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§ 17

Foresail area (jib and genoa)

How large will the wastage be with the overlap the customer wants?

Answer: A foresail is calculated from the fore triangle: I is the height from the deck to the forestay's attachment on the mast, and J is the distance from the mast's front edge to the forestay's attachment on the deck. The foot is about the forestay's length. LP is the perpendicular distance from the clew to the foot and is given as a per cent of J: a jib has LP about 100%, a genoa 130-155%. The area is approximately a triangle with the foot as base and LP as height.

Formula

Forlig LL ≈ √(I² + J²)

LP = J × LP% ÷ 100

Areal ≈ ½ × LL × LP

Fortrekant = I × J ÷ 2 (sejlets areal ved LP 100 %, uden overlap)

Symbols and units in the formula
SymbolMeaningUnit
iI (height of the triangle)m
jJ (base of the triangle)m
lp_pctLP as percentage of J (overlap)%
=Foresail aream²
=Fore triangle area (I × J ÷ 2)m²
=Forlig (≈ forstagets længde)m
=LPm

Validity and source

Source
Sejlmagerpraksis: areal ≈ ½ · LL · LP (målebegreberne I, J og LP)

Does not apply when

  • The finished sail is a bit shorter than the forestay because there must be room for fittings, pulley system and stretch. A foresail with curved aft edge (roach) or full foot has slightly more area. Always measure the boat before cutting.

Worked example

The calculator's own default values inserted. The figures are calculated with exactly the formula the tool uses.

Substituted

  • I (height of the triangle)10 m
  • J (base of the triangle)3.20 m
  • LP as percentage of J (overlap)150 %

Giver

  • Foresail area25.20 m²
  • Fore triangle area (I × J ÷ 2)16.00 m²
  • Forlig (≈ forstagets længde)10.50 m
  • LP4.80 m

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Sådan bruger du den

Når en kunde vil have ny fok eller genua, skal du regne arealet ud fra riggens I og J og den overlapning, kunden ønsker. Arealet bruges til dugforbrug, pris og til at vurdere, om sejlet passer til båden.

Step by step

  1. Find I og J i riggens data eller mål dem op på båden.
  2. Aftal LP med kunden: ca. 100 % for en fok, 130-150 % for en genua.
  3. Indtast tallene og aflæs forliget, LP og arealet.
  4. Brug arealet i sejldug-beregneren.

Typical errors

  • At bruge forstagets længde som forlig uden fradrag til beslag og rullesystem.
  • At forveksle LP med agterligets længde.
  • At måle J fra mastens agterkant i stedet for forkanten.

Examples from everyday life

Genua 150 %

En 30-fods båd har I = 10 m og J = 3,2 m. Kunden vil have en genua med 150 % overlap.

I (height of the triangle)
10 m
J (base of the triangle)
3.2 m
LP as percentage of J (overlap)
150 %

It means: Forliget er ca. 10.5 m, og LP er 4.8 m. Genuaen bliver ca. 25.2 m² mod fortrekantens 16 m².

Selvslående fok 105 %

Samme båd skal også have en selvslående fok med 105 % LP.

I (height of the triangle)
10 m
J (base of the triangle)
3.2 m
LP as percentage of J (overlap)
105 %

It means: Fokken bliver ca. 17.64 m². Det er kun lidt mere end fortrekanten.

Rule of thumb

Genuaens areal ≈ fortrekanten × LP%. En 150 % genua er ca. 1,5 gange fortrekanten.

How to check the answer

Ved LP 100 % skal arealet være tæt på fortrekantens areal.