What Heading Am I Really On ?

[pteron]

Originally Posted by btrayfors

Nope, in most situations it will mean that you'll travel on a more direct (rhumb line) course and take a shorter time with less fuel. The whole idea is to stay as close to the rhumb line as you reasonably can do, by making small corrections whenever you see the GPS bearing-to-mark change. This takes into account shifts in drift and set.

Bill

Sorry Bill but you are wrong. You will sail the shortest distance over the ground but the longest distance through the water.

To prove it imagine an English channel crossing, 60 miles at 5 knots boat speed. For arguments sake, assume an east going tide of 2 knots for 6 hours followed by a west going tide of 2 knots for 6 hours.

With your technique you will stem the tide in each direction to keep you on the rhumb line. Your true boat speed will drop to about 3.5 knots and you will travel further through the water.

The correct thing to do is aim straight across allowing the tide to sweep you first one way then the other. Your water distance will be 60 miles and your average speed 5 knots.

[boasun]

With the GPS set in magnetic mode, the COG has the Current & leeway buried in that course given... But what it does not show is the deviation of your vessel. And that can be, if your compass has NOT been compensated, a large difference between your compass and the GPS. So to reduce the error you need to have your compass swung and the deviation reduced to the least amount.
But you should be steering on you ship's compass and not the GPS... And keep your tool box, knives, belt buckles, and any other portable magnetized metals away from your Compass. For obivious reasons....

[DotDun]

Originally Posted by btrayfors

DotDun,

The rhumb line is the direct, constant bearing course from where you started to the next waypoint.

Not all autopilots implement XTE (cross-track error) corrections the same way. Most older ones, I believe, just reset the heading toward the mark. One newer one tries to get back onto the original rhumb line before resuming course and, in so doing, sometimes will drive you crazy with sudden substantial turns.

There's an excellent article on this in the April Cruising World: The XTE issue, autopilot behavior & electronics dollars | Cruising World

Bill

I've only owned 2 APs, both Raymarine, and both pay attention to XTE. It's hard for me to imagine how an AP can 'reset the heading toward the mark', as that would require an algorithm that calculates distance/angle/etc. Plus, the AP would then be taking responsibility for the rhumbline, i.e. not hit anything.

I agree, additional features like XTE have a 'cost'. The article you reference obviously covers the art of building AP software. How fast do you want the AP to return to the rhumbline? I have the RM SPx10, assumably it uses the same algorithms as the one cited in the article. And I do agree with the article that it *can* be annoying when you dodge a crab pot and tell the AP to resume the track. My chartplotter is a NavNet3D, which by the way, talked perfectly to the RM AP from day 1 on the N2K network. With my setup, when I dodge something, if the AP is 'too fast' trying to get back to the rhumbline, it's very simple to tell the NavNet3D to 'restart' the course, which sets a new rhumbline from the current position. Also, you can change the responsiveness of the AP (choices are 1-9, default is 5, I set mine to 2) to slow down the radical 'hunting for the rhumbline'.

But the bottom line is that XTE has many more positives than negatives. On the recent passage I cited, without XTE, one of two things would have happened:

1) The Yucatan Channel current (3.5kts) and then the opposite direction from the Gulf Stream loop current (1.5kts) would have pushed us way off course and added significant distance to the trip.

2) The grueling work for a helmsman to continually correct the heading to match the ever-changing current and maintain the rhumbline probably would have created a mutiny from my crew.

Understanding the information the AP has to work with goes a long way to accepting the anomalies of XTE.

[DotDun]

Originally Posted by wolfaroo

Navigating any passage over about one hour in duration in tidal waters by staying close to the rhumb line (and/or using XTE) without considering the wider strategical situation will undoubtedly mean you will; travel further/ use more fuel/take longer.

I don't believe this statement covers all cases. For the passage I cited, the current was aft of beam at all times with the stronger current the furthest aft. I didn't share the rhumbline going the other direction. Had I followed the current, it would have added at least 10% to the distance with less than 10% to the overall speed. The passage would have taken more time, hence more hours burning fuel.

[wolfaroo]

Originally Posted by DotDun

But the bottom line is that XTE has many more positives than negatives. On the recent passage I cited, without XTE, one of two things would have happened:

1) The Yucatan Channel current (3.5kts) and then the opposite direction from the Gulf Stream loop current (1.5kts) would have pushed us way off course and added significant distance to the trip.

2) The grueling work for a helmsman to continually correct the heading to match the ever-changing current and maintain the rhumbline probably would have created a mutiny from my crew.

With the greatest of respect:

1) whilst, yes, you would have been pushed 'off course' (off the rhumb line that is), it would have actually REDUCED the overall distance of the trip, therefore taken less time and/or fuel.

2) continually correcting for an ever-changing current in order to maintain the rhumb line, unless contrained by hazards etc, is adding significant distance/time/fuel consumption to your passage. If planned properly, the helmsman would have had to maintain a steady single heading only, thus making a mutiny unlikely!

Please rest assured I am trying to enlighten not to criticise and I therefore you read this with the spirit in which is was written.

[DotDun]

Originally Posted by wolfaroo

Please rest assured I am trying to enlighten not to criticise and I therefore you read this with the spirit in which is was written.

I understand and would appreciate more explanation.

[pteron]

Originally Posted by DotDun

1) The Yucatan Channel current (3.5kts) and then the opposite direction from the Gulf Stream loop current (1.5kts) would have pushed us way off course and added significant distance to the trip.

2) The grueling work for a helmsman to continually correct the heading to match the ever-changing current and maintain the rhumbline probably would have created a mutiny from my crew.

Understanding the information the AP has to work with goes a long way to accepting the anomalies of XTE.

Your boat sails in the water. Imagine you are trying to cross an enourmous fish tank. No currents or tides. You agree that the shortest distance is straight across?

Now imagine a giant moves the fish tank in a big circle 1000 miles in circumference but returns it to the same spot. Your shortest distance through the water is still straight across the fish tank, yet you have travelled over a thousand miles over the ground. You have used only the fuel needed to cross the tank and taken exactly the same amount of time to cross as you would have without any external movement of the tank.

The fish tank is your tide. It is the sum of the tides, i.e. where the tide would make you end up that is the only thing that matters (rocks aside!).

[DotDun]

Originally Posted by pteron

Your boat sails in the water. Imagine you are trying to cross an enourmous fish tank. No currents or tides. You agree that the shortest distance is straight across?

Now imagine a giant moves the fish tank in a big circle 1000 miles in circumference but returns it to the same spot. Your shortest distance through the water is still straight across the fish tank, yet you have travelled over a thousand miles over the ground. You have used only the fuel needed to cross the tank and taken exactly the same amount of time to cross as you would have without any external movement of the tank.

The fish tank is your tide. It is the sum of the tides, i.e. where the tide would make you end up that is the only thing that matters (rocks aside!).

Doesn't this assume there is an offsetting tide/current? In my case the initial and offsetting currents weren't equal. IOW, I would have been pushed further one direction than the other.

[pteron]

Originally Posted by DotDun

Doesn't this assume there is an offsetting tide/current? In my case the initial and offsetting currents weren't equal. IOW, I would have been pushed further one direction than the other.

You only need to counteract the vector sum of the tides. So if you would have had 3 knots pushing you to port for an hour and 1 knot pushing you to starboard for an hour, your resultant tide is 2 knots to port for one hour. You plot your course to correct for that resultant.

[btrayfors]

All very illuminating, gentlemen, but if anyone on this Board is smart enough to calculate -- in advance -- the average set and drift (current and leeway) for, say, a Gulf Stream crossing from Miami to Bimini without knowing precisely the set (current direction and speed), wind and sea conditions and resultant drift (leeway) which will obtain during the actual crossing, I'd like to hear about it.

As for me, I'll take she shortest GEOGRAPHIC route between waypoints, thank you. Then, should expected or actual conditions change, you're still on the shortest GEOGRAPHIC route (course over ground) to the waypoint.

You can on paper conjur up all kinds of scenarios with the direction and strength of tidal currents changing, boat speed constant or varying, wind direction and speed changing, etc., etc. Well and good, but when you're actually out there doing the crossing I think you'll generally want to stick close to the original rhumbline.

I suppose for one not-so-faint-of-heart it would be appealing to let a 3-knot cross current working on you for 4 hours take you 12 miles from the rhumbline in hopes that a reversal will take place afterwards -- and that the wind won't change and that boatspeed will remain the same, etc. -- but that kind of gambling is too rich for my tired blood, thank you.

JMO :-)

Bill

[pteron]

Originally Posted by btrayfors

You can on paper conjur up all kinds of scenarios with the direction and strength of tidal currents changing, boat speed constant or varying, wind direction and speed changing, etc., etc. Well and good, but when you're actually out there doing the crossing I think you'll generally want to stick close to the original rhumbline.

I suppose for one not-so-faint-of-heart it would be appealing to let a 3-knot cross current working on your for 4 hours take you 12 miles from the rhumbline in hopes that a reversal will take place afterwards -- and that the wind won't change and that boatspeed will remain the same, etc. -- but that kind of gambling is too rich for my tired blood, thank you.

Bill,

we do it every time we cross the channel. In many places tides are predictable to easily enough accuracy to make it no gamble at all. I think you are being disingenuous in saying 'hopes that a reversal will take place' - no hoping necessary at all, if you don't have the tidal data you can't correct for it.

You can obviously do whatever you choose, but it doesn't change the fact that you will use more time and fuel!

[wolfaroo]

Originally Posted by DotDun

without XTE, one of two things would have happened:

1) The Yucatan Channel current (3.5kts) and then the opposite direction from the Gulf Stream loop current (1.5kts) would have pushed us way off course and added significant distance to the trip.

2) The grueling work for a helmsman to continually correct the heading to match the ever-changing current and maintain the rhumbline probably would have created a mutiny from my crew.

Originally Posted by DotDun

I understand and would appreciate more explanation.

No worries, I will try my best - it might be difficult without diagrams!

I don't know the distances involved in your example but the same theory applies so it doesn't really matter. E.g. your destination lies 100nm due North (000T), boat speed through the water is 7kts, the rhumb line is 000T and we will encounter two significant tidal streams, the first flows west at 3.5kts for 5 hours, the second flows east at 1.5kts for 6 hours.

If you were to use XTE to stay on the rhumb line, you would have to first alter course to starboard by about 27 degrees to correct for the 3.5kts of westerly tidal flow (7kts boat speed, 3.5kts cross current from E to W). You are now steering along the rhumb line for 5ish hours with the boat heading 027T - your velocity made good towards final destination is now reduced because in order to travel 7 kts along the rhumb line 000T, you have to travel 7.8kts at the current heading of 027T - i.e. you have to travel 0.8kts further through the water.

After 5 hours you have travelled along the 000T rhumbline 31.4nm, and have travelled 35nm through the water.

The tidal flow now (instantaneously) changes to 1.5kts W to E (using simple tidal flow for easier example!). In order to remain on the rhumb line 000T, you alter heading to 348T (7kts boat speed, 1.5kts cross current from W to E). You now have to travel 7.2nm through the water on heading 348T in order to move along the rhumb line 7nm.

After 6 hours, you have travelled along the 000T rhumb line a further 40.8nm and through the water 42nm.

After 11 hours, total distance run through water = 77nm, total distance run along rhumb line = 72.2nm. We will assume remaining distance along rhumbline of 27.8nm is through slack water for this example.

Finally therefore total distance through water was 27.8 + 77 = 104.8nm

Now, instead of following XTE...

If you calculate the net or compound tidal flow before you set sail; 1.5kts east for 6 hours (+9kts) and 3.5kts W for 5 hours (-17.5kts) gives -8.5kts (flow E to W) and adjust heading accordingly (steer 005T) for the entire passage (helmsman less likely to mutiny!), whilst you won't actually stay along the rhumb line (you will drift angle ~19deg west away from rhumb line for 5 hours then angle ~17deg east back towards rhumb line for 6 hours), you have less water to travel through.

Rather than correcting against 17.5 kts of tidal flow in one direction and 9 kts in the other, you have effectively allowed the opposing flow to cancel each other out and are only correcting against the net flow of 8.5kts. The resultant heading is 005T and total distance through the water is therefore 100.5nm.

By not using XTE/correction to remain on rhumb line, the passage is 4.3nm less through the water, which means you order a cold beer at the marina bar 35mins earlier and have saved 4% on your fuel bill, not to mention your helmsman didn't create a mutiny

This may prove impossible to follow without a chart (it is so very much easier than I have made it sound here, apologies) and this example may not seem such a difference, but at slower boat speeds and/or larger tidal streams the differences can be substantial.

I hope this is of some help. I'm off to grab a beer!

Cheers.

[bangkaboat]

Wow!
Very simply, use a range marker(see chart), check to see whether compass, or AP's fluxgate, or whatever is giving you the most accurate fix.

For any new sailors reading this & becoming confused by types of north, etc., a simple trick I learned in the cg was:

Can (compass north)
Dead (deviation)
Men (magnetic north)
Vote (variation)
Twice (true north)
At Elections (Add going East)

Say it to yourself a few times & you'll never forget it.

[btrayfors]

Originally Posted by pteron

Bill,

..... I think you are being disingenuous in saying 'hopes that a reversal will take place' - no hoping necessary at all, if you don't have the tidal data you can't correct for it. .....

No, not disingenuous at all (and I really don't appreciate that implication).

I'm just reflecting the real world conditions in the waters I've sailed.

Even armed with tidal predictions and current predictions and pilot charts and other navigational trappings, there are many areas of the world where current behavior just isn't as predictable as it apparently is for you in the English Channel.

Some professionals make a living and a lifetime's work trying to predict the path of the Gulfstream along the U.S. East Coast, trying to work out it's location, it's average speed, it's width, and it's Eddys (counter-current loops).

Here are a few other examples, among many:

“ (Gulf of Mexico) Government officials and scientists from Mississippi to Florida are holding their collective breath to see whether a strong but unpredictable current in the Gulf of Mexico, known as the Loop Current, will continue to expand north toward Louisiana. Two days ago -- the latest time for which satellite data are available -- the current sat 125 miles south of the spill, its rotating tendrils licking at the slick's eastern edge.” NY Times 5/5/10

“Whitbread crews brace for 'tricky' Chesapeake Unpredictable currents, winds feared in stage here. April 18, 1998|By Ellen Gamerman | Ellen Gamerman, SUN STAFF

FORT LAUDERDALE, Fla. -- The boats in the Whitbread Round the World Race are making last-minute sail tests and weather checks before starting the sprint to Baltimore tomorrow, although they know there is only so much they can do to prepare for the fluky Chesapeake Bay.

"I think the shorter legs are actually more wide open in terms of outcome," said Paul Cayard, skipper of first-place Swedish boat EF Language. "There's potential for any boat in this leg to lead."

Crews are mostly worried about unpredictable wind and current conditions in the bay -- variables that could turn the leader of the 870-nautical-mile sprint into the loser by the finish. The boats are scheduled to arrive in Baltimore sometime Wednesday.”

“Tidal flow (in Cook Strait)

The tidal flow through Cook Strait is unusual. On each side of the strait the tide is almost exactly out of phase, so high water on one side meets low water on the other. Strong currents result, with almost zero tidal height change in the centre of the strait. Although the tidal surge should flow in one direction for six hours and then the reverse direction for six hours, a particular surge might last eight or ten hours with the reverse surge enfeebled. In especially boisterous weather conditions the reverse surge can be negated, and the flow can remain in the same direction through three surge periods and longer. This is indicated on marine charts for the region.[10]

There are numerous computer model representations of the tidal flow through Cook Strait. While the tidal components are readily realizable,[11] the residual flow is more difficult to model.[12]”

You get the point!

Hey, there's little question that IF you could reliably model the conditions you're going to encounter on each leg of the trip, THEN you could work out boat headings which would save some time (and fuel, if you're motoring). The savings would be greater for slower boat speeds, no doubt.

My contention, though, is that in the real world there are many many situations where this just isn't realistic, and sticking to the rhumb line is likely to be a better bet.

Bill

[DotDun]

Originally Posted by wolfaroo

I hope this is of some help. I'm off to grab a beer!

Cheers.

I appreciate your explanation, and I'll have a beer in a few minutes.

Given the only data available was as shown in the attachment (current expectations highlighted in yellow), you would have let the current take you north, expect the lighter south bound current to assist later all while maintaining the autopilot heading on the (ever-changing) bearing to the waypoint vs. staying on the rhumbline?

One other point to consider is the condition of the seas. If the wind is opposite the current, the ride is terrible and most will get out of the current as quick as possible.