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Varnishes vs Oils

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There are a number of Crustacea which habitually bore into wood. One type stands out pre-eminent on account of its ubiquity and the great damage done. These are the gribble, species of Limnoria, little creatures resembling miniature wood lice, usually between 3 and 4 millimetres long and with a semi-cylindrical body divided into segments. They have 7 pairs of short legs each ending with a sharp curved claw by means of which the animal holds onto the sides of the burrow. Beneath the hind end of the body there are 5 pairs of legs each carrying 2 broad plates which act as gills, these keep up a continuous movement during the life of the animal.

The gribble is recognised as a cold-water species which does not seem to survive for long if temperatures exceed 20 ºC.

Usually, one pair of Limnoria live together in a tunnel, the male following the female. The tunnels cross each other and as a result of extensive attack the timber surfaces are reduced to a sponge like appearance. They rarely bore deeper than 15mm below the surface of the wood but the entire surface may become infested. The erosive action of the sea on a surface thus weakened exposes fresh surfaces to attack, which is therefore continuous. The attack usually extends from a centre of infestation, as the young are unable to swim and the adults can only do so to a limited extent. The adult female lays 20 to 30 eggs which when hatched allow the young to burrow out from the sides of the tunnel made by the adults.

It has been known that 1sq. inch could contain 300 to 400 borers. The gribble will always make its first attack on new untreated timber.

The gribble can only exist in water with a salinity exceeding 16 – 20 parts per 1000 and can withstand daily exposure to air and thus survives in inter-tidal areas. The effect of moving a boat into fresh water for a sufficiently long period would be enough to kill the borers, although it would still be liable to fresh attack on its return to the sea. It is not known exactly how long timber must be out of saline water to kill marine borers but has been known that some species of shipworm can survive for 30 to 40 days. No species of timber in its natural condition is immune from the attack of marine borers but some have more resistant heartwood. Many timbers are readily attacked and can be totally destroyed within a very short time.

It is possible that your boat might be attached by molluscs called Martesia which are often found in localities where Teredo worms are prevalent. Martesia resemble elongated cockles, rather than worms, and do not bore into planking like Teredos but excavate burrows on the surface. They are not therefore such a hidden menace but, since they can grow to a length of two inches and a breadth of over half an inch while excavating burrows longer than themselves, they are a very definite one; the more so as they attack in large numbers.

Sea water in harbours and estuaries contains immense numbers of microscopic organisms and plants collectively known as plankton. If given a chance, many of these will settle on the hull of a boat and grow. They constitute a menace because, as they grow, they increasingly reduce the boats speed through the water and its manoeuvrability, and are a direct cause of increased fuel consumption.  A hull laden with fouling also presents the serious risk of the transfer of invasive plant & animal species from one region or country, to another, and is yet another solid reason why applying a good quality antifoul is an important consideration for all boat owners.  

The greatest menace among fouling organisms is the barnacle which can grow until it measures 3 inches across the base. Other animal shell growths include mussels and sepulidae. The latter form masses of coiled and tangled white shelly tubes, often described as “coral”.

Other growths are formed by ascidians or tunicates which, at the larval stage, resemble minute tadpoles. They are usually called jelly-bags or sea squirts. Yet others are formed by polyzoa, sometimes in the form of mats to which other organisms may attach themselves; and by hydroids. Those formed by the latter are scientifically known as tubulana, colloquially as roses. Weed growths include a green ribbon grass known as entermorpha; a brown weed called ectocarpus; and a red one called polisiphonia. They are distantly related to a slimy, moss like growth likely to be composed of vegetable growths and seaweed related organisms called diatoms. The slime may not adversely affect a boat’s speed much but weeds certainly do.