Water Conservation on Passage

Master Mariner Directory | Offshore Passage Planning · 2024

The watermaker is the obvious solution to offshore water management, but it is not the only solution, and for boats that have not yet fitted one — or whose watermaker has failed mid-passage — a clear understanding of water conservation turns a potential crisis into a manageable challenge. Many of the world's great passages were completed on tank water alone, and with good planning they still can be.

Conservation also matters on boats that do have a watermaker. A unit producing 15 litres per hour running two hours a day produces 30 litres — adequate, but not luxurious. The habits that stretch water on a tank-only passage are the same habits that turn a modest watermaker into more than enough.

Tank Capacity Planning

The starting point for any offshore water plan is honest calculation of capacity versus demand. The minimum daily water allowance for a person on passage varies significantly by climate: in the tropics, where sweating is constant and cooking in the heat requires hydration, 30 litres per person per day is a realistic minimum. In temperate conditions — the North Atlantic, the high-latitude Southern Ocean approaches — 15 litres per person per day is achievable without significant discomfort.

A two-person crew on a trade wind crossing from the Canaries to Barbados — roughly 2,800 miles, 18 to 22 days in typical conditions — needs a minimum of 1,080 litres (2 people × 30 L/day × 18 days) plus a 20 percent safety margin, arriving at approximately 1,300 litres of planning capacity. Most production cruising boats in the 38 to 45 foot range carry 200 to 400 litres in built-in tanks. The arithmetic makes clear that a tank-only Atlantic crossing either requires strict conservation discipline, supplemental storage, or rain catchment — and preferably all three.

For passage planning purposes, calculate required capacity at 30 L/person/day for tropical passages and 18 L/person/day for temperate passages, multiply by the passage duration in days plus 25 percent, and compare that number against your total carrying capacity. The gap tells you what your conservation and supplemental collection plan needs to close.

Rain Catchment: Boom Tent and Sail as Collector

Rain catchment is the oldest supplemental water source available to offshore sailors, and on tropical passages it is often the primary source of non-tank water for boats without watermakers. The yield from a single squall can be dramatic: a 10-minute rain event over a 40-foot sailboat with a boom tent and main boom catchment system can collect 50 to 100 litres, depending on rainfall intensity. In the squall-rich tropics — the doldrums, the Caribbean rainy season, the South Pacific convergence zone — this is not a rare occurrence.

A boom tent rigged fore-and-aft creates a gutter along the boom that channels rain into a collection point. The simplest systems are a length of canvas or Sunbrella with a reinforced gutter along the centreline that drains into a bucket or hose leading to the tank fill. More elaborate systems include a sump at the lowest point of the boom, a first-flush diverter that discards the first few litres of runoff (which contain salt and bird deposits from the sails and rigging), and a direct fill hose routed below.

The headsail can also serve as a collector. On a furling headsail, the sail foot creates a natural catchment when the sheet is eased to allow the water to pool at the tack fitting; a hose at the tack point captures the flow. The main sail's belly, if the boat is hove-to or motoring in calm conditions during rain, will drain water toward the boom regardless of boom tent rigging. On boats with a large genoa, the collection area available from the headsail catchment alone can exceed the boom tent in a heavy rain event.

First-flush discipline is important. The first five to ten litres of rain that hits a sail or boom carries whatever has accumulated on the surface since it was last wetted — salt spray, bird fouling, sail treatment products. Divert this initial flow overboard or into a separate bucket, then open the tank fill connection once the surface is washed clean. Collected rain water from a clean sail is safe to drink without further treatment in most offshore conditions; collected rain water in industrial port areas or from heavily bird-fouled surfaces should be treated before consumption.

The Deck Water Hierarchy

Offshore water management operates on a strict hierarchy of use. Drinking and cooking water is drawn from the tank; salt water is used for everything it can reasonably replace. The hierarchy is not complicated, but applying it consistently throughout a passage makes the difference between arriving in port with half your water remaining and making anxious calculations in the last week.

Cooking hierarchy: boil pasta and vegetables in seawater (properly used, seawater adds salt appropriately and requires less fresh water rinsing than land-based instincts suggest); rinse with the minimum fresh water necessary; reserve fresh water for drinking, coffee, rehydrating dried foods, and the final rinse of items that will have seawater residue affecting their flavour.

Washing hierarchy: use seawater with a deck bucket and seawater-compatible soap for the initial wash of everything, then a minimal fresh water rinse if required. In the tropics, many passage sailors make a final rinse with fresh water optional rather than standard — the seawater rinse is acceptable if the skin is then dried quickly.

The Saltwater Wash System

A dedicated saltwater wash system — a foot pump or electric pump plumbed from a seacock to the galley sink — is one of the most effective water conservation investments available. It costs less than $100 to install and reduces freshwater demand by approximately 40 percent on a passage where cooking and dish washing are the primary fresh water uses.

The system is simple: a manual foot pump draws seawater from a dedicated below-waterline intake into the galley sink, separate from the fresh water pump. All dish washing, pot scrubbing, and initial rinsing uses seawater; only the final rinse, if desired, uses fresh water from the tank. Seawater soap — specifically formulated to lather in salt water — is available in most chandleries and makes the seawater wash genuinely effective rather than merely redistributive.

The same saltwater system serves the heads — a seawater flushing toilet for the marine toilet requires no fresh water at all. This is the standard installation on most offshore boats, and it represents significant fresh water saving on a long passage where a single person might use 3 to 5 litres per day in toilet flushing alone.

Jerry Can Storage on Deck

Supplemental water carried in deck jerry cans extends range beyond what built-in tanks allow. A standard 20-litre jerry can per person per week of passage, carried in addition to built-in tank capacity, provides meaningful buffer for a long passage. The constraints are deck space, weight distribution, and container quality.

Container material matters. Ultraviolet radiation degrades many plastics, and the combination of UV exposure, seawater wash, and mechanical stress of offshore conditions will crack compromised plastic within a season. Use only food-grade HDPE (high-density polyethylene) containers rated for potable water storage and UV-resistant, or stainless steel jerry cans which are heavier but indefinitely durable. Avoid translucent containers — light exposure promotes algae growth in stored water. Opaque containers, filled and sealed in a clean environment, will hold potable water for several weeks without treatment.

Secure jerry cans as you would any deck cargo for offshore — doubled lashings, chafe protection at contact points, and accessible for regular checking. A 20-litre jerry can of water weighs 20 kilograms; a row of six on the side deck represents 120 kilograms that will shift in a knockdown if the lashings fail.

Treating Suspect Water

Water taken from marina docks, local filling stations, or onshore sources in developing countries requires treatment before it can be considered safe for drinking. The risks are real and the diseases consequential: giardia and cryptosporidium — protozoan parasites — are present in municipal water systems across much of the developing world and cause debilitating gastrointestinal illness that can persist for weeks.

Aquatabs (sodium dichloroisocyanurate tablets) are the simplest chemical treatment. One tablet treats 20 litres of clear water; cloudy water requires pre-filtering through a clean cloth and double dosing. The contact time is 30 minutes in warm water; at low temperatures allow 60 minutes. Aquatabs are effective against bacteria and most viruses but have limited efficacy against cryptosporidium — a protozoan with a chlorine-resistant cyst stage.

Iodine treatment (iodine tablets or 2 percent tincture of iodine) works on a similar principle and is similarly effective against bacteria and viruses but not cryptosporidium. Iodine has a distinctive taste that is acceptable in coffee but unpleasant in plain water; activated carbon post-filtering removes the taste at the cost of some of the disinfection effect.

The UV SteriPen — an ultraviolet light wand immersed in a water container — is effective against bacteria, viruses, and protozoa including cryptosporidium. It operates on batteries, treats one litre in 90 seconds, and leaves no taste. The limitation is turbidity: UV light is absorbed by particulate matter, so the water must be clear before treatment. Pre-filter through a cloth or sediment filter first in any water that is not visually clear. Carry spare batteries or a solar-charging rechargeable version for offshore use.

Giardia and Cryptosporidium Risk in Marina Water

Marina tap water in developed countries is generally safe to the standard of the national water supply — which in Western Europe and North America is high. Marina water in developing countries is a different matter. The municipal water supply may be technically treated but at insufficient chlorine levels to reach the tap reliably; the pipe infrastructure may introduce contamination post-treatment; the marina's own plumbing may have been contaminated by backflow. The cases of giardia and cryptosporidium among passage-making sailors are concentrated in marinas in Central America, West Africa, the Pacific Islands, and parts of Southeast Asia.

The conservative approach: treat all marina water outside Western Europe, North America, Australia, New Zealand, and Japan as suspect, and run it through a treatment process — UV SteriPen is the most reliable against all pathogens including cryptosporidium — before adding it to the drinking tank. Some sailors fill a dedicated jerry can for dock water and treat the can's contents before decanting to the main tank, keeping suspect water physically separate from the established tank water until it has been treated.

Dew Collection as Emergency Supplement

Dew collection is not a primary water strategy — it is a last-resort supplement when other sources are exhausted. In conditions of high humidity and clear nights, dew condenses on any cool horizontal surface; the amount collectible from a 40-foot boat's deck, cabin top, and sails is typically 1 to 5 litres per night in the best conditions.

The practical method: wipe horizontal surfaces with a clean cloth or absorbent towel at dawn before the dew evaporates, wring into a container, and filter through a fine cloth before drinking. The water is generally clean — atmospheric condensation does not carry salt or significant contamination unless the air is heavily polluted. In genuinely desperate circumstances, a sail stretched horizontally can be rigged as a dew collector; the pooled dew drains to the lowest point where it can be captured.

At 1 to 5 litres per night, dew collection extends survival but does not substitute for proper tank capacity planning. It is a tool of last resort, and its relevance lies primarily in abandoning-ship scenarios or extended calms where the passage is lasting much longer than planned.

Water Budget Per Person Per Passage Leg

A practical water budget for passage planning allocates allowances by category and checks the total against available capacity including expected rain catchment. The following is a realistic budget for one person on a tropical passage day:

Drinking and cooking: 8–10 litres. Personal hygiene (saltwater primary, fresh water rinse): 3–5 litres. Cooking water (seawater for boiling, minimal fresh water finish): 2–3 litres. Incidental (toothbrushing, medical, drinking cup rinsing): 1–2 litres. Total: 14–20 litres per person per day with active saltwater substitution.

Without the saltwater wash system and without discipline on cooking water, the same budget inflates to 25 to 35 litres per person per day — the difference between a 18-day crossing on 400 litres of tank water being comfortable or impossible for a two-person crew. The saltwater system and disciplined cooking are the highest-return interventions; everything else is margin.

Plan the budget before departure, communicate it to all crew, track consumption daily against the plan, and adjust early if the actual draw is higher than projected. Water rationing imposed in the final days of a passage because early days were unmonitored is avoidable and demoralising. A simple daily log — "tank level at 0800 each morning" — provides all the information needed to manage the resource through the whole passage.

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Frequently Asked Questions

What is a realistic daily water budget per person for an offshore passage without a watermaker? A practical minimum in tropical conditions is 30 liters per person per day for drinking, cooking, and basic hygiene without any saltwater substitution. With a disciplined saltwater wash system for dishes and deck showers, this can be reduced to 14–20 liters per person per day — enough to make a well-tanked cruising boat genuinely independent for multi-week passages.

How much water can rain catchment realistically provide on a passage boat? A boom tent rigged to funnel rainwater into the main tank can collect 50–100 liters from a single 10-minute squall on a 40-foot boat, making a well-placed rain catchment system one of the most reliable water sources in the trade wind belt. The sail itself can be adapted for catchment, but the boom tent method is cleaner and easier to rig without contaminating the sail fabric with whatever residue accumulates on a passage.

What types of jerry cans are safe for long-term fresh water storage on a sailboat? Only HDPE (high-density polyethylene) jerry cans should be used for water storage — they are food-grade, do not impart taste or chemical contamination over time, and resist UV degradation better than lower-density plastics. Cans designed for fuel must never be repurposed for water, and any can that previously held fuel cannot be adequately cleaned to food-safe standards.

How does a saltwater wash system reduce freshwater demand and what tasks can it realistically cover? A saltwater tap at the galley sink supplied by a small electric pump — or a bucket system on the more modest end — allows dishes, pots, and most food prep surfaces to be washed in saltwater with a final freshwater rinse, reducing freshwater demand for galley tasks by approximately 40 percent. Deck showers using a saltwater first-wash followed by a minimal freshwater rinse provide crew hygiene in a fraction of the water a conventional shower requires.

What are the risks of taking on water in developing-country marinas and how should sailors treat it? Marina water supplies in many developing-country ports carry giardia, cryptosporidium, and bacterial contamination at levels that make untreated use dangerous — the risk varies widely by location but cannot be assessed visually or by taste. Treatment options in order of reliability are boiling (certain but fuel-intensive), UV SteriPen treatment (effective against protozoa and bacteria), and chemical treatment with Aquatabs or iodine (effective against bacteria but less reliable against cryptosporidium cysts without adequate contact time and clear water).

What is the air draft and beam limit for sailboats transiting the Erie Canal and what does it require? The Erie Canal imposes a 15-foot 6-inch air draft limit at bridge crossings along the 363-mile route from Albany to Buffalo, a restriction that forces virtually all sailing vessels with a fixed mast to unstep the rig before entry. The controlling depth is 12 feet and the beam limit is 43.5 feet, which accommodates most monohull cruising boats but requires advance measurement and mast-unstepping arrangements at the eastern or western terminus.