Where Sits Her Waterline?

When painting the bottom outside portion of the hull, the part that lives submerged underwater gets covered with a special paint designed to inhibit marine growth.  Copper is toxic to marine life, especially as it oxidizes (rusts).  The sailors of old covered the ship’s bottom with copper sheeting and copper nails.

HMS Bounty arrives in Tahiti 1788 (Painting by Jon Clayton)
HMS Bounty arrives in Tahiti 1788 (Painting by Jon Clayton)

In Tahiti, HMS Bounty had a problem with her crew and the islanders pulling the nails from the ship’s bottom for their personal purposes of trade. Not good for a ship needing to sail back to the other side of the world, or even just to Pitcairn Island.


The green rust that forms on the surface of copper is called cuprous oxide.  In solution, cuprous oxide is used to clean algae from pools and ponds of water or the insides of water beds.  If you staple bare copper wire around a flower bed, snails won’t cross it. Today, boat owners paint copper on the bottom.  The price of the paint follows the price of copper.  Currently, copper is expensive.  A gallon of high copper concentrated paint sells for about $200.  Alternatives exists, some more environmentally friendly than others, and each U.S. state has its own environmental laws identifying which active ingredients they permit for use in marine bottom paint.

Painting the bottom of a boat, the boat owner must decide how far from the bottom of the boat’s keel to paint the bottom paint; where to draw the boot stripe, the strip of paint that separates the upper hull from the lower hull, usually a different color from the two.  For example, Kandu’s hull is white, her bottom is regatta red, and her boot stripe is forest green, matching her trim, the stripe along the upper hull that stretches just below her deck.  So where does the boat owner draw the line?  If it’s too high, exposed expensive bottom paint does little to protect the boat; if too low, marine life (algae, worms, barnacles, etc.) will form and have to be scrubbed off regularly, wearing down the shiny white expensive gelcoated surface (everything on a boat is expensive).  In determining the waterline, boat owners must take into account how much equipment and stores they plan to stow aboard their boat.  The more stuff you put in a boat, the lower the boat sits in the water, the higher the waterline must be.  We bought Kandu from a couple that had sailed to Australia and back.  When we purchased her, their waterline lay about 8 inches above sea surface, indicating that when they had Kandu fully loaded for their trip, she sat 8 inches lower.  So we went with the previous owners’ line, maintaining the bottom paint at their previous level.  For the four years we’ve owned Kandu, 6″-8″ of bottom paint has been exposed, but that’s now changing.

The darker part of the red bottom paint is the portion previously exposed.
The darker part of the red bottom paint is the portion previously exposed.
Close up, you can better see the cuprous oxide within the air exposed band of bottom paint.
Close up, you can better see the cuprous oxide within the air-exposed portion of bottom paint.

Over the past three weeks, we’ve been loading Kandu.  To state that not everything is loaded is an understatement.  Additionally, because of the heavier equipment loaded into our shower (our “rec room” storage), we’re currently listing (as in leaning) to starboard.  On the starboard side, we’ve about an inch of bottom paint exposed.  To balance her out, we have to find objects to stow on the port side, heavy ones.

Kandu's waterline today.
Kandu’s starboard waterline today.

Will we bury Kandu’s waterline or will we have her at the perfect level?  It’s too soon to tell.  By the end of next week, we’ll know.  Keep your fingers crossed.


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