What Size Outboard for an Inflatable Boat? Australian HP Guide (2026)

The right outboard is the smallest one that still planes your real load — not the biggest one the transom will take.

Most people over-buy or under-buy their first outboard for the same reason: they size the motor to the boat, not to the load. A 3.65 m inflatable catamaran with one person aboard and a 3.65 m catamaran with three adults, fuel, an esky and a bait board are two different boats. Horsepower has to answer the heavier one.

This guide matches outboard size to hull length and real Australian use — bay fishing, yacht tendering, family days, long runs across exposed water — and covers the two things people get wrong far more often than horsepower: shaft length and transom height.

How to size an outboard in four steps

1. Start with your loaded weight, not the hull length

Add up the worst-case day: crew, fuel, battery, safety gear, esky, anchor, catch. A 3.8 m inflatable that weighs 62 kg empty can easily be carrying 300 kg by the time it leaves the ramp. Planing is a power-to-weight problem, so that number drives everything.

2. Decide whether you need to plane, or just to travel

Not every boat needs to plane. A yacht tender running 400 m to shore is happy at displacement speed on 2.5–5 hp. A fishing boat crossing 6 km of open bay against an afternoon southerly needs to get up and stay up.

The practical rule

3. Check the boat's maximum rating

Every hull has a maximum rated horsepower and maximum load. Fit the largest motor you will actually use, at or under that rating. Overpowering an inflatable makes the bow light, the steering nervous and the warranty void.

4. Match the shaft to the transom

This is where most of the complaints come from — and it is almost never the boat's fault.

Shaft length and transom height explained

A short shaft motor is built for a 15 inch (roughly 380 mm) transom. A long shaft is built for 20 inch (roughly 508 mm).

Aerowave WaveRunner and Viper hulls use a 380 mm transom. That was a deliberate change from 405 mm: it puts more prop in the water, gives more bite and kills the ventilation and cavitation that plague boats where the prop sits too high.

What goes wrong when you mismatch

Long shaft on a short transom

The leg hangs too deep. You drag hardware, lose two or three knots, burn extra fuel and the motor works harder for the same speed.

Prop too high

The prop draws air off the surface, revs flare, and the boat feels like it is slipping in every turn. People blame the boat. It is almost always mounting height.

Rule of thumb: the anti-ventilation plate should sit roughly level with, or 25 mm below, the bottom of the hull at the transom.

Size-by-size recommendations

3.3 m — Aerowave AeroCat 330 / Viper 330

Two adults, protected water, easy to carry and store. 6 hp planes a light load; 9.9 hp is the confident choice once you add fuel, an esky and a second passenger. The Viper 330 is A$5,195 and the 9.9 hp package is A$8,995.

3.65 m — Viper 365 Open Bow

The all-rounder. 9.9 hp for bay work, 15 hp if you fish with two mates or run distance. Boat A$5,595.

3.8 m — AeroCat 380 / WaveRunner 380

Family and fishing size. 15 hp is the match: three adults on the plane, a genuine cruising speed and enough reserve for a headwind run home. AeroCat 380 A$4,535; WaveRunner 380 A$4,680; the WaveRunner 380 with a 15 hp four-stroke is A$8,924.

4.0 m — Viper 400 Open Bow / Viper 400 Sovereign

Four adults, longer crossings, more freeboard. 20 hp is the right answer here. Viper 400 Open Bow A$5,895, Sovereign A$6,970. The Open Bow with a Hidea 20 hp four-stroke is A$10,785; the Sovereign package is A$11,990.

Four-stroke, weight and starting

Four-stroke is the sensible default: quieter, cleaner, easier to live with and better on fuel. The trade-off is weight — a 15 hp four-stroke is a genuine two-person lift onto a transom in a swell, so plan the lift, not just the purchase.

Electric start

Under 15 hp, manual start keeps the package light. At 15–20 hp, electric start earns its keep the first time you have to restart in chop with wet hands.

What the motor sits on matters too

An outboard is only as good as the transom under it. Aerowave catamaran transoms are built into a 1500 GSM / 1.2 mm VALMEX® Heavy Plus hull (0.9 mm VALMEX® on the AeroCat range), a fabric with a realistic 10–12 year service life in Australian sun. A soft or flexing transom will out-argue any horsepower decision you make.

Buying it as a package

Buying the boat and motor together avoids the classic mismatch — right hull, wrong shaft. Every Aerowave package ships pre-matched for shaft length and transom height.

Air freight is 7–14 days with a A$810 customer share; sea freight is free and takes 30–40 days.

The short version

• Size to your heaviest realistic load, not the empty hull.
• 6–9.9 hp for 3.3 m, 9.9–15 hp for 3.65 m, 15 hp for 3.8 m, 20 hp for 4.0 m.
• Short shaft on a 380 mm transom. Always.
• Set the anti-ventilation plate level with the hull bottom and cavitation disappears.
• Four-stroke, and electric start from 15 hp up.

Still unsure? Tell us the water you run and the people you carry, and we will size it for you.