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News

World's fastest offshore racing boat back in the water with new high-strength stainless propeller shaft

Carpenter Technology Corporation : 28 January, 2000  (New Product)
For seventy-eight year old Howard Arneson, slicing through ocean swells in a powerful racing boat at speeds in excess of 100 mph is a piece of cake. He's most likely spent more time driving at those speeds than anyone in boating history. When he throttles up to 130 mph or more, it's still a ho-hum experience for this living legend in boating. Even at that impressive speed, the man who invented the famous Arneson surface drive propulsion system yearns for more thrust. His wish has been granted. Arneson has now reached 175 mph piloting the fastest offshore racing boat in the world. His own 46-ft. Skater is a new generation catamaran powered by a 4500 hp Lycoming gas turbine similar to that giving flight to a U. S. Army Chinook helicopter.
For seventy-eight year old Howard Arneson, slicing through ocean swells in a powerful racing boat at speeds in excess of 100 mph is a piece of cake. He's most likely spent more time driving at those speeds than anyone in boating history.

When he throttles up to 130 mph or more, it's still a ho-hum experience for this living legend in boating. Even at that impressive speed, the man who invented the famous Arneson surface drive propulsion system yearns for more thrust.

His wish has been granted. Arneson has now reached 175 mph piloting the fastest offshore racing boat in the world. His own 46-ft. Skater is a new generation catamaran powered by a 4500 hp Lycoming gas turbine similar to that giving flight to a U. S. Army Chinook helicopter.

Arneson, who holds patents for numerous boats and drive systems, had built 16 gas turbine powered racing boats before this record-breaking catamaran. When he designed the new Skater, he decided that he would get best performance by harnessing its awesome horsepower to just one propeller shaft.

'We were applying three times the power load that's normal to one shaft,' Arneson reflected. 'I guess, in a way, we were asking for trouble.'

The record breaking catamaran weighs more than 10,000 lbs. empty, and holds 1,000 gallons of fuel. However sleek the design, he knew that catapulting that much weight out of the water onto a racing plane in a few blinks of the eye would put an incredible strain on the drive system.

Arneson asked Zeiger Industries, Canton, OH, to make a propeller shaft for the Skater from 17Cr-4Ni precipitation-hardening stainless steel known for its good combination of high strength, high hardness and corrosion resistance. Zeiger, manufacturer of high performance marine components, fabricated a finished shaft measuring 40' long by 2' dia. in the center, tapering down to 1-7/8' at both ends.

At his San Rafael, CA, boat works, Arneson installed the shaft using a double cardon joint with a front-end drive, propeller at the other end and bearings at both ends. Then he lowered his powerful boat into the water and started cruising on the Pacific Ocean.

After 50 hours of running time at speeds of around 100 mph, the shaft broke off, dropping to the bottom of the sea with its propeller. Up to then, the shaft had not yet been subjected to the shear forces of the severe acceleration or sustained, record-setting high speeds Arneson had anticipated.

Zeiger's president, Don Zeiger, then suggested making the propeller shaft from Custom 465 stainless, a premium-melted, martensitic, age-hardenable alloy developed by Carpenter Technology Corp., Reading, PA. This alloy can reach a 260 ksi ultimate tensile strength when peak aged (H900 condition). In this condition, it has excellent notch tensile strength and fracture toughness.

When overaged 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.

Carpenter supplied Zeiger 2' round by 40' long Custom 465 stainless bar stock, with a ground finish. The stock was in the solution annealed/cold treated condition ready for a one-step hardening treatment.

Zeiger then selected condition H1050 from Carpenter's recommended heat treating schedule to get a hardness of RC 45-46 and maintain essential straightness. This condition provided the combination of high strength and stress corrosion cracking resistance required by this demanding application.

In a series of turning operations, Zeiger cut profiles on both ends of the shaft, threaded the propeller end and cut a spline on the other end to fit the transmission drive line. The fabricator then ground the OD and shipped the shaft to Arneson Research in California for assembly on the boat in the same manner as before.

Arneson took his Skater, refitted with the new Custom 465 stainless propeller shaft, out on the water and idled in a near dead float. Then he unleashed the explosive horsepower in the big gas turbine and felt his catamaran hurtle forward, accelerating to 100 mph speed in just 12 seconds.

He gradually throttled up to a record 175 mph and flashed around the ocean for several hours at that high speed. Arneson estimated that he had been cruising at sustained high speeds, 50 to 100 miles at a time, several times a week for four months at the time of this report... without any trace of a problem!

After a long period of service, Arneson removed the propeller to examine the shaft, particularly where it engages the spline. When the shaft was made of 17-4 PH stainless, he could see telltale marks and evidence that the spline was starting to twist from the powerful torquing action. In contrast, Arneson noted that the Custom 465 stainless shaft exhibited no marks at all.

'The spline was smooth,' he said. 'It looked like it was never used. That gave me assurance that I could run at high speeds for a long time without fear of the shaft breaking.'

Not incidentally, the high strength of the Custom 465 stainless allows Arneson, the boat designer, to load more horsepower on a single drive shaft.

The famous Arneson drive, now made under license by Twin Disc, Racine, WI, manufacturer of transmission systems, 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 such as U. S. 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 gear case) and cavitation, a detriment to 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 system, like Arneson's, 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 that 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 15 degrees of up/down motion. A horizontal steering cylinder allows 40 degrees of port/starboard trim.

Nominal chemical analysis of Carpenter's Custom 465 stainless is: carbon 0.02% max., manganese 0.25% max., phosphorus 0.015% max., sulfur 0.010% max., silicon 0.25% max., chromium 11.00/12.50%, nickel 10.75/11.25%, titanium 1.50/1.80%, molybdenum 0.75/1.25%, balance iron.
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