It's a bit like if you're pushing a boat through a river and measuring something about the surface - you don't want to measure the ripples from the boat.
这就有点儿像你将一艘船从河的这边推到另一边，并测量河面上的某些东西，但你不希望连同船儿泛起的涟漪也一并测量出来。
All sailboats need a keel of board for this reason: the narrow, flat surface of the board or keel produces little drag when the boat moves forward but resists motion sideways.
所有的帆船都为了这个理由需要某种龙骨：狭窄平滑的龙骨表面对于帆船的前行几乎没有阻力，但却能很好抗拒侧移力。
As I gradually master my buoyancy and hover 15 feet below the surface, I can see schools of small yellow groupers beneath the dive boat giving wide berth to a blade of a barracuda.
当我逐渐适应了浮力，盘旋下潜到水面以下15英尺以后，就看见成群结队的黄色小石斑鱼为了躲避梭鱼而藏在潜水船下方。
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