No watermaker
Four crew using three units each plus four units of shared daily use consume sixteen units per day. Seven days require 112 units before unusable water and emergency reserve are restored.
Boat Freshwater Provisioning & Tank Duration Calculator
Created by: Emma Collins
Last updated:
Estimate daily freshwater demand, watermaker contribution, departure requirement, duration, reserve, and shortage.
Boat Freshwater Provisioning & Tank Duration Calculator
BoatingEstimate daily freshwater demand, watermaker contribution, departure requirement, duration, reserve, and shortage.
What is a Boat Freshwater Provisioning & Tank Duration Calculator?
A Boat Freshwater Provisioning & Tank Duration Calculator turns crew count, passage duration, per-person use, cooking and washing demand, tank condition, reserve, and optional watermaker production into a transparent water budget.
Routine freshwater planning and emergency drinking-water planning are not the same. Water used for showers, dishwashing, deck rinsing, and flushing may be reduced, while safe drinking water must remain potable, accessible, protected from contamination, and available if pumps, power, plumbing, or a watermaker fail.
Tank nameplate capacity can exceed usable onboard water because of pickup position, heel, trim, sediment, isolation, leaks, gauge uncertainty, or deliberate reserve. The calculator therefore separates tank capacity, initial fill, unusable water, and emergency reserve.
Watermaker output is entered as a measured or documented net contribution for the intended operating hours. Production can fall with salinity, temperature, filters, membrane condition, voltage, pressure, maintenance, and feed-water quality, so it should not be treated as guaranteed.
How the Boat Freshwater Provisioning & Tank Duration Calculator Works
Daily demand equals crew multiplied by entered per-person consumption plus cooking and washing demand. Daily production equals watermaker output per hour multiplied by planned operating hours.
Net daily draw is demand minus production. Available routine water is initial water minus unusable water and emergency reserve. Dividing routine water by a positive net draw estimates duration.
Departure requirement multiplies the daily balance across the passage and then restores unusable water and reserve. If production equals or exceeds demand, arithmetic duration becomes unlimited, but storage, reliability, maintenance, energy, spares, and potable-water quality still limit the real plan.
net daily draw = crew demand + shared demand − watermaker production
Input Guide
- Enter crew and passage days, including realistic delay allowance.
- Separate drinking, cooking, and washing assumptions instead of copying a household average.
- Use the water actually aboard, not tank label capacity, for initial fill.
- Treat watermaker hours and output as conservative measured scenarios.
Example Scenarios
Partial production
A watermaker producing eight units per day reduces tank draw but does not eliminate the need for reserve, stored potable water, power, filters, maintenance, and a failure plan.
Delay scenario
Adding two passage days can expose a shortage that the original ETA concealed. Provisioning should be tested against delay, higher consumption, lower production, and tank-loss cases.
How to Read the Results
- Daily demand and production are shown separately.
- Duration uses routine usable water and positive net draw.
- Departure requirement includes the entered reserve and unusable quantity.
- Shortfall identifies how much additional entered-unit water the scenario requires.
Common Applications
- Passage provisioning
- Watermaker contribution checks
- Crew-size comparisons
- Delay scenarios
- Reserve and tank-loss planning
Planning and Safety Tips
- Carry independently accessible potable water.
- Measure real consumption during shorter trips.
- Verify tank cleanliness and water quality.
- Plan for watermaker, pump, power, hose, filter, and gauge failure.
Limitations and Assumptions
- No medical hydration target is prescribed.
- No potable-water quality or treatment advice is provided.
- Watermaker production and tank accessibility are not guaranteed.
Frequently Asked Questions
How much should each person drink?
Use appropriate health, climate, activity, and medical guidance. The calculator accepts the value but does not prescribe it.
Can washing water count as emergency reserve?
Emergency drinking-water planning should be separated and protected. Do not assume every tank or plumbing supply remains potable and accessible after a failure.
What if watermaker production exceeds demand?
The arithmetic shows no tank depletion, but storage, quality, energy, maintenance, feed water, and equipment reliability still constrain the plan.
Why subtract unusable water?
Tank geometry, heel, pickup location, contamination, isolation, and operating policy can make part of the nominal quantity unavailable.
Does the calculator include passage delays?
Only if they are included in entered passage days. Run explicit delayed-arrival scenarios.
Should tank gauges be trusted?
Verify gauges against known fills and consumption. A gauge reading is not proof of potable or accessible volume.
Sources and References
- Use the vessel capacity plate, owner’s manual, equipment manuals, tank documentation, and installation records applicable to the specific boat.
- U.S. Coast Guard Boating Safety Division resources, accessed July 16, 2026; https://www.uscgboating.org/.
- American Boat & Yacht Council standards information, accessed July 16, 2026; https://abycinc.org/.
- Manufacturer ratings are test-condition values; measured onboard performance and applicable standards take priority.