Racing boat steeled for action
Original article date: February 2000
For 78-year-old Howard Arneson, who invented the famous Arneson surface drive propulsion system, slicing through ocean swells at over 100mph is just a piece of cake.
The Arneson drive, now made under license by US-based transmission systems manufacturer Twin Disc, is probably the most popular propulsion system in high speed boating. It is used widely on racing boats, cruisers, patrol boats and yachts of all sizes. A variation of the drive is even used in military applications, including US Army tanks.
This is a surface drive that can be steered and trimmed in the water while the craft is moving. It runs with the propeller partly out of the water. This design reduces appendage drag (such as a dragging gearcase) and cavitation, which detrimentally affects efficient propeller performance.
The Arneson drive pivots port and starboard like a stern drive, working without a rudder. This pivoting controls the direction of propeller thrust, thus improving steering response. A boat with this drive can be trimmed while moving to get the right degree of submersion for the load and condition of the water.
With a surface drive like the Arneson, the propeller and drive train extend aft, not down in the water. This positioning reduces draft, along with the noise and vibration usually conducted from below through the hull. It also improves propeller bite. With the drive behind the transom, the designer of a high performance boat can place the engine as far aft as desired.
Finally, the hydraulic rams which are key to the Arneson drive are positioned outside the boat, freeing up space inside. Propeller depth is controlled by a vertical trim cylinder permitting 15deg of up/down motion. A horizontal steering cylinder allows 40deg of port-starboard trim.
Arneson has now reached 175mph piloting the 46ft Skater, a new generation catamaran powered by a 4500hp Lycoming gas turbine similar to that giving flight to a Chinook helicopter. The initial propeller shaft for the Skater was made by Ziegler Industries from 17Cr-4Ni precipitation hardening stainless steel, known for its good combination of high strength, high hardness and corrosion resistance. The finished shaft measures 40in long by 2.5in diameter in the centre, tapering down to 1.875in at both ends. The shaft is installed using a double cardon joint with a front-end drive, propeller at the other end and bearings at both ends.
But after 50 hours of running time at speeds of around 100mph, the shaft broke off, dropping to the bottom of the sea with its propeller. Up to then, the shaft had not been subjected to the shear forces of the severe acceleration or sustained, record-setting high speeds which Arneson anticipated. The President of Zieger – Don Zieger – suggested making the propeller shaft from Custom 465 stainless – a premium-melted, martensitic, age-hardenable alloy made by Carpenter Technology. This alloy can reach a 260ksi UTS when peak aged (H900 condition). In this condition, it has excellent notch tensile strength and fracture toughness.
When over-aged in the H1000 condition, Carpenter’s Custom 465 stainless provides a superior combination of strength, toughness and stress corrosion cracking resistance when compared with other high-strength PH stainless alloys.
Condition H1050 was selected from the heat treatment schedule to get a hardness of RC45-46 and maintain essential straightness. Test have shown successful performance of the shaft at speeds of 175mph, sustained for several hours.
- Carpenter Specialty Alloys
February 2000