A ship encounters a higher drag in a stratified fluid compared to a homogeneous one. Grouped under the same “dead-water” vocabulary, two wave-making resistance phenomena have been historically reported. The first, the Nansen wave-making drag, generates a stationary internal wake which produces a kinematic drag with a noticeable hysteresis. The second, the Ekman wave-making drag, is characterized by velocity oscillations caused by a dynamical resistance whose origin is still unclear. The latter has been justified previously by a periodic emission of nonlinear internal waves. Here we show that these speed variations are due to the generation of an internal dispersive undulating depression produced during the initial acceleration of the ship within a linear regime. The dispersive undulating depression front and its subsequent whelps act as a bumpy treadmill on which the ship would move back and forth. We provide an analytical description of the coupled dynamics of the ship and the wave, which demonstrates the unsteady motion of the ship. Thanks to dynamic calculations substantiated by laboratory experiments, we prove that this oscillating regime is only temporary: the ship will escape the transient Ekman regime while maintaining its propulsion force, reaching the asymptotic Nansen limit. In addition, we show that the lateral confinement, often imposed by experimental setups or in harbors and locks, exacerbates oscillations and modifies the asymptotic speed.

与均质船舶相比，船舶在分层流体中遇到的阻力更高。在相同的“死水”词汇下分组，历史上已经报道了两种造波阻力现象。第一个是Nansen造波阻力，它会产生固定的内部尾流，从而产生具有明显滞后的运动阻力。第二种是埃克曼波产生的阻力，其特征是由动力阻力引起的速度振荡，其起源尚不清楚。后者先前已通过非线性内波的周期性发射得到了证明。在这里，我们显示出这些速度变化是由于船在线性状态下的初始加速过程中产生的内部分散起伏凹陷所致。分散起伏的凹陷前部及其随后的小腿充当颠簸的跑步机，船舶将在该颠簸的飞机上来回移动。我们提供了船舶与波浪耦合动力学的解析描述，从而证明了船舶的非定常运动。通过实验室实验证实的动态计算，我们证明了这种振荡状态只是暂时的：船舶将在保持其推进力的同时逃脱瞬态Ekman体制，并达到渐近的Nansen极限。此外，我们表明，通常由实验装置或在港口和船闸中施加的侧向限制会加剧振动并改变渐近速度。这说明了船舶的不稳定运动。通过实验室实验证实的动态计算，我们证明了这种振荡状态只是暂时的：船舶将在保持其推进力的同时逃脱瞬态Ekman体制，并达到渐近的Nansen极限。此外，我们表明，通常由实验装置或在港口和船闸中施加的侧向约束会加剧振动并改变渐近速度。这说明了船舶的不稳定运动。通过实验室实验证实的动态计算，我们证明了这种振荡状态只是暂时的：船舶将在保持其推进力的同时逃脱瞬态Ekman体制，并达到渐近的Nansen极限。此外，我们表明，通常由实验装置或在港口和船闸中施加的侧向约束会加剧振动并改变渐近速度。