Introduction
This article was originally prepared as a response to a post to the
Trawler World mailing list on the subject of Twin Keels and Roll Reduction.
The following includes a brief description of the research done by Lord
Riverdale and the research of others on the benefits of Twin Keels and other
roll reduction strategies.
Please also see my web article
Beam vs. Ballast for a general discussion of how these two factors
relate to stability and roll motion.
The Research of Lord Riverdale
Naval architect Pat Bray and yacht designer Ted Brewer have been recent
proponents of Twin Keels; Ted Brewer for sail boats, and Pat Bray for
both sail boats and power boats. The many points offered by Bray and Brewer
in favor of Twin Keels for sailing yachts were originally researched by the
late Lord Riverdale, whose conclusions were published in a technical paper
offered to the Royal Institute of Naval Architects on December 15, 1967.
During the 45 years prior to the paper, Lord Riverdale had designed, built,
and sailed a series of twin keel sail boats dating from 1922 called
"Bluebird" and "Bluebird of Thorne." The last "Bluebird of Thorne" in
the series Lord Riverdale developed in cooperation with British designer
Arthur Robb.
The last Bluebird of Thorne, built in steel at 50 feet LOA, now resides
on nearby Lopez Island (among the San Juan Islands in Puget Sound).
During 1995 and 1996 I had the good fortune to be asked by Bluebird's
current owners, Jeff and Dianne Dyer of Lopez Island, to create a new
interior for the vessel to house the Dyer family and to re-design the deck
structures to allow for Jeff's height.
Working on the Bluebird of Thorne was a great opportunity to learn first
hand the incredible research efforts of Lord Riverdale. He made a
significant contribution to the design of twin keels over the course of 45
years of boat building, sailing, research, tank testing and development.
Those efforts reached no small degree of perfection with his last Bluebird
of Thorne.
Lord Riverdale offered the following definitions when referring to twin
keel yachts:
Twin Keel Type: Has no center keel. Ballast is
carried within prominent twin keels. May have either single or
twin rudders.
Bilge Keel Type: Has a center keel that carries most of
the ballast. Bilge keels long and shoal. Single rudder.
To this, I would add a sub-category to the "Twin Keel Type" that uses a
skeg / rudder combination on center aft; essentially a "third keel" that
houses and protects the propeller, and provides robust support to the rudder
(as with Boojum). This "three-keel" type is
possibly most appropriate for a small power vessel that has roll reduction
as its main focus. For a review of the rationale that went into the twin
keel arrangement given to Boojum, please see the article,
Boojum's Twin Keels.
A larger power vessel that makes use of twin engines might effectively
eliminate the center keel, and use twin rudders, as did Bluebird.
Due to being focused on Twin Keels for sailing vessels, Lord Riverdale in
his entire body of research did not see the need to address the benefits
that Twin Keels or Bilge Keels might offer in terms of roll reduction, since
that is mainly an issue for power vessels.
Benefits to Power Vessels and Motor Sailors...
Aside from the obvious benefits offered by twin keels published elsewhere
such as when taking the ground, a few points emerge from Lord Riverdale's
research that are more subtle, and that may be of some interest to those
experimenting with Twin Keels, as defined by Lord Riverdale, on power
vessels and motor sailors...
* Ballast Location: The keels that were employed
on Bluebird (after no small amount of testing) were fairly prominent
features, and contained the vessel's ballast. This was for the
sake of sailing performance on those vessels. For power vessels,
locating the ballast within twin keels has a further benefit: it
distributes the mass of the vessel farther from the centerline,
augmenting the transverse "roll moment" and providing an inherent
"inertial" resistance to rolling. This "inertia effect" is further
enhanced by the entrained mass of water in the boundary layer around the
keels. These effects were briefly mentioned by Lord Riverdale, but
were not of great import to his quest for sailing performance.
* Twin Keel Location: The most successful Twin
Keels were located approximately amidships, with the root of the keel
beginning roughly 45% of the DWL aft, and the largest section being
perhaps 60% or more aft. Forward twin keel locations were not
favorable, as that location tended to provide poor steering
characteristics and a poor wave form at speed. Aft locations were
not investigated, as they would not have located the ballast where
needed for the sake of proper trim.
* Steering Stability: Twin Keels, if located and
proportioned correctly, were observed to provide enhanced directional
stability, and an enhanced ability to heave-to during heavy weather.
Bluebird's Twin Keels were observed to provide improved seakeeping in
all conditions. These qualities are a benefit to power vessels as well.
* Toe-in: A small amount of "toe-in" seemed to
benefit the overall resistance upright as well as under sail. The
best results were achieved using a toe in of between 0.5 degree and 1.25
degrees. For Lord Riverdale's purposes, some amount of toe-in was
deemed to be desirable for better "lift" to windward when sailing.
While upright, the observed benefit to there being some amount of toe-in
was attributed to providing better alignment with the under water flow
lines.
This latter assumption is correct, as larger vessel flow analyses
have shown. The amount of toe-in is therefore a variable which
will be different with each hull shape, and which will vary with the
longitudinal location of the keels. It would be interesting to
test the effect of toe-in on yaw while rolling. It is questionable
whether any amount of toe-in will benefit a motor vessel, since one
would expect to observe that any amount of toe-in would enhance yaw, and
therefore be undesirable in terms of directional stability.
* Sectional Foil Shape: The Twin Keels chosen for
Bluebird were of an asymmetrical foil shape, flat outboard and cambered
inboard, much like an airplane wing. This was done in order to
achieve greater "lift" when sailing to windward.
It is questionable whether asymmetry would provide any benefit to a power
vessel. For a power vessel, one would expect that a symmetrical
section aimed at providing low drag would be the most appropriate. For
a power vessel, one would expect that the foil shape chosen should be "stall
tolerant" or able to take a large variation in the angle of attack, and
therefore within the 00xx family of NACA shapes. In order to contain
the ballast, and also for the sake of being "stall tolerant" one would also
expect that the foil would not be too "skinny."
For a motor sailor, as has been very adequately proven by Lord Riverdale
and others, an asymmetric foil shape will provide the greatest benefit, with
the same requirement for "fatness" to contain the ballast, and to be able to
tolerate a variety of angles of attack while rolling, so as to avoid a large
induced drag due to turbulence. For a motor sailor, one may make use
of other NACA foil sections to provide better lift characteristics, keeping
in mind the requirement of being "stall tolerant."
Other Research
In addition to the benefits that Twin Keels offer to sailing vessels as
researched by Lord Riverdale, there have been many tests done on power
vessels aimed at quantifying the effects of Bilge Keels, Twin Keels and
other stabilizing methods on roll reduction.
A few general observations about the most commonly considered methods
of roll reduction are as follows:
* Sails: Depending on sail area and wind strength, roll
reduction can be considerable. We might assume a roll reduction on
the order of 40% to 70% depending on sea state, etc. The sails and
the rig provide inertial damping even at anchor due to the weight of the
rig and its distance from the roll center (increased roll moment of
inertia). Sails can contribute significantly to propulsive
efficiency, at times completely eliminating the need for the engine.
Excellent synergy when motor sailing. Possible as a retro-fit,
depending on hull form and stability characteristics. Relatively
expensive. Somewhat complex. Fun!
* Active Fin Stabilizers: When sized correctly, Naiad
claims up to a 90% roll reduction for active stabilizers, depending on
vessel speed. Active stabilizers are most effective at maximum
vessel speed, less so at lesser speeds, minimally effective with no
forward speed. Active stabilizers provide some efficiency loss due
to frictional resistance, generally considered to be compensated for by
lesser overall resistance of the more stable vessel. Some power draw due
to the operation of the hydraulics to actuate the fins, which translates
into higher horsepower requirements and greater fuel use. Possible
as a retro-fit. Relatively expensive. Relatively complex to
install.
* Fixed "Twin" Keels: Depending on the twin keel
geometry, per research published in Marine Technology, roll
reductions have been observed on the order of 40% to 65%. Deeper
keels having greater area provide greater attenuation. Low aspect
ratio is considered a benefit due to being able to tolerate larger
angles of attack (while rolling) without stalling. Location and
geometry have been shown to be quite important for optimum vessel
handling and resistance, as noted above. Vessel speed does not appear to
be important to roll damping. Twin keels will add some frictional
resistance due to increased wetted surface area. Enhanced
directional stability, if proportioned correctly. Very unlikely as
a retro-fit. Relatively inexpensive. Relatively simple.
* Fixed "Bilge" Keels: Long, low aspect ratio bilge
keels, per research published in Marine Technology, have been
observed to offer possible roll reductions on the order of 35% to 55%.
Vessel speed is not important to roll damping. There is some added
frictional resistance due to increased wetted surface area. If
proportioned correctly, bilge keels offer enhanced directional
stability. Very common as a retro-fit. Relatively
inexpensive. Relatively simple to build.
* Paravanes: Per published data from various sources,
roll attenuation can be on the order of 40% to 60%. Vessel speed
does not appear to be important to roll damping benefit. There is some
speed and efficiency loss due to drag of the paravanes. Loss of
one paravane can impose possibly dangerous effects on stability, should
the vessel be caught in a beam sea with the one remaining paravane to
leeward (per research published in Marine Technology).
Paravanes are relatively easy to retro-fit. Relatively
inexpensive. Medium complexity in use.
* Passive Anti-Roll Tanks: According to published
research in Marine Technology, in some sea conditions, with
optimized tank / vessel design, roll reductions in both amplitude and
acceleration on the order of 50% to 60% have been documented. In
other sea conditions, the percentage of roll reduction appears to vary
considerably. Vessel speed does not appear to be important to roll
damping benefit. There does not seem to be any negative effect on
vessel speed or efficiency, except of course for the added displacement
required to carry the extra deadweight of the tank contents.
Anti-roll tanks seem to vary in size from around 1.5% to around 2.5% of
a vessel's displacement. If located higher, the overall weight may
be able to be less, since the tank will have a greater effect due to
being farther from the vessel's center of gravity. Similarly, if the
tank is able to be the full width of the vessel, its effect appears to
be greater and there may be the potential for a reduction in tank
weight. Space requirements are very difficult for small pleasure
vessels (say below 60 feet). Possible undesirable effects on
stability, depending on the vessel (large free surface effect).
Very unlikely as a retro-fit. Possibly noisy. Relatively
complex to design correctly (therefore expensive to design).
Relatively inexpensive to build. Relatively simple in use.
* Single Chine Hull Form: Some degree of roll
attenuation is contributed by the single chine hull form itself. A
single chine vessel appears to have roughly twice the roll damping
ability of a rounded hull form (per published model tests in Marine
Technology, performed on vessels having similar hull forms).
Roll amplitude will be less; roll
acceleration may be greater, rolling will decay more quickly.
This effect is viewed as being approximately similar to fitting long
shoal draft bilge fins on a rounded hull, except that bilge keels appear
to also reduce accelerations. Extremely unlikely as a retro-fit.
For new construction, chine shapes are relatively inexpensive by
comparison to rounded hull shapes, particularly in metal.
Extremely simple. Slightly greater wetted surface.
The research mentioned above and the percentages of roll reduction
claimed for the various methods have appeared in various issues of Marine
Technology, a publication of the Society of Naval Architects and Marine
Engineers, during the last five years. Past issues of Marine
Technology are available from SNAME at
http://www.sname.org.
The Measurements
In terms of roll attenuation, there are of course many variables.
What works well on one boat, may not be as effective on another boat.
For example, paravane size relative to boat size / displacement / righting
energy will definitely affect the results.
The percentages quoted above relate in many cases to roll amplitude,
which is only one component of rolling behavior... One can isolate
several components, as follows:
* Amplitude (measured in degrees)
*
Period (measured in seconds)
* Acceleration /
deceleration (a result of the above, measured in feet or meters per
second squared)
* Rate of Decay (number of cycles
to rest or to some other benchmark)
Among the behavior patterns directly observable from the numerous data
sources are the following...
The wildest rolling is referred to as synchronous rolling, i.e. rolling
in beam seas when the wave period is close to the natural roll period of the
boat in question. For example, when a given boat rolls to some extent
in harbor, say on receiving the wake of a passing boat, it might not do so
given a slightly different wake or wave pattern. Another boat that did
not roll so much at a given wake or wave pattern may roll wildly with a
different wave pattern or period.
The higher percentages for roll attenuation quoted above for any given
roll attenuation method (paravanes, keels, etc.) are from measurements of
the attenuation of synchronous rolling, and appear to have their
effect due to putting the boat out of sync with the wave pattern. The
lower percentages in the range of effectiveness quoted appear to be an
average of the overall effectiveness. In most of the published
research, the majority of measurements were of amplitude. The
next most common data quoted were measurements of acceleration.
It is interesting (and important) to observe that in some sea states
(wave period and wave height both considered) many of the "passive" roll
attenuation schemes will in fact at times slightly augment rolling.
This reportedly appears to be at small rather than large roll amplitudes,
appears to be random, and does not appear to be considered an issue.
Synchronous rolling is considered to be the main adversary, and all
methods mentioned appear to be effective there.
A Few Observations
Most commonly, research groups addressed the requirements of
commercial vessels, so tended to make use of anti-rolling tanks in
combination with relatively long and shoal "bilge keels" or paravanes in
combination with bilge keels. These vessels were relatively larger and
heavier than typical "trawler yachts."
Combined Methods
I find that the most interesting results appear to be obtainable when
two or more methods are used simultaneously, such as paravanes combined
with twin keels, or an anti-rolling tank with either, or say, sails in
combination with twin keels, etc. Several of the studies in
Marine Technology have been aimed at various combined methods.
In all cases reviewed, the tests showed that combining roll attenuation
strategies does appear to have a dramatically beneficial effect.
It therefore seems that a combination of strategies may offer the
greatest benefit aboard trawler yachts. A highly effective
strategy for trawler yachts might reasonably be the combined use of a
single chine hull form, twin keels, a modest get-home sail rig, and
paravanes for possible deployment in some conditions. This
combination may offer the possibility of an extraordinary degree of roll
attenuation, and may benefit rolling behavior over a wider variety of
conditions. In way of an actual example, this is the combination
of roll attenuation strategies given to the
Greatheart 48 and the Greatheart 60
power vessel designs.
A motor sailor with ample sail area can be an ideal platform for the
use of twin keels. In terms of resistance or propulsive efficiency, it
is interesting to note that, should one be concerned about losing some
speed to twin keels, that sails have the ability to recover that loss,
more or less in proportion to whatever extent the sail rig is employed.
Rather than losing a knot or so to various roll attenuation devices, one
can instead gain a knot or so...
Paravanes
It is unlikely that one would make use of paravanes at the same time
as sails, although paravanes could easily be provided on a motor sailor
for use when sailing is not optimum.
Attention given to reducing the drag of the paravanes themselves
should provide significant benefits. Paravane drag may be able to
be substantially reduced via the use of foil shaped paravane surfaces.
Such foil shapes can be easily machined and fabricated from aluminum.
I have designed such a set of low drag NACA 00xx series foil shaped
paravanes for Charles Vollum for use on the 25 foot
Boojum. Sea trials appear to verify the effectiveness of this
strategy.
Anti Rolling Tanks
On vessels above, say, 60 feet it is possible that the use of
anti-roll tanks may be preferred over the use of paravanes, primarily
due to the large forces involved in terms of being able to easily handle
the paravane rig. Anti-roll tanks operate by allowing water to
slosh (passive type) or by pumping water (active type) from side to side
out of sync with the wave induced roll of the ship. There are several
different styles of each. Anti-roll tanks must be designed
carefully, sized right, tuned to the ship, and then tweaked to match the
anticipated conditions.
For vessels below around 60 feet LOA, the space, weight and stability
requirements of anti-roll tanks may prove to be prohibitive. For
example, a 50,000 lb boat would need around a half ton of water in an
anti-roll tank located at least as high as deck level. For supply
vessels, research vessels or fishing vessels, all of which spend some
amount of time at sea while not making any headway, anti-roll tanks can
make a lot of sense. For example, per published data in Marine
Technology, the combination of anti-roll tanks and bilge keels have
been shown to be capable of reducing roll amplitude and accelerations by
as much as 90% in some sea conditions.
In spite of their potential benefits, as a retro-fit anti-roll tanks
are a very unlikely solution. During new construction however,
there may possibly be justification for their use, since they can then
be more gracefully incorporated into the design.
Active Stabilizers
Most larger yachts will favor active stabilizers... providing
an "off the shelf" solution that's very effective under way. When
the vessel is not moving forward however, active stabilizers don't
provide much, if any, benefit. Naiad has however developed a
system that is claimed to have reasonably good effect at anchor.
If first cost and maintenance concerns are less of an issue, then
certainly active stabies will be an excellent choice, and appear to
provide the ultimate in motion comfort under way. Per published
data mentioned above, active stabies appear to be capable of providing
roll attenuation roughly equal to that of Twin Keels or Bilge Keels in
combination with either anti-roll tanks, or paravanes.
What About Anti-Rolling Tanks for Smaller Boats...?
While the displacement penalty of an anti rolling tank may not seem to be
a big deal in some cases, the positioning of an anti-rolling tank is usually
very problematic in terms of the accommodations, deck space, etc.
Many anti-roll tank geometries have been tried. It seems that the
simpler the approach, the better. Active tanks that pump water around
do not appear to be optimum, since the water pumping requirements may be
quite extreme, therefore they tend to be expensive, noisy, and power hungry.
The most viable "active" system appears to be one that is configured as a
broad "U" tank that is joined by an air tube across the top, having a valve
that's controlled by the ship's gyro. It should go without saying that
these are complex and expensive to set up even aboard larger ships.
For the sake of simplicity and economy, it appears that passive tanks may
be the best. Among them the simple "H" tank seems likely to be the
most easily implemented with the least impact on the vessel layout, etc.
If the budget allows a slightly more sophisticated system, then there may be
other possible configurations, and with them, the possibility of improved
roll reduction as noted above.
The simplest tank, in the form of a broad "H" will have a large volume
tank (perhaps twice the volume of the contained liquid in its capacity) at
each end, joined by a somewhat narrower "slot" which forms the cross bar of
the "H." In some designs, the "cross bar" of the "H" may be less
narrow, and may instead be heavily baffled. There must be a good spot
onboard, ideally offering the full width of the ship, and as a rule of thumb
about 20% as high as that off the water.
Anti rolling tanks need to be planned quite carefully. The weight
of water in the tank must be able to be tolerated in terms of the stability
of the boat, as well as the "free surface" effect of the water as it sloshes
back and forth. The trick seems to be to get the slosh to happen out
of sync with the roll of the boat.
The stability of the boat must be known precisely in order to know the
correct proportions of the tank, the weight of water required for roll
reduction, whether the boat can tolerate the lessened stability effect of
the tank, etc. It is an unfortunate fact that quite a number of
builders (and no small number of designers) simply do not have any idea what
the stability figures are for their boats, nor are they especially
interested in investing money in the "design time" to find out.
This claim may sound surprising... I have to agree! If you
dare to know "the truth" by all means ask your builder or designer for the
vessel's "Weight Summary" and "Stability Report." In a shocking number of
cases, the information will simply not exist. In other cases, there
may perhaps have been an overly optimistic, inadequate, or incorrect Weight
Study...
At the very least, an offshore vessel should meet or exceed the minimum
Stability Criteria established by the IMO (International Maritime
Organization), including the IMO Extended Weather Criteria.
Alternate, and equally rigorous criteria are those within the US Code of
Federal Regulations (46 CFR), as used by the US Coast Guard.
These criteria do not offer simple "range of positive stability"
requirements. Both the CFR and the IMO provide for measurement of the
vessel's righting energy represented (by convention) as the measurement of
the area below the stability curve within various prescribed ranges of heel;
the initial GM of the vessel; the position of maximum righting arm
measured in degrees of heel; the amount of "reserve" righting energy
available to meet extremes of wind and wave; degrees of heel to deck
edge immersion and to downflooding. If thorough calculations show that
these criteria can be met with the tank in place, then the vessel may
possibly be a workable platform for an anti-rolling tank.
Given adequate knowledge of a vessel's stability, and given the
willingness to invest in the cost of planning a proper solution, anti-roll
tanks may possibly be able to be integrated into smaller craft. The
real benefit of a passive anti rolling tank appears to be to provide a
system that does not degrade the propulsive efficiency of the vessel in
order to achieve roll reduction. Several studies appear to have shown
the opposite, i.e. that if a vessel can be kept from heavy rolling,
efficiency seems to be increased.
Until they become commonplace, the cost of proper planning will likely
keep anti-rolling tanks from being a strong contender among the other roll
attenuation options mentioned here.
Michael Kasten
Port Townsend
- 2002
Copyright 2002 - 2006 Michael Kasten
Please also see my web article
Beam vs. Ballast for a general discussion of how these two factors
relate to stability and roll motion,
