Technical » Abbreviations, Definitions, Formulas and Terms
Release Date: 2/22/2013Abbreviations, Definitions, Formulas and Terms
I. Basics
Loa - Length overall
Lwl - Length at waterline
SA - Sail Area: Main + Jib (On this website 100 % fore-triangle area only)
Boa - Beam overall
Bh Beam hull: Hull beam width at waterline of a single hull of a catamaran or main hull (vaka) of a trimaran. Unless provided by the Designer, it will be approximated by the formulas:
Catamarans Bh = 9.85*sqrt (Displacement/Lwl)
Trimarans Bh = 15.2*sqrt(Displacement/Lwl)
Bcl - Beam centerline: Unless provided by the Designer, it will be approximated by the formulas:
Catamaran: Distance between hull centerlines: Bcl = Boa - Bh
Trimaran: Distance between outrigger centerlines: Bcl = 0.95*Boa
BdCl = Bridgedeck Clearance
Nom = Nominal Bridgedeck Clearance
BdCl v. Nom Bridgedeck Clearance v. Nominal Bridgedeck Clearance: The difference between a Catamarans Bridgedeck Clearance and the Nominal Bridgedeck Clearance for a catamarans of that Waterline Length (Lwl)
Boat Waterline Length |
Nominal Bridgedeck Clearance |
Less than 40 feet |
6.25% of waterline length |
Greater than 40 feet |
30 inches |
Catamarans in the database having a BdCl v. Nom result that is a negative number, such as -2, means that their Bridgedeck Clearance is 2 inches below the Nominal height for that specified Waterline Length (Lwl).
Catamarans in the database having a BdCl v. Nom result that is a positive number, such as +2, means the Bridgedeck Clearance is 2 inches above the Nominal height for that specified Waterline Length (Lwl).
*** See "Bridgedeck Clearance for Catamarans" on Technical page for more on this subject.
Disp. - Displacement (D): Designed Displacement, the Designer's target weight for the boat expressed in Long Tons. Lacking this information, the actual weight divided by 2240 lbs.
Bcl/Lwl Beam Centerline to Length at Waterline: The ratio of the Beam centerline to Length of waterline. For catamarans this should be 40% (expressed as .40 on this website), or greater, to avoid wave interference between the hull, as well as subsequent drag rise at speeds approaching hull speed.
Lwl/Bh Length at Waterline to Beam hull: The ratio of the Length of waterline to the Beam ratio of a single hull for a catamaran, or the center hull (vaka) of a trimaran. The following ratios indicate boat design:
16:1 - High Performance Racer
12:1 - Racer
10:1 - Racer/Cruiser
8:1 - Cruiser
The narrower hull will be faster. The fuller hull will be slower, but can carry more payload, for a given boat length.
N/A - Means data Not Available for this specification
II. Performance Indicators
BSpd - Base Speed: An empirically derived indicator of the speed a given boat could average over a 24 hour period (Best Days Run) under a variety of conditions. It can be used to compare speed potential of one or more boats. It has been used for handicapping boat races involving a variety of boat types.
"Base Speed: A Simple Measure for Estimating Multihull Performance",
Richard Boehmer, Multihull International, No. 225, (APR'89) pp. 108-110.
BSpd. = 1.7*(Lwl^0.5)*(SA^0.352)/((Disp.*2240)^.253)
KSP - Kelsall Sailing Performance Number: A measure of relative speed potential of a boat. It takes into consideration: Sail Area, Displacement and Length at waterline. The higher the number the faster speed predictionl for the boat.
= 0.5(Lwl*SA/D)^0.5
= 0.5(Waterline Length*Sail area/Displacement)^0.5
BN - Bruce Number: The Bruce Number is a power-to-weight ratio for relative speed potential for comparing two or more boats. It takes into consideration the Displacement and Sail Area of main and jib (100% fore-triangle only), no overlapping sails.
Chris White, "The Cruising Multihull", (International Marine, Camden, Maine, 1997), states that a boat with a BN of less than 1.3 will be slow in light winds, and a boat with a BN of 1.6, or greater, is a boat that will be reefed often in offshore cruising.
Derek Harvey, "Multihulls for Cruising and Racing", International Marine, Camden, Maine, 1991, states that a BN of 1 is generally accepted as the dividing line between so-called slow and fast multihulls.
BN = SA^0.5/(Disp.*2240)^.333
SA/D - Sail Area/Displacement: This ratio is similar to the Bruce Number, giving a power-to-weight ratio type figure for comparing two or more boats. Higher numbers indicate faster boats for a given length. However, in this ratio unlike the Bruce Number, main and genoa, or any combination of Sail Area can be used. When comparing any two boats using this formula, one should ensure that both the Sail Area and Displacement, including planned payload, are equivalent, to get a realistic comparison.
SA/D = SA/(Disp.*2240/64)^.667
TR - Texel Rating: The Texel Rating provides a handicap system for widely varying boats sailing together in a race. The formula is essentially the inverse of the Base Speed formula with constants applied to make the results useful. The Texel Rating system permits the calculation of time to sail a given distance. Thus, a boat with a higher Texel Rating can be expected to take longer from start to finish, than a boat with a lower TR.
TR = 100 /(0.99*(Lwl)^0.3*(0.88*SA)^0.4/(D)^0.3)*K,
where K is a correction for catamarans, based on the equation,
K = 1/(0.19*(0.88*SA)^0.4/D^0.36+0.91) if greater than 1 (otherwise 1).
(Note: These equations require metric specification inputs, i.e., meters, square meters and kilograms.)
The Europeans are using a different exponent for displacement in the formula that results in a 20 - 25% higher value.
***See "Texel Rating" on Technical page for more on this subject.
III. Stability Indicators
SSpd - Stability Speed: This figure is an indicator of the wind speed at which the windward hull of a catamaran, or the main hull of a trimaran, will begin to lift.
The formula does not take wave action, shortening sail or hiking-out into account. This figure is an indicator of the wind speed at which the windward hull of a catamaran, or the main hull (vaka) of a trimaran, will begin to lift. This formula includes a 40% safety factor for wind gusts but does not compensate for wave effects.
Multihull Designs by John Shuttleworth Yacht Designs, 1998, p. 37.
SSpd. = 8.23805*((0.5*Bcl*Disp*2240)/(SA*HCE))^0.5
The result of this calculation could be used as the first reefing wind speed since it has a 40% safety factor for unexpected gusts.
*** See "Stability Speed" on Technical page for more on this subject
KSI: Kelsall Stability Indicator
= 13.73(D*Bcl/2SA*Hcl)^0.5
= 13.73(Displacement *half Bcl/SA*Hcl)^0.5
Derek Kelsall says this is the wind speed, winds abeam and 100% of working sails or greater, in knots per hour at which the cat is likely to capsize. Many other factors affect the actual situation and hence he suggests that these figures are used for comparison of similar types only.
We do not include Hce in the list of specifications on the site, simply because we didn't think it was important to all potential site users. It is used in the Stability Speed (Sspd) calculation, however.
The data readily available for boats rarely includes mast height, but does include sail area. Thus, I have derived a method to approximate Hce from boat length and sail area. The derivation is included in the article"Analysis and Comparison of Cruising Multihulls." The resulting equations are:
For boats up to 40 feet Lwl: Hce = 2/3*(SA/Lwl) + 0.25*Lwl
For boats 40 feet Lwl and greater: Hce = 2/3*(SA/Lwl) + 10
The "40% of mast length + height of mast base above the water" is probably a good way of estimating Hce. If the sail were a triangle, it would be 33%. Given the shape of current multihull main sails, the 40% is possibly a better number. However, as stated above, mast length and height of mast base above the water are not readily available.
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