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Boat Displacement Calculator

Estimate hull displacement and DLR from dimensions

Displacement

14,892 lbs

DLR

155

Class

Light

Typical: 0.35 (racing), 0.45 (cruiser/racer), 0.55 (cruiser), 0.65 (motorsailer), 0.75 (trawler)

Displacement

14,892

lbs (6.65 long tons)

Kilograms

6,755

Volume

232.7 ft³

Waterplane

269.5 ft²

Displacement-to-Length Ratio

155

Light Displacement

UltralightRacing multihulls, dinghies
< 100
LightPerformance cruisers, racers
100–200
ModerateCruising sailboats
200–300
HeavyBluewater cruisers
300–400
Very HeavyTrawlers, workboats
> 400

Block Coefficient Guide

Racing sailboat0.30–0.40
Cruiser/racer0.40–0.50
Cruising sailboat0.50–0.60
Motorsailer / trawler0.60–0.75
Cargo ship / barge0.75–0.90

What You'll Need

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$18-$254.5
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3M Marine Adhesive Sealant 5200 Fast Cure White 3oz

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Shoreline Marine Bilge Pump 600 GPH 12V

$15-$254.3
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Star Brite Ultimate Aluminum Cleaner & Restorer 64oz

$18-$254.5
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3M Marine Adhesive Sealant 5200 Fast Cure White 3oz

$14-$204.5
View on Amazon

Shoreline Marine Bilge Pump 600 GPH 12V

$15-$254.3
View on Amazon

As an Amazon Associate, we earn from qualifying purchases.

Frequently Asked Questions

Q

What is the displacement-to-length ratio (DLR)?

The displacement-to-length ratio (DLR) compares a boat’s weight to its waterline length. It is calculated as displacement in long tons divided by (0.01 × LWL in feet) cubed. DLR classifies vessels from ultralight racers (under 100) to heavy bluewater cruisers (over 300).

  • Ultralight (DLR < 100): racing multihulls, sport boats, dinghies
  • Light (DLR 100–200): performance cruisers, racing keelboats
  • Moderate (DLR 200–300): typical cruising sailboats
  • Heavy (DLR 300–400): bluewater passage-makers, traditional designs
  • Very Heavy (DLR > 400): full-keel cruisers, trawlers, workboats
DLR RangeClassificationTypical Vessels
< 100UltralightRacing multihulls, sport boats
100–200LightPerformance cruisers, racers
200–300ModerateCruising sailboats
300–400HeavyBluewater cruisers
> 400Very HeavyTrawlers, workboats
Q

How does Simpson’s rule estimate hull displacement?

Simpson’s rule approximates the underwater volume by sampling cross-section areas at three stations (bow, midship, stern). The formula is Volume = (h/3) × (A_bow + 4×A_mid + A_stern), where h is half the waterline length. Displacement equals volume times water density.

  • Three stations: bow (narrower), midship (fullest), stern (slightly narrower)
  • Each cross-section area = Beam × Draft × Block Coefficient × station factor
  • Station factors: bow 0.6, midship 1.0, stern 0.8 (typical proportions)
  • Saltwater density: 64 lbs/ft³; freshwater: 62.4 lbs/ft³
  • More stations (5 or 7) increase accuracy but require hull line offsets
Q

What is the block coefficient and how do I estimate it?

The block coefficient (Cb) is the ratio of the underwater hull volume to the volume of a rectangular box enclosing it (LWL × Beam × Draft). Fine-entry sailboats have low Cb values (0.35–0.45), while full-bodied trawlers have high values (0.65–0.75).

  • Racing sailboat: Cb 0.30–0.40 — very fine entry, low wetted surface
  • Cruiser/racer: Cb 0.40–0.50 — moderate performance with some comfort
  • Cruising sailboat: Cb 0.50–0.60 — balanced comfort and speed
  • Motorsailer or trawler: Cb 0.60–0.75 — full hull for stability and cargo
  • Cargo ship: Cb 0.75–0.90 — maximizes cargo volume
Q

Why does displacement matter for boat performance?

Displacement directly affects a boat’s speed potential, stability, and load-carrying capacity. Lighter boats accelerate faster and can plane in moderate winds, while heavier boats carry more provisions and handle rough seas with a more comfortable motion.

  • Hull speed limit: 1.34 × √LWL (in knots) applies mainly to heavy displacement hulls
  • Light boats (DLR < 200) can exceed hull speed by planing or surfing
  • Heavier boats need more sail area or engine power to reach the same speed
  • Stability increases with displacement — heavier boats resist capsizing better
  • Payload capacity = displacement minus lightship weight (hull, rig, engine)
Q

Does salt water vs fresh water affect displacement?

Yes. Saltwater is denser than freshwater (64 vs 62.4 lbs per cubic foot), so a boat floats slightly higher in salt water. The same hull displaces about 2.5% less volume in salt water to support the same weight, meaning slightly less draft.

  • Saltwater density: 64.0 lbs/ft³ (1,025 kg/m³)
  • Freshwater density: 62.4 lbs/ft³ (1,000 kg/m³)
  • A 20,000-lb boat displaces 312.5 ft³ in salt vs 320.5 ft³ in fresh
  • Draft increases by roughly 0.5–1 inch when moving from ocean to lake
  • Load lines on commercial vessels have separate marks for salt and fresh water

Example Calculations

1Cruising Sailboat (35 ft LWL)

Inputs

Waterline Length35 ft
Beam11 ft
Draft5.5 ft
Block Coefficient0.55
Water TypeSaltwater

Result

Displacement67,078 lbs
Long Tons29.95
Kilograms30,425
DLR699 (Very Heavy)
Hull Volume1,048.1 ft³

Bow area = 11 × 5.5 × 0.55 × 0.6 = 19.97. Mid area = 33.28. Stern area = 26.62. h = 17.5. Volume = (17.5/3) × (19.97 + 4×33.28 + 26.62) = 5.833 × 179.7 = 1,048 ft³. Displacement = 1,048 × 64 = 67,078 lbs.

2Performance Racer (28 ft LWL)

Inputs

Waterline Length28 ft
Beam8.5 ft
Draft3.5 ft
Block Coefficient0.42
Water TypeSaltwater

Result

Displacement20,155 lbs
Long Tons9.00
Kilograms9,143
DLR410 (Very Heavy)
Hull Volume314.9 ft³

Bow area = 8.5 × 3.5 × 0.42 × 0.6 = 7.50. Mid area = 12.50. Stern area = 10.00. h = 14. Volume = (14/3) × (7.50 + 4×12.50 + 10.00) = 4.667 × 67.50 = 315 ft³. Displacement = 315 × 64 = 20,155 lbs.

Formulas Used

Simpson's Rule (3 Stations)

Volume = (LWL / 2 / 3) × (A_bow + 4 × A_mid + A_stern)

Approximates the underwater hull volume using cross-section areas at bow, midship, and stern stations.

Where:

LWL= Waterline length in feet
A_bow= Bow cross-section area: Beam × Draft × Cb × 0.6
A_mid= Midship cross-section area: Beam × Draft × Cb
A_stern= Stern cross-section area: Beam × Draft × Cb × 0.8

Displacement from Volume

Displacement (lbs) = Volume (ft³) × Water Density (lbs/ft³)

Converts hull volume to weight using the density of the surrounding water.

Where:

Volume= Underwater hull volume in cubic feet
Water Density= 64 lbs/ft³ for saltwater, 62.4 lbs/ft³ for freshwater

Displacement-to-Length Ratio

DLR = Displacement (long tons) / (0.01 × LWL)³

A dimensionless ratio classifying a hull as ultralight, light, moderate, heavy, or very heavy.

Where:

Displacement= Total displacement in long tons (2,240 lbs)
LWL= Waterline length in feet
0.01= Scaling factor for the cube

Understanding Boat Displacement and Hull Performance

Displacement is the single most important number describing a boat’s character. It determines how much water the hull pushes aside, which in turn governs speed potential, stability, comfort in a seaway, and how much gear you can carry aboard.

Simpson’s rule provides a practical estimation method when you know the hull’s waterline length, maximum beam, draft, and block coefficient. While full hydrostatic analysis requires detailed hull line drawings, the three-station Simpson’s approximation is accurate enough for comparing designs and verifying manufacturer claims.

The displacement-to-length ratio (DLR) normalizes displacement against waterline length, allowing fair comparisons between boats of different sizes. A 25-foot daysailer and a 45-foot cruiser can both have a DLR of 250, meaning they share similar weight-to-length proportions despite their size difference.

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Last Updated: Mar 25, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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