Tuesday, February 17, 2015

Rainwater Catchment

--Blogpost written by Bob

Each inch of rainfall will produce 1/2-gallon of water per square foot of catchment area.  To completely fill our 104 gallons of water storage with a 1-inch rainfall, it would take 208 square feet of catchment area.  

Rainwater has a much higher purity than reverse osmosis water.
(Animated gif courtesy of Ebony134 on photobucket.)

Rainfall in the Bahamas

During the winter in the Bahamas, the rainfall averages 1.5 to 2 inches per month.  So, ideally, filling our tanks every 2 weeks from 1-inch of rain may seem to be a logical design point for the catchment system.  However, the likelihood of having a 1-inch rainfall (in an area that only gets 1.5 to 2 inches per month) when you need it is very low.

The winter represents the least monthly rainfall for the year in the Bahamas.

Catching the Rainwater

The three easiest ways we considered to catch rainwater include: (1) use a temporary vessel (like an inverted tent) to catch and direct rain water into the water tank fill connections, (2) divert water from the deck drains directly to the water tanks, or (3) divert water from the aft starboard cockpit drain to a jerry jug (while plugging the aft port deck drain).

Of the three ways, diverting water from the aft deck drain to a jerry jug in the cockpit involves much less equipment--only a three way valve and a short length of hose (if I can figure a way to make water flow slightly uphill).

In this case, the entire platform area of the boat would be the catchment area.  My rough calculations would make the catchment area of the entire boat to be about 298 square feet.  So, for the more likely small amounts of rainfall, our rainwater catchment would produce the amounts of water in accordance with the following graph.

The relationship between rainfall amounts and the number of gallons
of fresh water we can capture on the entire deck surface of S/V Rainy Days

This would significantly contribute to our fresh water supply while cruising in the Bahamas and would have a better purity than RO (reverse osmosis) water.


I decided against using a catchment system from the deck drains because of the likelihood of contamination from small amounts of diesel fuel that may find it's way into the starboard deck drain when refueling.  The starboard deck drain is within 12 inches of (and downstream from) the primary diesel fill connection.

Therefore, I am leaning toward an inverted tent type of catchment system to be directed into each water tank's fill connection as required.  More on this in a future blog post...  

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