plane) produced a larger antiroll effect. Sensors that detect even small rolls control the preces-sion. Sperry installed some 40 gyros

on ships, including yachts, from about 1936 to 1967, but they spun relatively

slowly, and so had to be very heavy.

They also used a lot of power to keep spinning. Gyros gradually lost favor to fins and antiroll tanks Today, though,

modern high-performance materials, stress-optimized flywheel designs,

vacuum containers (to reduce air drag

that would slow down the flywheel),

and high-performance bearings allow

much higher flywheel speeds. Since the roll effect increases directly with flywheel speed, the antiroll moment available from these modern gyro

installations is much larger for a

smaller device, and the energy costs

of keeping it spinning are lower.

Computer control also allows design-ers more-sophisticated control

mecha-nisms that increase effectiveness.

There are at least two different groups

currently working on advanced gyro

roll-stabilization systems that will be practical for yachts and small ships.

To sum Up: paravanes and active

fins are common on power yachts, but

bilge keels, movable appendages,

steadying sails, antiroll tanks, and even

gyros all have a place and should be

considered for roll control. BB

About the Author: Chris Bariy is a

registered professional engineer, and

a naval architect with the United

States Coast Guard's Boat Engineering

Branch. The author's experience

includes commercial ship, feny,

fish-ing vessel tug, and workboat design with consultancies and shipyards in

both the U.S. and the U.K.

The opinions expressed herein are those of the author and do not

repre-sent official policy of the Coast Guard. The author would like to thank Rich-ard .,4kers of Ship iWotion Associates

(Portland, Maine) for his review of

this article. Please refer to Akers'

arti-cle "Motion Control" (PBB No. 61., page 82), for more on roll and other