2.2 DRIVE SYSTEM
The following notes describe the installation of
both the rotary and linear drive units.
2.2.1 ROTARY DRIVE UNIT
The rotary drive unit is coupled to the vessel’s
steering mechanism by a simple chain drive.
Most steering gear manufacturers supply
special autopilot drive attachments and many
include this facility as standard.
Fig. 7 shows recommended rudder
to
times for both planing and
displacement vessels up to 42 feet (13m)
The charts shown in Fig. 8 enables the chain
reduction ratio for optimum rate of rudder
application to be selected for both planing and
displacement vessels. To use the charts, it is
first necessary to determine the number of turns
of the driven sprocket when the rudder is driven
from
to hardover.
Example:
A 40 foot
displacement vessel
requiring two turns of the driven sprocket to
drive the rudder from
to
will
require a chain reduction ratio of approximately
(as indicated by the dotted line on the
chart). The table on the left hand side of the
chart gives suitable sprocket combinations. In
this example, the required reduction ratio of
would be best achieved by a 38 tooth sprocket
driven by a 13
on the drive unit.
It should be borne in mind that the
reduction ratios recommended are for the
‘average case’ and that vessels broadly
classified by length and hull type can vary
significantly in steering characteristics.
Selection of the correct chain reduction ratio is
not over critical however, and any slight
mismatch can usually be corrected later during
sea trials by an adjustment to the gain control
on the control unit.
Standard
pitch chain is recommended
for the chain drive. Sprockets of 13, 15,
17,
and 25 teeth are available as standard
accessories. Bore and
dimensions for
the drive unit sprocket are detailed in Fig. 4. If
sprockets other than those supplied by
Nautech are fitted, it is essential that bore and
dimensions specified in Fig. 4 are
strictly adhered to. The recommended driven
sprockets tabulated Fig. 8 are common
standard sizes and should be obtainable from
local suppliers of chain drive equipment. All
sprockets must be “keyed” and grub screwed
to their shafts, and finally secured with ‘Loctite’.
The drive unit is mounted by bolting to a
substantial frame member (Fig. 5). The
shaped mounting foot is secured by four
equally spaced
screws, and may be
rotated through 90 degrees to provide a more
convenient mounting position if required (Fig. 6)
some cases, it may be
necessary
to
fabricate a special frame to mount the drive
unit. It should be noted that chain tension can
exceed
and thus an extremely
rigid mounting structure is vital to maintain
good chain alignment. Installation failures
frequently occur in this area, and as a general
rule it is desirable to ‘over engineer’ the drive
unit mounting.
(0.127”-0
9 46-9 61 mm
G R U B S C R E W
(0.75”)
(0.5”)
Fig. 4
