Humminbird 987C SI COMBO User Manual

1.  First, determine the best location on the transom to install the transducer.

Consider the following to find the best location:

•  It is very important to locate the transducer in an area which is relatively free of

turbulent water. As a boat moves through the water, turbulence is generated by
the weight of the boat, and the thrust of the propeller(s) - either clockwise or
counter-clockwise. This turbulent water is normally confined to areas
immediately aft of ribs, strakes or rows of rivets on the bottom of the boat, and
in the immediate area of the propeller(s). Clockwise propellers create more
turbulence on the port side. On outboard or inboard/outboard boats, it is best to
locate the transducer at least 15" (380 mm) to the side of the propeller(s).

• The best way to locate turbulence-free water is to view the transom while the

boat is moving. This method is recommended if maximum high-speed operation
is a high priority. If this is not possible, select a location on the transom where the
hull forward of this location is smooth, flat and free of protrusions or ribs. 

• The hydrodynamic shape of your transducer allows it to point straight down

without deadrise adjustment.

• On boats with stepped hulls, it may be possible to mount the transducer on the

step. Do not mount the transducer on the transom behind a step to avoid
popping the transducer out of the water at higher speeds; the transducer must
remain in the water for the control head to maintain the sonar signal.

• If the transom is behind the propeller(s), it may be impossible to find an area

clear from turbulence, and a different mounting technique or transducer type
should be considered (see Trolling Motor Transducer Installation).

• The Side Imaging transducer must NOT have anything obstructing the ‘view’ of

the side looking beams, i.e. nothing can be in the line of sight of these beams
(not a hull, motor, or other transducer, etc).

1.  Remove the transducer mounting template from this manual. See Appendix A

for the Transducer Mounting Template. 

2.  Hold the template on the transom of the
boat in the location where the transducer
will be installed. Align the template
vertically, matching the lower edge of the
transom with the bottom corner of the
template. If your propeller moves
clockwise as the boat moves forward,
mount the transducer on the starboard
side, and use the bottom left corner of the
template. If your propeller moves counter-
clockwise as the boat moves forward,
mount the transducer on the port side, and
use the bottom right corner of the
template. 

3.  Using a pencil or punch, mark the three mounting holes on the transom. Do not

mark or drill any other holes at this time.

4.  Using a 5/32" (4.0 mm) bit, drill the three holes to a depth of approximately 

1" (25 mm). On fiberglass hulls, it is best to use progressively larger drill bits to
reduce the chance of chipping or flaking the outer coating. Use a marine-grade
silicone sealant to fill the drilled holes.

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