A lesser-known law can make a big difference. In contrast to Newton’s Three Laws of Motion, which almost everyone has heard of, very few know about Froude’s Law.

William Froude was a British naval engineer who determined that, all other things being equal, the ship with the longest waterline travels the fastest.

How does this relate to efficient swimming? A swimmer can in many ways affect the length of the body’s waterline. For one thing, most of the better competitive swimmers are tall.

Regardless of height, though, a swimmer’s arm or both arms can be extended as far as possible beyond the head. The legs can be extended straight as far behind as possible.

The elbows and knees can be locked to maintain this extension. And the hands can be lapped one over the other to decrease frontal resistance at the same time.

Without even considering propulsion, a glide position that follows Froude’s Law can make a swimmer more efficient.

The best glide position for a Crawl stroke is on the side, with the lower or leading arm fully extended and the legs extended together. The arm recovery and propulsive stroke rotate the body from one side glide to the other.

The other long-axis stroke, the alternating-arm Back stroke, is best performed similarly, rotating from fully extended positions on either side.

The short-axis strokes, which are performed with both arms (and legs) being simultaneous and symmetrical, use a long gliding position on the body’s front.

Both Butterfly and Breast strokes glide with legs and arms extended, their glide positions differing only in that the Butterfly glide keeps the arms at shoulder width. This slight sacrifice from optimizing Froude’s Law is more than made up for by a more efficient stroking effort initiated from this glide position.

I have used an easy experiment that a swimmer can follow to verify Froude’s Law. Just swim a single-arm Crawl, Back or Butterfly stroke with the non-stroking arm either up (extended ahead to lengthen the “ship”) or down (extended at the side toward the feet).

It takes fewer strokes per length to swim this drill with the arm up than it does with the arm down, thus confirming Froude’s Law.

Regardless of the propulsive effort used, whether competitive or not, an extended glide makes every stroke more efficient, whether one is familiar with Froude’s Law or not.

For more efficient swimming, be sure to take full advantage of it the next time you dive, push off, or stroke through the water.

Dr. Bob Colyer of Bluffton is an actively retired college professor, coach and author of “Swim Better: A Guide to Greater Efficiency for Swimmers & Instructors,” directed primarily to non-competitors. bobcolyer@yahoo.com