General 9

TECHNICAL LIBRARY

GENERAL ITEMS

GALVANIC CORROSION

This information in this article is supplied by James Glen - manufacturers of fasteners. For further information visit their web site at www.jglen.com

In addition to corrosion being caused by exposure of uncoated materials, it is also caused or enhanced by the combination of dissimilar or incompatible materials. The following chart gives guidelines for the selection of
materials or finishes based on this galvanic action:

BASE METAL	FASTENER METAL
BASE METAL	Zinc / Galv Steel	Aluminium	Steel / Cast Iron	Brass,Copper, Bronze, Monel	Stainless "4" Series	Stainless "3" Series
Zinc / Galv Steel	A	B	B	C	C	C
Aluminium	A	A	B	C	N/R	B
Steel / Cast Iron	A,D	A	A	C	C	B
Brass, Copper, Bronze, Monel	A,D,E	A,E	A,E	A	A	A
Stainless "4" Series	A,D,E	A,E	A,E	A	A	A
Stainless "3" Series	A,D,E	A,E	A,E	A,E	A	A

KEY

A ----The corrosion of the base metal is not increased by the fastener
B ----The corrosion of the base metal is marginally increased by the fastener
C---- The corrosion of the base metal may be markedly increased by the fastener
D---- The plating on the fastener is rapidly consumed, leaving the bare fastener material
E---- The corrosion of the fastener is increased by the base metal
N/R-- Not recommended

For further information see the items on Corrosion and Stainless Steel

MONOHULL & CATAMARAN CHARACTERISTICS

Library Catalogue

The decision to own either a catamaran or a displacement monohull can be very difficult - especially for those inexperienced on either type. The lists below show some of the characteristics resulting from the innate design differences applicable to vessels built to equivalent standards and for open waters.

MONOHULL

CATAMARAN

Slower in a straight line -----------------------------
Relatively large heel angles - less comfort ---------
The larger the heel the greater the righting moment
Poor visibility to leeward with jib / genoa set-------
Points higher ---------------------------------------
Less directionally stable - easier to tack ------------
Heavier with greater draft - less water access -------
Will sink if holed without repair below water line ----
Smoother ride in rough weather ---------------------
Stronger structure - sustain worse conditions -------
Sails can be set for peak wind gusts ----------------
Less manoeuvrable under engine power -----------
Less usable room for the same overall length -------
Smaller berthing space ------------------------------
Usually needs slipping to clean hull ------------------
Less maintenance for same length ------------------
Can recover from a capsize -------------------------
Operating costs too difficult to compare -------------

Generally faster on all points of sail
Sails flat with little heel - greater comfort
The greater the heel the smaller the righting moment
Good all round visibility in normal sail configurations
Not able to point as high
Directionally stable - special tacking techniques needed
Lighter with shallow draft - more water access
When properly constructed will continue to float if holed
Not as stable in rough conditions
Structurally weaker - more susceptible to weather damage
Sails need to be set for mean wind.
Fitted with 2 engines more manoeuvrable
Considerably more space for the same length
Larger berth space needed
Hulls can be easily cleaned with a moderate tide
More maintenance - two hulls and commonly two engines
Cannot recover from a capsize
Operating costs too difficult to compare

CIRCADIAN RHYTHMS

Library Catalogue

    Circadian Rhythms refer to a collection of biochemical systems built into the human body that have cyclical patterns recurring about every 24 hours. Examples include temperature regulation, digestion, hormonal releases and, most visibly, our sleep / wake patterns. These rhythms evolved in animal and plant life alike, indicating our synchrony with the cyclical rotation of the earth.
     Generally we are programmed for daytime activity and nighttime sleep. The brain physiologically induces two maximal opportunities for sleep each day - 0300 to 0600 and 1500 to 1700. Western culture is one of the few not to acknowledge this with some kind of siesta.
     A series of "time-isolation" studies were conducted in which volunteers were subjected to environments devoid of

'zeitgebars' (or time givers) such as sunlight, social cues and other factors from which clues about the time of day could be guessed. The most vivid results concerned the tendency of time isolated people to lengthen their normal 24-hour sleep / wake pattern to a 25-hour one. The remarkable stability of these rhythms in the absence of zeitgebars eventually led to the identification of a 'circadian clock' located in the hypothalamus. Further studies exposed volunteers to alternate daylight rotations.
Researchers found that under laboratory conditions their sleeping patterns and other circadian rhythms gradually adapted to the environmental conditions. This demonstrated that environmental cues act to pull circadian rhythms into alignment with into alignment with the day / night cycle.
There are therefore implications for a skipper when setting a watchkeeping roster.

VISION

Library Catalogue

This item has been abridged and adapted from an instructional item written for pilots. The information is relevant in the air, at sea and on land - especially when in control of a vehicle.
For related material see the item on Lookout and the Vision item in the section on Crew Fitness.

    Vision is vulnerable to just about everything : dust, fatigue, emotion, germs, fallen eyelashes, age, optical illusions, alcohol etc. In many activities our vision is altered by factors such as atmospheric conditions, dirty or distorted windows, acceleration, glare, heat & cold, light conditions and comfort. Most of all the eye is vulnerable to vagaries of the mind. We can 'see' and identify only what the mind lets us see. For example lack of concentration, daydreaming and staring into space does not allow the mind to register things that are in the field of vision - to say nothing of the things that might be in peripheral view. One function of the eye that is a constant problem (although a person is probably never aware of it) is the time required for accommodation. Our eyes automatically accommodate for or refocus on near and far objects. The change from something close like the speedometer in a usually dark panel to an object much further away takes at least a second, and sometimes more, for the eye to accommodate.
      Another focusing problem usually occurs where there is little or nothing to focus on at infinity. Although this is more relevant to an aviator it still has some effect - say when controlling a motor vehicle or boat. If the eye doesn't focus properly or at all, a person stares without the mind registering. This is called 'empty-field myopia'.
      Another sight inhibiting factor is caused by 'binocular vision'. To accept what we see we need to get vision from both eyes. If an object is visible to one eye but is not visible to the other because of some obstruction the total image is blurred and not always acceptable to the mind.
      Another inherent problem is that of a narrow field of vision. Although our eyes accept light rays from an arc of nearly 200 degrees they are limited to a relatively small area of 10-15 degrees in which they can actually focus on an object. Though we can perceive movement in the periphery we cannot identify what is happening. We tend not to believe what we see in the corner of our eyes. Aided by the brain this leads to 'tunnel vision'. This means that without relative motion or good contrast an object becomes much more difficult to see and register.
The eye is also limited by environment. Limited visibility actually means limited vision. In strong light glare can make objects hard to see. Furthermore some types of sunglasses can be useless in reducing glare and, in some cases, can obstruct clear vision. On the other hand on cloudy and / or hazy days a lack of contrast will make objects more difficult to see - especially when there is little or no relative movement and the background is of a similar colour to the object.
      Then there is the mind which through many factors can distract to the point of not seeing anything at all or lull us into myopia where we stare at something without seeing it. Perception is affected by many factors. We all tend to over estimate our visual abilities and misunderstand the limitations of our eyes. We need to appreciate that proper looking techniques do not happen automatically - they need to be consciously practiced.

PETROL v DIESEL

Library Catalogue

FOUR STROKE OTTO CYCLE FOUND IN MOST PETROL ENGINES

SOME DIFFERENCES BETWEEN DIESEL AND PETROL ENGINES

Fuel less volatile
Direct injection
No spark plugs
Higher compression ratios (typically 14:1 up to 25:1)
Mixture ignited by compression heat
More economical

Fuel more volatile
Either direct or indirect injection
Spark plugs required
Lower compression ratios (typically 8:1 to 12:1)
Mixture ignited by spark plug
Higher fuel usage.

FOUR STROKE DIESEL CYCLE - NOTE ABSENCE OF SPARK PLUG

Marine Watch Site Map

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