Tests Prove CLT Prop Characteristics

Tuesday, June 19, 2001
The CLT propeller recently underwent real-world testing, which proved, according to its designer and manufacturer that the propeller can help to reduce or eliminate hull-induced vibrations and noise levels. Independent studies were conducted aboard Superfast Levante, which demonstrated that: the cavitation level developed on the CLT propeller blades is negligible from the ship's structural integrity point of view; vibration and noise levels are excellent in terms of comfort on board; and the contractual ship speed was much better than predicted.

Superfast Levante is a 7,200 dwt RoRo cargo vessel delivered by Union Naval de Valencia (UNV) to Trasmediterranea, the major Spanish operator of RoRo vessels and ferries. The ship was ordered to operate between Cadiz and the Canary Isles.

Trasmediterranea has already fitted CLT propeller blades on several ships in its fleet. The decision to install CLT blades on Superfast Levante was based on the company's experience with these, and UNV's positive experience with CLT blades fitted on several chemical tankers and products carriers during the past six years.

Superfast Levante's twin propellers have a diameter of 15 ft. (4.5 m) and a blade area ratio of 0.48. Their CLT blades were designed by Sistemar and manufactured by Navalips, and are mounted on CP hubs from John Crane Lips.

The CLT blade design was carried out by direct calculation, using results obtained from model tests conducted at the Danish Maritime Institute (DMI) with stock conventional propellers. It was not necessary to carry out model tests with CLT propellers, as over the past 20 years Sistemar has developed a unique method for calculating their performance based on its own theories. Because of the high propulsion power installed on Superfast Levante (12,600 kW per shaft), the owner was quick to ensure that there would be no excessive propeller cavitation, which could cause unacceptable hull vibrations and erosion of the propeller blades. UNV therefore ordered a complete program of hull vibration measurements from the specialist company TSI.

The procedure — developed by TSI — needed no modifications to the vessel and consisted of measuring vibrations at the bottom of the ship, close to the propeller positions, and in the steering gear compartment. The company then analyzed the vibration levels obtained in these areas, in relation to varying propulsive powers achieved by altering the blade pitch while maintaining constant propeller RPM. The vibration measurements were performed in the ship's ballast condition, in an adverse sea state (Beaufort 5) and with no hallow water restrictions.

It is known that when a propeller is cavitating, the main frequencies appearing on the spectra of vibrations measured are the second, third and fourth harmonics of the propeller blade pass frequency. When there is no cavitation, the first harmonic is the major component.

The study showed that the second harmonic of the propeller blade pass frequency appeared only very intermittently, and that its influence on the vibration velocities in different structural points was much less than the influence corresponding to the first harmonic. Furthermore, the effects of the third and fourth harmonics were even less relevant.

From the vibration measurements carried out on the single-bottom close to the propeller position, the following deductions were made:

In most of the measurements carried out at 85 percent MCR, the vibration velocities were below five mm/sec; When the propulsive power was 100 percent MCR, the vibration velocities were below six mm/sec.

TSI concluded that: "The cavitation level developed on the CLT blades is negligible from the ship structural integrity point of view."

Vibration and noise levels measured in different parts of the superstructure and machinery spaces were also shown to be lower than those required by ISO 6954 rules, and therefore excellent in terms of comfort on board.

As agreed between Trasmediterranea and UNV, the results obtained from the speed trials were corrected and interpreted by the Danish Maritime Institute (DMI). The results show that, for the design condition at 85 percent MCR and with a service margin of 15 percent, the ship's speed is 22.4 knots. The contractual ship speed under these conditions was 22 knots. The results also clearly show that the result obtained was much better than predicted.

The results obtained in ballast condition (according to the analysis carried out by DMI) compared with the DMI predictions on the performance of the ship fitted with conventional and then CLT propeller blades.

Similar predictions correspond to the design condition.

The CLT propeller has been shown that it can also reduce fuel consumption approximately eight percent, or help to increase speed at the same consumption.

Sistemar is a Spanish company, formed in 1987, which specializes in ship propulsion — mainly in the propeller design field. The installed power on vessels fitted with CLT propellers ranges from 100 hp to 36,000 hp. The most recent reference for the propellers is on a pair of 35,000 dwt products tankers built in South Korea by Hyundai Mipo for the Soponata/Arminter consortium. Speed trials on the first ship are scheduled to be held in June. The shipyard ordered two 18 ft. (5.45 m)-diameter CLT propellers from Sistemar, propellers which were manufactured and delivered by Navalips, Cádiz.

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