We all know
the marine environment is intensely corrosive, and that it can have dire
consequences for our boats if left unchecked. The problem is that the
most dangerous type of corrosion to us is the hardest to track down, while
superficial corrosion is often heralded by rust stains. Given this background,
it is obviously vital we know when we can take liberties and when immediate
action is needed.
Marine corrosion
occurs for a variety of reasons, but one of the most common is the use of
dissimilar metals together, such as stainless steel fastenings and aluminium.
Think about all the fittings on a mast quietly corroding away - it's enough
to make you break out into a sweat, unless you've insulated them! The results
of corrosion of this nature is usually easy to spot on a mast, because the
paint will start to bubble and peel off, but usually it is not easy to deal
with if left for a long time.
Dezincification
occurs when the zinc used in the manufacture of copper based alloys such as
brass and some bronzes (manganese bronze) is eroded from the metal by galvanic
corrosion, leaving it in a weakened, brittle and porous condition This process
takes place relatively slowly but with time will seriously reduce the mechanical
strength, perhaps resulting in the complete failure of fittings like propellers,
'P' brackets and sea cocks, whether they be gate or ball valves. The first
symptom of dezincification will usually be seen as bright, clean, red or pink
patches on bronze fittings, caused by the surplus of copper which is left
behind. These are mostly easily spotted shortly after lifting out, and before
the metal becomes dulled. Apart from these visual indications, try tapping
the fitting lightly with a hammer. Dezincified bronze often has a 'dull' sound
- though not always, so treat this only as an indicator. Sea cocks and skin
fittings need removing before you can inspect them properly. Don't forget
that internal bronze or brass fittings exposed to sea water (ie heads and
cooling water fittings) are just as much at risk.
Contrary to some beliefs,
stainless steel is not immune from corrosion - even 316 which is regarded
as marine grade. It is now thought that, when the chromium and iron are amalgamated,
small chromium balls can form during the cooling prosess - they are less than
10 microns big. The surrounding free iron is then subject to corrosion and
a stress line can start. Worst of all is that such stress lines can become
fractures that can't be seen. The best precaution is to ensure that an appropriate
grade of stainless steel is used in the
first place.
ALPHABET
The nautical mile is defined as the distance at the earth's equator subtended by one minute of latitude arc from the earth's centre and is approximately 6080 feet. It is approximate because the distance from the exact centre of the earth to the surface at the equator varies slightly with a consequent small variation in the distance subtended by a fixed angle. Furthermore, because of the slightly flattened earth's surface at the poles, one minute of arc at the equator will subtend a greater distance than at the poles. Therefore distance can be approximately related directly to latitude anywhere on the surface of the earth with minor inaccuracies occurring mainly in polar regions.
Distance can only be directly related to longitude at the equator because, as we move away from the equator towards the poles, the distance between lines of equal longitude decreases until it is zero at the poles. Therefore, when measuring distances on maps such as a Mercator or Lamberts Conformal, distance can only be measured directly from the latitude scale i.e. along the vertical with the map oriented with north at the top. However, provided that a mathematical conversion is applied it is possible to measure distance along the longitude scale.
Knowing how a nautical mile is calculated allows us to calculate both the circumference and diameter of the earth. The circumference is 60 (nautical miles in a degree) X 360 (degrees in a circle) = 21,600 nautical miles-at the equator = 39,744 Km. (Mars circumference is 21,064 Km) Therefore the diameter of earth is 21,600 ÷ 3.1416 (Pi) = 6875 n miles =12,650 Km.
The accurate measurement of large distances on maps is quite complex because of errors caused when a spherical like shape is represented on a flat surface. However, provided a few basic precautions are taken by the mariner, inherent errors are negligible over a few hundred miles.
For those who think in kilometers a nautical mile is just a bit less than two Km. It is exactly 1.85 Km. Thinking in nautical miles is better for navigation because a nautical mile is a natural unit on most maps.
Apparent wind is that wind felt by an observer on a moving object. It is the combined effect of the artificial wind generated by movement over the earth's surface and the real wind generated by meteorological forces.
An observer in a boat travelling at 10 Knots in an environnment where there is no real wind will experience an artificial wind of 10K coming from the opposite direction to which the boat is travelling. This wind not being the real wind is an apparent wind of 10K. (Fig 1)
Assume that there is a 20K wind blowing
from abeam a boat which is moving at 10K. (Fig 2) The apparent wind is
the combination of the real wind and the wind generated by the boat speed.
By using a parellelogram of forces and pythagoras the apparent wind is
about 22K. Then by finding sin of 10/22 the apparent wind is about 27°
forward of the beam - quite a shift in direction from the real wind.
Note that, in this case, the apparent wind is
greater than the real wind. If the boat was travelling directly downwind at
the same speed in the same conditions the apparent wind speed would be only
10K.
On a sailing vessel it is the apparent wind
that determines what sails should be used and how they should be set.
Suppose a sailing craft is at sea in wind conditions forecast to be 40 Knots. What might be the maximum wind likely to be seen on the wind instruments?
Wind constantly varies in both strength and direction - a fact not so obvious in light and moderate conditions. Wind can and does gust up to 50% higher and lower than a forecast wind which perforce must be a mean of the expected winds. Therefore a forecast wind of 40K implies peak gusts up to around 60K.
Most sailing vessels have their wind sensors mounted on top of the mast and, in conditions where the mean wind is 40K, there will be large waves - often steep. As the boat goes over the wave crests the consequent pitching and / or rolling generates quite large and rapid mast movement - especially at the top where the wind sensor is located. The speed of the mast head (and wind sensor) can easily reach 20K.
If a peak wind gust coincides with the boat pitching or rolling into the wind, the wind reading on the instrument can then reach 80K - and this in conditions forecast for 40K.
Humans seem to have a knack of exaggerating danger escaped and tend to recall only the most extreme conditions encountered and, in the above example, the message often given or received would be that the wind was 80K. To compound this is the media propensity to also add their own exaggeration. A lack of understanding of apparent wind often leads to mariners, and especially sailors, complaining ignorantly about a poor weather forecast.
