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Old June 28th 04, 02:42 PM
Frank
 
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Default The future of yacht design - 10 myths scotched

http://www.eskimo.com/~mighetto/murrelet.htm

Myth number 1 - CENTERBOARDS COMPROMISE SEAWORTHINESS

This myth apparently comes from fatal capsize incidents in the 1880s
involving American center boarders. "Cutter cranks" claimed that such
incidents were not possible with the deep draft British boats and that
view prevails more or less even today. Even as late as 1996 experts
could be hired that would state in court that all centerboard boats
are dangerous, the danger stemming from the slot the board retracts
into and the notion that water can enter the hull through that slot.
Those experts were discredited by 1996 and by 1999, when Murrelet was
launched, the exact opposite notion - that attached foils that could
not be retracted, or give way, were dangerous - gained favor. This
controversy is fascinating and is discussed through out the Cruising
Log of the Murrelet.

Murrelet is a center boarder though this is not readily apparent.
Centerboards must cut down the middle of the vessel. Mac26x cruisers
avoid cutting accommodations down the middle through a power boat like
raised navigation table (see below). The rudders serve as lee boards
meaning that they, like the centerboard, also hold the vessel from
being pushed with the wind. For the most part Murrelet "goes where she
looks" but I notice some drift, like a plane in a cross wind in some
conditions. The centerboard's purpose is usually portrayed as to
reduce that drift, called leeway or crabbing.

In a Mac26x that is close hauled, excessive leeway is usually the
result of an overpowered main, rather than board extention or lack
thereof. Experienced deep keel racing sailors have a hard time with
this and insist on pulling the Mac26x boom in as close to the
centerline as possible to race into the wind, as would be done on a
heavy keel boat rigged for racing. But what this does on a light
cruising boat is put power in the main sail and that power is one that
is lateral. Easing the main puts the power back in the Genoa where the
power is more to the windward. The sails of a cruising sailboat are
cut differently than those of a racing sailboat. So it is important to
note that when racing a boat like the Mac26x which is designed as a
cruiser first and racer second. Sailmakers cut main sails on cruising
vessels for the work most common for a cruising vessel in the modern
age. Cruising sailboats often motor or motor sail to windward. Hence
the main sail is usually cut full to provide more power on other
points of sail where motor power is not as likely. The Mac26x can also
have excessive leeway close hauled if not on her optimum heel or if
one of the rudders is left in its up position. Contrary to what is
expected a fully extended centerboard may not be of value in reducing
leeway when close hauled. Its purpose on a Mac26x goes beyond notions
of purpose usually attributed to centerboards.

Centerboards were invented in China. When Europeans first began
settling the Americas, they were obliged to utilize the ports long
established there by the native American people. These all weather
ports for the most part were to shallow for the deep draft vessels
used by the British. Advancement in China's centerboard was an
absolute necessity for Europeans who wished to trade in the Americas
and in many ways centerboard designs represent the best of American
boat building innovation.

The first great ocean race featured two large American centerboarders
both bettering the deep keel British competition. It is interesting to
note that this was done in strong wind conditions. The centerboarders
reduced sail to maintain optimum heel where the external weighted keel
British vessel's captain believed that not advantageous.

Undoubtedly the notion that shallow ports could replace the
established deep water ones in Europe owing to faster sailing boats
advanced by the Americans must have been troubling to commerce barons
of the time. Owing to lumber, the majority of the vessels being built
for commerce were being built in the Americas and if the centerboard
caught on, the need for the established deep water ports in Europe
would come into question by upstart developers promoting shallow water
ports. An orchestrated campaign of disinformation (what we would call
FUD - fear, uncertainty, doubt) was then started I believe to convince
the professional captains not to favor centerboard designs. This was
highly successful. For example, the Spray was a center boarder.

Captain Slocum, probably the most famous American sailor
(Canadian/US), replaced Spray's centerboard with a solid plank keel
prior to circumnavigating. He then had problems holding course and had
to devise mechanisms to change sail plan (like bowsprits) so the
vessel would self steer which is an important feature for single
handling. Centerboard position will control weather helm and lee helm.
Hence a centerboard is not just a mechanism to reduce leeway but also
a valuable steering control. This control would have made Slocum's
circumnavigation easier. His erroneous example continues to be
followed and most US sailing schools will foster the myth that a fixed
keel or a fin keel with ballast as low as possible contributes to
stability and eliminates leeway (sometimes called crabbing) and is
more seaworthy than a centerboarded vessel.

In March of 2003 even the marketers of MacGregor yachts played into
this myth stating in a comparison of the X to the 2003 M model
powersailer that "The (X-boat) long centerboard trunk also required a
huge opening in the hull, in the most important structural area of the
boat (as well as creating a very large drag problem. See
Performance)". Hence this web page took on special significance.


Centerboards and Hull Structure
First, the centerboard trunk is part of the X hull, there is no
opening through the hull for the board as in the M. The photo to the
right shows that the X centerboard trunk is molded as part of the
hull. The only holes through the hull are for a lifting line (see
lower area of the photo - the blue line is the lifting line) and the
bolt through hull for the foil hanger (see upper portion of the
trunk.) Hover the mouse over different portions of the photo to
identify through hulls.
Second, there are no weak points on the X or M hulls. Both vessels are
produced in a manner where points of stress are reinforced with up to
17 layers of fiberglass. This reinforcement is not being done on hulls
of lengths under 30 feet by other US based manufacturers (such as
Hunter and the J-Boat manufacturers) and is likely the main reason
those manufacturers do not market their under 30 foot sail boats as
ocean going. Third, the X & M water ballast systems, because they are
layered into the hull, provide structural integrity far surpassing
that of other ocean sailboats. Third, the X centerboard, like that of
the third generation powersailer M daggerboard, will give way (break
off) during a hard grounding but the X centerboard may simply retract
itself where that is not likely with a daggerboard. Hence the M
centerboard trunk is reinforced. (Note a daggerboard is a type of
centerboard. MacGregor Yachts invented the swing keel - another kind
of centerboard - and moved away from daggerboards owing to a number of
owners who ran the boat up trailers with destructive results.)


Company founders made their fortunes on centerboard vessels in dancing
around this myth. If you ask an American sailor if a daggerboard and a
swing keel are types of centerboards you will usually be informed that
they are not. Ask an Australian and you will get the correct answer.
Daggerboards and swing keels are terms manufacturers use to avoid the
centerboard controversy. The current misperception is that weighted
centerboards or at least weight near the centerboard contributes to
stability and seaworthiness. Likely owing to dealer/distributor
influence, MacGregor Yachts company added 300 lbs of solid ballast to
the M in the form of resin in the water ballast tank and additional
solid weight around the centerboard trunk. This is not unlike what
fixed keel boat owners do when they find their vessel is a tippy
cruiser. When the Max26x was introduced her design took advantage of a
relatively unproven rigging invention that reduces or eliminates the
need for weight on an external foil or inside the hull. Today vary few
Mac26x cruisers and very few new model cruising sailboats do not take
advantage of this innovation. The X brochure states:


The roller furling system offers the single most effective way of
exactly matching the amount of sail area to the amount of wind. The
boat will sail very well with the jib or genoa completely rolled up,
partially unrolled, or completely unrolled to full size. The sail
keeps its shape no matter how much is rolled in. It is particularly
useful when sailing into the wind. If the wind becomes too strong, and
the boat is leaning too much, simply roll up some headsail and ease
the pressure on the rig. In many situations, the boat will actually
sail faster with less sail. All this can be done from the cockpit, and
it is not necessary for anyone to go to the foredeck to reduce sail
area.
Cranky is another word for tippy and there are two situations
(underway and at rest) to consider. When underway by sail the position
of the ballast need not be low in the vessel. Live ballast (crew) is
often put on the topside deck windward rail for example. When sailing,
the water ballast system of the X provides the same stability as a
keel boat with the caviat that rigging and sails also contribute to
stability. A vessel flying a single sail or in a reefed state is
naturally less tippy than otherwize.

At rest or under motor power, with no sails and rigging to provide
stability, weight low in the vessel is important to stabilize boats.
The deep draft British cutters, for example, were tippy when unloaded
and at rest. Hence the term cutter cranks as discussed above in
regards to the centerboard controversy. But the notion that this
weight should be on the centerline is incorrect.

The 1998 Hobart race showed that weight and foils at the centerline
weaken the structural area there. A weighted unyeilding foil on
centerline weakens the hull not only by touching down on the sea
bottom but just by wave action over a period of time.

In 1996 a long court case was resolved involving the centerboard keel
structure of a Bayliner Buccaneer 180. Water entered the bilges
unnoticed and an expert testified that the water could have entered
through the centerboard slot. The water contributed to a capsize and
the death of panicked children. The vessel did not sink. Bayliner was
believed responsible for not adequately explaining that water could
enter the boat by way of the centerboard slot.

MacGregor Yachts, after the Bayliner case was resolved (in Bayliner's
favor), instructed X owners how to make bilge water - which enters a
vessel in so many ways including simple condensation - visible before
it might contribute to a capsize. The modification ensures that excess
water spills onto the cabin floor. All X boats post 1998 have the
modification. X cruisers do not benefit from automated bilge pumps
because there is not enough bilge to store sufficient water to
activate them. A comparison of the X to the M is usefull.

The M probably should be fitted out with an automatic bilge pump. Like
the cutter-crank-FUDed centerboarders the M slot cuts through the
hull. This is in contrast to the X hull where only the board hanger
bolt and a hole for the lifting line are through hull holes. Yet the
need for a bilge pump on the M is not owing just to the fact that her
centerboard cuts through the hull but also to the deeper V hull shape
which can store possibly enough bilge water to contribute to capsize
before that water is noticed by the operator. She, like the 1998
Hobart boats which floundered, has solid weight at the centerline.
Such weight was believed to contribute to hull cracks in the 1998
Hobart race. More waterballast is also on centerline than on the X
cruiser.

The X is MacGregor's first off-centerline-water-ballasted vessel and
second of three powersailers but, given the above, the X has the more
advanced hull, water ballast, centerboard and keel configuration. The
photo below partially illustrates those features and raises questions
involving the keel.




The hull's fore-aft backbone extending longitudinally along the center
of the bottom and connecting the stem and the stern is often defined
as the keel. However, if the definition is modified slightly to remove
the word center some rather interesting conclusions can be reached.

Centerboards and Upwind Performance
"The hull's fore-aft backbone extending longitudinally along ANY
portion of the bottom and connecting the stem and the stern is defined
as the keel." By this modified definition, components of the water
ballast tank and hull sides on the X boats represent keels because
these are the vessel's true backbones. When on heel the part of the
tank in the water is the bottom of the vessel and it is that part of
the vessel where directional control attributed to a keel must be
located. The X hull form provides what is in effect a full keel at two
positions, each off the centerline. The M, owing to its rounding at
the sides, unless skeggs are added, does not. This explains the M's
main sail only mode of sailing in moderate winds. The X tracks better
owing to the hard chine aft and cods head forward and hence supports
large headsails upwind where these sails tend to force the M away from
windward. This is an important concept for performance under sail.

Under power, unofficial web sites claimed "The old (Mac26x)
centerboard trunk carried about 100 lbs of water, the new trunk
carries virtually none. The 26X boat, with its flatter bottom, was
slowed each time it came down hard off of a wave." This was the basis
for some dealers claims that the M would out motor power the X. Such
statements should be totally ignored. Remember that the X centerboard
slot would be covered by the board because it is retracted when
motoring at speed and that for the most part, at least in the context
of crashing over a wave, it is air and water tight. In theory, as
Mike, an M dealer pointed out, the X should better by nature of the
flatter hull.




However, the X has a cod fish like bow (the manufacturer calls this a
powerboat belly) when viewed from under the water (see above). This is
a feature of the advanced Dribbly hulls (ie Tasars) as well and that
could mean weight distribution or hull form itself rather than
centerboard slot turbulence might make the M faster under motor. Deep
V hulls are the most popular form of hull for a power boat. In the end
the manufacturer listed the M as slower under motor power (22 MPH)
than the X (24 MPH) when fitted with identical 50 HP engines.

Centerboards and Lee Helm
For a couple of years I tried to convince a fellow Mac26x owner that
his raking of the mast forward to induce weather helm was unnecessary
and even harmful to pointing. Forward raking is a standard technique
used on keel boats with lee helm to correct for unbalanced hull and
rigging designs. The technique is documented by many professional
riggers. Raking of the mast forward comes at a penalty involving
pointing into the wind and this owner soon was reporting that as a
problem. Later he convinced himself that a multihulled vessel (the
Corsair) was more to his liking.

The water ballast arrangement on the X is as far from centerline as
possible. This lowers the center of gravity and maximizes stability.
When either by crew weight or the press of the wind on the sails the
boat heels enough (about 11 degrees), the windward portion of the tank
is pulled from the water. This is exactly like a trimaran. So it isn't
surprising that my friend found a multihull to his liking. The off
centerline components of the water ballast system on the X in concert
with the sharp side chines of the hull are equivalent to the amas
(outer hulls) of a trimaran. Another way of looking at this is that
the water ballast tank is a catamaran hull within the X monohull. The
photo on the right demonstrates that the X when on plane with the
centerboard up creates a wake that is trimaran like.

There is very little difference between a dagger and a swing
centerboard on a boat that is balanced perfectly. But of course there
is the loss of the control over lee helm on centerboards that can not
be moved forward or aft as is the case with the M daggerboard when
crew has balanced the vessel less then perfectly. Lee helm is a
problem because it allows the vessel to fall off, heeling excessively
in more than moderate wind, with capsize then becoming a potential.
Ocean racers with dagger centerboards often also sport water ballast
systems that pump ballast forward and aft as well as side to side so
that perfect balance can be obtained. This type of control I view as a
replacement for a swing keel. The most popular production sailboat of
all time (though for reasons unknown no MacGregor yacht appears in
Royce Illustrated) is the Mac25. 7,000 were produced almost in Royce's
back yard, all sporting swing keels over 14 years. But MacGregor
Yachts advanced beyond that by removing weight from the foil and
prototyping with dagger and swing style centerboards. These
experiences were on boats where the water ballast was on centerline
and it appears that dagger fitted configurations are believed faster
because the smaller slot created less turbulence on down wing runs
where the board would be removed. This had also been observed on
Snipes and is documented in Royce Sailing Illustrated. Dingies with
dagger style centerboards (such as Snipes) were eventually fitted with
carpet and foam wedges so that the board slot would be free from
turbulence.

On the M. the trunk can be modified with carpet and foam wedges so
that it is turbulent free on down wind runs. This is done on Tasars.
Something similar might be done on the swinging centerboard of a
Mac26S if water entering the trunk really happened to the extent
necessary to impact performance. I don't think is does. On a Mac26x,
however, something very advanced has been implemented, something that
requires a different name, and something that, like off centerline
water ballast, is now being implemented on ocean racers. This is a
kind of centerboard called a canard.

Centerboard Slot, Jibing board, Forward Rudder and Canard
The nature of the X centerboard is not well understood. MacGregor
Yachts states in the X broshure that "A long, thin airfoil is far more
effiecient than a short, wide one. This is why racing sailboat keels
are deep, and why sailplane wings are long and thin. The relationship
between the fore and aft width of the board and its length is called
its aspect ratio. Most boats have keels with aspect ratios of 2 to 1
(meaning that the keel or centerboard is two times as deep as it is
wide). The MacGregor 26x centerboard has a ratio of five to one (it is
13" wide and 5' 6" deep). The high aspect ratio increases lift as the
boat sails into the wind and reduces drag. This is one of the major
reasons that the X will point closer into the wind and sail faster
than other trailerables."

It is important to note that the 26X trunk (often incorectly refered
to as the centerboard slot) is wider than it need be for the foil
which is hung in a way that enhances pointing. The foil is better, but
not perfectly, described as a jibing board.

Jibing boards, when cocked to windward slightly (see animation to the
right), provide much better pointing. The X centerboard is self
jibing, just like the M mast is self rotating. This jibing feature has
been portrayed as a defect because if the board is left down while the
vessel is stationary it can clunk about with wave action. I would much
rather lift the board and put out a second anchor, to prevent
swinging, than eliminate the jibing feature.

My conclusion from the animation below and the width of the
centerboard trunk (see above right) is that under sail there is very
little "centerboard slot" drag to be concerned about with the X. At
any heel over 11 degrees the slot is out of the water. The faster
speed of the M must come from hull form and on points of sail other
than close hauled, where the X will point higher owing to its jibing
board. It has yet to be demonstrated that the M will sail at 17 MPH
like the X (or a catamaran) but at some point of sail and in some sea
condition, I am expecting the M to better the X.

The Tasars, for example, loose speed noticeably when they tack or
jibe. This is likely owing to the hard chine and cods head structure
of the hull which when on a heel track the vessel in a streight line
that resists a turning action. A 26X with a standard jib shares that
behavior. It is eliminated by using a Genoa. The Tasar crews (which
are not allowed the use of a Genoa) work the behavior by making as few
tacks as possible in moderate and heavy wind and viewing each tack as
a loss of 10 boat lengths (a huge penalty)

Future 60 foot and single handed ocean racers are to have up to two
centerboards, the second being called a canard. The second centerboard
will be used just as the centerboards on the Mac26x cruisers are which
is for steerage by introducing weather or lee helm and like the Mac26x
off centerline water ballast likely plays a part in its desirability.

MacGregor Yachts' change in the water ballast system so that the
ballast was no longer on centerline represented a huge advancement in
the firm's monohull design. It created the equivalent of a twin-keeled
catamaran-like-structure in the X boat. (See animation showing 4
slices through the X hull at 11 degree lean.)

Because the rudders of the X are on the keel lines, the windward one
is not very rudder like when the boat is on heel. In winds over 17 MPH
you see behavior involving lateral drift supporting the conclusion
that the second rudder, the one on the windward, functions as a
centerboard. What is currently called a centerboard, or jibing board,
may (at least at planing speeds) be better described as a canard.




The canard is a controlling, rather than a weight bearing, foil. The
main foil on sailing surf boards and other craft that use canards are
mounted almost directly under the rider, supporting almost all the
weight of the craft. The weight bearing foil is designed to remain
submerged at all times, and it is guided in doing so by the canard
foil, which pops to the surface before planing and generally remains
there during subsequent operation. The importance of this foil in
sustaining a plane is not known. The X downwind "hang glider
configuration" does not use the centerboard to break plane and
sustained planing involves little effort. But for other points of sail
the centerboard is likely important in maintaining the planing mode of
sailing. In displacement mode sailing the centerboard acts more as a
rudder on upwind points of sail by jibing slightly. Johnathan McKee's
minitransat boat incorporated two concepts not unsimilar to the
Mac26x. First, a swing keel with both lateral and fore and aft
movement allowed McKee to balance the boat properly on all points of
sail. Second a canard, or forward centerboard (sometimes called a
forward rudder) also swings laterally to keep the foil vertical
allowing more efficient trim while heeled. Most of MacKee's boats in
the last 10 years, though not the one pictured above, and most of the
Trans Atlantic ocean racers have water ballast.


The behavior of the X described as "she is hard to knockdown,
ballasted or unballasted" likely supports the above. With all three
foils extended in normal operation, the aft lee one serves as the
weight bearing foil and primary lifting force for planing. When not
planing, the forward foil serves both as a rudder on upwind points of
sail by jibing slightly to the wind and to compensate for the weight
of water ballast by lifting the hull. The windward aft foil provides
additional lateral force to the lee side keel to prevent crabbing when
winds are over 17 MPH. When the operator is inattentive or
inexperienced, all three foils likely provide counter balancing forces
that prevent knockdown. Like three legs on a stool, any one of the
foils might function as the weight bearing one during an unplanned
action, propping the hull up and righting it. The point is that the
behavior with foils extended means X owners can expect fixed keel boat
behavior. Behavior like capsize preventing stability even when
surprised by unexpected wind while unballasted.

The additional observation that the Mac26x dances like a butterfly
when on the anchor supports the notion that the vessel is a form of
trimaran. Both a multihull and the Mac26x are pushed by wind or water
on the hull and sail off to one side until the anchor road pulls the
bow back repeating the process. Bow rollers help multihulls and Mac26x
cruisers reduce this behavior as will a bridle and or a stern anchor.
The point is that the behavior at anchor probably means X owners can
expect multihull behavior when underway as well. Behavior like 17 MPH
under sail.

Keelboat skippers capsize dinghies more often than novices (from what
I am told) so I imagine that the feel of a keel over time deadens ones
natural response to excessive heeling. During the 1880 incidents, only
the young Commodore Ralph Munroe saw that the capsizing center
boarders, unlike other center boarders such as Spray and McKee's mini
transat, had small amounts of freeboard. This means that the rail
could be buried causing sudden loss of stability.

 
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