April Propwash 2018 web.pub

Rudders can be used to produce small up or down force by angling the pivot relative to the transom. This is very helpful for adjusting bow steering in turning monos. Tilting the rudder pivot back forces the bow down while tilting it forward raises the bow. This effect can also be used in cats and hydros. Cats need to set in the turn without burying the bows. Hydros need to keep the transom from dragging as they slow down in a turn. The right rudder angle will help. For heat racing a wider wedge gives better control and allows aluminum rudders. The extra drag from a larger rudder won’t make a big difference at heat race speeds. For the highest speeds, a thin rudder has lower drag. Going from a thick aluminum wedge rudder to a thin, knife blade rudder added 6 mph to Mike Bontoft’s rigger at around 100 mph. Reducing the area is also important. Cutting 1/8” off the bottom of the same rudder added 2 more mph. (Continued on page #10) Two types of hydrofoils used on our models are rudders and turn fins. Both need to have as low drag as possible when running straight. Each develops lift when angled to the water flow. Since our speeds are high and our hydrofoils pierce the water surface, they are always ventilated and/or cavitating when they produce lift. Conventional airfoil shapes would need to be very thin to avoid this. Strength considerations alone require wedge shaped hydrofoils. Rudders simply produce a side force when turned. Another advantage of wedge shapes is that flow stays attached at small angles preventing sudden loss of turning force. Below are the beginning cavitation patterns on an airfoil rudder, flat plate rudder, and a wedge rudder at low angles to the water flow. Notice that the wedge is mostly cavitating at the trailing edge as you would expect. The airfoil and flat plate rudders are starting to cavitate over the whole low pressure side. The effect of ventilation on our boats would make the pictured patterns worse. PROPWASH April 2018 Page 9 Airfoil Rudder Cavitation Pattern Flat Plate Rudder Cavitation Wedge Rudder Cavitation Pattern Exaggerated Back Tilt on Rudder