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BRUNTONSAustralia

The physics of the wall

Hull speed: the wall your engine can't buy through.

A displacement hull rides between the waves it makes, and at 1.34 × √LWL the boat sits in a trough of its own making — pushing harder just digs the hole deeper. Enter your waterline length and see the whole story.

What the curve means for your wallet

Power required rises with roughly the cube of speed — then goes nearly vertical approaching hull speed. The practical consequences:

  • The last knot costs triple. Backing off from 98% to 90% of hull speed often halves the fuel flow for a few minutes on a day's passage.
  • A bigger engine rarely fixes a slow boat. The wall doesn't move — only the fuel bill does. The propeller and operating point decide what you get from the engine you have.
  • ~75% of hull speed is the sweet spot — the speed the fuel calculator and RPM chart assume for efficient cruising.

This is why correct propping matters more than horsepower on a displacement boat: the engine's job is to hold the sweet spot cheaply, and the propeller's pitch decides whether it can.

Hull speed

7.9 kn

Efficient cruise (~75%)

5.9 kn

5.9 kn sweet spothull speed 7.9 kn0 knspeed →power required →
Power demand rises with roughly the cube of speed, then goes near-vertical at the hull-speed wall — the last knot costs triple the fuel of the one before it. A bigger engine can't move the wall; a correctly matched propeller gets you to the sweet spot for the least fuel.

Know your sweet spot — now gear for it

The Builder sizes the propeller that holds your cruise speed at the lowest comfortable revs, verified by an engineer.

Build my prop spec0418 327 403