Monohull Turn Lift  This is an effect of water pushing on the rudder during a turn and lifting the boats stern. When a vee bottom boat turns it does not do it flat but instead tends to lean over on its side. When this happens it presents the rudder face at an angle which allows the water flow to lift the transom. To help compensate we add turn fins. These are placed perpendicular to the Vee bottom so that when the boat lays on its side in a turn the fin will be perpendicular with the water surface and not generate any additional lift.
 In this picture of my oval master you can see the turn fins and the rudder. I've banked the boat so you can see how it would look during a turn. Imagine how the water flow would want to lift the rudder out of the water. Again the turn fins help hold the boat down plus provide a fulcrum to help improve turning.
Hydroplane turn problems  3pt and rigger Hydros do not like to turn because they have so little hull in the water which leaves no fulcrum for the rudder to act against. To help compensate a turn fin is added to the back of one of the sponsons. I use the right sponson, some use the left, probably no right or wrong answer here. My theory is since we run in right hand ovals the right sponson turn fin keeps the right sponson pinned to the water. The turn fin should also be slightly angled to prevent it from lifting out of the water during a turn, the angle helps to force the sponson down onto the water surface.
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