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Tanking rainwater, harvesting what Mother Nature provides

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What is the issue?
Freshwater is a necessity for life aboard a vessel. Humans need approximately a ½ litre of water per day to survive and two litres to avoid thirst. Completely clean fresh water can be hard to come by in the tropics. This is particularly the case if you do not have an osmotic watermaker aboard, or choose to do without the associated cost and regular maintenance, and you are cruising in remote areas.

Why address this?
Without a means of provisioning fresh water aboard you will have to take the yacht alongside from time to time where good water may be acquired. This can be a challenge of itself as in some developing countries, and often many developed, municipal water supplies may still be unsafe to drink.

When good water is available it may be in locations where it is not possible to take the vessel in, which necessitates ferrying jerry cans back and forth via the tender. This is not a trivial task as you must row, or have the expense of motoring the jerry cans ashore and then carrying them to the water source. Then after refilling, return and decant these into the yacht's water tank. A large quantity of water may take an entire morning of hard slog in, as often as not, very high temperatures.

Other typical areas where it may be acquired on remote islands is from a large island cistern, typically beneath the roof of a church that offers the largest catchment roof for an island. But again this is often sitting open for long periods and is of doubtful purity. In very remote areas one may be forced to go to streams which is far from ideal. Animals are prevalent on most islands and their droppings wash into the streams and rivers causing unhealthy levels of E. coli bacteria.

It should also be noted that water can be scarce in many parts of the world, particularly during the dry season, so it seems a little discourteous to expect the local people to share their limited supplies with the visiting boatmen. Likewise, you cannot typically acquire drinking water in any quantity in deep ocean without the use of a watermaker.

Finding reliable means of easily provisioning fresh water is more than highly beneficial. Especially so for cruising sailors who use the natural elements to carry them around the world, they should look to make themselves as self-sufficient as possible with this simple key element of life.

How to address this?
Harvest as much rainwater as you can as there are a variety of easy means to do so. Rainwater harvesting is a low cost and low maintenance alternative to a watermaker Experience and even for those with a watermaker, a great way to supplement your supply. Even watermakers come up short in harbours, as you shouldn’t run harbour water through them, and they can break down so a backup system is important. Even if you don't want or need to use the rainwater harvested for drinking, you can certainly use it for washing, dishes and clothes and crew showers.

There is usually plenty of water around the trade-wind zones, high islands or atolls in the tropics, and the trick is to build catchment systems to tank as much rainwater as possible when the opportunity presents itself. There are many ways of harvesting rainwater, none are very complex or expensive.

There are ample opportunities to tank rainwater
Photo: CC0


The simplest, almost zero effort, approach is to trap the rainwater that runs off the deck and let it flow into the water tanks. This system is ready to go on vessels that have a solid toe rail. These are the small bulwarks where the deck and hull come together and run around the outside of the deck. They channel the water aft and overboard or sometimes out through a deck drain generally located a little aft of amidships at the lowest point under sail.

Our toe rail stood more than an inch over the deck and ran unbroken the whole way to the stern so it was a ready-made catchment system. All that was required was to throw a wet towel on the deck immediately aft of the water filler cap and remove the plug. The wet towel then acted as a dam in the scupper and the following rain would rise over the rim of the water tank filler inlet and flow directly into the fresh water tank.

You must, of course, take into consideration any local air pollution that may exist in the cruising area and allow a runoff to take out what is in the atmosphere, the deck and boat as a whole, before putting it into your tank. When the rain comes, depending upon how heavy the shower is, allow for what you think is an appropriate wash off period. Generally in the tropical or semi-tropical areas when it rains it rains hard so you can let the first half hour or so be the wash off before you start collecting. If you are in any doubt just scoop up some of the water and taste it for salt.

We found this such a useful approach that we set in place a permanent dam by laying down several layers of sealant behind the deck-filler. This, however, I would not recommend as it trapped seawater during normal operations which then evaporated and just caused a salt built up for when we needed to use it. The easy approach of simply laying down a towel and creating the occasional dam was the simplest and best approach.

If you want a little more control it is possible to insert a 'Y' handle ball valve below decks on the filler pipe as a first flush diverter to control where the inflow goes. This allows the run-off to be filtered and taken into another tank if the intention is to keep it separate or if it looks like it is not going to rain for long and the first flush, that would normally be discarded, could be saved for washing.

The joy of the deck or scuppers system is that it is always ready to go at anchor without setting anything up or taking it down. But it could not be used whilst making a passage and if your boat has perforated toe rails it is not possible to use this method.

Unfortunately downpours can be short or come with a squall so be prepared
Photo: CC0


Specially designed rain catchers, such as the RainSnare, can be purchased at a very low cost. They are typically unfolded, spread out and then tied up in place. When the rain falls upon them they drop into a gutter that is directed into a bucket or a jerry can. They go up when or just before the rains commence, come down once dry and take about two minutes to set up.

It is, however, perhaps just as easy to tailor one for your deck utilising an off the shelf low-cost poly-tarp available from any garden centre or DIY outlet. An ideal location to place a homemade water catcher is out of the way at the bow of the boat. After anchoring all you have to do is fit it as a matter of course and it also serves to shade the forward deck hatch when the sun is shining.

Cut the poly-tarp down to size and add a half-dozen tie-down grommets, battens and lift rings at each end, a plastic sink drain fitting through a reinforced hole with a pipe leading off it will bring the water into a jerry can. If you manage it well you may be able to switch out the filled jerry cans through the forehatch, or even lead the hose below decks, so there may be no need to go out into the rain. The big issue with these catchers is that tropical downpours or heavy rain and squalls often go hand in hand so coming up with a means to stop them flapping about and emptying is key. Laying a heavy stainless steel shackle near the drain should do the job. With this type of setup, you should be able to collect about 40 litres in a 'heavy' (6mm) rain squall.


With a little more effort you can convert biminis, sun covers, solar panels and awnings can be modified with through-hull fittings, gutters and hose pipes to catch water and channel it into tanks or jerry cans.

Practically all cruising yachts have a sun awning that can be very easily adapted to catch water if factored in from the outset. Each awning is designed to fit a specific vessel, but most will turn into an excellent water catcher by having a simple method of unhooking the awning’s apex and dropping it down plus a sewn-in plastic funnel and short hose lead at the lowest point when inverted. This then creates an instant water catchment area to collect water with a hose fitting ready to direct the water into the main tank or jerry cans.

The problem with using big awnings as water catchers are the squalls. In these conditions, large awnings flap madly and may need to be stowed. A well-fixed awning largely gets around this problem. So use a durable awning material along with multiple fixing points, plus copious short ties fixing it down along the lifelines and coachroof handrails.

A wide variety of ingenious means to harvesting rainwater will present themselves if you sit down and cast your eye about your vessel. Many more ideas will emerge if you make enquiries and of course look at what other people are doing around you External link.


You can also use a dedicated rain catcher to harvest water whilst underway. The ideal location is in the aft end of the cockpit and well out of the way of any light spray. Likewise, other permanently-mounted outside fittings such as biminis, shades, solar panels that have fitted rain catchers and are clear of salt, debris, bird droppings, and are clear of spray can still be used.

Most people use the mainsail to catch rain showers whilst making a passage. An adapted bucket placed at the gooseneck, as shown, can be used to funnel the rainwater directly into the main tank.

Collecting rainwater off the mainsail
Drawing: Tony Gibson

All that is required to achieve this is a plastic bucket modified into a funnel. Simply drill a hole into the bottom of a cheap plastic bucket and fit a nozzle for a hose pipe that can be fed to the main tank.
As the bucket is acting as a funnel it does not support much weight so a loop of light line is enough to hold it in place depending on the weather conditions. You can increase the efficiency of capture by modifying the bucket/funnel by applying a little heat so it fits snugly in under the gooseneck.
Please note

Don't be tempted to double it up as a funnel for diesel oil as this will taint the water.

Again allow an appropriate period to wash off all the salt and debris that is on the mainsail. Once the wash-off is completed, ease the halyards a tad and take up the topping lift until the sail complains. The boat will not sail as efficiently as the mainsail is starting to become bag-like but it will direct all the run-off to the gooseneck. This ocean fresh non-fluoridated or chemically treated water collected off the sails is always good and you never have concerns about it. This appears to be a far from elegant collection approach but it works surprisingly well when sailing in an area where showers are frequent.

Using an inverted sail cover to channel the raiwater
Drawing: Tony Gibson

An improved approach is to invert the vessel's sail cover, if it is made of impermeable material, and use this as canvas catchment drain to channel the water into the bucket. A rain catcher like this one is a dependable means of capturing freshwater. Properly tensioned so that the wind won’t throw it about, you can fill your tanks in a couple of hours in a tropical downpour. 150 litres of water in a single downpour could easily be captured.


Filling your tanks this way is daunting at first. It took me some time to get to grips with this as I was concerned that I would get my wash-off timing wrong and take some salt into the main tanks. What I would suggest is to experiment for a while with the process. You can use your test runs for bathing water or washing clothes – remember it is not the washing but the rinsing that takes all the water in this case. Trap the water in buckets and jerry cans for these purposes. Then when you are comfortable with the approach and quality of the drinking water, direct it into your main tanks.

If you have not got the capacity you may be able to do the laundry by plugging up the cockpit drains. Then bucket after bucket can be dipped into this for rinsing out and poured over the side. Another approach is to use an upright-stowed dinghy if you thoroughly clean it out.

Harvesting rainwater is so easily achieved and so useful, if not essential, that it should be common practice. Even if it simply reduces the need to call into port just to recharge your water supplies.

With thanks to:
Michael Harpur, Yacht Obsession.

Rain Snare

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