Cal's Evals » KD 860

Release Date: 12/30/2010

Kohler KD 860 Offshore Catamaran

The KD 860 is one of a family of novel multihulls designed by Berndt Kohler. It is a wide boat with trapezoidal (flat bottom) hull shape and incorporates anti-vortex panels in place of keels, daggerboards or centerboards. It is build from plywood and epoxy-fiberglass. The following figures show this boat in comparison with the average values of 22 other catamarans with length overall between 27 and 29 feet listed on the Multihull Dynamics, Inc. website.

Displacement: 2.94 tons [2.24 tons]

Length Overall: 28.22 feet [28.43]

Length at Waterline: 28.22 feet [26.77 feet]

Beam Overall: 18.37feet [16.62 feet]

Sail Area (Main plus jib): 398 sq. ft. [438 sq. ft.]

Displacement-Length Ratio: 65.41 [61.15]

Sail AreaDisplacement Ratio: 18.14 [25.18]

Beam between hull centerlines: 14.11 ft. [13.35 ft.]

Base Speed: 8.03 knots [8.77 knots]

Texel Rating: 144 [130]

Bruce Number: 1.07 [1.25]

Stability Speed: 21.93 knots [17.35 knots]

The numbers show that the KD 860 will be about 10% slower (Base Speed) but more resistant to hull lifting (Stability Speed) than the average of its peers. Both of these characteristics are because it is heavier and has less sail area than average. However, several elements make the speed comparisons questionable. Specifically, there is an assumption in the analysis on the website that hull shapes approximate a semicircle, which the KD 860 certainly does not. It has more wetted surface that would increase drag at low speeds.

At higher speeds, however the anti-vortex panels are claimed to reduce vortex-induced drag and hence improve the ability to sail to windward. How they compare with dagger boards and centerboards is not clear. To their advantage, these horizontally-oriented panels do not add to draft, a claim that is not available for daggerboards and centerboards while sailing. (Of course, the boards can be retracted for motoring.) More can be read about these devices on Kohlers website at http://www.ikarus342000.com/index.htm.

This boat is clearly in the cruiser category (see Classifying Multihulls in the Technical section of the Multihull Dynamics, Inc. website at www.multihulldynamics.com). In this length range, catamarans tend to be mostly cruisers or true racers. The list of other catamarans figuring into the averages above include the racers KL28 and Punch 8.53 with Base Speeds around 10.5 knots.

Calvin H. Markwood

Engineering Analyst Multihull Dynamics, Inc.

Contact Cal: multihull.analysis@comcast.net