Abstract: In a catamaran boat, including a propulsion system, and main hull positioned to travel above the water surface, the hull extending longitudinally forwardly, the combination comprising: two pontoons, each spaced below the main hull, the pontoons extending forwardly; support means carried by the main hull and carrying the pontoons, whereby the pontoons are at least mostly submerged below the water surface when the boat is loaded and at rest; the pontoons having associated chines positioned to permit the pontoons to plane on the water surface at typical planing speeds of the boat; the support means having streamlined leading edge structure and upper sponson structure; and the support means having lateral thickness at a longitudinal location spaced at a level above the pontoons and which is less than the maximum width of each of the pontoons at the longitudinal location.