How much weight does it take to sink your boat one inch? Enter waterline length, waterline beam, and a waterplane coefficient; get pounds per inch immersion and the sinkage from any load you add.
Use this when you are loading for a cruise, adding a generator or a larger battery bank, filling tanks, or planning where the boot stripe should sit, and you want to know how far the boat will settle.
Measure waterline length and beam at the waterline, not overall. The waterplane coefficient is the fraction of the LWL × BWL rectangle the actual waterplane fills; if you do not have a designer’s figure, the hint gives a typical range to start from.
Because the waterplane grows as a hull sinks, the result is best for small changes — a few inches, not a foot.
A floating boat sinks until it displaces its own weight of water. For a small extra load, the extra displaced volume is a thin slice whose area is the waterplane — the shape the hull cuts at the waterline. One inch of that slice is the waterplane area times 1/12 ft, and multiplying by the weight of a cubic foot of water gives pounds per inch.
Waterplane area is estimated as LWL × BWL × a waterplane coefficient. Salt water weighs about 64 lb/ft³ and fresh water about 62.4 lb/ft³, so the same boat sits slightly deeper in fresh water.
A_wp = LWL × BWL × Cwp (ft²)
PPI = A_wp × density / 12 (lb per inch; density 64 salt, 62.4 fresh)
sinkage (in) = added weight / PPI
Each row below starts from the default inputs and changes only the columns shown. Recompute any row by hand with the formula above.
| LWL | BWL | Cwp | Water | Added | A_wp ft² | PPI | Sink in |
|---|---|---|---|---|---|---|---|
| 30 | 10 | 0.68 | Salt | 500 | 204.0 | 1,088 | 0.46 |
| 30 | 10 | 0.68 | Fresh | 500 | 204.0 | 1,061 | 0.47 |
| 30 | 10 | 0.72 | Salt | 500 | 216.0 | 1,152 | 0.43 |
| 24 | 8 | 0.68 | Salt | 300 | 130.6 | 696 | 0.43 |
| 38 | 12.5 | 0.7 | Salt | 1500 | 332.5 | 1,773 | 0.85 |
| 18 | 6.5 | 0.75 | Salt | 400 | 87.8 | 468 | 0.85 |
With the calculator defaults (Waterline length LWL (ft) = 30; Waterline beam BWL (ft) = 10; Waterplane coefficient = 0.68; Water = Salt; Weight added (lb) = 500), the arithmetic runs like this:
waterplane area = LWL × BWL × Cwp = 30 × 10 × 0.68 = 204.0 ft²
PPI = area × density / 12 = 204.0 × 64 / 12 = 1,088 lb/in
sinkage = weight / PPI = 500 / 1088.0 = 0.46 in
Result: 1,088 lb/in immersion. Salt water. Waterplane ≈ 204.0 ft²; about 13,056 lb per foot of immersion. 500 lb settles her about 0.46 in (12 mm).
Now change Water to Fresh and leave everything else alone:
waterplane area = LWL × BWL × Cwp = 30 × 10 × 0.68 = 204.0 ft²
PPI = area × density / 12 = 204.0 × 62.4 / 12 = 1,061 lb/in
sinkage = weight / PPI = 500 / 1060.8 = 0.47 in
Result: 1,061 lb/in immersion. Fresh water. Waterplane ≈ 204.0 ft²; about 12,730 lb per foot of immersion. 500 lb settles her about 0.47 in (12 mm).
Moving the same boat from salt to fresh water drops PPI by about 2.5%, so the same load sinks her a little more. Boats that cruise from a river into the sea come up by a similar fraction of their displacement.
Water and fuel are the big variable loads on most cruising boats. Fresh water weighs about 8.3 lb per US gallon, and diesel roughly 7 lb, so a 100-gallon water tank going from empty to full is over 800 lb — run that through this page and you can see why the waterline creeps up at the start of a cruise.
If you are raising the boot stripe, measure how far the boat sits below the old line when loaded for a typical trip, then add a little for growth. PPI tells you how much weight each inch represents, which helps decide whether to move the stripe or move the gear.
Weight placed far forward or aft also changes trim, which this page does not model. A heavy anchor and chain in the bow can sink the bow more than PPI alone suggests while lifting the stern.
Units: Feet for hull dimensions, pounds for weight, inches for sinkage.
Where the model stops: It assumes a constant waterplane over the change, level trim, and a waterplane coefficient you supply. Large loads, flared topsides, and trim changes are not modeled.
Not a substitute: Use the designer’s hydrostatic data, the builder’s capacity plate, and a surveyor’s judgment for loading limits.
| LWL × BWL | PPI (lb/in) | 100 gal diesel (710 lb) | Six crew (1,100 lb) |
|---|---|---|---|
| 18 × 6.5 ft | 424 | 1.67 in | 2.59 in |
| 24 × 8 ft | 696 | 1.02 in | 1.58 in |
| 30 × 10 ft | 1,088 | 0.65 in | 1.01 in |
| 36 × 11.5 ft | 1,501 | 0.47 in | 0.73 in |
| 42 × 13 ft | 1,980 | 0.36 in | 0.56 in |
Pounds per inch is the quickest way to see whether a refit or a cruise load will put the boot stripe under water or drop the cockpit scuppers below the waterline. Add up the weights going aboard (a gallon of diesel is about 7.1 lb, fresh water 8.3 lb, an adult 160 to 200 lb), divide by PPI, and compare the result with the distance from the waterline to the lowest through-hull or scupper.
The figure holds for small changes near the design waterline. Many hulls flare above the waterline, so each extra inch takes a little more weight than the last; for large loads the estimate is conservative.
From the designer’s hydrostatics if available. Otherwise start in the typical range in the hint, then check it: add a known weight, measure how far the boat sinks, and solve backwards.
Most hulls get wider and longer at the waterline as they sink, so each additional inch displaces more water. That is why the page is best for small changes.
No. Displacement depends on the whole underwater volume. PPI only covers the slice near the waterline.
Yes, in different units. Ships quote tons per inch (or tonnes per centimeter); small craft are easier in pounds per inch.