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Twin Outboard Engine Installation on Moderate V-hull Boats

by James W. Hebert

Most classic Boston Whaler hulls from 18 to 27 feet are moderate V-hulls and have transoms that allow installation of twin engines. Twin engines require special consideration in their mounting and alignment to obtain optimum performance. We briefly discuss several of these concerns.


On single engine boats the outboard is mounted on the keel centerline. Twin engines are mounted symmetrical to the keel centerline but displaced from centerline. The amount of displacement varies with two principal factors:

The American Boat and Yacht Council has guidelines for the distance at which twin outboard engines are to be separated based on the size of the engines. Their recommended centerline spacing between outboards in a dual engine installation is:

These distances are recommendations to provide adequate clearance between the motors so that there is no interference when the motors are pivoted on their steering axis or on their tilt axis.

Recommendations of separation distances may also be made based on the deadrise angle of the hull. Because of the deadrise in a V-hull, the shaft length needed will change as the engine is moved outboard from keel centerline. The rate of this change is proportional to the deadrise angle. On a moderate v-hull with a deadrise of 18-degrees, for each inch the outboard is moved off keel centerline, the deadrise will cause the depth of the hull to decrease by

TAN(18°) x 1-inch = 0.325-inch

If an engine were mounted 13-inches off keel centerline, the deadrise would reduce the depth of the hull by

13 x 0.325 = 4.225 inches

Because outboard engines are generally available in shaft lengths of 5-inch increments (i.e., 15, 20, 25, 30-inch shaft), the vertical mounting height of the engine is often varied slightly by use of an optional mounting hole position on the transom bracket of the engine.

Boston Whaler has recommended a twin engine spacing distance of 29-3/4-inches for most of their moderate V-hull boats, such as the OUTRAGE 18, 20, 22, and 25. For hulls with an 18-degree deadrise, this distance results in a reduction of hull depth of

(29.75/2) X 0.325 = 4.83-inches

This is closer to the 5-inch difference in shaft length choices. For single engine installations where the recommended single engine shaft length was 25-inches, this explains the Whaler recommendation to use 20-inch shaft engines in twin installations.

How the 29-3/4-inch distance was determined is subject to some speculation. It does fit nicely with the 5-inch incremental shaft length. It may also have been arrived at experimentally from sea trials with twin engines.

The recommended spacing for twin engines on a Whaler Drive is believed to be 28-inches. On boats with aftermarket brackets such as an Armstrong bracket, consult the manufacturer for a recommendation.

When a boat goes onto hydroplane, the hull provides lift, which causes the hull to rise in the water and reduce its draft. This is also a consideration in the spacing of twin engines. Clearly, if the spacing were made too distant, the engines, which may have been set to a proper depth in the water at a static trim, may rise as the boat comes on plane. If the hull rise is too dramatic, this may cause the propeller running height to become too shallow.

Greater separation of the engines may be desirable to allow better low speed handling when trying to maneuver the boat with offset engine thrust; the farther off-center the thrust the more effective it will be at turning the boat. This may also have been an influence in the Whaler recommendation of 29-3/4 inch separation for twin engines.


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:

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.

Further Discussion

Questions and comments on this article can be found at a discussion that was reserved for that purpose.

DISCLAIMER: This information is believed to be accurate but there is no guarantee. We do our best!
This article first appeared May 27, 2006.

Copyright © 2015 by James W. Hebert. Unauthorized reproduction prohibited!

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