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Reference > 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 the sheet. For example, a foot block that turns a sheet 180А will see a load equal to twice the load on the sheet. A deck organizer, which turns a halyard only 30А, will see just 52% 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. ULDBs 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

30А 52% 90А 141% 150А 193%

45А 76% 105А 159% 160А 197%

60А 100% 120А 173% 180А 200%

75А 122% 135А 185%

Genoa System Loading

Because wind speed is squared, it is the most important variable and can greatly influence loading. Wind is the apparent wind and should be calculated for the specific sail being analyzed. For example, the #1 genoa on a 25' (7m) 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 45А, while a Kevlar #3 may deflect 75А 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 45А and .5 of lead car load when deflection is 60А

Genoa Sheet Load

English Metric

SL=SAЗV²З0.00431 SL=SAЗV²З0.02104

SL Sheet load in pounds Sheet load in kilograms

SA Sail area in square feet Sail area in square meters

V Wind speed in knots Wind speed in knots

Mainsheet System Loading

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 to 60'. Traveler car adjuster load is generally considered to be .2 times car load.

Mainsheet Load

English Metric

ML Mainsheet load in pounds Mainsheet load in kilograms

E Foot length of main in feet Foot length of main in meters

P Luff length of main in feet Luff length of main in meters

V Wind speed in knots Wind speed in knots

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

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:

LOA Length Overall - overall tip to tip length of the boat I₂ Height of staysail halyard above deck

LWL Length Waterline - length of waterline of the boat J Base of the foretriangle measured from the front of the mast to the intersection of the forestay and deck

DWL Design Waterline - theoretical waterline length of boat as opposed to LWL which is actual waterline length J₂ Base of staysail triangle

BMX Beam Maximum - width of the boat at the widest point P Luff length of the mainsail

BWL Beam Waterline - widest beam of boat at the waterline E Foot length of the mainsail

I Height of the foretriangle measured from the top of the highest sheave to the sheerline LP Shortest distance from headstay to the clew of the jib

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