Hydrofoils and Permatrims for Inflatable Catamarans: The Honest Guide to Stopping Cavitation and Aeration on Short-Shaft Outboards

If you run an inflatable catamaran or a monohull tender with a short-shaft outboard, there are two cheap upgrades that change how the boat behaves more than anything else you can bolt on for under three hundred bucks: a hydrofoil stabiliser and a Permatrim plate. Both sit just above the propeller on the outboard's cavitation plate, both stop the prop spinning air instead of biting water, and both turn a tail-happy, slow-to-plane setup into something that gets up cleanly and tracks straight.

We get asked about these almost every week — usually after someone has fitted a 9.9 or 15hp short shaft to a 3.3–3.8m inflatable cat and noticed the bow is up in the sky for the first 50 metres and the prop is whining instead of pushing. Below is the honest, unbiased version: what they actually do, when they're worth it, and where they don't help.

What's the difference between a hydrofoil and a Permatrim?

They look similar from the boat ramp but they're built for slightly different jobs.

Hydrofoil stabilisers — usually moulded plastic or composite wings that bolt to the top of the cavitation plate and extend outwards either side of the gearcase. They generate lift at the back of the outboard, which pushes the stern up, drops the bow, and gets the boat onto the plane faster. Cheap, cheerful, and start around AUD $50 at most Aussie tackle shops for the no-name versions, $90–$140 for the better-known brands (Doel-Fin, StingRay, SE Sport).

Permatrim — a New Zealand-designed CNC-machined aluminium plate that bolts to the underside of your cavitation plate, extending the plate's surface area rearward and outward. It's a single piece of solid aluminium, no wings, no flex, and it's model-specific (they make a different plate for every common Yamaha, Tohatsu, Mercury, Suzuki and Honda short and long shaft). Prices in Australia sit between $220 and $300 depending on motor size.

Hydrofoils give you a bigger visual change. Permatrims give you a more refined, factory-feeling result with no flex and no cracking failure mode.

Why your inflatable catamaran needs one (and why a tinnie often doesn't)

A glass-hulled tinnie with a long-shaft 30hp sits low at rest, has a deep V to bite the water, and the prop is well submerged the whole time. It planes by itself.

An inflatable catamaran is a completely different animal:

• The hulls are light. A loaded 3.6m cat with two adults and gear sits around 350–400 kg total. The transom is short, the tubes are buoyant, and the stern lifts easily.
• Most owners run a short-shaft (S) outboard — 15 inches — because the transom height suits it. That puts the cavitation plate fractionally higher in the water than a long-shaft setup.
• The wide, flat deck and twin-hull buoyancy mean the bow points to the sky on initial throttle. Until you're on the plane, the prop is partially exposed and gulping air.

That's the textbook scenario where a foil or a Permatrim earns its money on day one.

Cavitation vs aeration — and why it matters for cats

People use the words interchangeably. They aren't the same.

• Cavitation is when low pressure on the back side of a spinning prop blade turns water into vapour bubbles. Those bubbles collapse against the blade and over time pit the metal. You hear it as a sudden over-rev with no thrust gain.
• Aeration ("ventilation") is air from the surface getting sucked down into the prop because the cavitation plate is too high or the bow is angled too far up. Same symptom as the operator — engine revs climb, boat doesn't accelerate — but the cause is air, not vapour.

On a light inflatable cat with a short-shaft motor, aeration is the bigger problem. Every time you punch the throttle from rest, the stern squats slightly, the bow lifts, and the cavitation plate breaks the surface. Air rolls under it, hits the prop, and you lose drive until the boat finally levels out and the plate buries again.

A hydrofoil or Permatrim fixes this by physically extending the surface area above the prop. More plate area = harder for air to find its way down to the blades = the prop keeps biting water through the hole-shot.

The honest list of benefits

We sell boats, not foils, so there's no incentive to oversell these. Here's what genuinely changes after fitting one:

1. Plane time drops — typically from 4–6 seconds down to 2–3 seconds on a 3.6m cat with a 15hp short shaft. Big visible difference.
2. Bow stays lower in the hole-shot, so you can see over it. Big safety win, especially with kids on the bow seats.
3. Less aeration on tight turns — turning hard with a light cat used to lift the inside hull and air the prop. Plate area kills that almost completely.
4. More stable at low planing speeds — the boat will sit on the plane at 13–15 km/h instead of needing 18 km/h to stay up there. That's better fuel economy and a softer ride in chop.
5. Slightly better reverse control in tight spaces (more plate area, less prop air-bite when backing).

What they won't do:

• Add top-end speed. Most setups lose 1–2 km/h at WOT because the plate creates a small amount of drag.
• Fix a chronically over-propped engine. If your motor can't reach its top of rev range, the prop is wrong — a foil won't save you.
• Compensate for a transom that's the wrong height for the motor. Get the shaft length right first.

Hydrofoil or Permatrim — which one should you actually buy?

Honest answer: it depends on budget and how often you take the boat off the trailer.

• Weekend recreational use, 9.9–15hp short shaft, casual fishing and family runs — a $50–$140 hydrofoil is perfectly fine. They work, they're cheap, the worst-case is a wing snaps off in a strike and you fit a new one. We've seen the StingRay Junior and SE Sport 200 last years on cats.
• Charter, hire-fleet, working boat, or 20hp+ four-stroke — go straight to a Permatrim. The aluminium plate doesn't crack, doesn't flex, doesn't fatigue at the bolt holes. It's the one we recommend to commercial operators because it'll outlast the motor.
• You re-prop or change motors every couple of years — hydrofoils are universal-fit; Permatrims are model-specific. If you're swapping motors, a foil moves over more easily.

If the budget runs to it, our recommendation across the board for an inflatable catamaran or monohull tender is the Permatrim. The build quality is a noticeable step up, there's no flex through the plate, and the planing characteristics are more linear — no twitchy bow-drop, just a smooth lift onto the plane. The Kiwi mob that builds them have been at it for decades and the product reflects it.

Fitment — what's actually involved

Both are bolt-on. Both take 20–30 minutes the first time, 10 minutes after that.

• Hydrofoil — 4 stainless bolts through the cavitation plate, washers either side, marine sealant on the holes. Some models use clamps with no drilling at all.
• Permatrim — uses the existing cavitation plate's threaded holes if your motor has them (most do), or drills 4 small holes. The kit includes stainless bolts, washers and a fitting guide for your motor model.

Things to actually check before drilling:

• Make sure you have at least 25mm clearance between the new plate's trailing edge and the anti-ventilation (cavitation) plate's existing rear edge.
• Don't fit a foil to a motor that already has a high-performance prop and is running on the limit of its rev range — the extra drag will pull you under the rev band.
• Always use stainless fasteners with marine sealant. Galv bolts will rust through inside a season in saltwater.
• For aluminium Permatrims, use the supplied stainless bolts with the supplied nylon isolation washers to prevent galvanic corrosion between the aluminium plate and the gearcase.

Common questions

Will a hydrofoil void my outboard warranty? In Australia, no — fitting a hydrofoil to the cavitation plate is not considered a modification of the engine's serviceable parts under most manufacturer warranty terms. Tohatsu Australia, Yamaha and Mercury all have public statements confirming aftermarket foils do not void warranty unless they cause measurable damage. Permatrims similarly. Always check your dealer's written warranty if you're unsure.

Does it work on a long-shaft motor too? Yes — same principle, slightly less dramatic improvement because long-shaft motors are already running the prop deeper. Still worth fitting on any inflatable.

Will it help my electric outboard / trolling motor? No. These are designed around prop pitch and shaft length for displacement-speed operation. A foil adds drag with no planing benefit at trolling speeds.

Can I run a hydrofoil and a Permatrim together? No. They both occupy the same real estate above the prop and you'd just create turbulence. Pick one.

Where do I buy them in Australia? Hydrofoils — any major chandler (BCF, Whitworths, Marine Deals) or eBay AU. Permatrims — the manufacturer in NZ ships direct to Australia, and a handful of Australian dealers carry stock for the popular Yamaha, Tohatsu and Mercury short-shaft models. Allow 5–10 days delivery.

Our recommendation

If you own an inflatable catamaran or a monohull tender running anything from a 6hp short shaft up to a 30hp four-stroke, fit one of these. It's the single best-value upgrade you can make. Plane time drops, the bow comes down, aeration on the hole-shot disappears, and your fuel economy at cruising RPM improves.

For most owners: start with a $50–$140 hydrofoil and you'll feel the difference on the first run.

For commercial operators, charter fleets, or anyone who runs the boat hard several times a week: spend the $250–$300 on a Permatrim and forget about it for the life of the motor.

If you'd like a hand picking the right one for your specific motor and boat combo, drop us a line through the chat or our contact page — we run these on our own demo boats and we'll tell you straight which option suits your setup.