1967 Johnson Stern Drive Design Advantages - DL-168

August, 1967

SUBJECT: ADVANTAGES OF JOHNSON STERN DRIVE

Gentlemen:

Occasionally, questions arise concerning different aspects of our stern drive design and installation. Following are some of the inquiries most frequently received. The answers will help further enlighten you as to the many advantages of using stern drive power. You may wish to bring this information to the attention of your customer.

Q. Why do we mount our engines and drive units to the bottom of the boat without securing them solidly to the transom?

A. By using the stringers between the flooring and the bottom as a bed on which to cushion mount our engines and drive units, we are better able to isolate the annoying and damaging vibrations and distribute the thrust load over a larger area of the boat rather than on the relatively small transom area. The use of the neoprene transom seal assures a water tight seal with no metal to transom contact to transmit vibration. We are also able to make a finer degree of trim adjustment of prop shaft angle by raising or lowering the front of the engine using the screw-jack located on the front engine mount.

Q. Why do we use the ball gear type connection as a driveline rather than a splined shaft and universal joint?

A. Because of the rather limited working angle of a universal joint drive, the ball gear type drive has two distinct operational advantages.

First, the ball gear drive will allow the lower unit to be tilted a full 75°, allowing the gear housing to clear the water on most installations. This is recommended for mooring and is 25° more tilt than a universal joint will allow.

Secondly, the limited working angle of a universal joint restricts the steering angle of the outdrive unit reducing the maneuverability of the boat.

(over)

Additional advantages of the ball gear drive is that the driving force is distributed more evenly than with a universal joint and it requires far less maintenance when used in an exposed position.

Q. Why do we use a drive pin on the propeller shaft instead of a fixed spline?

A. The drive pin is engineered to protect the gearcase components and driveshafts from damage due to the fouling of the propeller by an underwater object. In addition to the drive pin, further protection is supplied by a cushion hub in the propeller. The hub is designed to slip momentarily on impact and absorb the force, rather than transmit the shock to the shaft and gears. The use of a plastic cap, secured by a cotter pin to hold the drive pin in place, makes removal or replacement of propellers a job easily done by anyone with a pair of pliers.

Q. Why do we have two separate cavities to fill with oil in the vertical drive unit?

A. If the entire unit was one oil reservoir, the loss of even a small amount of oil from the lower half would leave the upper shaft, gears, and bearings without adequate lubrication. By utilizing two oil reservoirs, we are able to lubricate and protect each section separately regardless of the oil level in the other.

Q. Why don't we have shock absorbers on our vertical drive unit?

A. Once a unit has been raised, due to striking an underwater object, the single action shock absorbers used on other stern drive units cannot effectively control the rate at which the unit drops back to normal operation position. The oil immersed series of clutch plates used by Johnson gives added protection by allowing the unit to rise on impact, while giving the operator control over the return to drive position by use of power tilt.

JRP/rkm