by Gregor Tarjan, Aeroyacht Ltd.
Catamaran Advantages vs. Monohulls: Efficiency
Catamaran efficiency can be best expressed in the manner in which certain performance levels are achieved. We will compare the catamaran’s Displacement/Length Ratio, Sail Area/ Displacement ratio and see how the all important stability is achieved in both monohulls and multihulls.
A catamaran’s Power to Weight ratio (or Displacement/ Length D/L Ratio) is one of the most important efficiency indicators of a multihull. The higher the number the slower the boat will sail. The driving power of any sail-driven craft is the energy of the wind acting through the Center of Effort (CE) of the sail plan. The weight of the catamaran is the weight of the structure of the vessel, including its gear and equipment.
D/L Ratio is the Displacement to Length Ratio
D/L= (Displacement in lbs / 2240) / (0.01 * LWL) 3
The higher the resulting number the less performance a catamaran will have. This formula is especially indicative for monohulls, yet comparing a catamaran will show the relative difference. The displacement length ratio is a measure of a boat’s speed potential. For displacement boats (most sailboats), speed potential is a function of waterline length (unless you are planing or surfing down a wave). Longer waterline boats can go faster. Lighter boats accelerate faster and reach hull speed with less wind. All else being equal, the boat with the lower D/L will be a better light air performer. Lower displacement catamarans will also make the yacht more sensitive to loading
A Catamaran’s Power to Weight Ratio or Sail Area Displacement Ratio
SA/D = SA / (Disp / 64)2/3
This ratio is an indicator of how much sail area a catamaran has relative to its displacement. A boat with a higher value will accelerate faster and get to hull speed with less wind. Again, all other boat parameters being the same, the vessel with the higher SA/D will be a faster a catamaran will be. This is the basic power to weight measure.
Catamaran Efficiency numbers will tell us everything about the potential speed. The two ratios together (D/L & SA/D) can give a good comparison of two boats’ speed potential relative to one another. Taken from the above examples, if our multihull has a SA/D of 27.7 and a DL of 117, and our monohull has a SA/D of 18.9 and a DL of 145, the multihull will clearly be a better performer.