Tech Analysis: Propellers

We take a look at one of the most hard-to-understand and secretive aspects of Powerboat P1 design – the propellers.

Much of powerboat design is what’s referred to as a ‘black art’. Water is one of the hardest materials to include in any form of technical analysis, thanks to its ever-changing and ever-fluid nature, and when you’re racing across unpredictable waves at 100mph, it becomes even more complex in behaviour. Perhaps the blackest of black arts in powerboat design, is the design of the optimum propeller. In fact, it’s closer to magic or alchemy than any form of science you or I may know.

Teams can often bring six propeller designs to a race, and will make their final choice as the boat is being craned into the water, hiding their design from the opposition in case it gives them any advantage. Even the widely acknowledged experts who lead the design of Powerboat P1 admit that choosing the right propeller for any given conditions is more a case of sticking your finger in the air, feeling which way the wind is blowing and taking a guess, rather than doing pre-race sums on a calculator.

But there are some over-riding principles to propeller design that at least give some guidance to making the right selection before the big race – these are the diameter of the propeller, the number of blades, and their pitch, rake and cup.

The diameter of the blade influences the acceleration of the boat – larger propellers provide better acceleration, but need more power from the engine to make a single rotation, as they effectively need extra force to cut through more water, thanks to their bigger size. The number of blades helps to influence the position of the boat within the water, with four blades providing less tail lift than six blades. More blades also help the propeller be more effective at cutting through the water, but increase drag.

The rake of the blade – or its curvature – helps to determine the lift of the bow, while the cup of the blades provides more or less grip in the water. Perhaps the most important aspect of propeller design, according to Craig Wilson – the team boss and throttle-man of the reigning Evolution class champion #99 King Of Shaves Fountain squad – is pitch.

“The pitch is the angle of the propeller as it comes through the water,” he explains. “A 30-inch pitch theoretically moves the propeller 30-inches through the water. The lower the pitch, the more acceleration you have. That helps in rough conditions, because you need to keep the revs up at the slower speeds, knowing you’ve got that quick push to accelerate you more quickly across the waves. When it’s flat you go for a larger pitch that helps the boat carry more top-end speed.”

Pitch is also affected by something called ‘slip’, which is a measure of how effective the propeller is at cutting through the water. More slip means the propeller is struggling to grip and cut through the water efficiently, like a sportscar struggling to get the grip down through the tyres if it’s suffering from smoky wheelspin.

Even the direction of rotation of the propellers plays a part. If the propellers rotate inboard – often the more popular choice – you get improved speed, but the boat will be less stable in the water. Outboard rotation allows the team to push the rear of the boat into the water and lift the nose, improving the ride.

In fact, all propellers on twin-engine powerboats are designed to act as a pair. If one propeller gets damaged, both get scrapped. And at a cost of between US$14,000 and US$20,000 a set, that’s not a light decision to have to make. Especially when producing the next set will throw up a new wealth of questions that are typical of ‘black art’ propeller design.