Our newly developed ENDP propeller has been shown to be effective in reducing propeller induced noise and vibration up to 40 – 60 %. The ENDP propeller can be found on high speed vessel, luxury yachts and fishing vessels where noise and vibration control are of the upmost important. Vessels with speed up to 39 knots have been successfully trialed using ENDP propeller.
The end-plate propeller (ENDP) developed by Prof. Kehr can eliminate sheet cavitation (Fig e.) typically found on the tip end-plate of Contracted loaded tip propeller (CLT). It means that ENDP not only retain the advantages of CLT propeller but also prevent drawbacks caused by tip end-plate to keep propeller silent. For horizontal shaft condition, remarkable high pressure still exists on the propeller face near the blade tip due to the effect of tip end-plate. The high pressure on the face near the endplate will retard the propeller inflow and force the inflow flowing toward the outside of end-plate, which will result in a positive angle of attack to the end-plate. As propeller operates with inclined shaft angles, a vertical velocity component will be induced by inclined inflow. Vertical velocity component will form an additional positive attack angle to end-plate when blade rotating to upwards circumferential position. The increment of attack angle due to inclined inflow will cause sheet cavitation to form easily on the outer surface of end-plate. Therefore, the end-plate of tip loaded propeller shall be designed as diffused type to avoid high attack angle to the endplate. ENDP is designed with a diffused endplate bent forward to pressure side. The improvement of cavity reduction and efficiency increment has been proved by experiments and sea trial.
As shown on Fig.e, a CLT propeller with 1.0o contracted endplate operates at KT/J2 =0.390 , σ=1.50, and horizontal condition, serious sheet cavitation can be observed on the outer surface of endplate. At same operating condition(Fig.f), sheet cavitation on the outer surface of endplate has been eliminated by using an ENDP propeller with 0.3o diffused endplate. Cavitation tests showed a huge improvement on cavity reduction, and proved that ENDP can eliminate the sheet cavitation typically be found on CLT propeller.
As propeller operates with inclined shaft angle, angle of attack to the end-plate will be increased due to inclined inflow. Serious sheet cavitation on the outer surface of end-plate will be induced even though the ENDP is designed with 0.3o diffused angle at the end-plate as shown in Fig g. If CLT propeller operates at same condition, cavity volume will be unacceptable for controlling noise and vibration. Prof. Kehr increased the diffused angle by considering the effect of inclined inflow, and the sheet cavitation on the outer surface of end-plate has been eliminated as shown in Fig h.
The conventional propeller (P4012) and ENDP propeller (ENDP401A) are designed with new foil and are designed for a same propeller loading. Because the propeller thrust of ENDP is mainly contributed by the pressure side, the back cavitation can be well controlled. Thus, the efficiency can be increased due to the possibility of reducing the blade area for the same cavitation behavior. Besides, ENDP can significantly restrain tip vortex cavitation. In Fig l, by comparing with P4012, the efficiency of ENDP401A does not decrease significantly as the propeller loading is increased. This shows that the ENDP propeller can delay the inception of thrust breakdown. It will be beneficial to increasing the acceleration performance of ship.
A 91-feet yacht equipped with a set of conventional or ENDP propellers is tested separately. The vibration spectrums are measured at the steering gear room. The position is top right side of the rudder. During the sea trial, the operating condition are the same for both sets of propeller. In Fig m, when the yacht equipped with conventional propellers, the first blade rate amplitude induced by sheet cavitation is 121.9 Gals. and the broad band amplitudes caused by tip vortex cavitation at high frequencies are nearly 30 Gal. In Fig n, when the yacht equipped with ENDP propellers, the third blade rate amplitude induced by sheet cavitation is 43.3 Gals, and the broad band amplitudes at high frequencies caused by tip vortex cavitation are below 11 Gals. From these sea trial results, it shows that the ENDP propeller can significantly restrain the tip vortex cavitation even with less blade number, and the reduction of back sheet cavitation is much better than the conventional propeller even with lower blade area ratio.
