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Old 08-02-2013, 10:31   #256
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Re: )

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Originally Posted by Dockhead View Post
Good, I think we agree about the testing.

For the one hour data there are two approaches -- 1. just use the UKHO tidal atlas; or 2. take an average of the one hour's 5 minute tides.

The former will be interesting because that is what real sailors really use in real life. The latter is interesting because it eliminates any random variation between what's in the tidal atlas (and how we read it; which diamond we choose), compared to the 5 minute data.

I guess the latter is probably the right approach.
I would think a reasonable test would be to use the hourly data from the tidal atlas to calculate your RYA and SWL CTSs and times required, then use the 5-minute data to plot the theoretical tracks over the ground with each of the respective CTSs, for the length of time calculated by each method. The one closest to the destination "wins".
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Old 08-02-2013, 11:42   #257
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Yes I'd agree , using real hourly tide data , this represents more what would really happen

No need to compute ground tracks. Just use the electronically calculated 5min CTS to reflect a " sort of " real life passage


That's the first test.

The next test is to perturbate the data to simulate real life and look at the respective errors circles

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Old 08-02-2013, 12:20   #258
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Re: Single CTS or follow the Courseline?

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No need to compute ground tracks. Just use the electronically calculated 5min CTS to reflect a " sort of " real life passage
My test is simply to compare the RYA and SWL single-CTS methods. The ground plot will show that steering the same course as the set changes will produce a curving path, and I hypothesize that the compounded errors will result in both arriving a significant distance from destination. It's done without consideration of common error sources such as leeway or steering error, and I would hazard a guess that it would amply demonstrate what you've been saying about relying on a single uncorrected course to destination.
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Old 08-02-2013, 15:11   #259
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Re: Single CTS or follow the Courseline?

A late contribution from me on the VMG question:

Thanks for posting that article, whoever did so.

What I reckon we need to optimise sailing to windward are two measures:

1) Instantaneous Velocity made good through the water in the direction of the instantaneous true wind direction, also as measured relative to the water
('True' as distinct from Apparent, rather than vs Magnetic)

It could be termed "Upwind Vmg(w)"
This is pretty useful as a general aid to optimising sail trim, effectively giving you a point on the polar diagram for the boat under present circumstances.

2) Same except over the ground.
It could be termed "Upwind Vmg(g)"

Seems to me this would be particularly useful when trying to continually re-evaluate the general strategy (not just picking the layline) to reach the top mark ASAP in a strongish current. (eg tack up the shallows inshore, or head up the middle of the harbour?)

Approach 1) has been available since the 1980s, requiring interlinked boatspeed and wind instruments, but leeway is a confounding factor, as is damping of the wind direction

(a better approach to the latter problem, which I suggested at the time and has finally been implemented this last decade, is to factor in the rate of change of heel angle in order to correct for the swing of the wind indicator at the masthead, rather than applying damping, which delays and degrades the delivery of data)

Approach 2) would require fairly sophisticated integration of the GPS with the wind instruments. I imagine it would be tricky to separate leeway from surface drift, tidal streams, ocean currents and other water-movement phenomena, but I guess a software-based 'lookup table' for leeway would do a reasonable job if the boat was kept in good underwater nick, trim and helming were consistent .... It all starts to make my brain hurt a bit.

Those who decided to incorporate Vmg (towards the next waypoint) into GPS without explaining the limitations in each and every manual must have been power boaters, I reckon, and/or marketers....

It would be a start if you could turn a dial to a particular True wind direction, (without fiddling around with waypoints) and get a real-time measure of Vmg in THAT direction.

Personally, when I'm cruising, I'd rather have a very simple GPS, one with a Lat/Long display only, and do everything else myself.

It's more interesting, there's usually plenty of time (there are always other things to do, but I'm not convinced they're more important) and it doesn't need to be done to three digit precision.

I'm horrified at the suggestion a cruising navigator should use a sharp pencil: i prefer to use a carpenter's pencil, and keep it nice and blunt, to remind me that the input data are generally inaccurate. I'd rather be approximately right than exactly wrong.
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Old 08-02-2013, 18:49   #260
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Re: Single CTS or follow the Courseline?

[QUOTE=Dockhead;1151173]
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Originally Posted by Pelagic View Post

I think you might profit from reading the 16 pages of comments

If after reading all that, you still believe in doing it that way, then I'll race you from Needles to Cherbourg at spring tide for pink slips You will use your art form, and I will do a precise CTS calculation. I'd love to have a 65' Van Hellman schooner
Sorry, Dockhead, don’t think I can handle another 16 pages of defining our race conditions....

I have always liked the Moody’s of the past, just not as much as my own SG.

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Old 08-02-2013, 19:05   #261
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Re: Single CTS or follow the Courseline?

There are algorithms in instruments like B&G that calculate leeway. A critical input for that is heel, for which you have great sensors mow like the Maretron heading sensor that includes roll & pitch.
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Old 09-02-2013, 02:20   #262
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Re: Single CTS or follow the Courseline?

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There are algorithms in instruments like B&G that calculate leeway. A critical input for that is heel, for which you have great sensors mow like the Maretron heading sensor that includes roll & pitch.
Indeed. My new B&G Zeus system calculates leeway; also current set & drift -- by comparing SOG & COG with STW and heading.

For that to work -- and I want it to work! -- you need really precise input data -- precise heading, and precise STW. I bought the best fluxgate compass based heading sensor you can buy -- the Airmar H2183 which is three-axis gyro stabilized. Now for STW a paddlewheel transducer is a poor source of data. So I'm hoping that the Airmar finally starts shipping their N2K ultrasonic DST sensor before I have to splash; if not I will probably use the old (and expensive) Airmar ultrasonic speed sensor.
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Old 09-02-2013, 02:23   #263
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Re: Single CTS or follow the Courseline?

I've now installed Admiralty Total Tides graciously donated by a forumite who is a professional mariner.

I'm now reading the manual from cover to cover before starting to play with it. I thought you guys might be interested to hear how tides are predicted:

"Harmonic Constants

The tidal heights and tidal rates for a particular port or stream can be broken down into different components each with a frequency (how often it repeats), an amplitude (the magnitude of the component) and a phase shift (an offset in time relative to the other components). These components are the harmonic constituents and individually they represent simple sinusoidal waveforms but when combined they form the complex waveform that describes a tide.
A special set of harmonic constituents have been defined for use in tidal prediction which correspond to the effects of the different factors that affect the tides (e.g. earth/moon, sun/earth, earth rotation and combinations of these).
These harmonic constituents are conventionally identified by name, for example, MM is the lunar monthly constituent, and K1 is the lunar diurnal constituent. The phase shift of a constituent can be inferred from its name.
The full list of valid harmonic constituent names is as follows:
Sa Ssa Mnum Mm Msf Mf 2Q1 sig1 Q1 rho1 O1 MS1 MP1 NO1 M1 chi1 pi1 P1 S1 K1 psi1 phi1 th1 J1 2PO1 SO1 OO1 KQ1 2MN2S2 3M(SK)2 2NS2 3M2S2 MNK2 OQ2 MNS2 MnuS2 MNK2S2 2MS2K2 2MK2 2N2 mu2 SNK2 NA2 N2 NB2 nu2 2KN2S2 MSK2 OP2 MA2 MPS2 M2 MSP2 MB2 MKS2 M2(KS)2 lam2 L2 2SK2 T2 S2 R2 K2 MSnu2 MSN2 KJ2 2KM(SN)2 2SM2 2MS2N2 SKM2 3(SM)N2 SKN2 MQ3 MO3 2NKM3 2MS3 2MP3 M3 NK3 MP3 SO3 MS3 MK3 2MQ3 SP3 S3 SK3 K3 4MS4 2MNS4 3MK4 2N4 3MS4 2NKS4 MSNK4 MN4 Mnu4 MNKS4 2MSK4 MA4 M4 2MRS4 2MKS4 SN4 3MN4 NK4 M2SK4 MT4 MS4 MR4 MK4 2SNM4 3MSN4 2MSN4 S4 SK4 3SM4 2SKM4 MNO5 2NKMS5 3MK5 2NK5 3MS5 3MP5 M5 MNK5 MB5 MSO5 2MS5 3MO5 3MQ5 2(MN)S6 3MNS6 4MK6 M2N6 4MS6 2NMKS6 2MSNK6 2MN6 2Mnu6 2MNKS6 3MSK6 MA6 M6 3MKS6 MSN6 4MN6 MNK6 2(MS)K6 2MT6 2MS6 2MK6 2SN6 3MSN6 MKL6 2SM6 MSK6 S6 2MNO7 4MK7 2NMK7 M7 2MNK7 2MSO7 MSKO7 5MK8 2(MN)8 5MS8 2(MN)KS8 3MN8 3Mnu8 3MNKS8 4MSK8 MA8 M8 2MSN8 2MNK8 3MS8 3MK8 2SNM8 MSNK8 2(MS)8 2MSK8 3SM8 2SMK8 S8 3MNO9 2(MN)K9 MA9 3MNK9 4MK9 3MSK9 3M2N10 6MS10 3M2NKS10 4MSNK10 4MN10 4Mnu10 5MSK10 M10 3MSN10 6MN10 3MNK10 4MK10 2MNSK10 3M2S10 4MSK11 4M2N12 4M2NKS12 5MSNK12 5MN12 5Mnu12 6MSK12 MA12 M12 4MSN12 5MS12 5MK12 3MNKS12 4M2S12 5MSN14 5MNK14 6MS14 "
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