TEREDO WORMS
Bahía de Caráquez, Ecuador, Sunday, October 28, 2007

Whilst Vilisar was on the makeshift tidal grid here in July and August we were surprised to see that the hull planks had been worn bare in places by the strong and silt-laden Rio Chone currents. We had put on lots of paint last summer, but it was largely gone now. What was left was thin and, as mentioned, gone altogether in places –amidships along the waterline where the current had strongest impact, for example. There were other vulnerable spots too: we had sustained some wood abrasion at the bottom of the out-hung rudder where we had backed over some rocks trying to get off the makeshift grid in July; one or two of the wooden plugs covering hull-plank fasteners (Vilisar was built using drifted-over galvanised nails) had fallen out or degenerated and a vertical wood seam in our heavy rudder was not totally covered by bottom paint any more either. Finally, a gap was showing between the wormshoe and the keel itself. (The wormshoe is a semi/sacrificial length of wood below the keel and separated from the keel by tar-impregnated felt. http://www.seatalk.info/cgi-bin/nautical-marine-sailing-dictionary/db.cgi?db=db&view_records=1&uid=default&Term=shoe.) We used Z-Spar Splash Zone (underwater epoxy putty) to fill every gap and abrasion we could find before coating with bottom paint.

A cruising friend who reads this blog regularly wrote to say that, if we had any bare wood at all we should definitely expect now to have been invaded by “shipworms”. The term is the colloquial expression for all types of marine boring “molluscs”, the most common of which is teredo navalis. My friend described working on the hull of a careened wooden sailing vessel in the South Pacific years ago. After wooding the hull (i.e., removing all the paint down to the wood planking), they used a blow torch to scorch the plank. In his words, the worms came pouring out of the planks like spaghetti. A pretty graphic description that kept both Kathleen and I mentally occupied in our nocturnal hours.

Researching teredo worms

I decided to do some research on the web. The teredo is plenty dangerous all right! The damage they can do is enormous. Worms once so seriously compromised the strength of 18th Century wooden dykes in The Netherlands. As a result, during a severe storm in 1733 the dykes collapsed and huge tracts of polder were inundated with salt water. All over the world, wooden harbour installations have been devastated and so have irrigation systems, dams, barriers, waterside docks, houses and facilities. Wooden ships have always been seriously vulnerable to teredos. On his third voyage to the Caribbean, for example, Columbus had to abandon two of his ships on the beach because their hulls were so riddled with teredo as to be totally unseaworthy.

Until recently, northern harbours with their cold waters were felt to be unsuitable for shipworms. It is true that cold water slows their growth. But apparently the grat northern harbours were actually being protected by pollution. Now that the EPA has been around for a generation, the waters are cleaner and the teredos are creating havoc. In one Massachusetts harbour, for example, new and treated wooden pilings were totally destroyed within two years. A local cruiser also recently told me the story of a temporary wooden grid built in Prince Edward Island to haul out fishing trawlers so new diesels could be installed. Given the short time span involved, the bents of the grid were left untreated. Two months later, when the time came to launch the first trawler, the scaffolding collapsed and pitched the trawler onto its side. The fast-working culprits? Right! Teredo navalis.

Teredos are technically not worms at all but molluscs whose shells have adapted to a boring and feeding regime. The shells are at the “head” and are used to grind away like a posthole digger. Interestingly enough (if everything about shipworms does not horrify you as a wooden-boat owner), some scientists believe that teredos can actually digest the cellulose of wood, a rare phenomenon in nature. Other scientists argue that they actually live from the saline water that flushes in and out of the hole; they excavate only in order to enlarge their habitat and not for food. Whichever it is, as it feeds and grows, teredo is also expanding its tubular nest. It follows the grain of the wood and rarely crosses over. Where there is one teredo, however, all the parallel grains are likely to be infested as well. As they progress, the teredos coat the tunnels walls with a calcium slime that hardens to keep the wood from collapsing on them. (Isambard Kingdom Brunel, the famous 19th-century, British engineer, modelled his Thames River tunnelling methods on the teredo and cemented the tunnel as he progressed to prevent collapse of the unstable soils.) The shipworm-infested wood appears healthy from the outside. But a good bump will often collapse things.

How to deal with the little beggars?

The Dutch rebuilt their dykes using stone and earthworks. In the old days, ship hulls were creosoted and heavily coated with tar. The tar tended to wear away with time, of course, making regular careening necessary to scrape off the flora and fauna, which often trailed for many yards behind large sailing vessels. Re-applying pitch and tar was a time-consuming solution. Eventually, the Royal Navy discovered that it could also prevent teredo worms by sheathing hulls with copper sheeting. Wormshoes were also developed, the tar-impregnated barrier being enough to keep the worms from attacking the keel from below. On the other hand, keels and wormshoes took the brunt of groundings and were hard to treat with tar when the vessel was careened in the sand.

Finally, anti-fouling paints, first containing tin additives (TBT being the most famous) and now copper, provided protection for the underwater wood. Of course, if these barriers were compromised (e.g., torn copper sheathings, erosion of the paints or serious damage to the wormshoe), shipworms could easily be back aboard the ship as stowaways. Creosoting the submerged portions of the hull was thought to be an effective countermeasure. This protection, one knows now, is fleeting at best. TBT, creosote, arsenic and other chemicals, indeed, might be of some benefit to the boat but are catastrophic for the environment and dangerous for the sailor and craftsman to boot.

There seem to be only two completely reliable ways of dealing with teredo navalis and his various shipworm cousins. Well, three really. First, there is substitution. Pilings and other harbour-side facilities are now mostly made of concrete or metals, while ships hulls are made of steel, aluminium, fibreglass or ferrocement. They all suffer from marine flora and fauna of various types, but not teredos. It’s us poor woodies that have still to worry about an invasion of our structural materials below the waterline.

Second, new barrier coatings have been introduced, most especially the various types of anti-fouling bottom paints. Nearly all surfaces in salt water need some sort of anti-fouling treatment to keep plant and mollusc growth from accreting; they slow vessels down and increase thereby fuel consumption. The Royal Navy learned to sheath their sailing ships in copper sheeting. It was effective but expensive and every captain had to worry about touching a coral head or otherwise grounding and compromising the integrity of the sheath.

The economic logic for commercial fleets? Hauling out, scraping and applying anti-fouling paint (and missing out on paid load-carrying) for a big ship is costly. The higher the price of fuels rise (crude oil hit nearly $90 a barrel this week!), the more attractive economically the maintenance work becomes. This remains true even as the high price of oil works its way through to the price of oil-based paints (The 4-litre can of Hempel self-polishing anti-fouling paint I purchased in Manta last year for $55 now costs $78, for example.) Tributyl tin (abbrev. TBT) was found to be a very effective additive to bottom paint. But its impact on the environment was almost catastrophic and an international agreement now prohibits its inclusion in bottom paints.

So, all the seagoing and shipbuilding nations are at present intensively looking for alternatives and for more far-reaching solutions. Copper is the best substitute for TBT so far. But it is not as effective, is more costly and has its own eco problems. Research is going forward on methods of reducing the times between repainting and cutting negative environmental ripple effects. “Hard” bottom paint (i.e., that does not slough off over time) is one approach. Extreme surface smoothness (e.g., with Teflon) as a means of preventing attachments is another. Using electricity to clean the bottom without a haulout is another.

For woodies like us, however, barrier paints are still the most readily available answer. But think of the cost even of this. In the last two years, we applied four Latino gallons (i.e., of 4 litres each) last year (i.e., 4 gallons @ $55 each = $220). This was barely enough to keep up with the erosion of the paint thanks to the strong, sand-laden currents here where we are anchored. We slapped on another 4 gallons (16 litres) this year again (i.e., 4 gallons @ $78 = $312). Exposing one’s bottom (you should pardon the expression) obviously has its costs. In our case, the costs were kept to a minimum by employing a makeshift tidal grid here in Bahía and doing most of the work ourselves. The grid cost us only $20 each year. Add Port Captain fees (yepp!), brushes, rollers, trays, thinners, etc. and our annual bottom painting in Ecuador has cost us about $250 last year and $350 this year. Only a few years ago we put on only 1.5 gallons costing $35 a US gallon and thought ourselves hard done by. To have sailed to Salinas, Ecuador, and done a haulout there, however, would have cost us at a minimum over $1,000. But I digress!

Is Vilisar infested with shipworm?

We don’t know. I looked as closely as I could at the bare wood while bottom-painting and saw no actual evidence of teredos. Physical inspection, however, may not be reliable since it is the minute larvae spawned by the bi-sexual parents that make a straight line (almost literally) for any bare wood they can get to. In many parts of the world this spawning activity is only in the spring and fall. (Nobody can tell me if or when it is here.) The offspring bore into the exposed surface and leave only a tiny pinprick of a hole. Even this hole they can seal off with mucous while they continue to bore into the planking. Inside however, they can grow to up to 60 cm (2 feet) in length in warm tropical waters! The entry holes are about the size of the LED indicator light on one of your electronic gadgets at home.

So barrier coats will stop them (anti-fouling paints or copper sheathing). But will poisonous paint, creosote or anything else kill them if they are already in there? It seems that there is only one sure-fire way to get rid of them if you have been infested. They both involve reducing the saline environment in which the shipworms thrive. Placing the boat in fresh or seriously brackish water for up to several months is one way. The other way is to haul the boat out of the water altogether.

The average salinity of sea water is 35 ppt (parts per thousand). Seawater is therefore about 3.5% salt. Because they get so much rain, the tropics and therefore the equatorial regions of the world tend to be less salty than more northerly waters. Teredos can actually survive low temperatures though their development will be slowed; they are found all along the coasts of England the Norway, for example. Happily, however, Mr. Teredo cannot survive if salinity drops below 12 ppt., i.e., about one-third of normal oceanic levels. It is for this reason that sailing ships that were not being employed were frequently anchored up large estuaries. The Baltic, as an example, is apparently too brackish for teredos and for this reason is an archeological treasure chest of sunken shipwrecks.

Rio Chone, where we are anchored, is a river estuary. It is tidal: we get high-high tides of about 3 metres above sea level at the full moon. There is also a very strong river current bringing fresh water (and silt) down from the glaciers and rains of the Andes. When the tide and river currents combine, we get an estimated 5 knots of current under the boat as we lie to our anchor. At high tides, there are standing waves indicating that the fresh river water is skimming over the incoming salt water from the river mouth. Our boat faces upriver into the current, for example, far more during a tidal cycle than it does downriver: it does so until about two-thirds of the way through the surge whereupon the river current overrides the incoming tide even on the flood and we are pointing back upriver.

Combined with the physical inspection of the hull while on the grid, this is the second indication that we may not have a teredo infestation. In other words, salinity levels may not be high enough for them to survive here. Evidence against this, however, is that our prop and bottom can become quite encrusted even within a few weeks with mollusc type creatures. We do not know if these are specifically adapted to estuary saline levels or not.

Salinity can be measured using conductivity and watermakers often come with some sort of testing device. Our friends David and Judy aboard S/V Revenir out of San Diego measured salinity way up river where they are moored and again at Puerto Amistad to see if they can make a comparison. Scientific, what? Dave thinks it is essentially the same as seawater. Bad news!

The bad news

Thirdly, I have been keeping an eye out for floating or beached wood that might have worm damage. The river and tides generally sweep the beaches clean every twelve hours so any evidence is taken away. We see floating logs at the monthly high tides and they look pretty solid, though we have no way of knowing how far they have travelled to get to the saline estuary, nor how long they have been immersed in saline water. Finally today, however, I found what I was looking for. Or, rather, what I was hoping not to find.

The poorest fishermen here in Bahía de Caráquez ply their trade on the river in narrow dugout wooden canoes. Just at the canoe’s waterline small logs are attached to the main body of the canoe on each side to provide more buoyancy and extra stability. Like the sailkng shps of old, the poorest fishermen here customarily use pitch or tar to coat the bottom of the canoe but apparently don’t worry much about the buoyancy aids.

So here is the perfect test. There is a canoe that is used daily for fishing in the Rio Chone quite close to our boat and anchored when not in use in the shallow water nearby (shallow enough so that the canoe is on the mud for at least part of the tidal cycle). In other words, except when it is exposed at low tide, the dugout is in the water near our boat all the time. The canoe itself is tarred thickly but the small buoyancy logs are not tarred at all. I have never noticed any worm damage till now, but then the dugout bottom – and more importantly the bottom of the buoyancy logs - are not exposed to view as long as the vessel is floating. The body of the overturned dugout was well covered in pitch but the outriders not. The canoe itself appears free of shipworm damage; the un-tarred buoyancy logs, however, are so riddled with worm damage that they have turned to powder as they dried out. To my mind that is a sign that the estuary is salty enough to support teredo life and there is a chance that we may have shipworm in the hull planking.

Next steps

We are leaving here about the end of October to sail to Western Panama. Upriver from Boca Chica lies David, Panama’s third-largest municipality. Near it is Pedregal, which lies some twenty-five miles up a swiftly flowing but serpentine (fresh water) river. This distance from the coast and the speed of the water flow led our friends Philip and Leslie on S/V Carina to estimate that the river is of very, very low salinity. This may be just the place for us. And there are some other rivers along Panama’s Pacific coast too. We are not sure how long it will take to rid ourselves of possible shipworms - one Lithuanian study said in a side remark that “several months are necessary”. The final alternative is to go some place like in Golfito or Punta Arenas in Costa Rica and actually haul out for a few months.

All of this would be of academic interest except that we live on a wooden yacht. The consequences of getting it wrong do not bear thinking about.