Industry standards, measurement method, acceptable tolerances and practical application to the electric motor KRONOS of Weenav.
What is shaft length?
The shaft length of an outboard motor is the vertical distance between two specific points: the lower face of the mounting bracket (the part that rests on the transom of the boat) and the anti-cavitation plate located just above the propeller.
It is this measurement that determines how deep the propeller will be submerged — and therefore whether your motor will operate correctly or not.
To remember
It is the height of your transom that determines the shaft length to choose — not the other way around. A correct pairing positions the anti-cavitation plate 0 to 50 mm below the hull, ensuring a continuous flow of water to the propeller.
Standards of major manufacturers
Yamaha, Suzuki, Mercury, Honda, Tohatsu… all the major manufacturers share the same industry standard. Lengths are expressed in multiples of 5 inches, converted to millimetres in the European documentation.
How to measure your transom
Before placing any order, measuring the transom is essential. It is done with the boat out of the water, along the central longitudinal axis (never on the sides).
Beware of deep V hulls
On a boat with a pronounced V-shaped bottom, never measure from the sides: this greatly underestimates the actual height. For inflatable and rigid-inflatable boats, take the bottom of the rigid keel as the low point, not the base of the tube.
Acceptable tolerances
In practice, the transom of a boat is never exactly at a standard dimension. The tolerances below define what is acceptable — and what is not.
Practical selection rule
📏 Golden rule: better too long than too short. The use of spacers is simple and inexpensive. A shaft that is too short, on the other hand, requires modifying the transom or returning the engine — which is cumbersome, lengthy, and costly.
If your measurement falls exactly on the boundary between two classes (for example, a transom of 510 mm), choose the upper class : a shaft L 508 mm, or even X 635 mm with a spacer depending on the hull configuration.
Conséquences d'un mauvais appariement
A poor choice of shaft length not only causes inconvenience: it can seriously damage your engine, your transom, and significantly degrade your range — particularly critical on an electric.
⚡ Application to the KRONOS range
The KRONOS is a 100% electric outboard motor designed to replace a petrol engine across a wide range of boats. It complies with standard manufacturer specifications, ensuring direct compatibility with existing transoms — without cutting or heavy modifications.
Selection procedure for our 100% electric KRONOS motor:
1 Measure the height of the transom
Central axis of the boat, hull bottom (tip of the V) to the top of the transom. In millimetres.
2 Identify the nominal class
Compare the measurement to the standard S / L / X / U / E grid to determine the base class.
3 Check the available margin
Allow for 20 to 50 mm drop below the hull bottom for the anti-cavitation plate.
4 In case of class boundary
Retain the upper class and plan for a riser wedge at installation.
5 When repowering
Also measure the outgoing motor shaft (distance bracket → anti-cavitation plate) for confirmation.
⚡The electric range at stake
On a petrol engine, raising the shaft by 1 to 3 cm gains a few km/h. On an electric like the KRONOS, this translates directly into range : drag costs watt-hours. On a well-designed planing hull, an optimised positioning of the anti-cavitation plate can recover 2 to 5% of range. Beyond that, you enter the cavitation zone and efficiency collapses.
Special cases
High-performance planing boats
On hulls that truly lift (sport, runabout, bass boat, offshore), the propeller only needs to be barely submerged in plane. Raising the engine reduces the drag of the gearbox and the shaft, resulting in an increase in top speed and range. This is the role of a jack plate (hydraulic lift plate): we keep a standard shaft, but we move it upwards.
Navigation in shallow waters
Marshes, deltas, fishing in shallow lakes. A short shaft allows for passage in shallow water, to raise quickly without touching the bottom, and to use the engine in prolonged tilt mode. This market, more common in the United States, also exists in the Camargue and in some inland French lakes.
Dinghies, tenders, small inflatables
The transoms of these vessels are designed low by construction. The short shaft (S) is simply the nominal pairing — not an exception.
Surface propellers
Architecture where the propeller turns half in the air, half in the water: used for racing, military, or large fast units. The shaft is deliberately shorter than the transom. A niche market, but worth knowing.
Our team is available to analyse your boat and recommend the optimal configuration.
Published on: 13/05/2026
Editor: Sophie Castelain