Theoretical Hull Speed

All displacement hull craft, which touring kayaks are, have an theoretical hull speed. This is the maximum speed through the water the hull can be driven efficiently.

The formula for finding hull speed: The square root of the waterline length multiplied by 1.35. It's actually slightly variable but usually considered to be 1.34 to 1.40. Most sources stick to the 1.35 figure.

The reason hull speed is related to waterline length is because of wave formation. The speed of waves or the speed of an object making waves (in knots*) is just slightly over the square root of the crest to crest distance between the waves. A displacement hull can still move efficiently when the stern wave is a little behind the hull, but sooner or later, a boat creating a hole in the water at the stern and a mountain of water at her bow will be litteraly paddling uphill. This just takes more and more energy.

* One knot is equal to 1.17 miles per hour.

Here is another formula said to be accurate to 1%.

V(mph) = .6818 * SQRT (LWL) * ((1.8533/SQRT (LWB/LWL)-1)

As in everything to do with boats this is not iron clad. It is extremely accurate repeatably and works with all displacement boats, monohull or multi-hull. It only takes into consideration the waterline plane as defined by your bow and stern waves, it does not reference your hull's resistance through the water.

Back to Technique

Back to Camp

HOME