May 10, 2009

Barnacles on my mind

WHEN YOUR AUXILIARY ENGINE seems to be working reasonably well, but the boat just isn’t going as fast as usual, start thinking about barnacles. Just a few barnacles can drastically reduce the efficiency of the average sailboat propeller.

It’s one of the facts of life that a big, slow-turning prop is (with a few exceptions) more efficient that a small, fast-turning egg-beater. But a big prop on a sailboat obviously causes more drag than a small one, so the size of the prop is a compromise, as usual.

In my experience, most props on cruising boats are fixed two- or three-bladers and, because they are comparatively small, they need to be very efficient to produce all the thrust the engine is capable of. That means their compound curves should be as smooth as possible to develop the necessary hydrodynamic “lift.”

Unfortunately, barnacles seem to have made it their mission in life to attach themselves with very strong glue to any nice, clean, smooth, hydrodynamically perfect propeller that comes their way, thereby destroying its efficiency. Given a chance, barnacles will colonize a propeller with such zeal that it resembles nothing more than a jagged lump of coral.

And such is their dedication to their mission in life that they will remain attached no matter how fast the propeller spins. It must take tremendous willpower not to get dizzy and fall off when you are whirred around at 1,500 revolutions a minute – that’s 25 times in every second – but somehow barnacles manage it. Perhaps they turn their little heads in the way prima ballerinas do when they spin on tippy-toe, but as far as I know nobody has ever verified this scientifically.

In the years I have been writing about sailing, I have come across many suggestions for preventing barnacles from attaching themselves to propellers. One friend of mine swore by axle grease that he stole from the marine railway when his boat was hauled out for antifouling. He would smear thick gobs of it on his prop just before launch time.

Other people advise you to use carnauba wax, or zinc paint. I myself have tried two coats of copper antifouling paint. I’m told the backroom boys are working on a way to apply a Teflon coating to a bronze propeller, so barnacles simply won’t be able to stick to it. But so far none of these remedies has worked satisfactorily in all waters. The very action of a propeller working in water quickly abrades whatever coating you apply.

There is one trick that really does work, and that is to tie a black plastic bag around the prop each time you reach your home mooring or slip. Some fanatical racers do that, but I’m sure you can see the problems, not the least of which is to remember to remove the bag before you set off again.

There is a theory that barnacles will not touch a prop that isn’t protected by a sacrificial zinc. Apparently the tiny electric currents generated in the bronze of an unprotected propeller are sufficient to deter them, and convince them to move to the boat next door whose propeller is nice and docile, thanks to its sacrificial zinc.

No doubt you can spot the problem, here, though. The electric currents that keep the barnacles away are also slowly eating the propeller away. Maybe if you can afford a new propeller every couple of years you can live a lovely life free of worry about barnacles. If not, you, like the rest of us, will just have to put up with an increasing number of barnacle squatters and a corresponding
decrease in motoring speed as time goes by.

You can, of course, dive and scrape off the barnacles from time to time if you sail in warm waters. But if you live in the frigid zone of Puget Sound like me, then grinding your teeth and swearing in a sailorly fashion seems to be all there is to do about it until the next haulout. I do find, though, that the occasional glass of port helps.

Today’s Thought
Though you drive away nature with a pitchfork, she always returns.
—Horace, Epistles, 1, x

“Why has your dog got such a flat nose?”
“He keeps chasing parked cars.”

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