While learning some students experience arm cramps and fatigue as they’re trying to work on different skills. One common denominator of the fatigue is that they grab the boom with their palms over and thumbs under, as if trying to squeeze juice out of it. When your palms are over the boom, your thumbs should be over it as well so your hand takes the shape of a hook. As a result, your forearm muscles stay relaxed and don’t cramp up, which staves off fatigue. Keeping your thumbs over the top also helps you hang your weight on the booms when accelerating and shifting gears. The underhand grip is used only with the front hand and most often when you’re sailing at full speed with the sail raked back because it’s a more comfortable and natural position. But even then, because the boom is angled toward the water, the back hand should be in the overhand and thumbs-up position. A good rule of the thumb is to use a thumbs-up, overhand grip when your booms are parallel to the water, and switch the front hand to an underhand position only when your booms reach a position diagonal to the water.
