NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Slip
From: George Huxtable
Date: 2003 Apr 24, 23:23 +0100
From: George Huxtable
Date: 2003 Apr 24, 23:23 +0100
Perhaps Doug Royer will explain why a day's "advance" of a vessel, calculated from shaft rotations, will be calculated and recorded in statute miles rather than in the nautical miles a simple-minded navigator might expect. He said- >Slip is determined by speed.So each time a vessel changes speed slip must be >recalculated. Then that will devalue, to some extent, the usefulness of shaft turns as a measure of distance travelled. I wonder if Doug can comment on my earlier question about the effects of weather. Does a ship's navigator expect to allow for the effect of heavy weather on the slip factor, when using shaft-turns? Or, may I add, different states of lading or fouling of the hull? In my early teens I spent a bit of time on an ageing 2000-ton coaster on a voyage between Liverpool and Belgium (coal-burner, triple-expansion, single screw: it was wonderful). There was a Walker log trailed from the taffrail and read, from time to time, sometimes in quite rough conditions. Off North Cornwall there was a lot of pitching, and on each big wave the screw would emerge and race away. It gave the engineers a lot of work to do in quickly closing and reopening the steam regulator. Under those conditions, there must have been much slip, and counting the shaft turns would have given little information about distance travelled. But the Walker log would have done the trick. Doug, is a taffrail log (or modern equivalent) ever used at sea these days, in the merchant service? My small boat usually trails a Walker log, but it gets brought in whenever I'm trailing a mackerel line, as otherwise the two get tangled. Food comes first! I can understand the use of "apparent slip", as Doug describes it, but "true slip" would need a knowledge of the speed of water under the ship's counter (where that body of water is carried along with the ship, to some extent). I don't see how a master would be expected to know that "wake factor" at the location of the propellor. This is the water that the propellor has to draw from, to then accelerate it and eject it astern. It would obviously be more efficient if the propellor could be working in "clean" water, water that was stationary when it met the blades. If it's moving forward, to some extent, with the ship, the propellor has to first bring that water to a stop, and then give it a backward velocity, which must take more power from the engine. To show how important this matter could be, in a vessel with a stern that's designed badly for power, I will quote from a paper by William Froude, the guru of tank-testing, in 1865, "Remarks on the mechanical principles of the action of propellers". The naval vessels he refers to are the "motor-sailers" of the day, three-masted square-riggers, with an auxiliary steam engine driving a propellor . The propellor could be hoisted out of the water when sailing, as it ran on bearings in a frame located between the ship's sternpost and the rudder-post. In use the screw was driven by a dog with could be pushed out to engage it from within the shaft-tunnel. You can see this clever arrangement if you ever visit HMS Warrior, which lies afloat in Portsmouth, not far from Victory, beautifully restored. I do recommend a look over Warrior, for anyone visiting England. But I digress... Here is what Froude had to say- "Behind the sternpost of any ship when in motion, and behind her square tuck if she has one, there is collected a mass of what is termed dead water; water which accompanies the ship almost without change. When the sternpost is thick, and the tuck broad, as in our timber line-of-battle ships, the volume of water thus carried is very large, insomuch that it was mentioned to me by a wardroom officer of such a ship that when the screw was hoisted out of the water, and the ship was under canvas and going seven knots, he had descended into the well and bathed there without inconvenience." This would be a terrible environment for a rudder to live in, and a great deal of "slip" would result. George. ================================================================ contact George Huxtable by email at george@huxtable.u-net.com, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================