If you go to an outboard engine dealer to buy a model, they will necessarily ask you for the length of the tree you need. This length depends on the height of your boat's transom.
Indeed, for the engine to be correctly installed, the propeller must be positioned under the hull and the anti cavitation plate (the metal plate horizontally above the propeller is more or less - we will see the exact settings in another subject - at the bottom of the hull.
Engine manufacturers offer several shaft lengths on their engines:
- short shaft
- long tree
- extra long shaft
- ultra-long shaft
This distinction allows you to choose your engine according to the height of your transom.
To measure a transom, measure the distance between the tip of the V at the center of the hull and the top of the transom at the center (not on the sides!).
The shaft length is then chosen so that it is longer than the table, but not too long. With an engine too short, the propeller will not be enough in the water, with an engine too long, it will be too submerged (CQFD !).
The shaft length is standardized. It progresses from 5" to 5" (about 12 cm).
| 15" | short shaft | 381 mm | for transoms less than 38 cm high |
| 20" | long tree | 508mm | for transoms between 38 and 51 cm |
| 25" | extralong shaft | 635 mm | for transoms between 51 and 63 cm |
| 30" | ultralong shaft | 762 mm | for transoms between 63 and 76 cm |
It is a good idea to check these measurements - as well as the denominations - with the engine vendor. Indeed certain brands allow themselves small deviations - often insignificant - but which can pose problem.
