Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 27-01-2013, 03:04   #481
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,737
Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

And con from the same source, with a generalized argument against anything based on lee bows:

This seemed appealing enough to my young, malleable, Naples sabot sailing around current-less basins in Southern California brain. I was willing to believe anything out of the Wizard of Zenda’s mouth, or written on the pages in front of me. The idea that a boat sailing alone with its bow slightly above the angle of adverse current would gain distance to windward makes sense, relative a stationary object like a shoreline or a mark anchored in the water. This idea is sufficiently flawed when you either of two things: add another boat to the situation, or remove the mark from the racecourse. At the risk of being flicked into oblivion as I tug on Superman’s cape, the lee-bow effect does not exist when racing against other boats.If two boats are sailing upwind on opposite tacks. According to Melges, the boat with its lee-bow facing the current will gain distance. to windward, and the boat with its windward hip facing the current will lose distance to leeward. In the following diagram. The boat gaining the mythical advantage is in red, as the current crossed the picture right to left. The critical issue is that two boats sailing together upwind are affected exactly equally by the current! Imagine that the two boats are dead in the water. There is no movement whatsoever up the racecourse or towards each other, but there are 2 knots of current running beneath them from right to left across the racecourse. As Mr Melges points out, the red boat is traveling over a distance to windward which happens to be to his left. Likewise, the green boat is traveling distance to leeward. However, there is no advantage to being the red boat or the green boat because both boats are moving at exactly the same speed in exactly the same direction, that of the current. Simply because the red boat is moving to windward, does not mean that the boat is gaining, it only means (in the demonstrated scenario) that it is gaining distance to the left. The green boat is likewise not losing anything, in fact it is gaining the same distance as the red boat toward the left.

June | 2008 | Campbell Sailing
__________________

__________________
Dockhead is offline   Reply With Quote
Old 27-01-2013, 03:18   #482
Moderator
 
Seaworthy Lass's Avatar

Cruisers Forum Supporter

Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,810
Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Dockhead View Post
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."
I have misunderstood the term lee bowing then.

I am not talking about pinching.
What I am talking about is what tack to be on if you want to sail your CTS but cant because the wind is too much on the nose. The tack to be on is the starboard one if the current is coming from the starboard side (going towards the port side). This seem counter intuitive, but will get you there quicker as you can point higher.

Not sure what you call this, but one tack will be much more favourable because of the current.
__________________

__________________
"The cure for anything is salt water: sweat, tears or the sea." Isak Dinesen
"To me the simple act of tying a knot is an adventure in unlimited space." Clifford Ashley
Seaworthy Lass is online now   Reply With Quote
Old 27-01-2013, 03:20   #483
LJH
Registered User

Join Date: Mar 2012
Location: Nova Scotia
Boat: Wauquiez Centurion 42
Posts: 274
Re: Lee bow

Quote:
Originally Posted by Dockhead View Post
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
Interesting read. I am sure there is another argument, or two in favour of lee bow.

Quote:
Originally Posted by Dockhead View Post
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?
I think that to get more precision would require more data and a faster sampling rate than an hour. I think that one inaccuracy with both the RYA and SWL method is that neither account for the actual tidal hour. What portion is spent with the first tidal current, i.e. if we were leaving at HWT, then we would only be spending 30 mins with the first current. As I have had no formal training, please correct me if this is assumption is wrong.

I am extremely interested in what you will discover playing with the Admiralty programme.

More precision will come with more expense (software and or data), time and effort.
__________________
LJH is offline   Reply With Quote
Old 27-01-2013, 03:21   #484
Moderator
 
Paul Elliott's Avatar

Cruisers Forum Supporter

Join Date: Sep 2006
Posts: 3,864
Images: 4
Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

The Lee Bow effect is what you get when you don't understand your planes of reference: Ground, for the mark. Water, for your boat and the wind it is sailing in. You need to take the "ground wind" and add the corrections for current to get the wind your boat is sailing in. Or, just consider the apparent wind as seen by your boat. There will be a favored course that gets you to the mark fastest, and this depends on the wind, the current, and your boat's performance (polars).

Hey, I'll bet we could come up with some triangles for this situation! (Duh!)
__________________
Paul Elliott, S/V VALIS - Pacific Seacraft 44 #16 - Friday Harbor, WA
www.sailvalis.com
Paul Elliott is online now   Reply With Quote
Old 27-01-2013, 03:25   #485
Registered User

Join Date: Feb 2012
Posts: 2,441
Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

For anyone who has got their head thoroughly around SwL's method, and preferably has some familiarity also with the RYA method:

Here's a scenario which I think addresses suggestions (some rather sweeping) that certain recent scenarios are borderline for likelihood.
In the interest of realism, this scenario includes elements which have all happened to me in real life, although never all on the same passage.

I apologise for the length: I'm hoping to trade on not having previously posted any scenarios, and trying to include enough background to limit those who delight in exploiting the inevitable omissions to shoot "what if" or "that's improbable" holes.

You are sailing two-handed (actually there's barely any breeze, and it's pouring with rain, so you're motoring).

Your mate wakes you, to explain that he will need to spend the next half hour in the cockpit negotiating a traffic separation lane (which he is dressed for, and you are not), and could you please finish working out the passage plan he has begun.

The plan's execution will start in an hour's time, at the beginning of your watch, when he expects to be well clear of the far boundary of the shipping zone. He has decided on a departure point for this current sailing portion of the passage.

He needs to take the chart the boat is currently on, (and will be for a couple more hours) which he can refer to under the dodger, but he reckons that's OK because he has plotted the current vectors far enough to get them onto the next chart, which you can use immediately.

He hasn't had time to finish the current vectors and decide which of two islands with sheltered anchorages would be the better destination. This is an important decision, because you're trying to minimise fuel usage, to avoid refilling the daytank.

During the afternoon watch you discovered a problem with the deck filler, and it's obvious lots of water has got into the main diesel tank. This needs to be sorted out at a quiet anchorage.

The area you're crossing the shipping channels to reach is a bank too shallow for (and forbidden to) shipping, with fast flowing tides clocking around, but it has major advantages, including few fixed dangers, other than well lit ones. The islands are also sufficiently well provided with lights and nav marks.

His good news is the rain is starting to ease, good forecast generally, and as far as he's been able to determine, the timing of the tides makes it look as though they may lend a major helping hand.

He wants you to work out a CTS and duration to each island, assuming indicative (constant) speeds of say both 2 and 4 knots, to find out which combo implies the most economical combination of speed and distance through water.

When he comes off watch, bringing the departure chart with him, you can plot the ground tracks, make any necessary tweaks, and make a final choice.

The excuse he gives for not having sorted it out already is that, when the tidal current diagram ran off the edge of the present chart and onto the next one (which is at a different scale), he had to transfer the overlapping tidal sets laboriously across, while steaming along at 6 knots and trying to keep on top of multiple crossing situations with shipping.

He reckons you'll have an easy time of it on your watch, pootling along and checking progress periodically against the plan, and to quit your moaning and put the kettle on.
- - - - -
What he wants to know ASAP is this: Can you provide the information he asks for with what he's left you with, or will you need his chart as well?

I have an additional question: would the RYA method or the SwL method be a better choice in this particular situation, and why?

Or do you have a better method than either of these, for the unusual and even intriguing situation this undoubtedly is?
 
(For brevity and to make the exercise easy to plot on squared paper I'll use coordinate notation like a simple graph:
(3,4) is a point 3nm East, and 4nm North of the bottom left corner of the chart)

Here's what your chart shows when it's handed to you: (see attached diagram which you can print out and use as a plotting sheet)

The vector for the third hour of current
comes onto your chart at (0,14)
(although this is not the start of the vector)
and ends at (3,15)

After consulting the tidal almanac,
you add to the diagram,
resulting in vector junctions as follows:

Fourth hour end: (8,14)
Fifth hour end: (11,11)
Sixth hour end: (12, 8)
Seventh hour end: (11,5)
Eighth hour end: (10, 4)
Ninth hour end: (8, 3)

There are islands at (2,27), and (11,0)

Can you work out a CTS and duration under motor for each island?
(assume constant speed, 2kn and 4kn, hence 4 options total).
If not, what information do you need?
- - - - - - - - - -
If this is too easy, and you want a small extra challenge:
which island minimises the fuel usage ?

Assuming you burn
2 units of diesel per hour at 2 knots
(2.8 units of diesel per hour at 3 knots)
4 units of diesel per hour at 4 knots
(7 units of diesel per hour at 5 knots)
(10 units of diesel per hour at 6 knots)
(15 units of diesel per hour at 7 knots: Black smoke pouring forth
Alright, we know already, our prop's pretty dirty, along with our bottom)

Dropping below 2 knots would not be considered prudent, to maintain a semblance of steerage way (eg to correct for current variations from "as predicted"), assist collision avoidance, and delay the onset of terminal "are we there yet" syndrome.

Given that we have full speed available (for emergency manoeuvres) and provided we are not trying to stem the tide thereby wasting precious fuel standing still, we feel 2 knots is perfickly OK.
In the words of a skipper I once navigated for, "It's not your boat, it's not your problem." <tongue poke>

If you want to get really fancy, you could check whether there's a better tabulated speed than either of the values already suggested.
Attached Thumbnails
Click image for larger version

Name:	Plotting Sht for Current Sailing Scenario AT1.PNG
Views:	53
Size:	16.9 KB
ID:	53540  
__________________
Andrew Troup is offline   Reply With Quote
Old 27-01-2013, 03:25   #486
Moderator
 
Seaworthy Lass's Avatar

Cruisers Forum Supporter

Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,810
Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Paul Elliott View Post
The Lee Bow effect is what you get when you don't understand your planes of reference: Ground, for the mark. Water, for your boat and the wind it is sailing in. You need to take the "ground wind" and add the corrections for current to get the wind your boat is sailing in. Or, just consider the apparent wind as seen by your boat. There will be a favored course that gets you to the mark fastest, and this depends on the wind, the current, and your boat's performance (polars).

Hey, I'll bet we could come up with some triangles for this situation! (Duh!)
Exactly!!!

That's how I interpreted lee bowing, ie what tack to be on, not whether or not to pinch
__________________
"The cure for anything is salt water: sweat, tears or the sea." Isak Dinesen
"To me the simple act of tying a knot is an adventure in unlimited space." Clifford Ashley
Seaworthy Lass is online now   Reply With Quote
Old 27-01-2013, 03:34   #487
Moderator
 
Seaworthy Lass's Avatar

Cruisers Forum Supporter

Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,810
Re: Lee bow

Quote:
Originally Posted by LJH View Post
I think that to get more precision would require more data and a faster sampling rate than an hour. I think that one inaccuracy with both the RYA and SWL method is that neither account for the actual tidal hour. What portion is spent with the first tidal current, i.e. if we were leaving at HWT, then we would only be spending 30 mins with the first current. As I have had no formal training, please correct me if this is assumption is wrong.

I am extremely interested in what you will discover playing with the Admiralty programme.

More precision will come with more expense (software and or data), time and effort.
Yes, you are right, my method "assumes" the current for the last tidal hour is constant, whereas it is varying. So my CTS is still an approximation, just a much better approximation than the RYA who totally IGNORE the current in the last portion of the journey if their D (on the rhumb line) falls closer to B before it rather than after it.
__________________
"The cure for anything is salt water: sweat, tears or the sea." Isak Dinesen
"To me the simple act of tying a knot is an adventure in unlimited space." Clifford Ashley
Seaworthy Lass is online now   Reply With Quote
Old 27-01-2013, 04:00   #488
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,737
Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Seaworthy Lass View Post
Exactly!!!

That's how I interpreted lee bowing, ie what tack to be on, not whether or not to pinch
The pinching part is a narrow assertion for the lee bow effect. Some people have conceded that the broad assertion (you correctly describe it) is mythical (as Paul does), but say there really is a narrow case where it works -- the pinching case.

Our own Nick of Jedi, master mariner, also argues that it is entirely a myth:

Differences Between Ground, Apparent, and True Wind Direction
__________________
Dockhead is offline   Reply With Quote
Old 27-01-2013, 04:13   #489
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,737
Re: Lee bow

Quote:
Originally Posted by LJH View Post
Interesting read. I am sure there is another argument, or two in favour of lee bow.



I think that to get more precision would require more data and a faster sampling rate than an hour. I think that one inaccuracy with both the RYA and SWL method is that neither account for the actual tidal hour. What portion is spent with the first tidal current, i.e. if we were leaving at HWT, then we would only be spending 30 mins with the first current. As I have had no formal training, please correct me if this is assumption is wrong.
That had me laughing. NONE of us has any formal training -- we are a bunch of amateurs stumbling together towards some kind of understanding. Blowing cobwebs off high school trig, in some cases (like mine).

As to WHICH tide we have -- hour tide data is given as the AVERAGE rate and set for that hour. The currents vary more or less smoothly, and a series of whole hours tide data will work perfectly.

The problems in all the methods arises if you have a half hour (say), so a large partial hour. SWL takes the average of the whole hour, but the first half hour will be different from the second half hour, so there's a built in error.

We could use one hour tide data and get much better results if we interpolate that the rate at the transition between each hour will be the average rate of the two adjacent hours. That's what I did with the last but one problem.

RYA don't even calculate the partial hour. Incidentally, RYA will produce EXACTLY the same result as SWL if you were to do two of them, one through the whole hour before, and one through the whole hour after, and calculate a proportionate share of the difference. The only thing is that this is vastly more laborious than SWL's ingeniously simple method.

You correctly point out that we are ignoring possible partial tide hours at the beginning of the passage. This must be fudged with both methods, to avoid significant errors, but it's an easy fudge so, I think, not a big problem.


Quote:
Originally Posted by LJH View Post
More precision will come with more expense (software and or data), time and effort.
My hunch is we don't need more data; just better methods.
__________________
Dockhead is offline   Reply With Quote
Old 27-01-2013, 04:19   #490
Registered User

Join Date: Dec 2009
Posts: 3,016
Images: 4
As a racer ... There are a couple of rules ... More like guidelines (heh) ... Sail the longer tack first ... Unless you know something about the changing wind: then sail toward the new wind. These are true under the effect of any current so long as you do the following:

First displace the mark by the reciprocal of the sum of all the current vectors. Which is what you have been discussing here at length.

I think I got that right. But happy hours here in the PI may have interfered.
__________________
daddle is offline   Reply With Quote
Old 27-01-2013, 04:39   #491
Moderator
 
Seaworthy Lass's Avatar

Cruisers Forum Supporter

Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,810
Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Dockhead View Post
The pinching part is a narrow assertion for the lee bow effect. Some people have conceded that the broad assertion (you correctly describe it) is mythical (as Paul does), but say there really is a narrow case where it works -- the pinching case.

Our own Nick of Jedi, master mariner, also argues that it is entirely a myth:

Differences Between Ground, Apparent, and True Wind Direction
I think the broad one will work beautifully (tacking down current). The narrow one won't! When does pinching on anything ever work well

Will read Nick later, have a guest arrived by dinghy for coffee.
__________________
"The cure for anything is salt water: sweat, tears or the sea." Isak Dinesen
"To me the simple act of tying a knot is an adventure in unlimited space." Clifford Ashley
Seaworthy Lass is online now   Reply With Quote
Old 27-01-2013, 04:42   #492
Moderator
 
Seaworthy Lass's Avatar

Cruisers Forum Supporter

Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,810
Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

Quote:
Originally Posted by Andrew Troup View Post
For anyone who has got their head thoroughly around SwL's method, and preferably has some familiarity also with the RYA method:

Here's a scenario which I think addresses suggestions (some rather sweeping) that certain recent scenarios are borderline for likelihood.
In the interest of realism, this scenario includes elements which have all happened to me in real life, although never all on the same passage.

I apologise for the length: I'm hoping to trade on not having previously posted any scenarios, and trying to include enough background to limit those who delight in exploiting the inevitable omissions to shoot "what if" or "that's improbable" holes.

You are sailing two-handed (actually there's barely any breeze, and it's pouring with rain, so you're motoring).

Your mate wakes you, to explain that he will need to spend the next half hour in the cockpit negotiating a traffic separation lane (which he is dressed for, and you are not), and could you please finish working out the passage plan he has begun.

The plan's execution will start in an hour's time, at the beginning of your watch, when he expects to be well clear of the far boundary of the shipping zone. He has decided on a departure point for this current sailing portion of the passage.

He needs to take the chart the boat is currently on, (and will be for a couple more hours) which he can refer to under the dodger, but he reckons that's OK because he has plotted the current vectors far enough to get them onto the next chart, which you can use immediately.

He hasn't had time to finish the current vectors and decide which of two islands with sheltered anchorages would be the better destination. This is an important decision, because you're trying to minimise fuel usage, to avoid refilling the daytank.

During the afternoon watch you discovered a problem with the deck filler, and it's obvious lots of water has got into the main diesel tank. This needs to be sorted out at a quiet anchorage.

The area you're crossing the shipping channels to reach is a bank too shallow for (and forbidden to) shipping, with fast flowing tides clocking around, but it has major advantages, including few fixed dangers, other than well lit ones. The islands are also sufficiently well provided with lights and nav marks.

His good news is the rain is starting to ease, good forecast generally, and as far as he's been able to determine, the timing of the tides makes it look as though they may lend a major helping hand.

He wants you to work out a CTS and duration to each island, assuming indicative (constant) speeds of say both 2 and 4 knots, to find out which combo implies the most economical combination of speed and distance through water.

When he comes off watch, bringing the departure chart with him, you can plot the ground tracks, make any necessary tweaks, and make a final choice.

The excuse he gives for not having sorted it out already is that, when the tidal current diagram ran off the edge of the present chart and onto the next one (which is at a different scale), he had to transfer the overlapping tidal sets laboriously across, while steaming along at 6 knots and trying to keep on top of multiple crossing situations with shipping.

He reckons you'll have an easy time of it on your watch, pootling along and checking progress periodically against the plan, and to quit your moaning and put the kettle on.
- - - - -
What he wants to know ASAP is this: Can you provide the information he asks for with what he's left you with, or will you need his chart as well?

I have an additional question: would the RYA method or the SwL method be a better choice in this particular situation, and why?

Or do you have a better method than either of these, for the unusual and even intriguing situation this undoubtedly is?
 
(For brevity and to make the exercise easy to plot on squared paper I'll use coordinate notation like a simple graph:
(3,4) is a point 3nm East, and 4nm North of the bottom left corner of the chart)

Here's what your chart shows when it's handed to you: (see attached diagram which you can print out and use as a plotting sheet)

The vector for the third hour of current
comes onto your chart at (0,14)
(although this is not the start of the vector)
and ends at (3,15)

After consulting the tidal almanac,
you add to the diagram,
resulting in vector junctions as follows:

Fourth hour end: (8,14)
Fifth hour end: (11,11)
Sixth hour end: (12, 8)
Seventh hour end: (11,5)
Eighth hour end: (10, 4)
Ninth hour end: (8, 3)

There are islands at (2,27), and (11,0)

Can you work out a CTS and duration under motor for each island?
(assume constant speed, 2kn and 4kn, hence 4 options total).
If not, what information do you need?
- - - - - - - - - -
If this is too easy, and you want a small extra challenge:
which island minimises the fuel usage ?

Assuming you burn
2 units of diesel per hour at 2 knots
(2.8 units of diesel per hour at 3 knots)
4 units of diesel per hour at 4 knots
(7 units of diesel per hour at 5 knots)
(10 units of diesel per hour at 6 knots)
(15 units of diesel per hour at 7 knots: Black smoke pouring forth
Alright, we know already, our prop's pretty dirty, along with our bottom)

Dropping below 2 knots would not be considered prudent, to maintain a semblance of steerage way (eg to correct for current variations from "as predicted"), assist collision avoidance, and delay the onset of terminal "are we there yet" syndrome.

Given that we have full speed available (for emergency manoeuvres) and provided we are not trying to stem the tide thereby wasting precious fuel standing still, we feel 2 knots is perfickly OK.
In the words of a skipper I once navigated for, "It's not your boat, it's not your problem." <tongue poke>

If you want to get really fancy, you could check whether there's a better tabulated speed than either of the values already suggested.
Nice exercise! Will do it after I have made coffee for unexpected guest and drawn up the graph paper on 2 bits stuck together. The longest bit in these exercises is drawing up the paper LOL .
__________________
"The cure for anything is salt water: sweat, tears or the sea." Isak Dinesen
"To me the simple act of tying a knot is an adventure in unlimited space." Clifford Ashley
Seaworthy Lass is online now   Reply With Quote
Old 27-01-2013, 05:00   #493
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,737
Re: Lee bow

Quote:
Originally Posted by Seaworthy Lass View Post
Yes, you are right, my method "assumes" the current for the last tidal hour is constant, whereas it is varying. So my CTS is still an approximation, just a much better approximation than the RYA who totally IGNORE the current in the last portion of the journey if their D (on the rhumb line) falls closer to B before it rather than after it.
Fairly minor detail, but not exactly the right interpretation of RYA. You are supposed to "inflate" (or "deflate") the vector triangle to lay the destination. So you are not ignoring the current during the last partial hour, you are not calculating any current for the last hour (of which the last partial hour is part), and instead you substitute the average of the whole passage.

RYA can also be used as you suggest -- to steer to a point short of the destination, less than an hour away. This is actually pretty reasonable. 30 or 40 minutes might be a stretch, but 15 or 20 minutes would mean bugger-all after all the accumulated errors of the passage. Only I wouldn't steer to point on the rhumb line -- I would choose some full hour passage destination (easy to do since you've got a solved tidal vector line already) uptide of the destination, and then solve it completely and perfectly as a separate destination, forgetting "B" entirely.

I can say from practical experience that no matter how precise your result was, you have abandoned any constant heading by the time you're 20 minutes off at the latest, and have gone over to matching COG with BTW on the plotter, or just put the pilot on "track" mode. So steering to a point somewhere a little uptide and 20 minutes off, rather than to the destination itself, is perfect, if that's an easy whole hour solution per RYA. And it is not rare to be an hour off and abandon your constant heading -- if you are missing your mark by the time you are an hour off, then you just work out a simple course to a point a mile or so uptide and Bob's your uncle. If you have not had any big mid-course corrections, and you are not somewhere downtide, then winging the last hour does hardly anything negative to your overall efficient passage.

It's important to emphasize that errors which bring you uptide of your destination are painless unless very large indeed; downtide even a mile can really suck. Just think about it -- even just 4 knots of tide, boat speed 7, SOG downtide = 11 knots; SOG uptide = 3 knots. So making good a couple miles error with those speeds is 11 minutes downtide, and 40 minutes uptide. It is a truly massive difference.

A cardinal value in this whole exercise, from a practical point of view, is to NOT arrive downtide of your destination. Or on a point of sail you can't sail.
__________________
Dockhead is offline   Reply With Quote
Old 27-01-2013, 10:20   #494
Moderator
 
Seaworthy Lass's Avatar

Cruisers Forum Supporter

Join Date: Oct 2008
Boat: Aluminium cutter rigged sloop
Posts: 12,810
Andrew Troup's real life scenario

Hi Andrew
Brilliant exercise . Devilishly clever! You all thought I was sneaky LOL, but Andrew is in a whole different category.

I took about quarter of an hour to compute the figures, but I had to stick bits of paper from an exercise book together LOL. The current data was, however already presented rather then hunting it up, so in reality the exercise would have taken longer.
It took ages writing it all up to post here though.

SWL method results:

ISLAND ONE (2, 27)
At 2 knots:
Would never make it.

At 4 knots:
Estimated CTS = 355 degrees true
Estimated time taken = 3 hours
Estimated fuel burned = 12 units

ISLAND TWO (11, 0)
At 2 knots:
Estimated CTS = 181 degrees true
Estimated time taken = 5.2 hours (5 hours 12 minutes)
Estimated fuel burned = 10.4 units

At 4 knots:
Estimated CTS = 164 degrees true
Estimated time taken = 3.74 hours (3 hours 44 minutes)
Estimated fuel burned = 15 units

I don't know the capacity of the day tank, but on the basis of the SWL method, I would head to Island One on a CTS of 355 true at 4 knots as it gives the shortest time with very little more fuel used than traveling 5.2 hours to Island Two.
Island Two at 4 knots would be the worst option.



RYA method results:

ISLAND ONE (2, 27)
At 2 knots:
Would never make it.

At 4 knots:
Estimated CTS = 355 degrees true
Estimated time taken = 3 hours
Estimated fuel burned = 12 units

SAME RESULT AS THE SWL METHOD. The RYA method will always give a good result if D coincides with B

ISLAND TWO (11, 0)
At 2 knots:
Estimated CTS = 156 degrees true
Estimated SMG = 2.6 knots
Estimated time taken = 3.4 hours = 3 hours 24 minutes
Estimated fuel burned = 6.8 units

At 4 knots:
Estimated CTS = 161 degrees true
Estimated SMG = 2.9 knots
Estimated time taken: 3.1 hours (3 hours 6 minutes)
Estimated fuel burned = 12.4 units

Using the RYA method, I would select Island Two travelling at two knots as the time taken was roughly the same for all 3 options, but the calculated fuel used was the least for this option. The CTS does not look instinctively wrong given the relative position of B from A.
I would be stuffed heading off in this direction .


Nice example Andrew
You are so much better than I am at setting examples!
Thank you for taking all the trouble to work this out and write it up xxxxxx

What did everyone else get?
And if you didn't do the calculations what choice of island and boat speed would you have made based on my calculations above?
Attached Thumbnails
Click image for larger version

Name:	image.jpg
Views:	50
Size:	163.0 KB
ID:	53557  
__________________
"The cure for anything is salt water: sweat, tears or the sea." Isak Dinesen
"To me the simple act of tying a knot is an adventure in unlimited space." Clifford Ashley
Seaworthy Lass is online now   Reply With Quote
Old 27-01-2013, 13:59   #495
Registered User

Join Date: Feb 2012
Posts: 2,441
Re: Inaccurate RYA Teaching : CTS - Quest For a New Method

I can see a few wee problems with your from-the-hip analysis, Seaworthy, but lets see what others come up with independently, before we start interrogating any suspects.

Please remember the scenario is not the usual sort of textbook simplified abstraction, and may have some curly aspects.

One assumption we have to make for simplicity is that the tide diamonds tell the same story, at any given time, everywhere the ground track might go.

In terms of a previous abstraction: the checkered tablecloth representing the ocean is inelastic and does not rotate. All the squares remain the same size, oriented the same way.

-- - - --

It might help to try for the mindset of someone who's actually trying to make it to shelter in earnest, using their wits instead of their diesel.

The amount of fuel in the daytank is one of the things our intrepid sailors will have to assess against the favoured scenario; the brief here is to come up with the best preliminary options in terms of minimum fuel burn.

When they do take that further step based on our analysis they may consider the reliability of outcome, and that might tip them into choosing an option which our analysis would suggest is not optimal.

You are not being asked to evaluate such considerations, and that would need more info, but feel free to discuss the issues, if they interest you.
__________________

__________________
Andrew Troup is offline   Reply With Quote
Reply

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off




Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 00:08.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.