Duhh!! watch what??

Okay, I had to check Chapman Piloting: A nautical mile is 6,076.12 feet or 1,852 meters., an international standard; for practical purposes, equals one minute of latitude.

p.626 in the appendices.

Wyatt

During the 1980's (and I believe since) speed trials were held annually at Portland, Dorset, UK. The 'course' originally consisted of a series of buoys set in a 1nm diameter circle. A run constituted entering the circle via one pair of buoys and exiting via the corresponding pair, directly opposite. This was later refined to a 'bow-tie' shaped layout either side of 90degrees to the wind allowing boats to choose their best course for sheer speed between a close reach and a broad reach. It is many years since I visited a speed week, but the camaraderie and enjoyment of comparing approaches to a common goal was very memorable. I don't know exactly what timing method was employed but I do know that 'gate boats' were involved.

With modern portable GPS's it should be possible to arrange a set of buoys, entered as waypoints on several GPS units. Then each boat could carry such a unit for their run and hand it over to a support boat at the end of the run for their attempt to be downloaded to a laptop before the GPS unit is passed over to another competitor.

Alternatively, if the distance between buoys is known exactly, synchronised watches on two properly stationed gate boats plus a radio link is all that is required (plus some mathematics).

Just my twopenn'orth....

You hear that guys !

1 nm is 6076 POINT ! 12 feet.

So if I'm flying at 3000 foot altitude how much is that in nautical miles. NO calculators please. Also why is horizontal speed measured in nm and vertical speed measured in ft/sec. When flying to the moon; do you measured that in feet/sec, nm/h or normal miles / hour ; afterall it vertical speed from the beginning and not over land or sea later on ? Or even the distance between city A and B is 1000 normal miles; how long does it take an airplane travelling at 600 knots take to cover the distance in a straight line. Or this one. A biplane has a speed over land of 80 knots and has a vertical speed of 20 feet per second. How much is his resulting airspeed ? If his wings break of at 150 ft/sec airspeed will this plane survive his dive ?

Say a ship is 300 ft long and travelling at 10 knots; how long does it take to pass the harbour entrance bouy from bow to stern ?

Now think about doing all these calcs in the SI system where only meters and Meters/sec are allowed to be used. Wow, you could do all these calculation right from your head within seconds ! But the French invented it so there MUST be SOMETHING wrong with it.

Wouter

With modern GPS units such a complex measurement area is not needed.

Just set the GPS unit to record location points at some small interval like anywhere from 1 to 10 sec and just blast around for a while strying to get the best speed out of the boat.

Read the track in a PC and look for a 500 mtr stretch where the average speed is highest. This will also negate any negative effects of a windshift etc.

See here an example :





We made this track with a 150 US$ handheld GPS unit (Garmin Geko 201) with intervals set at 10 seconds. However It can log 10,000 points with 1 second intervals = just shy of 3 hours logging at a very high rate. Don't use single points or momentary speed readings they are to undependable. Select a series of points on one track and average the result this is very accurate.

Interesting to note is that in this test the boat achieved the same top speeds on all possible courses. Upwind, downwind and on a reach. The differences were less than 1 km/u = less 1 foot/sec for you imperial guys.

Highest average speed reached over a 10 second stretch was 30 km/u or 16.2 knots.

Wouter

Hi Wouter,

I use SI units for most engineering calculations at work and then convert back to INCH|POUND when required. There are fewer constants and conversions to remember so I am less likely to screw it all up. BUT - for navigation I use nautical miles.

When you are trying to figure the distance between 2 points on the surface of the earth you start with the lattitude and longitude and then use "the sailings". Which type you use will depend on the distance, direcion and desired accuracy of the calculation. Unless you are only concerned with a very short distance you would not use a plain sailing. You would typically use a mercator sailing to find the course and distance. The table (table 6 in Bowditch) to get the meridional parts is all based on degrees and minutes. This makes using nautical miles the most convenient.

Now, you could get really clever and do your mercator sailings purely by calculation, based on some trig and the WGS ellipsiod. But, how do you calculate distance on the surface of an ellipsiod and not use polar coordinates at some point? If you are going to use degrees and minutes to measure angle then nautical miles require the least conversion.

Aircraft navigation uses nautical miles for the same reasons. The speed over ground is used for navigation.

If I am measuring distance in nautical miles, it makes sense to measure speed in the same units.

Talk to you later
-colin

Steve- I have not logged in for a long time. I just saw your message from December.

Colin,

The methodes do not chance when using a different unit. A 2,207 lbs carton of milk is handled exactly the same way as a 1 kg cartons of milk which is handled the same as a 1 liter carton of milk.

So I really don't see why using nm is more "convenient" than another unit. Afterall the distances between longitudal lines isn't constant as well so how handy is nm here. Also a change of the distance between latitudal lines can easily be expressed by any other unit. One could even change the distance between these lines to reflect say 100 km.

Actually, lately more and more systems are in use where the position is given by a short code that is referenced to a series of square block along a given longitudal line. What issue is there to make these block say 100 x 100 km or even 10 x 10 km or any other measure. As you say inside these blocks a mercator projects is practical and sufficiently accurate.

Why do we have to devide the globe of the earth in to 1/60 and 1/360 parts ? For decades now landsurveyers have used a 400 degree system to measurement angles and distances because that is more practical than 360 degrees. A change of reference like that can easily be made in global navigation as well. Why not devide angles in portions of 10 ? Subdevide these another 10 portions each and so on. Much easier to use than 1/360 then 1/60 again 1/60. And the methodes of navigation won't change.

No seriously, the use of nautical miles is tradition based. That is all. When I navigate I only use the degree system. Hell I find it more easy to just express my speed in bow seconds or bow minutes per hour or day. I don't even do nm conversions.

They only conversion I do regulary, but for other reasons, is m/s and knots (and back) just because the conversion is so simple. 1 m/s = 1.9438 knots. = say 2 knot with a 3 % error. So if I'm travelling at 20 knots ; I know immediately that I'm doing just better than 10 m/s (actually 10.3 m/s). That is a speed measure I can directly relate to wether I'm on land or water. On my 5 mtr long cat it means two boats lengths travelled per second. Now do the same using nautical miles and a 16'2" long hull. I wish you alot of fun dividing the numbers by 6076.12 (= feet per nm) and 12 (inches per feet)

Wouter

The video of the tornado shows very gusty winds. tough to sustain a steady speed. My contention is that beach cats without hydofoils will never go much faster than the record set by Hobie Alter on a 16 in huricane gulch -circa 1971 or so. Maybe in a speed ditch you could get a cat to add more than a few knots of speed, but not in open water for 500 meters. For efficiency the best wind is barely single trapeze weather. Drag is at a minimum and apparent wind is kicking in.
On another note i think Finnian set the record because he finally got the perfect board shaped by Richard Greene who knows what to shape and makes it come out perfect. also it was great conditions.
The other thing is a radar gun or equivilent is a good way to measure speeds.

As a qualified and licensed surveyor (now known as "land information systems") I would suggest that you keep your references to methods of determining degrees and distances to a subject that you are familiar with Wouter, you obviously are not intimately familiar with the methods used in land tenure or surveying in general, which is not unusual as few people outside the profession are, but in our profession we find that "engineers" feel that they know all there is to "surveying" when in actual fact, they are greatly mistaken.

Well, you guys do use arcs of 400 degrees instead of 360, don't you. At least I have that bit of info from a dependable source.

And if you don't believe me than here : http://www.absoluteastronomy.com/encyclopedia/g/go/gon.htm

Maybe its use never made it too Australia.

But whatever you do don't call me a lier or misinformed.

I never said that I had intimate knowlegde of land surveying I just used this example to show that it is not uncommon to get of the traditionally inspired path when that gives advantages. Don't some engineers use metric time as well. Where 100 seconds are 1 new minute and where a 100 minutes are in a new hour.

And I also never said in any way :"that "engineers" (I) feel that they (I) know all there is to "surveying" when in actual fact, they are greatly mistaken". Don't put words or meanings in my mouth.

Wouter

Watch this thread, you may learn something.

Read this as well : http://www.absoluteastronomy.com/encyclopedia/M/Me/Metric_system_in_the_United_States.htm

Wouter

A radar gun would certainly give you peak speed (if it happened to be pointing at the boat at the time) but I thought a lot of this thread was about average speeds? (Ignoring the spurious arguments about units of measure)

A GPS or timer over a known distance is the most obvious way to find out average speeds.

Hi Mary, I haven't logged on this week until today. This has turned into quite a thread!
My idea, being the lazy type, is to measure the distance between 2 existing marks, and then have a stopwatch (or two) on the boat. Using the gps to measure the distance between the marks would seem to be the most accurate method, and then once the co-ordinates are posted here we can have 20 pages of arguments about the calculations
I refuse to believe that I can't average more than 20mph over a mile or so on a reach, but the data won't lie I guess. Maybe it just "feels" like 30 mph?
Al

The measured distance, say 500 meters certainly does make sense. you could then just state your time like in swimming or running.
How accurate is a gps? Measuring distance over water and getting your gates set on the water can't be a very exact science. If you wanted to go all out you could do gps speed, timed speed, and average the results. not sure if it is possible to train a radar gun on a craft for a full 500 meters and save all that data.

That video of the tornado sure shows alot of wasted energy. if only it was all going towards forward motion.

Good evening.
Unless I have to go back to the Maritime Academy again, I am sure that each degree of latitude equals to 60 minutes equals to 60 miles; therefore one minute equals to one nautical mile or 2,000 yards.
So long and fair winds

Take care

Rafael

One nautical mile equals to one minute of a meridian arc; every 60 minutes is one degree and you get the rest.

Regarding maximun Catamaran speed; John McKnight and a few others can attest I broke world records when I got caught in Gale force winds ( 40 + knots w/gusts over 50 ); I can attest the speed achieved was VERY SCARY to say the least.
After this I told John I will stop my research work; now I see a cloud in the sky and I do not go. I will just leave it to Steve Fosset.

The biggest problem with GPS is that the American military incorporate a built in "error" for readings from their satellites for positional readings available to the "general public" (different for military forces). This error only allows an accuracy no greater than + or - a 5 metre radius circle (at best), that is why for the accuracy required for surveying when using GPS, an independent "radio base station" with known XYZ coordinates has to set up and used in conjunction with many multiple GPS readings so that the error can be (almost) negated. For general use, particularly in determining speed over the surface of the earth, the results can only be taken as an indication, not fact. For long or medium distances at sea (in particular) this degree of accuracy is not a problem, but for short distances ie 500 metres, it is just not accurate enough for a precise result.

That is not true Darryl. The shift is not that quick; it takes a while for the position to swing about significantly while a typical record speed run will be done in under 50 seconds. In that time frame the shifted is neglectable. Also on the such a short 500 mtr stretch the position error is as good as the same at the start and finish so the COVERED distance will be 500 mtr with a very high accuracy. Otherwise the differential GPS methode like you describe can't work, you should have know that as a certified landsurveyer.

Sure the WHOLE course may move about by 5 mtr on the GPS during the hours of all the record attemps but that is pretty unimportant. All you need is a central bouy, a stop and go at the bouy and the assigment to all sailors to hold their max speed for at least 60 seconds. The GPS travelled distance readouts (not momentary speed !) from stop-and-go to the 500 mtr mark after that can then be used to calculate speed with an accuracy of at least 2 decimals.

Simple, easy and can be done by everyone with a GPS with a PC port and tracking ability (garmin 201 Geko = 145 US$)

Wouter

"neglectable" ??? -

Sorry Wouter - surely you mean negligible?