Fly-by-wire?

[RupertW]

Re: No, No, No!

Your banana shaped course assumes that the tide is in the same direction throughout the crossing. A typical sailing crossing has the tide first one way, then the other, and describes an S shape.

You work out how the tide nets out over the whole of your estimated passage time, then set your course to the east or west of your target port, as appropriate. You cover less distance through the water this way compared to sticking to a straight line over the ground, therefore arrive sooner.

Of course, you recalculate every now and again if your speed is different to your estimate.

<hr width=100% size=1>

 

[qsiv]

Re: No, No, No!

I'd argue that you should set up a series of waypoints describing the curved course (my software would do them at about 15 minute intervals), and then get the pilot to follow these points (I euphemistically call these 'leglets'). In a better scenario, one would simply recalculate the optimum route and update the pilot with the course you require on a more or less constant basis, whether you describe this course as a CTS, an AWA or TWA requirement is a separate issue!

<hr width=100% size=1>

 

[Twister_Ken]

Hear, hear. nm

1

<hr width=100% size=1><A target="_blank" HREF=http://www.writeforweb.com/twister1>Let's Twist Again</A>

 

[bedouin]

Re: Yes, Yes, Yes

If you excuse me saying so - that is a typical powerboater approach.

For a (slower) sailing yacht tide streams become a lot more important. We tend to calculate the compass course that will bring us to our destination. allowing for all tidal streams.

That leads to the shortest Water track, hence shortest time, even though the ground track will be longer.

<hr width=100% size=1>

 

[stuartw]

Re: Yes, Yes, Yes

If you excuse me saying so, that is a typical yottie thing to say.

I am only too well aware of varying tidal streams and the effect it has on slower moving boats, particularly with large keels.Do you really think that a motor cruiser is not effected by tidal streams or wind.
But what you are doing is setting a course to counteract that stream, no matter which direction it is coming from. In other words you are manually steering to counteract it. Therfore your rudder is always set other than dead ahead. This is precisely what the autopliot does for you. It is heading for a waypoint, which is your fixed peg in the ground. So if the XTE increases which it surely does, then the rudder is set to counteract it. Even if the tidal stream alters, then the XTE will increase the other way, and the rudder will be set to counteract the offset.
What YOU are doing, is thinking way ahead and trying to counteract what the tidal effect is going to do. What the autopilot is doing for you is to incrementally readjust your rudder dynamically, and in constant conditions, it will be doing exactly the same as you, but doing it earlier.
If you have a Autopilot with GPS then try it, & prove me wrong.

<hr width=100% size=1>

 

[Twister_Ken]

Same hymn sheet.

Could be we are both signing from the same hymn sheet here. As I understand it, what you are saying is if I were to set an interfaced autopilot to steer to my final waypoint in a x-tide situation, it would attempt to keep XTE at 0 by always steering to keep me on the rhumbline. Yes?

Effectively, therefore, the AP is steering me into a small component of the tide. Yes?

In which case I'm sailing into the tide at all points of the x-ing, except slack water. Put another way, the AP/GPS is compensating for the gross tidal offset. Yes?

In which case I'm sailing through more water, than if I disconnected the AP/GPS interface and merely set a compass course on the AP to take me to my final destination, adjusted for the nett tidal offset.

On a typical x-channel trip of approx 12 hours (one tide cycle), which is what a 5 kt sailing boat would take for Needles/Cherbourg. I might experience say, 10 miles of east going current and 12 miles of west going. Therefore I would set a compass course which would take me 2 miles east of my destination. The tide will take me 10 miles east and 12 miles west and Chbg will be on the nose when I get there. (For the sake of simplicity I've left leeway out of this)





<hr width=100% size=1><A target="_blank" HREF=http://www.writeforweb.com/twister1>Let's Twist Again</A>

 

[bedouin]

Re: Yes, Yes, Yes

I think we are talking at cross purposes then let me start from scratch to explain:

When setting out e.g. to cross the channel, I would not set the Autohelm to follow the GPS at all. Rather I would set it to the pre-determined course that, once allowing for the tidal streams.

The result would be that with a perfectly balanced boat, and perfectly calculated course, the rudder would never deviate from straight ahead.

If I understand your reasoning your idea would work correctly if there were a constant tidal stream throughout the crossing; that may be a reasonable assumption on a Motorboat doing 15+ kts, but my typical channel crossing time is 12 hours.

The autopilot can only react to the conditions you are experiencing now - it can never think ahead and allow for changes to the stream caused either by the tide turning or by the stream changing as you get closer to land.

I cannot think of any way of steering to GPS on a cross-tidal channel crossing using the "steer to GPS function".

<hr width=100% size=1>

 

[stuartw]

Exactly! - well almost

Yes I fully agree with your synopsis. However I do not agree with the fact, by steering to your final WP with a fixed bearing offset, you will have covered less water. The AP will in fact be doing exactly the same, but doing it differently.
If you agree in principal that the AP can be effective, then please tell me, why don't the die hard yotties who have an AP with GPS input, use it? There does seem some reluctance to either accept the principal that it works or even try it.
It would seem the original question is in fact satisfied with the use of AP + GPS (XTE)

As I previously stated, I have talked to many yotties, who would not dream of setting up a multi waypoint plan, even with the benefit of electronic chartplotters. Seemingly, they go from one new waypoint to the next, and use a windvane input to keep the boat correct to the wind.
I would like to think this discussion has opened a few eyes, but somehow I get the feeling that they've done it the traditional way for so long, it is difficult to change.
Whether we agree that you or the AP can get you to the final WP covering less water or not, surely it must take the strain out of constant re-assessment of the situation.

<hr width=100% size=1>

 

[RupertW]

A pedantic reply

Quick check on the maths of the argument (someone please correct me if I'm wrong)

Scenario - 50 miles at estimated 5 knots, so about 10 hours. 4 hours X-tide one way at average 2 knots, 6 hours tide the other way at average 2 knots.

The first boat looks at the net tidal effect of 2 hours x 2 knots and aims the boat 4 miles off the target point to reach it. This boat therefore has to fight the X-component of the tide for 4 miles, and covers 50.2 (I've left out the workings, but it's basically drawing a triangle) miles at 5 knots through the water to reach the target port.

The second boat keeps a zero XTE and therefore has to fight the X-component of the tide for 8 miles one way, then 12 miles the other, and covers 53.8 miles through the water and therefore reaches the target port over half an hour later.

At 10 or 12 knots boatspeed the differences are far smaller.

<hr width=100% size=1>

 

[goupil]

Re: Same hymn sheet.

Keep your line:
That's just the way I used to Xing, sophisticated GPS or not.
As long as wind is steady.

<hr width=100% size=1>Brittany Yacht brokerage & assistance : PM

 

[stuartw]

A manipulated reply.

Nicely done, to take a set of figures to prove an argument. You've conveniently forgotton about that bit in the middle, where there may be slack water, so correction isn't necessary. Yes I do understand what you are trying to say, but life, tides and weather cannot be packaged into a neat algorithm.
There may be the odd occaison when your scenario IS true, but you should know better than me about the dynamics of wind and tide. The chances are, that there are varing current eddies, veering winds which can knock you off your intended course. So you have to re-plan for a new WP, or a new bearing. Your maths is correct, but your scenario is far, far from reality.
No, I'd rather let the AP take the strain, and if I arrive 1/2 later so what. At least I have't had to struggle with re-assessing the situation every few miles.


<hr width=100% size=1>

 

[david]

Re: A pedantic reply

Excellent answer Rupert, both Bedouin and Twister Ken tried but failed to get stewartw to see the light, so let him waste fuel /forums/images/icons/smile.gif

<hr width=100% size=1>David

 

[RupertW]

Re: A manipulated reply.

Fair points, although by going for an average I was taking slack water into account.

It's only about half an hour, as you say, and I think we all do the bits we enjoy, and are lazy about the rest. I like passage planning (and re-planning), but I'm lazy about things like adjusting mast-bend, or storing the boat for perfect trim, or putting up spinnakers - all of which will probably add more than half an hour to my time.

I wonder what the offshore racers amongst us think about the priority of net-tide planning - they'll know far more than me about how important or relevant it's likely to be be compared to other factors.




<hr width=100% size=1>

 

[AlanPound]

Re: A pedantic reply

I felt that Twister Ken's answer was very clear.

The argument for a single heading all the way across is simpler still perhaps -

"The shortest (water) distance between two points is a straight line (forget great circles for the moment). The fact that the water moves in the interim is irrelevant, provided you end up exactly where you want to go."

The reason that everyone sticks to the 'traditional' or 'conventional' method is simple. It happens to be the best one (ie. the optimal solution was found a long, long time ago).


<hr width=100% size=1>

 

[Rich_F]

All this talk of auto-helming to apparent wind angle. Does it work?

I've got an ST2000+, but it's not currently interfaced to the wind instruments. Is it worth the effort, or am I likely to be disappointed with the result?

[I would prefer to get wind-vane self-steering, but maybe the autohelm could be a stop-gap until I win the lottery].

Rich

<hr width=100% size=1>

 

[Twister_Ken]

Re: A manipulated reply.

As an ex -race navigator (pre the sophisticated course computers that we see nowadays) nett tide planning was important, but so was leeway, and more importantly on longer courses, predicted wind behaviour. If there was any chance the wind would head you on the course to the next mark, the priority was always to make your ground to windward early, so that you are not forced to tack on the header later. And let's not forget leebowing either if all else was equal.

<hr width=100% size=1><A target="_blank" HREF=http://www.writeforweb.com/twister1>Let's Twist Again</A>

 

[david]

Re: A pedantic reply

Yes Alan, Twister Kens answer is very clear, and so is yours. It's a shame stewart won't admit he's wrong wrong wrong...

You can't learn um nuffin these days can ya? /forums/images/icons/smile.gif

<hr width=100% size=1>David

 

[david]

Ssssshhhh Don\'t mention leebowing

Ssssshhhh Don't mention leebowing or you will wake NigeCh up /forums/images/icons/smile.gif/forums/images/icons/wink.gif

<hr width=100% size=1>David

 

[qsiv]

Yes it certainly does work - mine is a 7000 system, driven off the masthead wind instruments does just fine. I use most of the time, as I am to greater or less extent single handed most of the time, and the pilot allows me to do other important things like pilotage, nav, keeping watch, running galley, sorting out arguments between children and other routine matters on passage. It's only on the odd passage race that I find I get to helm for any real periods. Upwind it's as good as some, downwind its probably not as good as someone you would entrust with the helm in a breeze.

<hr width=100% size=1>

 

[Rich_F]

Thanks - sounds like I should wire it up and give it a try.

I have been a bit suspicious of the concept, thinking that the masthead movement due to the yacht rolling would cause the autohelm to weave around. Sounds like I should give Raymarine more credit!

Cheers,

Rich

<hr width=100% size=1>