Outboard shaft length decides where the propeller sits in the water. Too short and the prop draws in air and loses grip; too long and the motor drags, sprays and steers poorly. Getting it right takes one measurement of your transom.
How to measure your transom
- Find the centre of the transom, where the motor will mount.
- Measure straight down from the top of the transom to the bottom of the hull (the keel line), not along the slope of the transom.
- Match that measurement to a shaft length below.
Shaft lengths
| Transom height | Shaft | Model-code letter |
|---|---|---|
| About 15″ | Short shaft | S (Suzuki, Tohatsu), MH/EH (Mercury) |
| About 20″ | Long shaft | L (Suzuki, Tohatsu), MLH/ELH (Mercury) |
| About 25″ | Extra-long shaft | X (Suzuki), UL (Tohatsu), XL (Mercury) |
When the motor is mounted, the anti-ventilation plate (the flat plate just above the propeller) should sit level with the bottom of the hull or slightly below it. Sailboats and boats with an outboard bracket often need a long or extra-long shaft because the mounting point sits higher above the water.
Signs the shaft length is wrong
- Too short: the propeller ventilates (draws air) in turns and chop, the engine over-revs as the prop loses grip, and the water intake can suck air, which risks overheating; see outboard water pump impeller.
- Too long: extra drag and spray, lower top speed, heavier steering, and a gearcase that sits deep enough to hit bottom in shallow water.
Fine-tuning the height
Most outboard clamp brackets have more than one set of mounting holes, so the motor can be raised or lowered slightly on the transom, and a jack plate allows finer adjustment on larger boats. Move in small steps and test: the aim is a clean grip in turns with as little of the gearcase in the water as the boat allows.
Kickers, sailboats and inflatables
Kicker motors on larger boats and motors on sailboats often need a 20-inch or 25-inch shaft to reach the water from a high transom or bracket; see kicker motors. Inflatable boats and dinghies usually take a 15-inch short shaft; see outboard motors for inflatable boats.
Reading the model code
The shaft letter is in the model code printed on every listing. For example, the Suzuki DF9.9BTL is a long shaft and the DF9.9BTX5 an extra-long shaft; the Tohatsu MFS20EEFS is a short shaft; the Mercury 15MLH is a long-shaft tiller model.
Shop outboards by horsepower
Each page lists the shaft length of every model: 2.5 HP · 4 HP · 6 HP · 8 HP · 9.9 HP · 15 HP · 20 HP · 25 HP · 30 HP.
Still unsure? Send us your transom measurement and we will recommend the right shaft. Shipping is a flat $80.00 per order with 15-day free returns.