N e w Or M o d i f i e d Blades Improve C-P Propeller Performance
As early as the 1950s, controllable- pitch propellers accounted for a substantial and steadily growing share of the propeller market worldwide. KaMeWa AB in Sweden claims to hold some 30 to 50 percent of today's CPP market, and has delivered to date more than 4,000 C-P propellers. Most of these are still in operation.
The development of propellers and auxiliary equipment has made progress which can be applied to old ships to save fuel and/or improve performance.
During the 60s and 70s, many ships were built with propulsion machinery and propellers adapted to higher outputs and speeds not considered economical today. Most of these ships are now operated at reduced ship speed and power. In general, the propellers no longer produce the highest possible efficiency given these altered conditions. In many cases, considerable fuel savings can be achieved by modifying the propeller blades or by replacing the existing blades with new ones optimized for the new operating conditions. Among alterations that may be considered are the following: • Reduction of the propeller diameter This may be considered for geared medium-speed machinery where the number of engines utilized per shaft has been reduced, where one engine per shaft is used instead of two, two instead of three, etc.
• Increased propeller diameter An increase in diameter may be considered if the new and reduced power can be utilized at the shaft speed that is reduced more than corresponding to the cubic root of the power ratio (Pn:i = "propeller law"). This condition may often apply for plants with directly coupled, lowspeed diesel engines. To prevent propeller-induced vibration from increasing due to reduced tip clearance, blades of the highly skewed type may be considered.
• Reduction in blade area and/ or thickness Such reductions contribute to increased propeller efficiency by reducing friction between the water and the blades. In most cases where propeller power is reduced, blade area may also be reduced without risk of harmful cavitation.
• Changes in radial pitch distribution and/or blade section camber These alterations may be considered, in combination with some of the changes discussed above, if the original blades are operating with an unfavorable load distribution. Highly Skewed Blades For severe propeller-induced vibrations, propeller blades of the highly skewed type are recommended. These blades yield vibration levels that are only 30-50 percent of those experienced with conventional blade designs.
Since 1978, KaMeWa has delivered some 250 propellers with highly skewed blades. Most of these were for newbuildings, but about 20 percent were for retrofits—replacing existing blades of conventional design. The high-skew blades are characterized primarily by their low vibration impulses. As a rule, the HS propeller will result in vibration levels in the hull only 30-50 percent of the levels occurring with conventional blades. Despite this very low excitation level, the HS blades yield propeller efficiencies equal to those of conventional propellers.
Because of low vibration impulses, accompanied by an improved comfort level in ship's accommodations, HS propellers are fast becoming standard for all types Box 10326 Bi Telephone (20 TWX 810-733-!
of passenger ships as well as naval ships where low noise and vibration levels are vital.
In all cases when the old conventional propeller blades have been replaced by new HS blades, vibration and noise levels are reduced. For a copy of the complete report and free literature on KaMeWa propellers,