Aquamet, 316, bronze. The right choice depends on your use case — not the price list.

Introduction

Pick the wrong shaft material and you’ll know within three seasons — either from a corroded shaft, a bent one, or an unnecessarily expensive stern-gear bill. Here’s how to choose.

Aquamet 22 — the premium choice

A specifically marine-engineered precipitation-hardened stainless. 50% stronger than 316, with excellent seawater corrosion resistance. Expensive but worth it on commercial vessels and premium pleasure boats.

316 Stainless

The industry workhorse. Good corrosion resistance, moderate strength. Watch for crevice corrosion under cutless bearings and inside the shaft taper. Great value in the 25–50mm range.

Manganese bronze

Traditional sailboat material. Soft but immune to seawater corrosion. Easy to repair-weld. Softer than stainless so wears cutless bearings faster — a trade some sailors accept for the longer-term corrosion story.

Nickel-aluminium bronze

Harder and stronger than manganese bronze. Suits high-torque commercial applications. More expensive but longer-lasting under heavy duty.

Decision matrix

Pleasure boat, moderate budget: 316 stainless. Pleasure boat, premium: Aquamet 22. Commercial workboat: Aquamet 22. Classic sailboat or traditional restoration: manganese bronze.

A note on galvanic compatibility

Mixing dissimilar metals in a seawater drive train is a battery waiting to corrode something. Keep shaft, propeller, cutless bearing, and bronze fittings on a sensible galvanic series — and bond and anode everything properly.

Need advice on your boat’s stern gear or propeller? Call Brian directly on 0413 771 090 or use the contact form — we respond the same day.

Grease is simple and cheap. Oil runs cleaner and longer. Here’s when each one wins.

What they do

Both oil and grease seals form a barrier between the moving shaft and the stationary stern tube. The lubricant lowers friction and keeps the sealing elements running smoothly. Beyond that, they differ.

Grease-type (traditional)

A grease cup on deck feeds grease down a line to a sealed stern-tube chamber. Grease the cup periodically; grease pushes out through the shaft seal. Simple and robust, works on any size shaft, but you must remember to grease it.

Oil-type (Super Seal and similar)

A sealed reservoir of marine oil feeds the stern tube. Oil runs cleaner than grease, cools better, and lasts longer between services. More components, more cost, but less daily attention.

Dripless face seals

A separate category: carbon face against a stainless rotor, water-cooled. No grease or oil in the normal sense. Longest-lasting of the three but needs well-aligned, straight shafts.

Which to pick

Casual pleasure boat: grease is fine, cheap, forgiving. Commercial / high-hours: oil-type pays back in less service time. Modern pleasure boat where bilge dryness matters: dripless face seal.

Need advice on your boat’s stern gear or propeller? Call Brian directly on 0413 771 090 or use the contact form — we respond the same day.