This is the question that stalls one order in three, and a tape measure settles it. Outboard shaft length is not a preference: it is a measurement, dictated by the height of your transom.
Getting it wrong breaks nothing but spoils everything: too short and the propeller ventilates, churning air at the first wave; too long and it drags, costing you in range what it never gives you in thrust.
The measurement, in thirty seconds
Put the tape on the top of the transom, where the motor will sit, and run it straight down to the bottom of the hull, vertically. That is your transom height.
Compare that figure directly with the shaft lengths. Take the closest, rounding up rather than down: a propeller running slightly deep works better than one that lifts out of the water.
The available lengths
| Length | Shaft | Suits |
|---|---|---|
| Short · S | 39 cm | Tenders, small inflatables, low transoms |
| Long · L | 52 cm | RIBs, yacht tenders, most sailing boats |
| Extra-long · XL | 64 cm | High-freeboard sailing boats, workboats |
What happens when you get it wrong
- Too short. The propeller ventilates: it draws air instead of water. The motor races, thrust collapses, and the noise gives it away at once. In a chop it never stops.
- Too long. The propeller and leg drag through the water permanently. Nothing breaks, but you carry needless drag every mile — range lost, on every outing, for good.
If in doubt
Between two lengths, take the longer one if you go out in open water, the shorter if you stay in sheltered waters. And if your measurement falls exactly between two, send us a photo of your transom with the tape in place: it beats any table.
Frequently asked questions
Can I change the shaft length later?
No: the length is part of the motor, it does not extend. That is why the measurement is taken before ordering, not after.
What if my transom is raked?
Measure vertically, not along the transom. What counts is how deep the propeller sits, not the length of the board.
Does a tilting bracket change the measurement?
Yes, if it raises the motor’s seat. Measure from where the motor will actually rest under way.
In short
A tape measure, one vertical reading from the top of the transom to the bottom of the hull, and the length chooses itself. Thirty seconds that decide the comfort of every outing you make.
Unsure about your boat? A Momentum dealer will check it with you, or send us the measurement.
