Cal's Evals » ECO 7.5m Coastal Cruising Catamaran

Release Date: 1/13/2011

ECO 7.5 Coastal Cruising Catamaran

 

The ECO 7.5 is one of a family of novel multihulls designed by Berndt Kohler. He has designed this small coastal cruiser that has a single berth aft in each hull plus a settee/berth in the salon for a total of four. The construction is as expected for his designs, plywood and epoxy skin over wood bulkheads and stringers. It is a popular design for self-builders who have a limited budget. The ECO 7.5 hulls start with a sharp vee at the bow, then transition into a flat bottom plus chines toward the stern. The hulls are symmetrical with retractable rudders on each hull and a dagger-board in the port hull.  The headroom in the hulls is six feet plus in the center of the hulls where the head is located on the port side and a galley on the starboard side.


The plans show a conventional fractional rig with a fully-battened main. An alternate rig is a wishbone gaff with a roller reefing jack stay for fast main sail operation.  Several options are available for auxiliary power. More details are available on Bernds website at
www.ikarus342000.com/.

 

The following figures show this boat in comparison with the average values of 17 other catamarans with waterline length between 22.5 and 25 feet, listed on the Multihull Dynamics, Inc. website.

 

Displacement: 1.28 tons [1.8 tons]

 

Length Overall: 24.6 feet [26.19 feet]

 

Length at Waterline: 24.6 feet [24.12 feet]

 

Waterline Length to Beam Ratio: 9.0 [9.67]

 

Beam Overall: 14.76 feet [12.15 feet]

 

Sail Area (Main plus jib): 308 sq. ft. [339 sq. ft.]

 

Displacement-Length Ratio: 42.99 [64.73]

 

Sail AreaDisplacement Ratio: 24.42 [23.99]

 

Beam between hull centerlines: 11.8 ft. [12.15 ft.]

 

Base Speed: 8.46 knots [8.16 knots]

 

Texel Rating: 135 [144]

 

Bruce Number: 1.24 [1.21]

 

Stability Speed: 16.03 knots [17.52 knots]

 

 

 

The numbers show that the ECO 7.5 will be nearly 4% faster (Base Speed) but slightly less resistant to hull lifting (Stability Speed) than the average of its peers. Both are mainly due to its light displacement. The dagger-board and spade rudders should support good performance and handling. Its Base Speed is just above the trendline for 958 catamarans on the Multihull Dynamics, Inc. website.

 

Another interesting comparison is with two trimarans nearly the same size, the Farrier F-25A and the Dragonfly 800. These boats have nearly the same displacement but 20% greater sail area. Their Base Speeds are half a knot higher (over nine knots) and stability speeds nearly the same, 17 knots. The combination of larger spacing between outriggers (six and one half feet greater than the beam between hull centerlines of the ECO 7.5) compensates for their larger sail area to keep Stability Speed up while taking advantage of the added sail power.

 

Care will need to be taken to avoid overloading the ECO 7.5. Koehler uses terms on his website like whole family and extended cruising that should be viewed with caution. It looks like a boat for the whole family (four) for an afternoon and extended cruising for two. This is true for any boat in this category.

 

The Stability Speed will most likely be higher than estimated above because the main sail has a very large, rounded roach that will serve to lower the height of the center of effort of the sails.

 

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).

 

 

Calvin H. Markwood
Engineering Analyst
Multihull Dynamics, Inc.

Contact Cal: multihull.analysis@comcast.net