Installing an outboard motor on your boat requires making sure the transom height is properly matched to the motor’s shaft length. Measuring your transom correctly is crucial for optimal motor performance. This guide will teach you how to accurately measure your boat’s transom height.
What is Transom Height?
Transom height refers to the distance from the bottom of the boat hull up to the top of the transom, measured along the boat’s centerline.
- For single motor boats, measure at the exact center
- For twin motor boats, measure at each motor’s mounting location
Why Transom Height Matters
Matching your outboard motor shaft length to the boat transom height is critical for:
- Keeps propeller submerged and prevents ventilation
- Allows motor to be trimmed properly
- Reduces drag and improves performance
- Prevents engine overheating
- Avoids damage to boat transom
Below are the standard transom heights for different shaft lengths:
| Shaft Length | Transom Height |
|---|---|
| Short (15″) | 15″ |
| Long (20″) | 20″ |
| Extra Long (25″) | 25″ |
How to Measure Transom Height
Measuring transom height is simple if you follow these steps:
- Identify the boat’s centerline – For single motor boats, this is straight back from the middle. For twin motors, measure at each mounting location.
- Measure from the bottom of hull to top of transom – Use a tape measure pressed vertically against the transom. For vee hulls, measure to the lowest point.
- Account for trim tabs or blocking – If present, measure from the bottom edge of these instead of the hull.
- Consider transom angle – Angled transoms may require adding 1-2 inches to allow for the slope.
- Compare to shaft length needs – Choose an outboard motor that matches your transom height.
Adjusting Transom Height
If your transom height doesn’t match your desired outboard motor, you have a few options:
- Use a jack plate – Attaches below transom to adjust motor height
- Modify the transom – Cut down or build up transom to required height
- Get an outboard with a different shaft length – Easiest option
- Add a trim wedge – Small block to increase transom angle
Carefully measure your transom first before deciding on adjustments. Consulting with a marine mechanic is also recommended.
Conclusion
Measuring transom height correctly allows pairing your boat with the optimal outboard motor. Take your time, measure accurately, and use the right shaft length motor. This will provide the best performance and avoid potential engine issues.
