Harken Block Loading Formulas
SELECTION GUIDE
Harken's formulas for sizing blocks, genoa leads and traveler cars: block load by angle of deflection, genoa sheet load, mainsheet load and the standard rig dimension abbreviations.
DocumentHarken Block Loading Formulas
FormatPDF · 1 page · 2.5 MB
LanguageEnglish
Block Loading vs Angle of Deflection
| Angle of deflection | Load factor |
|---|---|
| 30° | 52% |
| 45° | 76% |
| 60° | 100% |
| 75° | 122% |
| 90° | 141% |
| 105° | 159% |
| 120° | 173% |
| 135° | 185% |
| 150° | 193% |
| 160° | 197% |
| 180° | 200% |
Block load = line load × load factor for the angle the block turns the line. A footblock turning a sheet 180 degrees sees twice the sheet load; a deck organizer turning a halyard 30 degrees sees 52 percent.
Boat type
- Formulas assume a medium-displacement monohull.
- Multihulls and boats with canting keels or water ballast carry sail very high in apparent wind – calculate with that wind speed (e.g. a trimaran with its blade jib in 25 knots true at over 15 knots boat speed, nearly 40 knots apparent).
- ULDBs are tender and reef early; a ULDB will probably drop its #1 genoa at about 15 knots apparent.
Genoa System Loading
| English | Metric | |
|---|---|---|
| Sheet load | SL = SA × V² × 0.00431 | SL = SA × V² × 0.02104 |
| 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 |
- Wind speed is squared, so it is the key variable: use the apparent wind the specific sail is carried in (a #1 genoa on a 7 m (25') boat might only see 15 knots; a #3 blade on a maxi could see 40).
- Genoa lead car load = sheet load × load factor of the sheet deflection. Most #1 genoas deflect about 45 degrees; a #3 may deflect 75 degrees or more.
- Lead car adjuster tackle load: about 0.3 of lead car load at 45 degrees deflection, and .05 of lead car load at 60 degrees (as printed; the drawing on the same page shows .5 × car load at 60°).
For typical cruising monohulls with fixed keel and Dacron sails, sheets and halyards. For other types, ask Harken for technical assistance.
Mainsheet System Loading
| English | Metric | |
|---|---|---|
| Mainsheet load | ML = E² × P² × 0.00431 × V² / (√(P² + E²) × (E − X)) | ML = E² × P² × 0.02104 × V² / (√(P² + E²) × (E − X)) |
| 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 | Aft end of boom to mainsheet attachment, feet | Aft end of boom to mainsheet attachment, meters |
- Less widely accepted than the genoa formula: a rough guide only, for offshore boats of 9–18 m (30–60').
- Assumes standard roach of 7.5%. For large-roach sails such as flattops, multiply by the roach percentage – with 25% roach, multiply the result by 1.25.
- Traveler car adjuster load is generally taken as 0.2 × car load.
Rig Dimension Abbreviations
| Abbr. | Meaning |
|---|---|
| LOA | Length overall – tip-to-tip length of the boat |
| LWL | Length waterline – length of the waterline |
| DWL | Design waterline – theoretical waterline length, as opposed to LWL, the actual waterline |
| BMX | Beam maximum – width at the widest point |
| BWL | Beam waterline – widest beam at the waterline |
| I | Foretriangle height, top of highest sheave to sheerline |
| I2 | Height of staysail halyard above deck |
| J | Foretriangle base, front of mast to forestay/deck intersection |
| J2 | Base of staysail triangle |
| P | Luff length of the mainsail |
| E | Foot length of the mainsail |
| LP | Shortest distance from headstay to the clew of the jib |
Need a part that isn't listed? Ask us to source it and quote the part number from the PDF.
Products in this Selection Guide
