When the skipper comes to the bridge and wants to know why the XTE is whatever it is, then your mates start driving via the ECDIS.
I could honestly care less about cross track error unless a shoal is nearby. I’m usually more questioning the amount of leeway the mates are holding in open ocean to stay on the trackline and sacrificing speed to do so. It’s a big ocean, we can steer for the waypoint after this eddy gives a 1 knot boost for a few hours.
Setting the XTD (cross-track) distance is important because the area inside is what’s going to be checked when the route safety check is done. Skipping this step is how you wind up on Varn Bank.
So the XTD has got to be inside all hazards, aids etc on the approach and inside the ports. But in the open ocean I use 5 miles so the check will find all hazards within that distance.
Most mates seem to stay within a mile or two at sea but some get compulsive about it.
And if anything happens and you’re outside the cross track you’re fucked worse than if you’re inside it.
Staying within meters of the track gives mental simulation on boring watches. Unless there’s a reason not to be, like crossing the Gulf Stream, why not?
Well, the mate’s instructions are to follow the voyage plan. Which would be to obey COLREGs with a 2 mile CPA and follow the track-line. In practice in mid-ocean a XTE of under two miles has always been acceptable in my experience. I usually kept it to less than1 or 1-1/2 miles. That keeps it within 2 miles. I don’t object if mate wants to keep in within meters but it seems unnecessary. Maybe an inability to be aware of or avoid compulsive behavior?
As far as XTD, that controls the area that’s been checked to be free of hazards. I think a 3 miles XTD should be fine in mid-ocean. My thinking on the 5 miles is the mate has a XTE of 2 miles and than needs a 2 mile CPA the ship will be 4 miles off track. Than add a mile for charting/position errors.
I would correct for leeway unless the captain explicitly told me not to. I don’t think the argument that it saves fuel holds up because thinking in terms of vectors the increase in SOA is going to be canceled by the increased distance.
I am typically more concerned with speed and a mate holding 8 degrees of leeway against the current can mean the difference of 1-2 knots in some spots. This is open ocean of course.
I’m not sure. In practice if I was getting set excessively I’d split the difference and maybe take off half or less and wait to see what happens. If I was within a 1-1/2 miles or so I’d be satisfied if the ship was running parallel to the track. Except of course if the capt instructed otherwise.
I’ll take being set 8 miles off of the track line and maintaining speed any day unless we are near some sort of obstruction or coastline. You can always shape up on the waypoint after the current or wind abates. You never get that speed back without burning extra fuel. Maybe that’s just from the tight schedules I keep but it is my philosophy.
Same here, but I’ve sailed with a Captain that would have your ass if you were more than 500 yds off it too.
I don’t really think he’s a good example since he also got angry about ships getting in front of us when our ECDIS track clearly showed them where we were going. (Yep, he actually thought other ships could see it on theirs)
While it’s not necessary it helps keep one alert/awake. Mental engagement plays a big part in alertness.
When I see someone that’s just cruising along at a mile or more off track it makes me wonder if they have any idea where they are. If they’re on watch completely oblivious to their position, what else aren’t they paying attention to?
I used to obsess about COG vs CTS, not so much about XTE. Every half hour I’d lay an ERBL down our track to figure out the average course, then compare the reciprocal to CTS and make sub-degree course adjustments. This was indeed more to keep myself occupied and make pretty track lines than it was about efficiency, in the same genre as my obsession with perfect constant-radius turns.
I had a nagging suspicion that this wasn’t the most efficient approach, but got lost when trying to compute the problem for a transverse current of sinusoidal speed, because that relies on mathematical tools that I’ve neglected to maintain. However, when I reduced the problem to an imagined current of constant velocity that reverses instantly, the answer became rather obvious.
Let me put it thus: In a vector triangle, do you want to be the hypotenuse or the leg?
Okay, I realize that my rationale might have been a bit cryptic, so here’s an example: You’re making 10 knots STW and CTS is 0 degrees. The tidal current sets E and W at 1 knot.
Case 1: You maintain a heading of 0 degrees. For the first six hours, your COG is 5.711 degrees and your SOG is 10.05 kts. For the next six hours, your COG is 354.289 degrees. You end up back on your intended track, and VMG has stayed 10 knots throughout.
Case 2: You bear first starboard then port by 5.711 degrees to maintain a COG of 0 degrees. Your SOG drops to 9.95 knots, which is also your VMG.
Of course, my example has extraneous precision, but that’s needed to illustrate the point. Also, things get vastly more complicated once the current sets slightly from ahead or astern, not to mention when you consider partial tidal periods, non-tidal current components, etc. I actually considered this for a thread of its own, but here we are, and I don’t have so much to add on the subject anyway.
Yeah, I’m not sure at this point. My first thought was it’d be a wash. There are two different cases, the ship get sets back in a tidal type or non-tidal current change and it doesn’t.
Without digging it too deep, now that continuous positioning is available I’d put some weight on the fact that the two mile wide “highway” is more or less the standard. In an ocean crossing however it doesn’t seem like 8 miles is significant but if it’s the case it’s faster what’s the best practice? Why stop at eight? I’d like to see things on a firmer basis before I depart from SOP.
The charterer does look at fuel, also trim (down by the head), min ballast, some look at min fuel ROB to reduce displacement I’ve never seen a recomend to open the XTE, why not?
Another issue is I use the computer to generate the fastest route. The initially generated route is usually is made up of many short legs. Are we saying that is not in fact the fastest?
I smooth out the many short legs to longer legs and the total time increases by a few minutes. Which I ignore because a new weather update changes the time by far more, most often by several hours depending on the weather situation.
Does the program take into effect the rudder efficiency? All that turning moment has to come from somewhere. At what rudder angle does it start becoming a meaningful input to the problem?
The auto pilot takes care of that. There are settings for the precision of the course keeping. At sea the fuel saving mode is used which alllows the ship to depart more from the course.
Yes, solving the same triangle twice, once for distance then as a vector. I agree with the solution of case 1 and 2.
So is there a quantifiable balance between fuel-saving steering mode and the increased path length it would generate, all else being equal?
No, that’d be above our pay grade. The manuals would be too big to be useful for the crew. There are three modes, depending on the requirements for precision and when to use them.
The one time I needed help with an advanced auto -pilot feature the techs asked me to email a scan of the course recorder record to them.
Shipping is by far the cheapest mode of transport on the planet – but because the individual dollar amounts are enormous there is plenty of motivation to put computers to work in optimizing this stuff. I predict that at some point the autopilot will be talking to the course planner and the ship’s handling database and AIS and radar and maybe even a smart version of the dumb pelorus. And likely have an eye on the Chief’s sleeve oil account as well.
Google adaptive autopilot, here’s a hit with that and our old friend the Kalman filter.
Again, what doesn’t make sense to me is if I use the routing software to generate the fastest route, it generates the actual route to follow. It’s not a series of headings to use.
I guess the argument would be that the weather data is insufficiently granular to capture the ideal route. Seems like that would lead to the question of what is the optimum allowable XTE?
My observations would lead me to believe that there is a lot of randomness in current, eddies and so forth, they could be ignored, wind not so much, not much change in wind over short distances and time.
My guess is two miles XTE is going to be in the ballpark.
EDIT: Eight miles doesn’t seem unreasonable either on a long leg. Seems like returns diminish the greater the XTE.
The chief’s “sleeve oil” has already gone out the window with the introduction of fuel meters with shoreside monitoring.