NWS Homepage -
http://www.nws.noaa.gov
NWS Marine Forecasts - http://www.nws.noaa.gov/om/marine/home.htm
NWS Marine Text Products - http://www.nws.noaa.gov/om/marine/home.htm#text
NWS Marine Radiofax Products - http://www.nsw.noaa.gov/fax/marine.shtml
NWS Voluntary Observing Ship Program - http://www.vos.noaa.gov
AMVER/SEAS Homepage - http://seas.amverseas.noaa.gov/seas/
U.S. NAVY AND OTHER WEATHER INTERNET SITES
See these sites for further links
Naval Oceanographic Office -
http://www.navo.navy.mil/
Navy Fleet Numerical -
http://www.fnmoc.navy.mil/
International Ice Patrol -
http:// www.uscg.mil/lantarea/iip/home.html
National Ice Center -
http://www.natice.noaa.gov
WMO Homepage -
http://www.wmo.ch
JCOMM GMDSS -
http://weather.gmdss.org/
USCG Maritime Telecommunications - http://www.navcen.uscg.gov/marcomms
Sunday, May 4, 2008
Saturday, May 3, 2008
Sea State And Wind
The sea state descriptions that follow are for steady state conditions, which result when the wind has been blowing from the same direction for a relatively long time over a sufficiently long fetch (the distance the wind has blown across the water without interruption). For a given wind speed and duration, the longer the fetch, the greater the sea disturbance. Depth of water also effects the appearance of the sea waves running into shallow water become steeper and are more likely to break, which may result in an overestimate of wind speed.
The sea criterion is for use over relatively deep water only. There are other conditions when wind speed may be higher or lower than indicated by the sea state. Heavy rain and floating ice will have a dampening effect on the sea surface. Wind blowing against (opposite) a tide or strong current causes a greater sea-wave height and sea-disturbance than normal, while wind blowing in the same direction as a tide or strong current causes a smaller sea-wave height and sea-disturbance than normal. There is a lag period between the wind increasing or decreasing, and the sea wave height rising or falling. This is especially pronounced during a sudden change in wind speed. The presence of swell may cause more whitecaps to form, because wind waves have a greater tendency to break when superimposed on the crest of swell.
To distinguish sea from swell, remember that sea waves are generated by the wind blowing at the time of observation, or in the recent past, in your local area. Swell waves have travelled into your area of observation, after having been generated by winds in other areas (sometimes thousands of miles away). As sea waves move out from under the wind that produces them and become swell, their character changes. The crests become lower and more rounded, and they move in trains of similar period and height. Swell is more symmetrical and uniform than sea,andwill have a longer period.
Alto - Cloud prefix meaning middle level.
Altocumulus - Middle level white or gray patch, sheet, or layer of cloud, composed of rounded masses, rolls, etc., which mayor may not be merged. Mainly composed of water droplets, sometimes partly fibrous (ice).
Altostratus - Middle level grayish or bluish sheet or layer of striated, fibrous, or uniform appearance.
Anemometer - An instrument for measuring wind speed and direction. Typically 3 or 4 rotating cups measure speed, and a vane indicates direction.
Beaufort Wind Scale - A numerical scale of wind force originally designed by Admiral Francis Beaufort in the mid-19th century. It consists of sea-state descriptions correlated with ranges of wind speed.
Cirro - Cloud prefix meaning high level.
Cirrocumulus - High level thin white patch, sheet, or layer of cloud, composed of very small elements like ripples, grains, etc.
Cirrostratus - High level transparent, whitish cloud veil of fibrous (hairlike), or smooth appearance, often producing a halo.
Cirrus - High level clouds of ice crystals in the form of delicate white filaments, or white patches or narrow bands, with fibrous appearance of silky sheen.
Cloud Height - The height of the base of the cloud or cloud layer above the sea or land surface.
Cloud Layer - A grouping of clouds whose bases are at approximately the same level.
Convection - Vertical air movement in unstable air masses resulting in the development
of cumulus clouds.
Cloud Type - A cloud form identified as distinct according to the World Meteorological Organization.
Crest - The highest part of the wave.
Cumulonimbus - A heavy, dense cloud with considerable vertical extent, in the form of a mountain or huge tower. Part of the upper portion may be smooth or fibrous.
Cumulus - Detached clouds, generally dense and with sharp outlines, developing vertically in the form of rising mounds, domes, or towers, of which the bulging upper part may resemble a cauliflower. The sunlit parts are brilliant white, the bases can be dark and nearly horizontal.
Fetch - The distance the wind has blown across the water without interruption.
Fracto - Cloud prefix meaning torn, ragged, or scattered appearance due to strong winds.
Freak Wave - A wave of great height and steepness, much higher than other waves in the prevailing sea or swell system.
Gust - Sudden brief wind increase followed by lull or slackening. One nautical mile per hour or .5 meters/second.
Lenticular - A type of cloud formed in the ascending portion of an airstream, which remains stationary while the air blows through it.
Nimbostratus - Gray cloud layer, often dark, thick enough to block out the sun, which appears diffuse by falling precipitation.
Ripple - A small wavelet that forms at wind speed of 1-3 knots.
Sea - Locally generated waves produced by the wind.
Sea Disturbance - Waves, whitecaps, spindrift, foam, etc. on the ,Sea surface.
Strato - Cloud prefix referring to cloud sheets or layers.
Stratocumulus - Lower level gray or whitish patch, sheet, or layer of cloud, usually with dark parts, with rounded masses or rolls, which may not be merged.
Stratus - Generally gray lower level cloud layer with a fairly uniform base, which may produce drizzle or snow grains.
Swell - Ocean waves which have traveled beyond the generating area, which have riot been produced by the local wind. They have longer periods than sea, and are more regular and uniform.
Trough - The lowest part of the wave.
True Direction - Direction measured in degrees clockwise from true north, where north is 0°.
Wave Height - Distance from trough to crest, averaged for the. better formed waves in the group.
Wave Length - Distance from trough to trough or crest to crest for adjacent waves.
Wave Period - Time, in seconds for the passage of successive wave crests. Normally computed as an average value for several waves.
White Cap - The breaking crest of a wave, usually white and frothy.
Wind - The horizontal motion of the air past a given point.
Wind Direction - The true direction from which the wind is blowing at a given location.
Wind Speed - The rate at which the air is moving horizontally past a given point. Wind speed estimates are usually made by relating the state of the sea to the Beaufort Scale of wind force.
How To Use A Sextant
A sextant is not difficult to use but it takes practice to get a sight quickly and accurately, especially aboard a bouncing vessel. The instrument is held vertically in the right hand and the sighting is made through the telescope. The horizon is observed in the horizon glass while the celestial object is found in the mirror and positioned such that it is in line with the horizon. In the case of the Sun or Moon, the edge of the disk is placed on the horizon. If the lower edge is used, the sight is referred to as a lower limb sight. An upper limb sight is less often used with the Sun but is often necessary with the Moon since the lower edge may not actually be a circular one, depending on the phase.
Once the body is lined up properly, the sextant is "rocked" or pivoted as if the top of the index arm were attached to the rod of a pendulum and the arc were at the bottom with the swinging action. This is done to insure that the sextant is held vertically when the sight is taken. As the rocking is done, the celestial body will seem to trace an arc with respect to the horizon. The sextant is vertical or plumb when the body is at the bottom of the arc. The sight is then marked, the observer says "mark" to his timekeeper or observes the time himself.
The angular height of the celestial body is read on the arc and on the micrometer drum. The arc displays the degrees whereas the drum displays the minutes and tenths of minutes (or in some cases minutes and seconds). An arrow on the index arm points to the degrees on the arc. The degree is chosen that rests just to the right of the arrow. If the arrow pointing to the micrometer drum lies between two minutes, an estimation is made as to how many tenths of the way between it is or sometimes a vernier is available on the index arm for that purpose.
There are several techniques of getting the celestial body in the field of view, an important step in sextant use that I skipped over quickly a couple of paragraphs ago. In sighting the Sun, assuming reasonably good sea conditions, the observer can get the horizon under the Sun in the glass and then move the index arm back and forth, homing in on the glare surrounding the Sun until the Sun's disk is seen. Filters will be needed in front of the index mirror to protect the eye from the Sun's brightness. Also, filters may be necessary in front of the horizon glass if the Sun's sparkle on the water is too bright. A second method is useful for non-glaring objects such as the planets, stars and daytime Moon. Hold the sextant upside downin the left hand and sight through the glass toward the celestial object. Then move the index arm until the horizon appears in the mirror. The advantage in this method is that it is easier to find the celestial object by direct observing and leave the easily found horizon line for the moving mirrors. Once the object is reasonably well lined up with the horizon, the sextant is turned right side up and the final adjustments with the micrometer drum are made.
If some mathematical calculations are made ahead of time, the rough altitude of the celestial body can be figured allowing a third method to be used. This involves presetting the sextant to the prefigured altitude and then scanning the horizon with the horizon glass until the celestial body comes into the field of view of the mirror. The rough azimuth of the body can also be prefigured so that the area of scanning can be limited. For this method, the sextant would be held right side up the whole time. Some practical hints on using the sextant are in order especially if the instrument represents a considerable investment and happens to be the only sextant aboard. A lanyard attached to the sextant and to the observer saves accidental dropping of the instrument, either to be damaged on the deck or to be lost to Davy Jones Locker. Wrapping oneself around the shrouds when taking a sight over the rail saves the navigator from the same fates.
Sighting when the ship gets to the top of a wave is important to insure that the real sea horizon is used rather than the closer top of a nearby wave. The real sea horizon can vary in distance depending on the height of the observer's eye but corrections for this can be made.
Once the body is lined up properly, the sextant is "rocked" or pivoted as if the top of the index arm were attached to the rod of a pendulum and the arc were at the bottom with the swinging action. This is done to insure that the sextant is held vertically when the sight is taken. As the rocking is done, the celestial body will seem to trace an arc with respect to the horizon. The sextant is vertical or plumb when the body is at the bottom of the arc. The sight is then marked, the observer says "mark" to his timekeeper or observes the time himself.
The angular height of the celestial body is read on the arc and on the micrometer drum. The arc displays the degrees whereas the drum displays the minutes and tenths of minutes (or in some cases minutes and seconds). An arrow on the index arm points to the degrees on the arc. The degree is chosen that rests just to the right of the arrow. If the arrow pointing to the micrometer drum lies between two minutes, an estimation is made as to how many tenths of the way between it is or sometimes a vernier is available on the index arm for that purpose.
There are several techniques of getting the celestial body in the field of view, an important step in sextant use that I skipped over quickly a couple of paragraphs ago. In sighting the Sun, assuming reasonably good sea conditions, the observer can get the horizon under the Sun in the glass and then move the index arm back and forth, homing in on the glare surrounding the Sun until the Sun's disk is seen. Filters will be needed in front of the index mirror to protect the eye from the Sun's brightness. Also, filters may be necessary in front of the horizon glass if the Sun's sparkle on the water is too bright. A second method is useful for non-glaring objects such as the planets, stars and daytime Moon. Hold the sextant upside downin the left hand and sight through the glass toward the celestial object. Then move the index arm until the horizon appears in the mirror. The advantage in this method is that it is easier to find the celestial object by direct observing and leave the easily found horizon line for the moving mirrors. Once the object is reasonably well lined up with the horizon, the sextant is turned right side up and the final adjustments with the micrometer drum are made.
If some mathematical calculations are made ahead of time, the rough altitude of the celestial body can be figured allowing a third method to be used. This involves presetting the sextant to the prefigured altitude and then scanning the horizon with the horizon glass until the celestial body comes into the field of view of the mirror. The rough azimuth of the body can also be prefigured so that the area of scanning can be limited. For this method, the sextant would be held right side up the whole time. Some practical hints on using the sextant are in order especially if the instrument represents a considerable investment and happens to be the only sextant aboard. A lanyard attached to the sextant and to the observer saves accidental dropping of the instrument, either to be damaged on the deck or to be lost to Davy Jones Locker. Wrapping oneself around the shrouds when taking a sight over the rail saves the navigator from the same fates.
Sighting when the ship gets to the top of a wave is important to insure that the real sea horizon is used rather than the closer top of a nearby wave. The real sea horizon can vary in distance depending on the height of the observer's eye but corrections for this can be made.
Friday, May 2, 2008
1. Gyrocompass repeaters reproduce the indications of the master gyrocompass. They are __________.
- a. accurate only in the Polar regions
b. accurate electronic servomechanisms
c. hand operated
d. accurate only if the vessel is underway
- a. 0900, 24 January
- b. 2100, 24 January
c. 2100, 25 January
d. 0900, 26 January
- a. directly with the temperature of the air mass
- b. directly with the pressure gradient
c. inversely with the barometric pressure
d. inversely with the absolute humidity
- a. range of the tide
- b. plane of the tide
c. stand of the tide
d. reversing of the tide
- a. come left
- b. come right
c. continue on the present course
d. wait until the lights are no longer in a vertical line
- a. the channel boundaries
- b. that vessels must stop
c. the bridge is about to open
d. that traffic is approaching from the other side
- a. 119° at 0.9 knot
- b. 172° at 1.1 knots
c. 225° at 0.6 knot
d. 340° at 0.4 knot
- a. interrupted quick flashing
- b. composite group flashing
c. Morse (A)
d. quick flashing
- a. magnetic latitudes
- b. magnetic declinations
c. dip
d. isogonic lines
- a. bell alarm to warn the user
- b. lighted alarm signal to warn the user
c. alternate signal keying system
d. view finder for each station
Sextant Adjustment
Once the sextant is obtained, adjustments to the mirrors may be necessary to reduce the index correction to a minimal amount. One or two adjusting screws are located on each mirror for this purpose. Each mirror should be perpendicular to the sextant frame and when the sextant is set at zero the two mirrors should be parallel to each other. Three tests are involved. The first test is for perpendicularity of the index mirror. Hold the sextant on its side (with handle down) and with the index arm set to 35°.
Place your eye close to the sextant near the index mirror so that you can see the sextant arc in the mirror (reflected) and also just to the right of the mirror (direct). If these two images are not in a straight or continuous line, the mirror is not perpendicular to the frame. Adjusting the screws will bring the images in line.
The second test is for perpendicularity of the horizon glass. Actually, the "glass" is only half glass with the right half of the frame filled with a mirror. The horizon is viewed through the glass, the reflected image of the celestial object viewed in the mirror. If this horizon glass is not perpendicular to the frame, the error is referred to as side error. If a star is viewed both in the glass and in the mirror with the sextant set near zero, by adjusting the altitude, the star should pass over itself, become superimposed. If instead the reflected image of the star passes to the right of the direct image, side error exists and can be minimized by adjusting the two screws at the base of the horizon glass. Other celestial bodies may be used for this test as well as reasonably distant terrestrial objects.
The third test is for parallelism of the index mirror and horizon glass when the index arm is set exactly at zero. If at this setting the horizon or a celestial body appear higher or lower in the mirror than in the glass, the mirrors are not parallel and should be adjusted until they are. This error is called index error.
This is an error in the sextant itself and can be found by setting the sextant to read exactly zero and observing the sea horizon, a distant mountain top (a reasonably flat one), or a celestial object. At zero reading, the objects observed should appear the same height in the horizon glass and mirror.
Thursday, May 1, 2008
USCG Deck General Questions
1. A wildcat is a __________.
a. deeply-grooved drum on the windlass with sprockets which engage the links of the anchor chain.
b. winch that is running out of control due to a failure of the overspeed trips.
c. line that has jumped off the gypsy head while under strain.
d. nylon line that parts under strain and whips back in a hazardous manner
2. A crack in the deck plating of a vessel may be temporarily prevented from increasing in length by __________.
a. cutting a square notch at each end of the crack
b. drilling a hole at each end of the crack.
c. slot-welding the crack.
d. welding a doubler over the crack
3. The turning circle of a vessel making a turn over 360 degrees is the path followed by the __________.
a. center of gravity
b. bow
c. bridge
d. centerline
4. The pivoting point of a fully loaded vessel with normal trim proceeding ahead at sea speed is __________.
a. right at the bow
b. one-third the length of the vessel from the bow
c. one-half the length of the vessel from the bow
d. two-thirds the length of the vessel from the bow
5. Under title 46 of the United States Code, the person in charge of a documented vessel who fails to report a complaint of a sexual offense may be __________.
a. fined up to $5,000
b. imprisoned for up to one year
c. charged with accessory to sexual assault
d. All of the above
6. The stowage factor for a cargo is based on __________.
a. one short ton
b. one short metric ton
c. one long ton
d. one long metric ton
7. You are in charge of a U.S. documented vessel. Under title 46 of the United States Code, if you fail to report a complaint of a sexual offense, you may be __________.
a. criminally charged and jailed
b. civilly penalized
c. held personally liable by the victim and sued
d. All of the above are correct
8. You are planning the stowage of two incompatible products on your multiple-product tankship. What will NOT provide the minimum required segregation?
a. Empty tank
b. Solid (non-intercostal) bulkhead
c. Diagonally adjacent tanks
d. Tank containing a third cargo compatible with the other two
9. An embarked Pilot __________.
a. is a specialist hired for his local navigational knowledge
b. is solely responsible for the safe navigation of the vessel
c. relieves the Master of his duties
d. relieves the officer of the watch
10. The explosive range of a fuel lies between the lower explosive limit and the __________.
a. flash point
b. ignition temperature
c. upper explosive limit
d. fire point
a. deeply-grooved drum on the windlass with sprockets which engage the links of the anchor chain.
b. winch that is running out of control due to a failure of the overspeed trips.
c. line that has jumped off the gypsy head while under strain.
d. nylon line that parts under strain and whips back in a hazardous manner
2. A crack in the deck plating of a vessel may be temporarily prevented from increasing in length by __________.
a. cutting a square notch at each end of the crack
b. drilling a hole at each end of the crack.
c. slot-welding the crack.
d. welding a doubler over the crack
3. The turning circle of a vessel making a turn over 360 degrees is the path followed by the __________.
a. center of gravity
b. bow
c. bridge
d. centerline
4. The pivoting point of a fully loaded vessel with normal trim proceeding ahead at sea speed is __________.
a. right at the bow
b. one-third the length of the vessel from the bow
c. one-half the length of the vessel from the bow
d. two-thirds the length of the vessel from the bow
5. Under title 46 of the United States Code, the person in charge of a documented vessel who fails to report a complaint of a sexual offense may be __________.
a. fined up to $5,000
b. imprisoned for up to one year
c. charged with accessory to sexual assault
d. All of the above
6. The stowage factor for a cargo is based on __________.
a. one short ton
b. one short metric ton
c. one long ton
d. one long metric ton
7. You are in charge of a U.S. documented vessel. Under title 46 of the United States Code, if you fail to report a complaint of a sexual offense, you may be __________.
a. criminally charged and jailed
b. civilly penalized
c. held personally liable by the victim and sued
d. All of the above are correct
8. You are planning the stowage of two incompatible products on your multiple-product tankship. What will NOT provide the minimum required segregation?
a. Empty tank
b. Solid (non-intercostal) bulkhead
c. Diagonally adjacent tanks
d. Tank containing a third cargo compatible with the other two
9. An embarked Pilot __________.
a. is a specialist hired for his local navigational knowledge
b. is solely responsible for the safe navigation of the vessel
c. relieves the Master of his duties
d. relieves the officer of the watch
10. The explosive range of a fuel lies between the lower explosive limit and the __________.
a. flash point
b. ignition temperature
c. upper explosive limit
d. fire point
Choosing a Sextant
In choosing a sextant, the biggest decision is quality versus purchase price. There are two basic "grades" to investigate, plastic and metal, with the corresponding retail price ranges varying.
The gap here is considerable the buyer should be better armed to choose the side of the gap best suited to him or her.
The plastic sextant's one advantage is price. This should be weighed against several disadvantages. First, since plastic will expand and contract with varying temperatures, the index correction (instrument error) is constantly changing. This can be partially compensated for by obtaining an index correction each time a set of sights is taken. However, the navigator may find that even between the first and last sight during a twilight series, the change can be considerable. Remember, a minute of error in sextant altitude directly corresponds to one nautical mile on the plot.
Secondly, plastic sextants weigh less than a pound and some varieties offer considerable wind resistance, all making it more difficult to hold the sextant vertical when sighting in breezy conditions.
Thirdly, the quality of the components is less, the filters, the mirrors, the zero to three power viewing scopes. And lastly, the life of a plastic sextant is shorter, depending on the amount of use. Filters break off, the plastic gearing wears down, the micrometer drum develops slop.
Any of the plastic sextants make excellent teaching aids where principle, not accuracy, is important. Also, as a back-up sextant, the micrometer plastic sextants can be very valuable. However, as a primary sextant when the celestial fix is important, I would strongly recommend investing in the better grade.The advantages of a metal sextant are obvious after reading the disadvantages involved in using a plastic sextant. Index correction is always the same unless the sextant is dropped or mirror adjustments are made. The weight (2 to 4 pounds) and open work frame reduce windage problems, the better optics and filters give guaranteed accuracy and the life of the instrument is indefinite as long as care is exercised in usage and storage.
The metal frame may be made of either brass or an aluminwn alloy, lightening the weight from roughly four to three pounds. The size of the frame varies too, also changing the weight. I find that the lighter sextants are easier to hold for a length of time.
The telescope power varies from three to eight power, the advantage of the higher power being mainly its ability to pick up the light of a star earlier in the twilight when the naked eye still cannot see it. The disadvantage of greater power is reduced field of view, and this becomes critical when the navigator is trying to keep the celestial body in the field while bouncing around on a small vessel. A four power scope is a good compromise:
Lighting is another option. Of course this isn't needed during the day but near the end of twilight, it is convenient to press a button or turn a switch to illuminate the arc and micrometer drwn. The battery case, wires and bulb socket arel all subject to corrosion at sea and batteries tend to wear down when most needed so often this "luxury" is questionable. The extra cost for lighting will finance an inexhaustible supply of penlight flashlights which will clip on to clothing or store in the sextant case. Cases usually come with the sextant and are included in the price.
Second hand metal sextants are a rarity and often not much of a price bargain. Some sextants are sold as "antiques" and application of this title prices them beyond their useful value. The true antiques, the vernier sextants or octants, are nice as display items but the difficulty of reading a vernier versus a micrometer drum is a big disadvantage. Sometimes Navy surplus sextants can be found at reasonable prices. In any of these situations, instrument cleaning or mirror resilvering may be necessary but this cost will be minimal compared to the price of the instrument.
The final choice of instrument to buy comes down to how much you can afford, how essential celestial navigation is to your voyage, how comfortable the instrument is to use, and how experienced the navigator is in handling the sextant. Guaranteed accuracy ratings become less important when rough sea conditions and navigator inexperience prevail.
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