Continued from WebLetter 12

Page 2 of 4

In the examples shown by Plate 33, a standard planing type boat is used as an example. While there are other straight-line shaft installations that are somewhat different, the principles are similar and can be extrapolated to various types of boats. In the illustration a few standard dimensions and clearances are given for an inboard installation. Note that the motor is directly located over the point indicated on the plans if building a new boat and if noted by the designer. In other cases no point will be given or available, and a "guesstimation" will be required. Usually with the motor located at the central location, a slight shift fore or aft will not have any severe consequences. However, if the point is known where the center of gravity of the motor should fall, then try to set the motor as close as possible to this point. The center of gravity of the motor may be indicated on an installation drawing provided by the motor manufacturer, or you can determine it for your motor by balancing it or lifting it with a chain hoist.

With most installations it will be desirable to get the engine as low in the boat as practicable in order to reduce the shaft angle. However practical limitations must be considered. The engine should not be subjected to excessive bilge water, and some clearance is desirable for access to certain motor components. Then too, a drip pan may be fitted under the engine and space must be provided for this. In some cases, the boat's structure may interfere with the motor installation as well. While entire structural members must not be cut away, some portions can sometimes be cut away and then reinforced to restore the member's original strength.

The position of the rudder can vary with certain designs, while in other cases, the designer provides quite specific size and location requirements for this component. With most boats solid water must be kept over the rudder which means that in most cases the rudder MUST be located under the boat. If placed too close to the transom, air may be sucked into the rudder causing poor steering and loss of control on turns. Sometimes the rudder can be located somewhat outboard from the transom, in which case a plate extended out from the transom over the rudder will cure this problem. It is also possible to set the rudder too far forward which will cause the shaft angle to increase. In general locate the rudder inboard as far as practical and at least so the aft edge of the blade is completely under the boat whenever possible.

The desirable distance between the propeller and rudder is at least 4" or as required to provide room to remove the propeller without first removing the rudder. Clearance between the bottom of the boat and the propeller should normally be at least 2" or 10% of the propeller diameter (see Chapter 9) whichever is least. While it is possible to "cheat" on these clearances somewhat, especially on small boats or other cases where cramped conditions apply, it is best to strive for the minimum tolerances noted for general practice. Then again, on very high-speed competition type boats, the distance from the rudder to the propeller is often increased.

Plate 34 - Typical components used in straight shaft centrally located inboard installations. The "bob-tail" version is primarily used in competition craft where a transmission is not necessary.

Continued