Found this interesting article on another board
TOE-IN and TOE-OUT
With twin engines a new consideration enters the rigging problem: alignment of the propeller shafts of the engines with respect to each other. This is known as toe-in or toe-out. A twin engine installation is said to have toe-in when the alignment of the propeller shafts is not parallel, and the shaft angles converge somewhere ahead of the engines. If the shaft angles converge astern of the engines, it is known as toe-out. Because of the difficulty in actually measuring the propeller shaft angles of twin engines, this adjustment is generally referred to in terms of the difference in distance between the horizontal spacing of the the tips of the gear case nose cones and the centerlines of the propeller shafts. Typical values of toe-in or toe-out are less than one inch difference.
The ultimate goal of toe-in or toe-out adjustments is to cause the propeller shaft angle at high speed to become the most effective at propelling the boat. Adjustment of the propeller shaft angles is made experimentally and in consideration of several factors:
convergence of the wakes
alignment with water flow
counter rotation propellers
Engine alignment will effect how far behind the boat the wakes of the twin engines will converge. In some instances there may be objectionable effects if the wakes converge too close to the boat. Such a convergence can produce loud exhaust sounds, and it may also lead to problems with propeller ventilation. For these reasons, the propeller shaft angles are often set with some toe-in, which tends to keep the wakes from converging until farther behind the boat.
In a V-hull boat the water flowing off the V-hull may not be perfectly aligned with the fore and aft centerline of the keel, but may be projecting outward at a slight angle. In order to align the propeller shafts with this slightly angled flow of water, some toe-in adjustment is often made. In installations where the engines are mounted on a set-back bracket, the water may have returned to a more parallel alignment by the time it reaches the gear cases, and a smaller allowance for toe-in may be more appropriate.
Literature from OMC provided with their Adjustable Tie Bar Kit mentions the following regarding toe-in and toe-out:
"The adjustment feature [of the tie bar] also allows you to create a toe-in condition between dual engines (bring the forward portions of the engines together slightly). This is very important if your boat has a deep V shaped hull. Because water does not pass straight to the rear off the bottom of a V shaped hull, but slightly towards each side of the center of the hull, a slight toe-in of the engines will align the engines' lower unit (gearcase) with the true direction of the water flow. This alignment with water flow allows the propeller to grip the water cleanly, more efficiently, resulting in improved boat performance.
"The precise amount of toe-in required depends upon the degree of V-shape of the boat's bottom. As the V-shape gets deeper or steeper, the amount of toe-in must increase. Proper toe-in adjustment can result in better boat performance and lower steering effort. The correct amount of toe-in for your boat and engine combination can only be achieved by making successive runs with varying degrees of toe-in."
The OMC literature goes on to mention that 1/2-inch of toe-in is appropriate for many V hulls and suggests starting from that point.
Specific advice for Boston Whaler boats is relayed via Boston Whaler Customer Service Representative Chuck Bennett. He recommends as follows:
"Toe in: Measure the distance between prop centers and forward edge of the gearcase at the ventilation plates. The measurement at the forward edge of the gearcase it to be 1/2" to 3/4" less than the prop center measurement."
It has also been speculated that twin engine installations with counter-rotating propellers will tend to create a situation where the propeller thrust may act to push the engines apart (i.e., create a reduction in the propeller shaft angle or cause a slight toe-in). The amount of this effect may be influenced by the nature of the engine mounts. If the engine mounts are flexible, it is more likely to occur. Thus, the static alignment of the engines may be set to be even or to have a slight toe-out. This will be overcome at speed and the propeller shafts will toe-in slightly or align to parallel when the counter-rotating propellers push the propeller shaft tips slightly apart.
