D/L
Displacement / length ratio
Displacement / length (D/L) compares immersed weight to waterline length. Higher D/L means more volume for a given LWL. FairHelm judgment: many Scandinavian cruisers sit heavier than charter production boats of the same LOA; that is often intentional for payload and motion.
Formula
D/L = Displacement_lb / (0.01 * LWL_ft)^3
Displacement in pounds. LWL in feet. Keep both imperial for the classical form.
History
D/L appears in design texts and broker sheets as a shorthand for 'heavy' versus 'light' hulls. It grew with imperial yacht measurement culture and remains quoted even when yards publish metric specs.
Rule of thumb
Under about 200 is often called light; 200-300 moderate; 300+ heavy for cruising monohulls. Ultra-light racers sit far below. Treat bands as conversation starters, not class rules.
Limits
Wrong LWL (using LOA) skews the cube term badly. Half-load versus light-ship displacement changes the ratio. D/L ignores beam, ballast, and sail plan. A heavy D/L boat can still be tender if ballast is low.
Boat examples
- Hallberg-Rassy 36
Expect a heavier D/L than a same-LOA charter hull designed for marina berths and light crews.
- Sweden Yachts 390
Swedish performance-cruiser lineage: confirm published displacement basis before ranking against HR.
- Linjett 40
Coastal Scandic design: D/L helps explain payload and motion claims in owner forums.
FAQ
- Why use LWL and not LOA?
- The classical formula uses waterline length because wave-making and immersed volume scale with LWL. LOA includes overhangs that do not carry the same immersed length at rest.
- Is a high D/L always better for offshore?
- Not always. Payload and motion can improve, but so can marina costs and light-air needs. Match D/L to your passage profile and crew size.
- My listing only shows metric tonnes. Can I convert?
- Yes: convert mass to pounds and LWL to feet, then apply the formula. Do not mix metres into the imperial expression without converting first.