Propeller Blade Tip Clearance; Propeller Shaft Coupling - Mercury MerCruiser Product Applications Manual

Propeller Blade Tip Clearance

Sufficient clearance is required between the propeller blade tips and the boat bottom to
prevent a water-hammering action against the hull. Insufficient clearance will contribute to
excessive noise and vibration and could result in cavitation problems. A clearance of 10–
15% of the propeller diameter is typical for most boats.

Propeller Shaft Coupling

Since the engine has flexible mounts, some provision must be provided in the propeller
shaft and coupling to accommodate for slight engine movement. On recreational boats,
this is typically provided by the use of a flexible stuffing box and a single strut bearing. In
these cases, a rigid propeller shaft coupling can be used. Rigid propeller shaft couplings
can be ordered with the engine or purchased through Mercury Precision Parts and
Accessories. Alignment with these couplings is critical; See Section 3 — Engine.
Typically, a minimum distance of 20 times the propeller shaft diameter should be provided
between the transmission output flange and the first fixed bearing to minimize vibration. If
this distance is not available, a flexible coupling can be used. Flexible couplings provide
vibration isolation and are more tolerant to engine misalignment. They also reduce the need
for periodic realignment of the engine.
If an aftermarket coupling is to be used, the flange must conform with the specifications on
the installation drawing. The transmission output flanges comply with:
• Velvet Drive 71C and ZF 45C—SAE J756 Type 410.
• All other transmissions—SAE J756 Type 5.
All coupler bolts must be SAE Grade 8 (Metric Grade 10.9) or better, with a shoulder (grip
length) long enough to pass through the coupler and transmission output flange. All coupler
bolts must be torqued.
90-863758060 AUGUST 2006
Minimum Distance Between Engine Centerlines
Model
All Except 8.1S
8.1S
a
a - 10–15% of propeller diameter minimum
Transmission and Drive Line
Measurement
838 mm (33 in.)
927 mm (37½ in.)
14885
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