NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: awareness / actually: speed from engine RPM
From: Herbert Prinz
Date: 2003 Apr 24, 05:54 +0000
From: Herbert Prinz
Date: 2003 Apr 24, 05:54 +0000
George Huxtable wrote: > Here I disagree with Herbert. The two, driving propeller and idling > patent-log, are not the same at all. I did not say they are the same. I said that the patent log is also based on the principle of a rotating propeller screw, therefore suffers just the same from slip (amongst other problems) and has to be calibrated for varying conditions, too. > Those that read the account from the > 1890s of a cable-ship in a storm, discussed recently here, will recall that > it took all the power available simply to hold station against the wild > weather. Presumably, the shaft-counter will have been clicking away without > the vessel making headway at all. I did not follow that discussion, but I get the idea. I bet that if the master of the ship would have found the time to bring out a patent log in said conditions, the log would also have been clicking away due to wind induced surface current. And so would any other fancy modern instrument regardless of what principle it is based on, because they all measure the speed relative to water in close vicinity of the hull. But there is a lot of water flowing under the keel if you go full power against the wind without making headway! Clearly, any method will fail in extreme circumstances. This does not mean that it is useless on a regular basis. The slip factor is certainly not a constant. Think of what happens when you reach hull speed. It depends on wind and sea state, but also on speed, draft, trim and foulness of bottom, in short on everything that affects the resistance of the hull. It must be determined empirically for various conditions; using it certainly requires that the master is familiar with his vessel in these conditions. In actual practice, nobody bothers with formulas for propeller pitch, slip, and so forth. Rather, one directly produces curves or tables that show speed versus engine rpm. The proper procedure for doing so using the Measured Mile markers that I mentioned in my earlier post can be found in nautical handbooks. Although these markers are not US Coast Guard maintained aids to navigation, they are charted and whoever pays for their upkeep must think that they are still useful. Bowditch, 1909, considers counting prop revolutions a "valuable check on the patent log" and a "means of replacing it, if necessary". 75 years later, DMAHC Pub. 9 says "This method of determining speed is widely used in the merchant marine." (Article 616, p. 155). Calibration techniques are described in appendix U. Herbert Prinz