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Rhumb Line, Vol 2 Issue 2 -- New at www.great-lakes-sailing.com March 07, 2014 |
DearRhumb Linea course that keeps a constant bearingThe Rhumb Line for Great Lakes Sailing is clear: to provide a comprehensive listing of ports around the Great Lakes basin and articles that cover a broad range of topics of interest to sailors. The purpose of Rhumb Line is to keep you up-to-date with new additions to Great Lakes Sailing in a brief, easy-to-scan and concise manner. I value not only your interest but also your time. February 2014 This past month and a half has been busy. Great Lakes Sailing was present at the Toronto Boat Show and participated in the seminar program. Meeting folks who love to sail the Great Lakes or who hope to some day is always fun. And a special thanks to the Nautical Mind bookstore for providing the draw prize for those who attended. Also this past month, we spent time in England, welcoming a new grandson. (Funny how 6 pounds, 15 ounces can refocus you!) While in England, I followed the footsteps of many sailors before me as I travelled to and explored Portsmouth - the home of the Royal Navy. Portsmouth is also home to the oldest continually-commissioned warship in the world - HMS Victory. She is the flag ship of the First Sea Lord (the Chief of Naval Staff) and coincidentally, he was on board the same day we were. If you ever grumble a little about close quarters on your boat, HMS Victory was 227 ft/69 m in length and 800 men lived aboard her! Two new ports have been added – Hamilton, ON and the small harbor of Suttons Bay, MI. Some technical adjustments were made to the site as well. (I find it interesting how hours and hours of work can be reduced to 10 words!) A new powerpoint presentation on the Great Lakes has been developed to use with some up-coming speaking engagements. There is an update about water level measurements on the Great Lakes, something we all have to be concerned about. Some have noted that there is a difference between the charted visibility ranges for lights and the ranges given in Great Lakes Sailing's notes. The discussion below will explain why you will never see the light at the range distance shown on your chart – never. 1. New Ports (a). Hamilton ON is situated at the western end of lake Ontario. It is the 3rd largest city in Ontario and the 5th largest city on the Great Lakes. The port is a major Great Lakes harbour and the city is an integral part of the Greater Toronto Area (GTA). A major industrial city, Hamilton has a very active and attractive recreational waterfront. Click here to visit Hamilton, ON (b). Suttons Bay MI is a picturesque village located about ½ way down the west shoreline of Grand Traverse Bay (Lake Michigan) at the head of Suttons Bay. The surrounding area - Leelanau County - –is the largest cherry growing region in the United States. Vineyards abound as well. Today, tourism is the foundation of the local economy and Suttons Bay MI works hard to make visitors feel very welcome. Click here to visit Suttons Bay, MI 2. Water Levels on the Great Lakes Could there be a silver lining in the cloud that is this long, hard winter with continual snowfalls and bone-cracking temperatures? Indeed there is! The Great Lakes Environmental Research Laboratory based in Ann Arbor, MI reports that the frigid polar temperatures we have experienced have resulted in the most extensive freeze-over of the Great Lakes since 1979. That year, a record 95% of the lakes were frozen over. This winter, 91% of the surface of the lakes is ice-covered. One immediate effect of this is the slowing of the rate of evaporation. As well, the much colder water will slow evaporation when the ice cover recedes and melts. Another aspect of that silver lining is the significant snowfall that the Great Lakes basin has received. In many areas, that snowfall has reached record levels. Once the ice melts, that snow cover will melt into the lakes, raising the water levels. In addition, the runoff from surrounding lands will add a significant amount of water to the lakes. Any heavy rains in the spring can only add to that. Indeed, concerns are being voiced about the increased risks of flooding. Water levels on Lake Superior are about 33 cm/13 in, above its long-term avarage depth for the first time in 15 years. Lakes Michigan and Huron are projected to be about 30 cm/12 in above last year's level, although still 30 cm/12 in below their long-term averages. Lakes Erie and Ontario are projected to be above their long-term averages as well. So, as you stare out your window at all that snow, remember that, for us boaters, there is indeed a silver lining in this winter cloud. 3. Visibility Range of Charted Lights If you have noticed, when Great Lakes Sailing (GLS) identifies lights, the range at which it becomes visible is usually noted. However, if you compare the GLS range with the range shown on your chart, you will see that the numbers are not the same. For example, the entrance light for Providence Bay on Manitoulin Island is shown on charts as 11 Statute Miles (SM) while GLS shows it as 9.2 SM. Why this difference? Its not that I can't calculate (although I had math teachers who might disagree!). Here's the reason. The charted visibility range is called the nominal range. The nominal range depends only on the candlepower or luminous intensity of the light. It does NOT take into account the curvature of the earth's surface, the height of the light or the height of the viewer's eye. These elements constitute the geographic range of the light. (Note: GLS bases all its calculations on the height of the light only. Because GLD does not take the the height of the viewer's eye into account, in effect, the viewer will see the light a bit further that GLS states.) There is a specific mathematical formula to determine the geographic range. To find the geographic range in Statute Miles, the formula is 1.345 x square root of the height in feet. EG: Providence Bay Light is 47 feet. Geographic visibility in SM is: 1.345 x √47 = 1.345 x 6.86 = 9.2 SM To find the geographic range in Nautical Miles, the formula is 1.169 x square root of the height in feet. 1.169 x √47 = 1.169 x 6.86 = 8.0 NM So, as you can see, the geographic range falls short by nearly two statute miles from the nominal range. Standing on the deck of your boat, you will never see a light at its nominal range.
Thanks for reading Rhumb Line. Your opinions, thoughts and comments do matter. If you have any comments or questions, please feel free to contact me here at Rhumb Line or at Great Lakes Sailing If you like this newsletter, please do a friend and me a big favor and "pay it forward." If a friend DID forward this to you and if you like what you read, please subscribe. You will find a subscribe button on most pages of the site. |
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