sail-world.com
 
 
News Home Video Gallery Newsletters Features Magnetic Is RW FishingBoating MarineBusiness-World Photo Gallery
Event Calendar

 

Sail-World.com : The Necessary Sextant and How to Choose It

The Necessary Sextant and How to Choose It

'Captain Nemo taking a star sight'   
GPS’s have sneaked under the guard of ancient mariners, making their wonderful navigation skills almost a thing of the past. Sailors set out on journeys across the world with no skills but how to read the GPS and Cmap. Where once the ocean mariner used vigilance and good seamanship to weather storms and miss the rocky bits, now satellite telephones and helicopters give the hesitant sailor the confidence to go where no bad seafarer should be.

However, what happens when your computer crashes at sea? Or worse, seawater fouls the instruments? Or what about the possibility of lightning strikes? – leaving you without any electronics at all?

(The two cruising boats we have spoken to who have been hit by lightning in the last couple of years have been adamant that no lightning protection would have protected their boat from the lightning that struck them in the ocean. Both had all instruments fried, even though they had disconnected all gear. One did have lightning protection, the other not, but each agreed that in the event it would not have mattered.)







Global_Challenge-’Vince_taking_a_star_sight’ -    
This makes the knowledge of how to use a sextant, and the carrying of the appropriate tables, one of the most excellent safety measures any cruising boat can have. If you are convinced to carry a sextant – or didn’t need convincing - the following is some sound advice that comes from Rob Landis of Celestaire , a company which specialises in navigation equipment, on how to choose the best sextant for your purpose – and budget.


Selecting a Marine Sextant

OVERVIEW

The first choice to make is between plastic or metal construction. Today's low cost metal sextants offer high accuracy and ease of use. These reward the beginner's efforts, and satisfy the professional's demands. Plastic models are perfect for lifeboat provisioning, and for restricted budgets. They are also acceptable to some experts who don't mind making frequent adjustments. The following characteristics of sextants should be considered.

NEW OR USED?

Older sextants tend to have smaller mirrors and scopes which make them harder to use. Spare parts and maintenance are also more uncertain. Avoid discontinued models (ie. those not shown in this catalog), and those greatly out of date. Purchase only from someone you know and trust, or a reputable dealer. You will find that today's low cost metal sextants are very competitive with expensive used ones.

ACCURACY

For all practical purposes, metal sextants are error free when compared to the many uncontrollable errors which may exist from such things as refraction, oblateness of the earth, and data tabulation. Generally, a minute of arc (one mile) is about the best anyone can hope to achieve. For these reasons, undue emphasis should not be placed on extreme accuracy guarantees. Plastic sextants commonly exhibit errors in excess of 5 minutes, even when great care is exercised. Although this is sufficient to make landfalls; precision navigation is difficult, and student progress may be retarded.

MIRRORSIZE

The size of the mirrors on a sextant generally vary directly with the quality of the instrument. Large index and horizon mirrors are desirable because larger mirrors admit more light, making it easier to obtain sights in marginal conditions. Larger mirrors also lessen the possibility of losing the image as the body is brought down to the horizon.

WEIGHT

Sextants are available with their major metal parts made of either aluminium or brass. The alloys of each metal are both suitable for use at sea. Some people feel that the heavier weight of a brass sextant provides greater steadiness and hence more accurate readings, especially if it is windy. Others find that the lightweight models are less tiring to their wrist and arm and that the reduced fatigue gives better results. As the observer develops proficiency and speed in sight taking, fatigue becomes less of a factor. Lightweight plastic models can be difficult to use facing into a stiff wind because they tend to 'flutter'.

SCOPES

A 3.5 (or 4) x 40 scope is a good choice for stars. The large objective 40mm lens admits a great deal of light. The 3.5 to 4 power magnification helps you find and maintain stars in view in both calm or pitching seaways. A 6x30 or 7x35 monocular of greater magnification is well suited for sun sights, or the greater heights of eye associated with large ships.The increased magnification allows the sun's diameter to appear larger, and better defines a more distant horizon. This helps the navigator determine the point of tangency of the sun's limb and the horizon. The increased magnification however makes finding and holding sights more difficult on a moving deck. A Sight Tube of zero magnification affords a wider field of view for rough weather, horizontal angles, and finding stars. If your sextant is to have only one scope, a 3.5x or 4x would be the logical choice for yacht sized vessels.


Views -    
















HORIZON MIRROR

Many sextants have an option of either the traditional (half-silvered) horizon mirror or what is called a 'whole horizon mirror'. With the traditional mirror, the horizon glass is divided vertically into two halves producing a 'split image.' The half nearest the frame is a silvered mirror and the other half is clear glass. In some cases this clear glass is eliminated. A later development in sextant technology is the whole horizon mirror. Using specially coated optics, the whole horizon mirror superimposes both the horizon and the celestial body on the entire mirror with no split image. This greatly simplifies 'bringing down' the celestial body and makes it easier to hold the body in view. A draw back to this system is a very slight reduction in light transmission and reflection which may affect marginally lighted observations. Some feel these two aspects are a 'trade off; that is, one can more quickly take observations with the whole horizon mirror, and be finished before marginal conditions occur. In general, people on stable platforms such as large ships tend to favor the traditional horizon mirror while those on yachts tend to favor the whole horizon mirror.

ILLUMINATION

Sextant lighting is the least needed feature on a sextant, since a flashlight should normally be available in any event for recording observations.

VALUE

Contrary to the adage that you get what you pay for; global exchange rates, tariffs, and labour costs have combined to produce variations in value. In this monetary respect only, we would rate the ASTRA IIIB sextant highest, and the Tamaya sextants lowest in value for the metal sextants. The Davis Mark 15 is the best in value for the plastic models.

ASTRA IIIB -    

THE ASTRA IIIB SEXTANT $795

The Astra IIIB has somewhat revolutionized celestial navigation. Never before has such a high quality, accurate metal sextant been available at such a low price. Over 18,000 sextants of this model have been sold in the US alone in the past 20 years; far more than any other serious sextant. Its popularity has spread throughout the rest of the world as well, making it the the most recognized (and supported) sextant worldwide.

Its low price, excellent optics, and choice of horizon mirrors and other accessories makes it the perfect selection for beginners. Yet, we know several professional navigators who prefer the ASTRA IIIB merely because they can leave their expensive models at home without sacrificing noticeable accuracy. The Astra IIIB is made by the Changzhou Celestaire Instrument Co. in China, with whom Celestaire is joint-ventured, and it is produced in accordance with our specific quality guidelines.

Construction Details

The frame is made from lightweight aluminum alloy which resists corrosion. Aluminum alloy is not new to sextant construction. It has long been used by m




by Rob Landis/Sail-World Cruising

  

Click on the FB Like link to post this story to your FB wall

http://www.sail-world.com/index.cfm?nid=24879

8:19 AM Fri 16 Jun 2006 GMT






Click here for printer friendly version
Click here to send us feedback or comments about this story.

Click for further information on
Practical

Related News Stories:

31 Jan 2014  Ten boat safety checks every skipper needs to make
05 Jan 2014  In-mast furling - is it the 'no-no' we've always believed?
13 Nov 2013  Product of the week: One Sailor MBS for single-handed docking
29 Oct 2013  Get your DSC-equipped VHF hooked up right
14 Oct 2013  Evolution Autopilot - top honours in Marine Electronics Awards
30 Sep 2013  Product of the Week: Safer and easier docking - SlideMoor
17 Sep 2013  Mountain bike torch great for tough sailing conditions
16 Sep 2013  Product of the Week: Drawer fridge for cool-keeping and accessibility
25 Aug 2013  Rig your own sailing boat? Yes, you can!
23 Aug 2013  Product of the Week: The LED ringed switch
MORE STORIES ...

News - World & Australia











HUD Vision: An interview with Afterguard Marine’s Alex Moret *Feature by David Schmidt, Sail-World USA Editor, Seattle












Rolex China Sea Race Day 1 by Rolex China Sea Race,


America's Cup: Expected de Ridder penalty should be reduced *Feature by Richard Gladwell/Sail-World.com/nz, Auckland, NZ


Challenge takes the 2014 Quantum Sails Sydney 38 Victoria State Titles *Feature by Rowan Simpson, Melbourne, Australia




































Congressional Cup - Victory for Taylor Canfield and crew by Rich Roberts (As Amended by ISAF),




SLAM Combined High Schools Championship - Makin and Turner lead
Brisbane to Gladstone Yacht Race fleet on weather watch
Asia's best to compete at 13th Samui Regatta
Image Gallery: Stratis SL33 flies on the Waitemata
Clipper Round the World Yacht Race - PSP Logistics prepares for USA
Hamilton Island junior sailors to try first ever regatta
ISAF Sailing World Cup Hyères - Crème of the crop to compete
Congressional Cup: Luna Rossa makes podium in first event
Int 14 World Championships 2015 launch new event website
Sail Port Stephens 2014 - Easy-going Commodore’s Cup day one
Ha Ha – Laughing all the way to Audi Hamilton Island Race Week 2014
Congressional Cup: World top rankers finish that way in Long Beach
Newcastle on a plate, in a glass, on a hill, by the beach…
America's Cup: Gino Morrelli outlines the new AC62 design
Doyle Sails New Zealand signs Andrew Brown as One Design Manager
America's Cup: Dean Barker's Blog - A sail with the Duke and Duchess *Feature
50th Congressional Cup: See the delayed coverage and media conferences
Tough Conditions for the start of the SLAM Combined High Schools Champ
Be safe on the water over the long weekends
Clipper Race 10 Day 27: Closing stages - Qingdao to San Francisco
Husband and wife teams winners in DSS Autumn Two-Handed Series   
Sail Port Stephens Trophy winners announced   
Red shirts prevail at Sail Port Stephens   
Audi IRC Australian (Keelboat) Championship 2014 - in profile   
Canfield, Williams, Bruni, Swinton in Congressional Cup final four   
PWA World Tour - A taste of things to come at La Torche + Video   
Women's International Match Racing Series kicks off in June   
SLAM Combined High Schools Championship – Opening Day   
Sail Port Stephens 2014 Day 2 - Four seasons in one day   
Clipper Round the World Yacht Race - Frustrating conditions   
Abell Point Yacht Club welcomes revival   
Sydney to Mooloolaba Yacht Race comes to a close   
Congressional Cup - Canfield leads toward Congressional Cup sailoffs   
America's Cup: Coutts claims ISAF Jury on a crusade, backs Kiwi report *Feature   
Clipper Round the World Yacht Race - Will light winds hamper progress?   
Les Voiles de St. Barth - Preparing to rock Saint Barthélemy waters   
Clipper Round the World Race - OneDLL and Qingdao now in San Francisco   
America's Cup: Emirates Team NZ up against funding hurdle for year *Feature   
50th Congressional Cup: View the Live racing here   
Royals go match racing on the Waitemata: Image gallery - 2   


For this week's complete news stories select    Last 7 Days
   Search All News
For last month's complete news stories select    Last 30 Days
   Archive News







Sail-World.com  


















Switch Default Region to:

Social Media

Asia

Australia

Canada

Europe

New Zealand

United Kingdom


http://www.sail-world.com/event_images/image/Twitter_logo_small.png http://www.sail-world.com/event_images/image/FaceBook-icon.png  http://www.sail-world.com/event_images/image/RSS-Icon.png

United States

Cruising Northern

Cruising Southern

MarineBusiness World

PowerBoat World

FishingBoating World

 

Contact

Commercial

News

Search

Contact Us

Advertisers Information

Submit news/events

Search Stories/Text

Feedback

Advertisers Directory

Newsletter Archive

Photo Gallery

 

Banner Advertising Details

Newsletter Subscribe

Video Gallery

Policies

 

 

 

Privacy Policy

 

 


Cookie Policy

 

 



This site and its contents are © Copyright TetraMedia and/or the original author, photographer etc. All Rights Reserved.  Photographs are copyright by law.  If you wish to use or buy a photograph contact the photographer directly.
XL VIR AUS