Boat Transom Angle Calculator

REF: EN-204-GEOMETRY

Use this tool to precisely calculate your vessel's transom rake angle using simple rise-and-run measurements. Determining the exact transom angle is critical when mounting outboard motors, selecting wedge shims, installing trim tabs, or ensuring proper propeller thrust geometry relative to the waterline.

Understanding Transom Angles & Rake

The transom angle (or transom rake) is the backward tilt of a boat's stern relative to a true 90° vertical plane. Most standard outboard motor brackets are engineered around a standard transom angle of 12° to 15°. Getting this angle right ensures your engine can trim properly both inward (for rapid planing) and outward (for maximum top-end speed).

Why Transom Geometry Matters:

• Trimming Capacity: If your transom angle is too steep (under 10°), the outboard motor may not tuck in far enough, causing slow planing, excessive bow rise, or porpoising.
• Wedge Shims: Boats with shallow or custom transom angles often require transom wedges (typically 5° cast aluminum spacers) to adjust the mounting angle to match standard outboard brackets.
• How to Measure: Place a straight edge vertically down from the top edge of the transom, drop a plumb line from the top edge, and measure the horizontal offset distance (Run) at a known vertical depth (Rise). Trigonometric arctangent ($ \theta = \arctan(\text{Run} / \text{Rise}) $) converts these lengths into degrees.