30-second answer
Horsepower is useless when the propeller sits at the wrong height. Measure the transom before ordering an outboard.
Best forBuyers making a propulsion fit decision before choosing a brand
Watch outDo not turn a general guide into a final specification without checking the exact boat and local conditions
Why shaft length matters
The anti-ventilation plate needs to sit close to the bottom of the hull. Too short and the propeller can ventilate in chop or turns; too long and the extra drag, poor trim and deeper gearcase compromise handling and beaching.
Small inflatable catamarans and conventional tenders do not all use the same transom height. Never infer shaft length from overall boat length or from another model in the same showroom.
Measure before you buy
- Stand the unladen boat level on firm ground.
- Measure vertically from the top centre of the transom mounting surface to the lowest running surface beside the transom.
- Ask the boat builder for its stated shaft requirement and compare that wording with the outboard manufacturer's definition.
- Repeat the check if a transom riser, jack plate or unusual catamaran tunnel is fitted.
Common warning signs
- Engine races without a matching increase in speed: possible ventilation or mounting-height problem.
- Heavy spray around the leg: the engine may be mounted too low.
- Poor turning grip: confirm engine height, trim, propeller and loading rather than assuming more horsepower is needed.
- The clamps do not sit fully on the mounting face: stop and confirm the transom arrangement.
Before ordering
Get the required shaft length and maximum engine weight in writing for the exact hull variant. Then confirm the selected outboard's actual dry weight, starting system and tiller configuration. Dealer terminology can differ, so dimensions are more reliable than labels alone.
How this guide was made
Written and reviewed by Gabriel Machuret. BuyDinghy compares published manufacturer and Australian distributor material, labels missing specifications instead of estimating them, and separates editorial judgement from verified facts. Read the methodology or report a correction.