Changes

The Speed/Length Ratio (SLR) is the boat’s maximum velocity in knots divided by the square root of the LWL in feet. For example, with an LWL of 54 ft 04 in and a maximum speed of 9 knots, a boat's SLR is 1.22. Typically a boat is at its most fuel efficient at an S/L between 1.1 and 1.2. SLR is closely related to the Prismatic Coefficient.

Prismatic Coefficient (Pc) is a dimensionless coefficient of form (glad you understood that!), allowing comparisons with other boats even of different size. It is the ratio of the under body volume to the volume of a prism having a length LWL, and a section equal to the boat’s maximum midsection. It indicates the fineness of the ends compared to the midsection. In general, it is a reasonable measure of the wave resistance of a boat, and thus related to the amount of power required to drive it forward. In aircraft design it has been found that the Sears-Haack body shape is least susceptible to wave drag. This is a canoe shape ill-suited to a small live-aboard. Larger boats like the 83-ft Wind Horse are exploring semi-canoe shapes.<ref> FPB 83 – Wind Horse, http://www.setsail.com/dashew/FPB83_Intro.html</ref>

Block Coefficient

Block coefficient is the volume of a hull as a proportion of the volume of a rectangular block having the same length, width and depth. The higher the coefficient, the lower is the propeller efficiency.

Midsection Coefficient is the area of the midsection, divided by the beam on the waterline multiplied by the (draft plus the freeboard).

The wetted area (WP) of the boat’s hull is an indicator of friction through the water. WP is very important in a submarine but less so in a surface ship, where wave resistance is more important.

A/B, the ratio of the area above the water to the area below the water, is a measure of stability. It is a gross rule of thumb that is easily misused. Stability can be better predicted using computer programs that consider many factors. A lower ratio, say below 2.5, is inherently more stable than a top-heavy boat with a high A/B ratio of say 3.0 or more.

[Based on sail boats]

Ballast/Displacement Ratio = Bt/D * 100

Ballast displacement ratio is the boat’s ballast divided by the boat's displacement converted to a percentage. This ratio indicates the resistance to heeling (stiffness). An average ratio is approximately 35%. A higher ratio indicates greater stiffness.

Displacement/Length Ratio = D/(0.01 * LWL)^3

Displacement to length ratio indicates if the boat is a heavy (results greater than 300), medium (200-300) or light (75-200) cruiser. Displacement is in long tons (2240 lb). Note that ranges for sail boats are different (325-400, 275-325 and 200-275 respectively). A D/L of 280-350 indicates a boat is a heavy cruiser suited for serious offshore work.

[Based on sail boats]

Capsize Risk = B/(D/[0.9*64])^0.333

The Capsize Risk is a seaworthiness factor derived from the [http://www.sailingusa.info/formula.htm USYRU] analysis of the disastrous 1979 FASTNET Race, funded by the [www.sname.org/ Society of Naval Architects and Marine Engineers]. Values less than two are good for sail boats. No comparable data exists for displacement boats.

Length/Beam Ratio = LOA/B

A lower Length to Beam (L/B) number indicates a beamier boat. Boats with a wider beam have better initial stability, and more interior room. They have worse ultimate stability, and high inverted stability, meaning it is hard to turn them upright. A beamier hull (L/B ratio below 2.7) has more room inside, but is less efficient and pounds much more going into head seas.<ref> Downeast Lobster Boats, http://www.downeastboats.com/hulldesign.html </ref> A narrow hull has better directional control and steers better. For boats from 30 to 50 feet in length a hull with an L/B ratio above 3.0 is more efficient and pounds less into head seas.

Comfort factor = D/(0.65*(0.7*LWL+0.3*LOA)*B^1.33)

This Comfort Factor, developed by Ted Brewer, predicts the overall comfort of a sail boat when it is underway.<ref> Ted Brewer Yacht Design, http://www.tedbrewer.com/yachtdesign.html</ref> The formula predicts the speed of the upward and downward motion of the boat as it encounters waves and swells. Faster motion makes passengers more uncomfortable.

The higher the number, the more resistant a boat is to movement, and the more comfortable it is. Obviously bigger boats give a better ride; although the formula favours a narrow beam. Use with caution analysing power boats.

Bulbous bows were developed in the 1950’s for large cargo vessels, to improve their penetration of the water, and reduce fuel consumption. The underwater bulb creates a wave 180 degrees out of phase with the original bow wave. This cancels or reduces the bow wave. The first merchant vessel with a bulbous bow was the <i>Yamashiro Maru</i> delivered in November 1963 by the Mitsubishi Heavy-Industries, Ltd. Nagasaki Shipyard in Japan.<ref> Ripples in Time, Bulbous Bow – Introduction of wave-making resistance reduction technology, http://www.nykline.co.jp/english/seascope/200010/</ref> Today all the largest ships, including Nimitz-class aircraft carriers, have bulbs.<ref> Reagan Takes a Bow, http://www.nn.northropgrumman.com/Reagan/About_the_ship/Bow.htm</ref>