Mailport January 01, 2001 Issue

Mailport 01/01/01

True Wind Correction
The obvious answer to Wayne Richard’s question on the back page of June’s issue is that if you are doing 5 knots at 45° to the wind you are going upwind at .707 x 5, or 3.5 knots and therefore the true wind speed is 3.5 knots less than the apparent. The .707, of course, is the leg of a 45° right triangle when the hypotenuse is 1. This is as easy, if not easier, than using your mysterious 25%.

Cameron Gibb
Annapolis, Maryland


Pin Point Accuracy
Although I agree with your muted criticism of the government’s insistence of the use of SA and with your last paragraph, I have some concerns about other sections of your August 1 editorial.

Specifically, I do not agree with DGPS being a “government boondoggle.” International requirements (SOLAS, IALA) for safety-critical navigation, such as electronic harbor navigation, require an accuracy of better than 10 meters coupled with an integrity/warning service. GPS as provided by DOD does not provide either of these requirements. Many government departments try to solve a new requirement by inventing and installing a totally new system, costing billions and taking many years. The US Coast Guard saved uncountable taxpayer money and time using existing radio beacon stations around our waterways—the sites, the antennae, the transmitters, all existed. The electronics needed to provide the correction system were quickly developed and tested. The Coast Guard system is so good that it will probably be adopted as a nationwide system, again saving money by using former Air Force transmitter sites that are no longer needed. By contrast, the FAA has made two attempts to develop the equivalent system for airports, with at least one contract canceled after the expenditure of millions. To many of us, the Coast Guard is underfunded, undermanned, overtasked, and underappreciated!

Although of much lesser importance, I come up with somewhat different answers to your two plot comparisons, as you’ll note from the attached worksheet (see box below). For the first (an office building) plot I get lat/long differences of approximately 50 ft. and 10 ft., while for the second (a ship pier) plot I get 71 ft. and 154 ft.—not surprising, since, as you say, plotting a position on a chart is not very accurate.

Brian Pollard
Mount Desert, Maine


With regard to the greater issue of whether differential GPS still has something to offer over conventional GPS now that SA has been removed, the answer is yes. Is DGPS worth the extra cost and hassle for a now-modest increase in precision? In most cases, probably not.

DGPS is able to provide a more accurate fix because the local DGPS beacon sends out subtle correction data for the GPS signals as they are received in that area. These subtle errors exist not because they are artificially introduced, as was the case with Selective Availability, but because of small errors in the satellite ephemeris and the delaying and refraction effects caused by the atmosphere. Since two different locations on earth have differing amounts of atmosphere between them and any particular GPS satellite, errors will be different at the two locations.

The challenge then is to take DGPS into the mainstream and get wider coverage. This is being accomplished by WAAS, which stands for Wide Area Augmentation System. This system was developed to serve the precise GPS altitude needs of modern aircraft. A good source of information on this is www.raytheon.com/c3i/c3iproducts/c3iatc/atcnls01.htm. Raytheon was the primary contractor on this system. It should go online this fall, and WAAS-enabled units should be out soon.

The genius of WAAS is that it works with the conventional GPS constellation of satellites. Ground stations at beacon sites collect information on the subtle errors seen from the satellites and send this data to a central uplink station. The uplink station sends all of the errors from all of the stations up to another satellite which then beams these back down from space. A WAAS DGPS receiver simply listens for the normal GPS signals, determines which station is closest, and then applies that station’s correction data, which it also receives via satellite signal. Now there are no places that are out of range of the beacon, and you don’t have to hassle with tuning in new beacon stations or special antennae. With such systems being hardly more complex than today’s GPS receivers, it is a fair bet that all handhelds will soon go this route.

Harry C. Ragland
N38 37' 57.6"
W90 12' 58.0"
676 ft


We look at WASS next month.


About finding the GPS longitude coordinate to be about 200 feet different from your charted location…you put the difference to a thick pencil mark, but it may be something else.

What is the horizontal datum of the chart you used? If it is the earlier NAD 27 datum, there’s your 200 feet. When they re-did the horizontal datum (NAD 83) they found the earlier survey’s longitude was off. If you used an NAD 27 chart but an NAD 83 GPS, the numbers won’t agree. Prior to electronic navigation, a couple hundred feet didn’t make any difference. Electronic navigation changed all that, and now 200 feet is important.

I suspect that you did use an old chart. The present chart for Portsmouth is Chart 13221, 1:40,000, NAD 83. In your column you said you plotted your position on a 1:47,000 chart. That sounds like an old chart.

A second point: You refer to a 1:47,000 chart as a large scale chart. I would call it a medium scale chart. A 1:12,000 harbor chart would be large scale, and even 1:20,000 would qualify. At the other end of the argument, a 1:80,000 chart is a small scale chart. For understandable reasons, many people get it backwards, calling 1:80,000 large scale and 1:12,000 small scale. A friend told me how to remember which is which: Large scale, large detail. Corny, but it works.

Bob Hale
Seattle, Washington


We used a Chart Kit corrected to October 1985, without a text block indicating horizontal datum.


Your points on turning off GPS SA are all good. But, the really big item is having accurate velocity measurements available for the first time. GPS speed measurements with SA would wander 1 or more knots much of the time. Now, set it down and it will say 0.0, or maybe 0.1. This now makes the GPS a much better tool for the obvious things like course over ground. And, if one spends a few hours in a place without currents, you can calibrate your knotmeter (mine sure needed it) and while you are at it check the compass. Back in the current you can accurately measure current/wind drifts.

When I get into my kayak I can measure the current in a few seconds and tell how long a paddle I am into. This never worked well with SA.

Bob Larkin
Corvallis, Oregon


Hose Tests
Regarding the “8-Way Hose Test” (September 2000), your investigation into why the test may have produced misleading results failed to investigate a number of other things that happen in discharge lines.

Chemicals tested for in the SeaLand test are products of the anaerobic decomposition of sewage. The question that would be better answered is: What kind of decomposition happens inside typical marine sanitation hose? There are two possible answers: anaerobic decomposition and aerobic decomposition. Each does very different things and therefore will produce different results when testing.

Anaerobic decomposition depends on a lack of oxygen and is a two-part process carried out by two different types of microorganisms. In the first stage, a group of microbes referred to as “acid formers” break down the volatile portion of the sewage solids (volatile solids) into volatile fatty acids (VFAs). In the second stage, microorganisms referred to as “methane formers” utilize the VFAs produced in the first stage. They use them as a food source and reduce them to methane, carbon dioxide, hydrogen sulfide and others. VFAs are a primary source of offensive odors.

Aerobic decomposition requires oxygen. Gases generated by aerobic decomposition are odor-free, such as carbon dioxide, nitrogen gas, and water vapor.

The key to why expensive SeaLand hose works so well is that their toilet systems are closed and particles are not broken down. Vacuum systems move big lumps of sewage from the bowl into the hose where anaerobic decomposition begins almost immediately. Gases produced by this process are intense, offensive and toxic. Most other marine toilets break down the waste into smaller particles and when connected to a vented holding tank, with sufficient oxygen supplied, aerobic decomposition occurs.

I hope in the spirit of being an independent and fair media, you will investigate hose quality again. The conclusion you should reach is that SeaLand hose is needed only with their vacuum systems. I would rather fix the problem for the boat owner from its root and advise them to avoid anaerobic decomposition.

Vinod Mehta
VP Engineering, Raritan Engineering
Millville, New Jersey


In theory, you may be right, but most holding tanks are not sufficiently well vented for aerobic decomposition of waste; hence the need for odorproof hose. If aerobic decomposition is indeed the answer, the real root problem may be the kind of holding tank systems builders install in boats.


Lavac Vacuum Toilet
After cruising for two years with a Lavac onboard my Westerly Conway, I found the head to be one of the most reliable bits on the boat—despite my children’s best efforts to plug it. It is simple and foolproof, and the Henderson pump can handle just about anything, including a half-roll of paper, but don’t let the anti-siphon vent get clogged!

Byron Sachs
Via email

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