Dave Gerr - Boat Mechanical Systems Handbook-How to Design, Install, and Recognize Proper Systems in Boats

Constant Velocity and

Universal Joints

So far we’ve been talking about standard
straight-shaft inboard installations. Being
simple and rugged, they have much to rec-
ommend them. But when there is room
enough and budget enough, I like to install
double constant-velocity (CV) joints with an
independent thrust bearing. Usually, I go
with an off-the-shelf package like Aquadrive
(Figure 3-2) or PYI’s Python-Drive. Spicer
and CON-VEL are two other sources for CV
joints. Figure 3-3 shows the workings of a
CON-VEL CV joint.
Using double CV joints has several
pluses. They allow more flexibility (no pun
intended) in engine location. They also
accept what for straight-shaft systems
would be unimaginable misalignment, so
shaft-alignment problems are virtually un-
heard of.
In addition, because systems like Aqua-
drive allow so much flex, the engine can be
“floated” on extremely soft mounts that further
isolate engine vibration from the hull.

The Independent Thrust

Bearing

It’s important to remember that the indepen-

dent thrust bearing in this system plays a big

role in permitting such soft mounts. In standard

straight-shaft installations, the thrust from the

propeller is transmitted along the shaft and

taken by a thrust bearing in the marine gear.

(By the way, this is why you can’t just use a car

transmission in a boat; auto transmissions have

Chapter 3: Transmission Geometry, CV Joints, Stuffing Boxes, and Engine Mounts and Beds

Figure 3-2.

Aquadrive system

(Courtesy

Aquadrive)

Figure 3-3.

CON-VEL CV joint

(Courtesy

CON-VEL Inc.)