Allen Stark

November 10th, 2007, 03:49 PM

I was asked on the SDK thread about my past posts on speed and height.Here is a more complete explanation.There are 2 main forms of drag affecting swimmers:form drag and wave drag.Wave drag only occurs at the surface so it is not a factor when swimming underwater.Lack of wave drag is why SDK can be so fast even though it is less propulsive than full stroke.

Form drag is from how much water you push in front of you and pull behind you. Improved streamlining decreases form drag.There are many things we can do to decrease form drag:good body position,shaving down,technical suits,losing weight,etc.For a given shape form drag resistance increases as the square of the velocity.

Wave drag comes primarily from pushing your bow wave. There is very little drag from this until you exceed your "hull speed" at which point you are climbing up on your bow wave.At this point resistance goes up as the cube of velocity so it rapidly becomes the primary resistance.

The formula for hull speed is:hull speed(in knots)=1.34times the square root of the length at the waterline(in feet)(for a swimmer that is the height)This is why longer boats(and taller swimmers) are faster.

For example I'm 5'8" (or 5.67 ft) so my hull speed is 3.19 Kt.A knot is 1 nautical mile per hr or about 1.67 fps so my hull speed is 5.32 fps.This is doing 50 yd in 28.19 sec.Going faster than that requires disproportionally more power than going slower than that(at the surface).

What can you do to decrease wave drag?You can be tall(or at least swim tall),you can stay underwater,or you can swim slower.Obviously swimming slower is no help in a sprint,but it does mean that even pacing will use less energy than going fast for part of the race.

Here is a table I calculated of height and hull speed

Height Hull Speed(feet per sec) Time for 50 yd

5' 5 fps 30 sec.

5'3" 5.12 fps :29.29

5'6" 5.24 fps :28.62

5'9" 5.36 fps :27.98

6' 5.47 fps :27.42

6'3" 5.59 fps :26.83

6'6" 5.71 fps :26.26

Form drag is from how much water you push in front of you and pull behind you. Improved streamlining decreases form drag.There are many things we can do to decrease form drag:good body position,shaving down,technical suits,losing weight,etc.For a given shape form drag resistance increases as the square of the velocity.

Wave drag comes primarily from pushing your bow wave. There is very little drag from this until you exceed your "hull speed" at which point you are climbing up on your bow wave.At this point resistance goes up as the cube of velocity so it rapidly becomes the primary resistance.

The formula for hull speed is:hull speed(in knots)=1.34times the square root of the length at the waterline(in feet)(for a swimmer that is the height)This is why longer boats(and taller swimmers) are faster.

For example I'm 5'8" (or 5.67 ft) so my hull speed is 3.19 Kt.A knot is 1 nautical mile per hr or about 1.67 fps so my hull speed is 5.32 fps.This is doing 50 yd in 28.19 sec.Going faster than that requires disproportionally more power than going slower than that(at the surface).

What can you do to decrease wave drag?You can be tall(or at least swim tall),you can stay underwater,or you can swim slower.Obviously swimming slower is no help in a sprint,but it does mean that even pacing will use less energy than going fast for part of the race.

Here is a table I calculated of height and hull speed

Height Hull Speed(feet per sec) Time for 50 yd

5' 5 fps 30 sec.

5'3" 5.12 fps :29.29

5'6" 5.24 fps :28.62

5'9" 5.36 fps :27.98

6' 5.47 fps :27.42

6'3" 5.59 fps :26.83

6'6" 5.71 fps :26.26