30-fods sejlbåd
En sejlbåd har 8,5 m vandlinje.
- Waterline length (LWL)
- 8.5 m
- Fart-længde-forhold (skrogfart = 1,34)
- 1.34
It means: Skrogfarten er ca. 7.08 knob, altså 13.1 km/h.
§ 18
Hull speed for a displacement hull
How fast can the vessel sail before it gets stuck in its own wave?
Answer: A displacement hull pushes a wave in front of it. At hull speed the wave length equals the waterline length, and the boat gets stuck in its own wave trough. More speed costs disproportionately much engine power. Hull speed is used when you need to assess engine choice, propeller and what the customer realistically can expect of speed.
Formula
Skrogfart (knob) ≈ 1,34 × √(LWL i fod)
1 m = 3,2808 fod, og 1 knob = 1,852 km/t
I meter: skrogfart ≈ 2,43 × √(LWL i m)
| Symbol | Meaning | Unit |
|---|---|---|
| lwl | Waterline length (LWL) | m |
| faktor | Fart-længde-forhold (skrogfart = 1,34) | |
| = | Speed | knob |
| = | Speed in km/h | km/t |
| = | Waterline length in feet | fod |
Validity and source
- Source
- Froudes lov: skrogfart ≈ 1,34 · √LWL(ft) knob = 2,43 · √LWL(m) knob
Does not apply when
- Light, narrow hulls can exceed hull speed and planing boats are not bound by it. For a heavy sailing boat 1.34 is a ceiling, not a cruising speed. A cruising speed under engine is typically some way below.
Worked example
The calculator's own default values inserted. The figures are calculated with exactly the formula the tool uses.
Substituted
- Waterline length (LWL)8.50 m
- Fart-længde-forhold (skrogfart = 1,34)1.34
Giver
- Speed7.08 knob
- Speed in km/h13.1 km/t
- Waterline length in feet27.9 fod
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Sådan bruger du den
Når en kunde spørger, hvor stærkt båden kan sejle, eller når du vurderer motor og propel, giver skrogfarten den realistiske øvre grænse for et deplacementskrog.
Step by step
- Find vandlinjelængden (LWL).
- Brug faktor 1,34 for skrogfarten.
- Aflæs farten i knob og km/t.
- Sammenlign med det, kunden forventer.
Typical errors
- At bruge længden over alt i stedet for vandlinjelængden.
- At bruge reglen på planende både.
- At blande meter og fod.
Examples from everyday life
Lille spidsgatter
En lille spidsgatter har 5,8 m vandlinje.
- Waterline length (LWL)
- 5.8 m
- Fart-længde-forhold (skrogfart = 1,34)
- 1.34
It means: Den kan sejle ca. 5.85 knob. Mere motor hjælper ikke.
Økonomisk marchfart
Kunden med 8,5 m vandlinje vil sejle billigt på motor og spørger om en fornuftig fart. Du bruger et fart-længde-forhold på 1,1.
- Waterline length (LWL)
- 8.5 m
- Fart-længde-forhold (skrogfart = 1,34)
- 1.1
It means: Ved forholdet 1,1 sejler båden ca. 5.81 knob, og brændstofforbruget er langt lavere end ved skrogfart.
Rule of thumb
Skrogfart i knob ≈ 2,4 × √(LWL i m).
How to check the answer
Fire gange så lang vandlinje giver dobbelt skrogfart.