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Harken Loading Formulas - Tech Info

Harken Loading Formulas

Block Loading vs. Angle of Deflection: Load on a block is a combination of the load on the line passing through the block, plus a block-loading factor, which is determined by the angle by which the block turns a sheet. For example, a footblock that turns a sheet 180-degrees will see a load equal to twice the load on the sheet. A deck organizer, which turns a halyard only 30 degrees, will see just 52 percent of the load on the halyard.

Boat Type: Most load formulas assume a medium displacement monohull, but you can easily correct for other boat types. Multihulls have great form stability and speed and will often carry sails very high in the apparent wind speed, so calculations must be done with this wind speed in mind. ULDB's are typically tender and often change sails or reef quite early, so loading may be done at relatively low wind speeds. For example, a modern trimaran may carry its blade jib in 25 knots of wind at speeds over 15 knots for an apparent wind of nearly 40 knots, whereas a ULDB will probably remove its #1 genoa at about 15 knots of apparent wind.

Angle of deflection	Load factor	Angle of deflection	Load factor	Angle of deflection	Load factor

30º	52 %	90º	141 %	150º	193 %
45º	76 %	105º	159 %	160º	197 %
60º	100 %	120º	173 %	180º	200 %
75º	122 %	135º	185 %
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Genoa Loading System

Because wind speed is squared, it is the most important variable and can greatly influence loading. Wind speed ( the apparent wind) should be calculated for the specific sail being analyzed. For example, the # 1 genoa on a 25 ft ( 7 m) boat might only be carried in 15 knots of wind while the # 3 blade on a Maxi-boat could well be carried in 40 knots.

To calculate loading on a genoa lead car, multiply sheet load by the load factor of the sheet. Most #1 genoas will deflect about 45-degrees, while a #3 genoa may deflect 75-degrees or more.

Lead car adjuster tackle load is dependent on the angle of deflection of the sheet in the lead car, but is generally assumed to be .3 of lead car load when deflection is 45-degrees and .5 of lead car load when deflection is 60-degrees.

Genoa Sheet Load
Imperial		Metric

SL = SA x V² x 0.00431		SL = SA x V2 x 0.02104
SL	Sheet load in pounds	SL	Sheet load in kilograms
SA	Sail area in sq. ft.	SA	Sail area in square meters
V	Wind speed in knots	V	Wind speed in knots
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Mainsheet Loading System

The formula for mainsheet loading is not as widely accepted as that for genoa sheet loads and should only be used as a rough guide for offshore boats from 30 ft to 60 ft (9 to 18 m). Traveler car adjuster load is generally considered to be .2 times car load.

Mainsheet Load

Imperial

Metric

ML = E² x P² x 0.00431 x V²
(vP² + E²) x (E - X)

ML = E² x P² x 0.02104 x V²
(vP² + E²) x (E - X)

Mainsheet load in pounds

Mainsheet load in kilograms

Foot length of main in feet

Foot length of mains in meters

Luff length of main in feet

Luff length of main in meters

Wind speed in Knots

Wind speed in knots

Distance from aft end of boom to mainsheet attachment point in feet

Distance from aft end of boom to mainsheet attachment point in meters

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Rig Dimensions

The following abbreviations are often used to describe various measurements on a sailboat. Precise technical definitions exist for each abbreviation, but the following is a list of simple descriptions.

Descriptions

LOA

Length Overall- overall tip-to-tip length of the boat

LWL

Length Waterline- length of waterline of the boat

DWL

Design Waterline- theoretical waterline length of boat as opposed to LWL, which is actual waterline length

BMX

Beam Maximum- width of the boat at the widest point

BWL

Beam Waterline- widest beam of boat at the waterline

Height of the foretriangle measured from the top of the highest sheave to the sheerline

Height of staysail halyard above deck

Base of the foretriangle measured from the front of the mast to the intersection of the forestay and deck

Base of staysail triangle

Luff length of the mainsail

Foot length of the mainsail

Shortest distance from headstay to the clew of the jib

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Harken Loading Formulas

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Pictures and text courtesy of Harken Sailboat Equipment

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