A "settled wind" means the same as a steady wind: one whose direction has settled. (For some reason, the best sources I can find for this are translation sites.) This is as opposed to shifting winds - those whose direction keeps changing, making them hard to predict and to steer by. The important thing about wind is usually its direction, not its strength, until you get up to really strong winds.
When the wind direction keeps changing, the waves tend to be more confused and harder to steer in. From this article about waves:
Also, a change in wind direction over existing waves can create confusion and hence larger waves. If a wind has been blowing northeast over an open body of water for three days and suddenly switches to northwest over that same body of water, new wavelettes will form within the existing system of waves. The energy of both systems will multiply to create larger waves.
Whereas waves rolling before a settled wind - one in a single fixed direction - will be able to move clearly and smoothly without interference. If the wind keeps blowing in the same direction for a prolonged period, these waves can increase in size due to positive feedback.
This is the situation which the speaker in your quote is using as a simile: her blood is rushing in the direction her "spirit's set", smoothly and without hesitation, just as waves roll in the direction of a steady ("settled") wind.