April Propwash 2018 web.pub

Curved or bent turn fins have a vertical section to provide side force and a curved or bent section at the bottom for the down force. Adjusting the fore and aft angle adjusts the amount of down force. Usually the fin is set close to vertical with the bottom section horizontal. Very slight adjustments will fine tune the boats ride and cornering. A threaded strut can be used on the turn fin bracket to adjust the side to side angle and prevent flexing. A modification of the curved turn fin is used on some Eagle riggers. Unlike most turn fin designs, this Eagle turn fin is designed to flex. When loaded the angle increases increasing both the side and down force. It returns to a neutral position in the straight for lower drag. The angled turn fins are more tolerant of left turns than the curved fins. However, the boat will still flip with any but gentle left turns. High Speed Boat Design - Part 4 (Continued from page #10) Turn Fins are more complicated. They need to produce side force and down force to maintain the turn and hold the boat flat. Again, ventilation means all the forces are generated on the high pressure side with an air filled cavity on the low pressure side. An idea of what’s happening can be seen in this picture of a hydro turning hard. The pressure side creates spray because the water pressure leaving the turn fin is higher than air pressure. You can just pick out the cavity created on the low pressure side. There are two basic turn fin designs. The simple angled design produces a force angled down and toward the turn center. Adjusting the angle adjusts the proportion of side to down force. Twenty degrees from vertical is a good starting point. PROPWASH Page 10 April 2018 Knife Blade Rudder Hydro Cornering. Photo courtesy of Jerry Dunlap Angled Turn Fin Side Angled Turn Fin Anderson Sport 40 Side View Anderson Sport 40 Turn Fin