How do you estimate a hull's wetted surface area?

The Denny–Mumford formula adds a side term, 1.7 × waterline length × draft, to a bottom term, waterline length × beam × block coefficient. A 10 m hull, 3.2 m on the waterline with 0.9 m of draft and a 0.80 block coefficient, is 15.3 + 25.6 = 40.9 m², the Hull Surface Area Estimate calculator's own example.

What is the block coefficient?

The hull's displaced volume divided by the box around it: waterline length × beam × draft. Fine, slender hulls have low values and barge-like hulls high ones. Take it from the hydrostatics of the design, or from a similar hull you have built.

How much antifouling do I need?

Multiply the area to coat by the number of coats, divide by the coverage per coat from the data sheet, and divide by one minus your application loss. 42.95 m² for two coats at 9 m²/L with 20% loss is 11.9 L per hull.

Why add an allowance above the waterline?

Boats float lower when loaded and in waves, so bottom paint is carried above the design waterline. The allowance on Setup adds that band as a share of the wetted area; set it from your yard's practice.

Does it include the keel and rudder?

No. The formula covers the hull's canoe body. Fin keels, rudders, skegs and shafts add area that needs coating; add them as a separate job or raise the allowance.

Does the workbook use macros?

No. It is a plain .xlsx with ordinary formulas, tables and native charts. The Verify sheet re-checks the formulas against fixed test jobs and shows Self-check: PASS.

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Hull Surface Area Excel Template

Hull wetted surface area = 1.7 × waterline length × draft + waterline length × beam × block coefficient (Denny–Mumford). Give every job a row and see each hull's wetted area, the area to coat with your allowance above the waterline, and the litres and cost of barrier coat and antifouling the job needs.

Free, and a plain .xlsx with no macros to enable. A CSV with live formulas and a printed how-to guide come with it.

Overview

This hull surface area template is a working Excel workbook for boatbuilders, yard estimators, paint shops and naval architects. Each job is one row with the boat model, its waterline length, beam, hull draft and block coefficient, the number of hulls, and the bottom coating system: a barrier coat and an antifouling, each with its coats. Each coating's coverage per coat and price per litre come from Setup. The workbook gives each hull's wetted surface area from the Denny–Mumford formula, the area to coat with your allowance above the waterline, and the litres and cost of each coating for the job after application loss. It ships with eight jobs at a fictional boatyard, so every tile and chart is filled in when you open it.

In the example a 10 m work barge, 3.2 m on the waterline with 0.9 m of draft and a full 0.80 block coefficient, has a 15.3 m² side term and a 25.6 m² bottom term: 40.9 m² of wetted surface. With a 5% allowance above the waterline and 20% application loss, three coats of epoxy barrier and two of hard antifouling on two barges take 53.7 L and 23.9 L, 3,281 in all. A harbour tug's 125 m² takes 54.7 L for three coats of antifouling. Across the eight jobs 21 hulls carry 598 m² of wetted surface and need 470 L of coating, 20,661 in all.

Wetted area here is a parametric estimate for quoting and ordering; keels, rudders and skegs are extra, and the hydrostatics should confirm it. The download is a plain .xlsx with no macros. A CSV with live formulas is also included for other tools; the dashboard, charts, dropdowns and self-check are in the .xlsx.

What this template includes

Setup sheet with your yard and currency, the allowance above the waterline and the application loss, and the bottom coatings with their coverage per coat and price per litre
Jobs table, one row per job: model, LWL, BWL, draft, block coefficient, hulls, barrier coat and coats, antifouling and coats, with a Check column
Per job: side and bottom terms, wetted area and area to coat per hull, barrier and antifouling litres for the job, coating cost and cost per hull
Dashboard with four tiles (hulls, wetted area, coating to buy, coating cost), wetted area per hull split into side and bottom terms, coating litres by job, and the job table
Method sheet with every formula, and a Verify sheet that re-checks the formulas against an independent reference and the Hull Surface Area Estimate calculator (Self-check: PASS)
CSV with live formulas for the Jobs table; the dashboard, charts, dropdowns and self-check are in the .xlsx

Suggested use case

Use it to quote bottom painting before a hull is modelled, to order barrier coat and antifouling for a batch of new builds, or to compare the coating cost of hulls of different form.

How to use it

On Setup, type your yard and currency, the allowance above the waterline and the application loss, and list your coatings with their coverage per coat and price per litre.
On Jobs, give each job a row: model, LWL, BWL, draft, block coefficient, hulls, and the barrier coat and antifouling with their coats.
Fix anything the Check column names.
Open the Dashboard: wetted area by hull, and the coating and cost each job needs.

Frequently Asked Questions

How do you estimate a hull's wetted surface area?: The Denny–Mumford formula adds a side term, 1.7 × waterline length × draft, to a bottom term, waterline length × beam × block coefficient. A 10 m hull, 3.2 m on the waterline with 0.9 m of draft and a 0.80 block coefficient, is 15.3 + 25.6 = 40.9 m², the Hull Surface Area Estimate calculator's own example.
What is the block coefficient?: The hull's displaced volume divided by the box around it: waterline length × beam × draft. Fine, slender hulls have low values and barge-like hulls high ones. Take it from the hydrostatics of the design, or from a similar hull you have built.
How much antifouling do I need?: Multiply the area to coat by the number of coats, divide by the coverage per coat from the data sheet, and divide by one minus your application loss. 42.95 m² for two coats at 9 m²/L with 20% loss is 11.9 L per hull.
Why add an allowance above the waterline?: Boats float lower when loaded and in waves, so bottom paint is carried above the design waterline. The allowance on Setup adds that band as a share of the wetted area; set it from your yard's practice.
Does it include the keel and rudder?: No. The formula covers the hull's canoe body. Fin keels, rudders, skegs and shafts add area that needs coating; add them as a separate job or raise the allowance.
Does the workbook use macros?: No. It is a plain .xlsx with ordinary formulas, tables and native charts. The Verify sheet re-checks the formulas against fixed test jobs and shows Self-check: PASS.