A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sumner's Line (Navigation question)
From: George Huxtable
Date: 2006 Feb 10, 12:03 -0000
From: George Huxtable
Date: 2006 Feb 10, 12:03 -0000
This is a response to Bill's postings about Sumner, from George. Come on, Bill, if you are really interested in what Sumner did and said, READ HIM IN THE ORIGINAL ACCOUNT, not in a secondhand tinkering in Bowditch. It's easy to get access, now. Frank Reed pointed to where it could be downloaded from http://www.hti.umich.edu. But I recommend the Vanvarenbergh and Ifland paperback, "Line of Position Navigation", stocked by Ken Gebhart at Celestaire, which was only $13.99, and includes a lot of stuff from St. Hilaire thrown in for good measure. Bill has written "It is a tough read, and I find it somewhat ambiguous." I disagree. Sumner seems to me the archetypal American Practical Navigator. His booklet is not full of fancy mathematics, but common sense, clearly expressed. (St Hilaire is MUCH harder going!). Sumner's chart, plate III, plots his "true position as afterwards proved", obtained from hindsight, which puts his position, at the time of taking the sight, as just about 10 miles short of the Smalls light. Though he couldn't know that, of course, until the lighthouse was seen. Bill makes guesses as to his vessel's speed. No sailing ship ever did 25 knots, of course. An ordinary cargo carrier of the 1830s would, I suggest, be happy with a speed of 12 knots under best possible conditions, and usually much slower. Sumner's ship, presumably carrying cotton, screwed (compressed) into her hold, would have been no speed merchant. Sumner was under short canvas, and was in no hurry to get into the bottleneck of St. George's Channel, fringed by steep-to rocks, in such bad visibility: quite the reverse. He had been marking time over the previous night, and was still feeling his way. Many Nav-L members must be familiar with sailing in bad visibility (though perhaps not so much unless they sail in the waters around Britain). Not only can't you see the horizon, but without sight of land or seamarks or other vessels, it's hard to be certain just how far you CAN see. Combined with a gale and those rocks ahead, everything was conspiring to present Sumner with an unpleasant surprise, unless he was very careful. He was. As I see it, everything fits together nicely, in Sumner's account. In one respect, Sumner was lucky, and took advantage of that luck. The point he needed to aim for, and aim for very precisely, was the Smalls lighthouse. If he had missed it, and sailed past into those rocks, he would have been in serious trouble. And his position line, taken from the Sun observation, happened to pass exactly through that spot. So then, all he had to do was so sail along the appropriate line. It didn't matter how far away from the Smalls he was, along that line; all he had to do was to stick to it (tide permitting) and he would arrive there some time. What if his Sun sight had put him on a position line that would take him, say, 10 miles East of the Smalls? He would then have to steer somewhere to leeward (to port, or North) of that position line, to aim for the light. No problem about that, of course, it's always easy to bear-away. But by how much should he diverge from the direction of that line? He had no way of telling, unless he could discover how far away from the Smalls his ship was. So that would have been a much trickier problem, which by luck he managed to avoid. His experience provided a useful example, for his booklet, of what a Sumner Line could do, but things could not be expected to work out so nicely every time. By the way, in a previous message (2 Feb), I suggested that Sumner had not obtained any sights for the previous 1500 miles. His example on page 14, which refers to that Smalls-light episode, states that the run was "between 600 and 700 miles without any observation", so clearly I got that a bit wrong. George. contact George Huxtable at firstname.lastname@example.org or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. | Fred wrote: | | > Well yes, he was courting disaster had he kept his course up to the | > moment of spotting Small's light, lacking the LOP. But lacking the | > LOP, I expect that he would have stood off or similar, and lost time. | | If we believe that his true position was 8 miles/minutes south of his DR | position, the distance to Small's lighthouse would increase by 20+ nm. | That, added to the 27+ nm from his DR to Small's gets close to 50 nm. Let's | assume our Harvard man took a little over 20 minutes per calculation and he | was making top speed during the calculations. So a little over an hour in | calculations, and a little under an hour in reaching Small's after the LOP | was confirmed. That's almost 25 kn, and fails my common sense test. (But | if I could keep up, I would grind for that skipper any day ;-) I therefore | imagine you are correct in your assessment--stand off instead of barreling | headlong into rocks at 25 kn.
| | Bill |