Part 2: Horsepower and Lightweight; Hull Form
By George L. Petrie — June 2002
First, what do we mean by "fast"? Speed is a relative term, Peters explains. Boats come up on plane when their speed-to-length ratio is greater than 2.5 or so; speed-to-length ratio is determined from speed (in knots), divided by the square root of length (in feet). So a 115-foot yacht (100 feet on waterline) going 40 knots has a speed-to-length ratio of 4; it is up on plane, and we would agree it is a "fast boat." Now compare that to a 20-foot runabout (16 feet on waterline) going 16 knots; its speed-to-length ratio is also 4, meaning the hull is up on plane, but we would not think of it as a "fast boat."
The point is, a large yacht may be considered fast even if it has barely reached planing speed, while a small boat must go at much higher speeds (proportionally) to be considered fast. But no matter the size, there are similarities in what it takes to make a boat go fast.
Horsepower and Light Weight: Clearly these are important factors; more horsepower translates to more thrust, hence greater speed. And less weight means the hull has to produce less dynamic lift to get up on plane, so induced drag is reduced and the boat can go faster with the same power.
In theory, speed varies as the cube of horsepower/weight, meaning it would take eight times the horsepower to double the speed of a given vessel (assuming you could increase the power without increasing weight). In reality, because of the dynamics of high-speed planing hulls, speed actually varies more closely as the square of the horsepower/weight ratio.
Hull Form: In calm water, the most efficient planing surface is a flat board; flip the kitchen table upside down and strap on an outboard motor. But a perfectly flat hull has poor directional stability and would jar your fillings loose the first time you hit a wave. So virtually all hulls have some degree of deadrise in the bottom. It's a tradeoff; more deadrise delivers a softer ride in rough seas, but sacrifices top end speed in calm water.
The figures below show four typical hull configurations. The modified-V hull strikes a balance, with high deadrise sections forward for a smooth entry and flatter sections aft to provide an efficient lifting surface. It is typical of many sportfishing yachts and express cruisers. The deep-V carries high deadrise all the way to the stern, softening the impact under extreme conditions when the entire hull may become airborne. It's the hull form that brought Richard Bertram's Moppie decisive victory in the legendary 1960 Miami to Nassau Race, and is favored for boats that must go fast even in high seas. The stepped-V and offshore catamaran are race-bred adaptations we'll discuss later in this article.
This article originally appeared in the February 2003 issue of Power & Motoryacht magazine.