Inaccurate RYA Teaching : CTS - Quest For a New Method

[Paul Elliott]

Originally Posted by Seaworthy Lass

If the Expedition software doesn't tell you what tack to be on with a headwind, this would be one way the software could be improved. [...]

Note: Computers don't tell you what wind is ahead, but your eyeballs can. Watching the pattern of wind on the water ahead is absolutely critical in a race and software is useless in this instance to tell you what to do .

Trust me, Expedition knows all about tacking on headwinds! Expedition also knows all about the (forecast) winds ahead. The forecasts don't handle the conditions we see in nearshore sailing, where the headlands create those interesting eddies, funnels, and dead-zones, but at sea they usually get the big patterns right. The navigator and crew are always fine-tuning for actual conditions, but Expedition (and other programs) can do a good job of working out your strategy through the next 3-5 days. You will re-run the simulation whenever you get data from a new wind forecast model run, or you deliberately diverge from the suggested route.

Of course, there are times when I decide that I know more than expedition, and adjust a route to minimize the risk when a forecast might miss a shift in the highs / lows. Or, when I just say "Screw it! I want to sail in *that* direction!". Then again, I'm not a rock-star pro racer, I just like to sail.

[Dockhead]

Originally Posted by Paul Elliott

Trust me, Expedition knows all about tacking on headwinds! Expedition also knows all about the (forecast) winds ahead. The forecasts don't handle the conditions we see in nearshore sailing, where the headlands create those interesting eddies, funnels, and dead-zones, but at sea they usually get the big patterns right. The navigator and crew are always fine-tuning for actual conditions, but Expedition (and other programs) can do a good job of working out your strategy through the next 3-5 days. You will re-run the simulation whenever you get data from a new wind forecast model run, or you deliberately diverge from the suggested route.

Of course, there are times when I decide that I know more than expedition, and adjust a route to minimize the risk when a forecast might miss a shift in the highs / lows. Or, when I just say "Screw it! I want to sail in *that* direction!". Then again, I'm not a rock-star pro racer, I just like to sail.

I wonder if this program can work out routing for passages through areas with complex rotary currents, like the Channel Islands? I've no method for that, and it bothers me. I think it's not solvable without crunching a lot of numbers.

[Seaworthy Lass]

Originally Posted by Paul Elliott

Trust me, Expedition knows all about tacking on headwinds! Expedition also knows all about the (forecast) winds ahead. The forecasts don't handle the conditions we see in nearshore sailing, where the headlands create those interesting eddies, funnels, and dead-zones, but at sea they usually get the big patterns right. The navigator and crew are always fine-tuning for actual conditions, but Expedition (and other programs) can do a good job of working out your strategy through the next 3-5 days. You will re-run the simulation whenever you get data from a new wind forecast model run, or you deliberately diverge from the suggested route.

Of course, there are times when I decide that I know more than expedition, and adjust a route to minimize the risk when a forecast might miss a shift in the highs / lows. Or, when I just say "Screw it! I want to sail in *that* direction!". Then again, I'm not a rock-star pro racer, I just like to sail.

Sigh, so no possibility of Scotch there either LOL?

Bewitched has just finished telling me that the computation for CTS (mathematically computed or otherwise) when you have variable cross current is not utilised by Expedition. I am saying it is and that if it needs to be worked out manually I have a better method than the one the RYA is teaching .

Has anyone played with Expedition? Does it tell you what tack to be on with variable cross current when the wind is directly on the nose?

[Dockhead]

Originally Posted by Seaworthy Lass

Sigh, so no possibility of Scotch there either LOL?

Bewitched has just finished telling me that the computation for CTS (mathematically computed or otherwise) when you have variable cross current is not utilised by Expedition. I am saying it is and that if it needs to be worked out manually I have a better method than the one the RYA is teaching .

Has anyone played with Expedition? Does it tell you what tack to be on with variable cross current when the wind is directly on the nose?

That's a question about lee-bowing. An interesting and complicated question. I would like to know the answer (I don't). Maybe we should start a separate thread.

[Paul Elliott]

I'm pretty sure that Expedition will give you appropriate tack / course information in the presence of arbitrary currents. This is because it doesn't try to solve the overall triangulation / path minimization problem we have been discussing, but instead it uses an iterative technique. Expedition calculates "isochrons", which are contours of constant time (as "isobars" are contours of constant pressure). It simulates your boat's performance (from the boat polars, and the forecast wind and current) on various points of sail for, say, 15 minutes. It sees how far you get, then from each of these (hundred?) endpoints it runs another 15-minute multi-course simulation. This continues until one or more of the endpoints reaches your goal. Expedition chooses the fastest route among the many it has run.

As this method would explode exponentially, Expedition trims the obviously bad routes as it goes. It also somewhat intelligently chooses which routes to try; it doesn't blindly try sending you all around the circle on 360 one-degree routes. But it does keep enough outliers in play so as not to miss a non-obvious route.

So, it's going to semi-monte-carlo it's way through whatever random wind and current you throw at it. It's not a precise mathematical solution, more of an iterative brute-force "try this and see what happens" approach. Something computers are good at!

[Seaworthy Lass]

Originally Posted by Dockhead

English sailors are light years ahead of us on this (since they deal with strong currents every day ); no one there is unaware of the power of constant heading passages. They debate a much more complicated issue -- lee bowing.

The question is whether you get a lift by tacking so that the tide is always on your lee bow.

I confess that I have no idea whether it works or not. There is a ferocious debate which has gone on for decades.

The ground track is dramatically shortened if you use the lee bow technique, which leads to some claims that this gives you a left -- this is obviously bogus to anyone who knows even a little about constant heading steering. But others say that is not the source of the lift.

Another theory is that it increases true wind (the wind will be reduced if you are being swept away from the direction of the wind, by the tide), and you get a lift that way. But we rarely have a shortage of wind in the English Channel; that "lift" would more than likely just mean you have to put a reef in.

So I can't say. It bothers me a lot that I don't know the answer.

Originally Posted by Dockhead

That's a question about lee-bowing. An interesting and complicated question. I would like to know the answer (I don't). Maybe we should start a separate thread.

It's quite simple. You sail to the wind relative to your total boat displacement (due to boat speed along your CTS plus current along its direction), so yes, you will point higher on one of the tacks .

That is the debating team side I am on (I hate being on the losing side LOL).

Dockhead, did you rework your data for the RYA result in Example 4? You were about 3 degrees off I think, making my result even more favourable (regardless of whether 5 minute segments of tide data are used or not) .

[Dockhead]

Originally Posted by Seaworthy Lass

It's quite simple. You sail to the wind relative to your total boat displacement (due to boat speed along your CTS plus current along its direction), so yes, you will point higher on one of the tacks .

That is the debating team side I am on (I hate being on the losing side LOL).

Dockhead, did you rework your data for the RYA result in Example 4? You were about 3 degrees off I think, making my result even more favourable (regardless of whether 5 minute segments of tide data are used or not) .

Well, I know the technique perfectly well. I just don't know whether it works or not. I have seen a lot of sophisticated arguments on both sides and can't choose between them. What's really complicated is that some arguments both pro and con are stupid, specifically caused by failure to understand water tracks. Then there is a whole higher level of arguments (on both sides!) that don't suffer from this. Here for example is a very sophisticated argument that the whole thing is a myth: Destination One Design - Preparation


As to the RYA solution for your problem, as I wrote, I got bored and didn't finish calculating it. It would have needed some recursive jiggering of the passage time and I couldn't be bothered. You worked it, and I'll take your word for it.

Even this is another little argument in favor of SWL -- it's also dead simple to calculate with math. During this whole debate I have not done a single plot -- I don't have a protractor here. All my calcs were done purely with math.

As I've said before, I think there is a third, almost absolutely precise method lurking in our unformed thoughts, having to do with sine wave development of currents, and requiring even less information -- just limits and timing of limits of the streams.

I'll let that ferment in the back of my brain for a few days and see what I can do with it when I'm on the boat next weekend and have the Admiralty tide program to play with. Maybe I'll write an app with it.

Did you play with that Belgian spreadsheet model, by the way?

[TeddyDiver]

Originally Posted by goboatingnow

Therefore it boils back to a discussion about the accuracy of tidal time boundaries. I would argue that both SWL and RYAs error circles lie WITHIN the overall passage error circle ( ie an EP work up ). Hence in that regard either method is suitable. aND neither gives more precision .


Dave

Agree completely!

[Paul Elliott]

Originally Posted by Dockhead

I think there is a third, almost absolutely precise method lurking in our unformed thoughts, having to do with sine wave development of currents, and requiring even less information -- just limits and timing of limits of the streams.

Sure, merely work out the equation for the current's displacement as a function of time. You know the distance and positions of A and B, and you have stipulated a fixed boat speed, so you know the boat's course vector as distance vs time. Then solve for current time = course time. This completes the triangle and you get your CTS.

The equation for the current will probably require more than a sine/offset/multiplier though, at least in my neck of the woods where we have two unequal tides per day. I doubt if the results would be worth the effort, since the actual current is non-uniform. It would be fun to try, especially if you used a curve-fitter for the current equation. It's all a bit over my head, mathematically, though. An iterative computer program would be more my speed.

And at the end of the day, we want something we can use when the batteries go dead!

[Seaworthy Lass]

Originally Posted by Dockhead

Well, I know the technique perfectly well. I just don't know whether it works or not. I have seen a lot of sophisticated arguments on both sides and can't choose between them. What's really complicated is that some arguments both pro and con are stupid, specifically caused by failure to understand water tracks. Then there is a whole higher level of arguments (on both sides!) that don't suffer from this. Here for example is a very sophisticated argument that the whole thing is a myth: Destination One Design - Preparation

Originally Posted by Seaworthy Lass

It's quite simple. You sail to the wind relative to your total boat displacement (due to boat speed along your CTS plus current along its direction), so yes, you will point higher on one of the tacks .

My reason is correct I think. It is purely because the wind is hitting the sails differently on the two tacks so that you can point higher and make better progress on one of them.

Originally Posted by Dockhead

As to the RYA solution for your problem, as I wrote, I got bored and didn't finish calculating it. It would have needed some recursive jiggering of the passage time and I couldn't be bothered. You worked it, and I'll take your word for it.

Even this is another little argument in favor of SWL -- it's also dead simple to calculate with math. During this whole debate I have not done a single plot -- I don't have a protractor here. All my calcs were done purely with math.

Yes, it is . Makes it easy to check your chart computed result if you want to, but I think it is more important to check it by drawing up the ground track on the chart as then not only can you see that you arrive at your destination as predicted, but you can also check for obstacles along your track.

Originally Posted by Dockhead

As I've said before, I think there is a third, almost absolutely precise method lurking in our unformed thoughts, having to do with sine wave development of currents, and requiring even less information -- just limits and timing of limits of the streams.

I'll let that ferment in the back of my brain for a few days and see what I can do with it when I'm on the boat next weekend and have the Admiralty tide program to play with. Maybe I'll write an app with it.

Yes, that would be brilliant! The actual CST could be calculated precisely by differential equations and if sinusoidal tide data could be used it would give a perfect result. One I think anyone either motoring or sailing in tidal waters would find very very useful . It would be a worthwhile thing, I would certainly buy the app.

My method is only useful if you have to make the calculations without access to a computer/tablet.

Originally Posted by Dockhead

Did you play with that Belgian spreadsheet model, by the way?

It didn't seem to work, but I haven't had time to really play with it. Will do that after sail repairs are done. Maybe I need to use my Netbook, not the iPad.

[mcarling]

To see that a single CTS will not always be optimal, we can run a simple thought experiment. First, plot or calculate a single CTS using any method. Then consider the possibility that parallel to and near the CTS is a more favorable tidal stream (perhaps due to different depth). If the distance to the more favorable tidal stream is small enough and the additional SOG from the more favorable tidal stream is great enough, then it follows that first steering over to the more favorable tidal stream and adjusting course once there is faster than the single CTS.

[Seaworthy Lass]

Originally Posted by mcarling

To see that a single CTS will not always be optimal, we can run a simple thought experiment. First, plot or calculate a single CTS using any method. Then consider the possibility that parallel to and near the CTS is a more favorable tidal stream (perhaps due to different depth). If the distance to the more favorable tidal stream is small enough and the additional SOG from the more favorable tidal stream is great enough, then it follows that first steering over to the more favorable tidal stream and adjusting course once there is faster than the single CTS.

Yes, I definitely agreee. . The CST is only one factor in route planning.

Current will vary with depth, so that if you are close to land there will be considerable advantage sailing a certain route depending if you want to minimise or maximise the tidal effect.

See this response of mine to Bewitched a few posts back:

Originally Posted by Seaworthy Lass

Bewitched, are you convinced yet that the quickest way to sail a leg with variable cross current is by starting with a single computed CTS (if water depth is constant, so advantage can't be taken of less or more current in certain areas and you don't need to tack or gybe) and then thinking about leeway, wind in certain areas, etc to determine the best route?

This initial CTS determination will be the quickest way of getting from A to B if you are motoring in calm water with variable cross current. The quickest way is NOT following a straight track on the GPS!

[Dockhead]

Originally Posted by Paul Elliott

Sure, merely work out the equation for the current's displacement as a function of time. You know the distance and positions of A and B, and you have stipulated a fixed boat speed, so you know the boat's course vector as distance vs time. Then solve for current time = course time. This completes the triangle and you get your CTS.

The equation for the current will probably require more than a sine/offset/multiplier though, at least in my neck of the woods where we have two unequal tides per day. I doubt if the results would be worth the effort, since the actual current is non-uniform. It would be fun to try, especially if you used a curve-fitter for the current equation. It's all a bit over my head, mathematically, though. An iterative computer program would be more my speed.

And at the end of the day, we want something we can use when the batteries go dead!

It's probably worthwhile having both something we can do with a protractor (and I am one of those who tries never to sail anywhere without paper charts on board, and it goes without saying, paper tide atlases), as well as an ace electronic tool.

I have a hunch that a really excellent tool could be built without the math needing to be all that sophisticated. Just need to model the current development from slack to slack artificially as a sine wave, and I think that would already be a great leap forward -- much better than any of the methods using hourly tides.

As Dave has correctly pointed out, we also need to careful of False Precision. My hunches about the limits of this are different from his, but we need to transition from hunches to calculations to probe this further.

[Seaworthy Lass]

Originally Posted by Dockhead

Well, I know the technique perfectly well. I just don't know whether it works or not. I have seen a lot of sophisticated arguments on both sides and can't choose between them. What's really complicated is that some arguments both pro and con are stupid, specifically caused by failure to understand water tracks. Then there is a whole higher level of arguments (on both sides!) that don't suffer from this.

Here for example is a very sophisticated argument that the whole thing is a myth: Destination One Design - Preparation

ROFL
Had my good laugh for this morning reading that link! This technique has nothing to do with pinching. I am talking about what tack to be on NOT about pinching.

Why on earth would you be on port tack in this example

The whole principle is that you want to be on the other tack so that you can point higher and get there quicker

[Dockhead]

Pinching up into the current is one of several different interpretations of the lee bow effect. Here is the pro:

"Lee-Bow Effect: This brings up the famous lee-bow effect. If the current is coming at you at an angle that is very close to the course you are sailing and if, by pinching just a little bit, you can get your lee bow into the flow of the current, the movement of the water is going to push against the hull, the keel, and the rudder, and it is goign to drive you up to windward even though you are going slower over the bottom. If you are on the other tack the current is going to be hitting you broadside and pushing you down. If you can get the lee-bow effect to push you to windward, I feel you also increase the wind pressure on the sails. If I am on the tack that goes across the current I feel I am losing speed and distance to the mark. That is why, unless there is an obvious way to get out of the current entirely, or at least to a slower flow, I think you should always make your longest tack to the next mark sailing in the lee-bow position. And I would do this even if it meant pinching a bit to do so."