When Deep Water is Shallow

[leong]

Everyone knows that if you paddle at a steady pace in deep water and then suddenly enter shallow water, the boat slows down for the same effort. The extra drag slowing the boat down is due to a subtle interaction of the boat’s turbulence and bow wave with the bottom. Knowing how much the boat will slow down is interesting; especially useful for kayak racing. To this end, I did a little study and the results are in the attachment. Here are some (perhaps surprising) results:

1. The percentage slowdown is a function of both speed and water depth.

2. At 5.75 mph:

there is a 14% slowdown at a water depth of about 2 feet.

there is a 4% slowdown at a water depth of about 3.5 feet.

there is a 1% slowdown at a water depth of about 6 feet.

there is no slowdown at water deeper than 13.8 feet.

3. At 4 mph:

there is a 14% slowdown at a water depth of about 1 foot.

there is a 4% slowdown at a water depth of about 1.7 feet.

there is a 1% slowdown at a water depth of about 3 feet.

there is no slowdown at water deeper than 6.7 feet.

As discussed in the attachment, these results depend on the concept of Critical Depth and the Schlichting method. I’m not a naval architect and I have very little formal training in hydrodynamics; however, I think my results are correct. If you know better please let me know.

CAM-able Leon

Mathcad - Schlichting method for Shallow Water Slowdown.pdf

[Paul Sylvester]

Leon,

Such a lonely topic that seems over my head......

In current I sometimes head to the shallows. May rethink that now to avoid that 14% slowdown??

[leong]

Leon,

Such a lonely topic that seems over my head......

In current I sometimes head to the shallows. May rethink that now to avoid that 14% slowdown??

Paul,

It’s a tradeoff between current slowdown vs. shallow water slowdown. For example, last year I competed in the Great Stone Dam Classic race, starting upriver on the Merrimack River in Lawrence. At the start everyone was lined up in the middle of the river and at the start immediately paddled over to the left bank, apparently, to be in the slower current. I stayed in the center for a while and then paddled over towards the left bank, but to the right of the other competitors; i.e. I wanted to be in slightly deeper water. It seems there was some optimal point where the increased speed of the slightly deeper water overcame the slowdown due to the opposing current. It was easy to determine where that point was based on my relative speed with respect to the racers to my left. I think it helped, at least I came in first for true sea kayaks (two training surf skis did beat me).

Leon