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Engine Mounting Height

Guidelines for outboard transom mounted engines

by James W. Hebert

This article presents information from Mercury regarding their recommendations for engine mounting height. The recommended mounting height varies with the anticipated speed range of the boat, the objective of the installation, and whether single or dual engines are used.

Chart: Engine mounting height as a function of boat speed

LEGEND

A—This solid line is recommended to determine the outboard mounting height dimension. Use transom mounting bolt holes that will position outboard nearest to the recommended height. After engine break-in (if necessary), raise or lower outboard one position at a time to attain best performance. IMPORTANT: Increasing the height of outboard will generally provide the following:

  1. Less steering torque,
  2. more top speed,
  3. greater boat stability, but
  4. will cause more prop "break loose" which may be particularly noticeable when planing off or with heavy load.

B—These broken lines represent extremes of known successful outboard mounting height dimensions.

C—This line may be preferred to determine outboard mounting height dimension if maximum speed is the only objective.

D—This line may be preferred to determine outboard mounting height dimension for dual outboard installation.

E—Outboard mounting height (height of outboard transom brackets from bottom of boat transom). For heights over 22-inches (560 mm), a propeller that is specifically designed for surfacing operation, such as a "Chopper" series, is usually preferred.

Comments or Questions

If you have a question about this article, please post it to the message thread in the PERFORMANCE forum reserved for that purpose.


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Author:James W. Hebert
This article first appeared May 18, 2008.