
.....daggerboards and centerboards
Any object floating on the water
will tend to be blown across the water in a leeward direction, or away
from the direction which the wind is blowing. To prevent this from
happening to a sailboat, and to enable it to sail more in a direction into
the wind, a DAGGERBOARD or CENTERBOARD is used. A daggerboard or
centerboard is a thin vertical plate sticking out the bottom of the boat
that gives the boat a "grip" on the water thereby preventing or at least
minimizing leeward movement. Contrary to popular belief, the daggerboard
or centerboard does not keep the sailboat from tipping over. While
daggerboards and centerboards are not strictly a part of the rigging in
terms of its "transmission" function, they do help greatly in converting
the power of the wind in the sails to forward motion. And because
daggerboards and centerboards often require fittings and lines to make
them operable, they are relevant to our discussion on rigging.
What's the difference between a
daggerboard and a centerboard? Well, a daggerboard can be likened to an
actual dagger that fits into its sheath or case. The "case" in the
instance of the daggerboard is referred to as the DAGGERBOARD TRUNK. The
daggerboard slips in and out of the daggerboard trunk, usually in a
vertical direction only. The purpose of the daggerboard trunk is to
provide bearing for the board and keep water out of the boat. Once the
daggerboard is in the trunk there is very little movement of the
daggerboard. The daggerboard can be made out of sheet metal (usually
aluminum) or wood. The hole in the bottom of the boat where the
daggerboard passes through is usually a fairly tight fit around the
daggerboard. There is very little adjustment with a daggerboard. Once it
is down in position, the position can only be varied up or down.
 |
FIG. 1O-1 - A typical centerboard trunk in a
small sailboat. Part of the centerboard can be seen protruding out
the top of the trunk. The centerboard pennant passes through a
sheave on the forward trunk upright and is knotted to the
centerboard. This arrangement forms a "tackle" to ease the effort
required to lift the board. A cleat on the centerboard trunk is used
to secure the pennant. |
A centerboard on the other hand is
hinged at its forward portion on a CENTERBOARD PIN. The centerboard pin is
usually a bolt or rod about which the centerboard pivots up or down in the
CENTERBOARD TRUNK (see Fig. 10-1). The centerboard trunk is similar in
function to the daggerboard trunk. However, with a centerboard, it remains
in the boat at all times in its trunk, while the daggerboard is removable
through the top of the daggerboard trunk. Whereas the daggerboard moves up
and down vertically in the slot of the daggerboard trunk, the centerboard
"swings" about the centerboard pin. Because it can swing, the position of
the centerboard can be varied to suit different courses of sailing, and
changes in the loading and balance of the hull. From this standpoint, a
centerboard is more efficient than a daggerboard. However, the slot for
the centerboard where it passes through the bottom of the boat must
necessarily be longer than for a daggerboard, and this adds to the
resistance of the hull thereby slowing the boat down somewhat. One
drawback of daggerboards is that should the boat sail into shallow waters
or accidentally come up onto a beach, the daggerboard will not pivot back
and up into the trunk as will a centerboard. The damage to the boat in
such an encounter could be extensive, whereas with a centerboard, it would
pivot back and up into the trunk with probably no damage at all.
When daggerboards or centerboards
are made from wood, they have a tendency to float up out of the water just
like a wood rudder. There are several ways to prevent this. Naturally, the
board could be made from metal (aluminum or steel) or fiberglass, but this
is not recommended unless the boat is originally equipped this way, or the
designer specifies it. With daggerboards, spring stainless steel "clips"
are available, which fit onto the board making the fit so tight in the
trunk that the board cannot float up. Another method is to use an elastic
"shock cord" across the top of the trunk slot secured at each end to screw
eyes each side of the trunk. With both types of board, a removable "pin"
can be used through the trunk sides and board that will "lock" the board
in position. The boat shown in Fig. 6-8 features such a pin for the
daggerboard trunk. By providing a series of holes in the board, the pin
arrangement could be used to lock the board in various positions to suit
sailing conditions. On centerboards of wood, a common method is to put a
weight on the board. This can be a lead weight built right into a hole in
the board, or a specially fabricated casting designed to fit the bottom of
the board. The weight will pull the board down in position and then a pin
arrangement can be used to lock the board in position.
Daggerboards are simpler in nature
than centerboards, and therefore require few if any fittings other than
those described above. Centerboards, however, can get more complex,
especially the bigger the board gets in size and weight. The line used to
raise and lower the centerboard is known as the CENTERBOARD PENNANT. On
lightweight boards, the line may be attached by a knot through a hole in
the board, and lead to a cleat somewhere on the trunk or hull structure.
On heavier boards, the centerboard pennant can be rigged in a tackle
arrangement to decrease the effort required to raise the centerboard.
Pulleys or sheaves can be fastened to the centerboard and/or to the trunk
to come up with the proper tackle.
On many of the trailerable cruising
sailboats from about 17' in length and up, the centerboard is often a
piece of steel plate or even a casting of steel or lead. Sometimes the
latter are referred to as "retractable keels" because they also add to the
stability of the boat. In either case, the weight of the unit is often
such that a tackle will not be practical. When this is the case, a winch
is used with a wire rope pennant to raise and lower the centerboard. The
winch is best a two-way type with positive ratchet action on both the
"down" and "up" directions. Worm-gear winches are available for use on
boat trailers that are suitable for centerboard use, as are more deluxe
types especially designed for centerboard use. All components used in such
an installation should be of extra heavy duty and arranged so no jamming
or fouling of the gear will result in use.
As noted previously, the
centerboard or daggerboard trunk helps keep water out of the boat. This
means that no holes or leaks should be permitted in the trunks AT LEAST
BELOW THE WATERLINE. Sometimes this is a problem with centerboard trunks,
especially at the centerboard pin. The centerboard is constantly "working"
and causing stress on the pin that tends to loosen and let water seep into
the boat. There are so many variations in the design of centerboard pin
mechanisms (probably in an attempt to make a good one that won't leak)
that it is not possible to specify a "cure-all" which will work in all
cases. If the pin area is leaking, check to make sure that the bolt is
tight, and if it has worked to where it fits too loosely in the hole, it
should be replaced with a larger pin. Also check the gaskets, if so
equipped, to see that they are in good condition, and replace if
necessary. The structure of the centerboard trunk should be checked to see
that all members are tight and well fastened. Sometimes a leaking pin is
the result of a weakened trunk structure. If this is the case, repairs or
replacement of structural members may be in order. Sometimes a little
fiberglass tape and resin can work wonders in patching up leaks and
reinforcing weak members, but make sure the areas to be fiberglassed are
clean and dry first. Because the inside of the trunks will not be
accessible after assembly, it is advisable to fiberglass the inside prior
to assembly to seal the trunk and protect against abrasion by the
board.
In use of the boat, sometimes there
will be a vibration or "chatter," especially noticeable when the boat is
moving along at top speed. This chatter is usually caused by an unfair
centerboard or daggerboard, and sometimes an unfair rudder. What this
means is that the edges and general sectional shape through the board have
not been finished correctly. Obviously a board with a square edge forward
or aft will not go through the water as effectively as a board with edges
that are shaped or "faired." With sheet metal centerboards or
daggerboards, about the only fairing that can be done is to radius the
edges. If wood is used, a better "shape" can be incorporated. One method
of fairing a wood board that gives good all around results is shown in
Fig. 10-2. While very fine edges give least resistance, it is best not to
make them "razor sharp" because they damage too easily in use. It is
possible to build up the correct "shape" to any board by using foam shaped
to suit and covered with fiberglass. If fiberglassing the outside of the
hull, it is best to tuck fiberglass cloth into the trunk to protect the
exposed plywood edge.
FIG. 10-2 - A section through a "faired" wood
daggerboard. The same principle can be applied to rudders and
centerboards. Note that the thickest part is located approximately
1/3 the width of the board aft of the forward or leading edge. Edges
should be radiused slightly to prevent
damage. |
With daggerboards that tend to bind
or jam in use, a little paraffin will usually smooth things out. Sometimes
the board will warp or the slot will not be quite the same shape as the
board. This may require reshaping or replacing the board. Sometimes the
board will fit too loosely in the slot. This will cause working at the pin
on centerboards, and sometimes banging from side to side. Small blocks or
metal angles can be used to make the fit at the slot tighter. Some device
is often provided, especially with steel centerboards, to limit the
forward travel of the centerboard to keep it from banging against the
forward portion of the centerboard trunk. This is a good idea because
pennants sometimes break, or a lock pin will fail, and the board will fall
forward at a crashing speed.
Chapter
11 Contents |