Single CTS or follow the Courseline?

[Dockhead]

Originally Posted by Lodesman

If you're going slower or faster than planned, that should be apparent on your log - you would want to rerun all the numbers anyway. Unless you're going the planned speed, the plan is garbage.

Not at all garbage. A course to steer of, say 65, does not suddenly become say, 90, because of a knot or even three knots of difference in speed. As I wrote -- if you're faster, even quite a bit faster, it's all good. If you're slower, then you're still headed in generally the right direction, you just need to add correction.

If you're talking about Andrew's multi-waypoint scheme, his plan is also not garbage, because his plan also has you going in generally the right direction. He will, however, have to throw out his waypoints and start over again, in order to correct his course (and sailing pure CTS we also start over again in such a case).

[Lodesman]

Originally Posted by Dockhead

Not at all garbage. A course to steer of, say 65, does not suddenly become say, 90, because of a knot or even three knots of difference in speed. As I wrote -- if you're faster, even quite a bit faster, it's all good. If you're slower, then you're still headed in generally the right direction, you just need to add correction.

If you're talking about Andrew's multi-waypoint scheme, his plan is also not garbage, because his plan also has you going in generally the right direction. He will, however, have to throw out his waypoints and start over again, in order to correct his course (and sailing pure CTS we also start over again in such a case).

Maybe we want to run some scenarios (aaarrrggghhh!!!)

I think it depends largely on how big the difference is. If the current to boat speed ratio was such that you require a single-CTS solution, then I think it would be very susceptible to incorrect data inputs. If you were planning for 5kts boat speed with a cross-current of 3 kts, but only got a boat speed of 4 kts, it knocks your SMG from 4 kts down to 2.6 and you would need to steer over another 12 degrees to maintain your CMG. I don't see how you can realistically hope to salvage the current plan, when it only takes a few minutes to work out a new one?

[jackdale]

Originally Posted by Lodesman

Maybe we want to run some scenarios (aaarrrggghhh!!!)

It might also be worthwhile for the scenario writer to provide the variation and a deviation card (you do have one -right?)

I have posted Rousmaniere's estimates of leeway. http://www3.telus.net/jackdale/navle...CTS/Leeway.pdf
So the wind direction and speed along with a vessel description would be beneficial.

[Dockhead]

Originally Posted by Lodesman

Maybe we want to run some scenarios (aaarrrggghhh!!!)

I think it depends largely on how big the difference is. If the current to boat speed ratio was such that you require a single-CTS solution, then I think it would be very susceptible to incorrect data inputs. If you were planning for 5kts boat speed with a cross-current of 3 kts, but only got a boat speed of 4 kts, it knocks your SMG from 4 kts down to 2.6 and you would need to steer over another 12 degrees to maintain your CMG. I don't see how you can realistically hope to salvage the current plan, when it only takes a few minutes to work out a new one?

I will do some scenarios as soon as I have a little time. I now have all the tools -- I received AllTides with the 5 minute tide data donated by a kind CFer, and besides that I have the Neptune passage planner. So we can do some really interesting scenarios!

We may have a semantic difference -- I don't propose to "salvage" a plan if you get off track; of course you have to make a new plan. I have been writing that all along. I simply mean that the old plan has served its purpose and has gotten you towards your goal -- in that sense it was not "garbage".

Your calculations about Speed Made Good would apply to a rhumb line passage, but don't apply to a CTS passage, which will achieve a much better SMG than that, with a big variable current.

[Lodesman]

Originally Posted by Dockhead

Your calculations about Speed Made Good would apply to a rhumb line passage, but don't apply to a CTS passage, which will achieve a much better SMG than that, with a big variable current.

Boat speed vector is still present in the CTS calculation. I think SMG will be affected to near the same degree regardless of the swing in current.

[jackdale]

Originally Posted by Dockhead

Your calculations about Speed Made Good would apply to a rhumb line passage, but don't apply to a CTS passage, which will achieve a much better SMG than that, with a big variable current.

I am not sure what this means.

You depart using the CTS. The boat's knotmeter will show the boat speed (S). If you happen to have a GPS running the SMG and the SOG should be about the same. If the current is with you the SMG/SOG will exceed the S. If the current is against S will exceed SMG/SOG.

[Paul Elliott]

SMG is Speed Made Good towards a destination or waypoint. This is not the same as SOG. When steering a CTS route, unless the currents being compensated for are nil the COG will *not* be directly towards the destination, so SMG will be less than SOG.

Am I missing something? I thought this had all been discussed and agreed upon.

[Seaworthy Lass]

Originally Posted by Lodesman

If you were planning for 5kts boat speed with a cross-current of 3 kts, but only got a boat speed of 4 kts, it knocks your SMG from 4 kts down to 2.6 and you would need to steer over another 12 degrees to maintain your CMG. I don't see how you can realistically hope to salvage the current plan, when it only takes a few minutes to work out a new one?

Originally Posted by Dockhead

Your calculations about Speed Made Good would apply to a rhumb line passage, but don't apply to a CTS passage, which will achieve a much better SMG than that, with a big variable current.

Dockhead is correct, calculations made beforehand for SMG only apply when you have reached your destination unless you have a current constant in speed and direction for the entire passage resulting in travel along the rhumb line .

Originally Posted by jackdale

You depart using the CTS. The boat's knotmeter will show the boat speed (S). If you happen to have a GPS running the SMG and the SOG should be about the same. If the current is with you the SMG/SOG will exceed the S. If the current is against S will exceed SMG/SOG.

No, the speed over ground is unlikely to match the VMG on the chartplotter when travelling on a precomputed CTS with variable cross current.

Think of the case where there is little or no current at the start of a journey with the current expected to increase significantly along the way. Your compass heading may well be 10 or 20 degrees off the bearing of the rhumb line and so your initial SMG (displayed as VMG on the chartplotter) will be considerably less than your SOG.

SMG relates to the destination point. SOG doesn't.

Originally Posted by Paul Elliott

SMG is Speed Made Good towards a destination or waypoint. This is not the same as SOG. When steering a CTS route, unless the currents being compensated for are nil the COG will *not* be directly towards the destination, so SMG will be less than SOG.

Am I missing something? I thought this had all been discussed and agreed upon.

You are missing nothing

[Dockhead]

Originally Posted by jackdale

I am not sure what this means.

You depart using the CTS. The boat's knotmeter will show the boat speed (S). If you happen to have a GPS running the SMG and the SOG should be about the same. If the current is with you the SMG/SOG will exceed the S. If the current is against S will exceed SMG/SOG.

SMG and SOG will be the same only on the rhumbline. A CTS boat will not be on the rhumbline. SMG will be less than SOG at first for such a passage while the boat is travelling away from the rhumbline, then will pick up SMG as it turns towards the rhumbline. The total average SMG over the passage will be higher than a boat following the rhumb line.

Therefore, you can't say about a boat on a CTS passage that x degrees of error in steering will give you y knots of lost SMG at "a" boat speed and "b" current speed. That calculation only applies to a rhumb line passage. The calculation will be more complicated for a CTS boat, and the effect will be less.

[Lodesman]

I didn't realize there was such confusion over the term SMG. Sorry, I'm a traditionalist - to me it's the same as SOG. Starpath Online Classroom Glossary

A CTS boat may not be following a rhumbline, but it will follow a COG - the speed at which it follows the COG, otherwise known as SOG or SMG will be affected by a change in boat speed - I cannot imagine that it will not be affected to the same degree that a NCL boat would be affected by such a change in boat speed.

[Dockhead]

Originally Posted by Lodesman

I didn't realize there was such confusion over the term SMG. Sorry, I'm a traditionalist - to me it's the same as SOG. Starpath Online Classroom Glossary

A CTS boat may not be following a rhumbline, but it will follow a COG - the speed at which it follows the COG, otherwise known as SOG or SMG will be affected by a change in boat speed - I cannot imagine that it will not be affected to the same degree that a NCL boat would be affected by such a change in boat speed.

You are probably using the term more precisely than we are.

I think what we mean should really be called "Speed Made Good Towards the Waypoint".

I think it is traditionally called "Rate of Advance", not so?

Here, however: http://nauticalinformation.blogspot....h-respect.html

Speed Made Good is contrasted to Speed Over Ground, and exactly in our context.

[Lodesman]

Originally Posted by Dockhead

You are probably using the term more precisely than we are.

I think what we mean should really be called "Speed Made Good Towards the Waypoint".

I think it is traditionally called "Rate of Advance", not so?

Here, however: Gary's Nautical Information: COURSE AND SPEED MADE GOOD

Speed Made Good is contrasted to Speed Over Ground, and exactly in our context.

"Speed of advance (SOA)" I think.

I didn't see the contrast between SMG and SOG in the link - he's describing the same thing.

[Lodesman]

Ok a quick problem on calculator - sorry no diagrams. No destination, this is just speed made good comparison. You want to go east, current runs south, 3 kts first hour, 2 kts second hour. Boat speed 5 kts. Adding the two tide vectors gives me a 5nm south vector, to which I connect boat speed vector (2 hrs @ 5 kts = 10nm). Pythagoras tells us the resultant vector is 8.66nm long for an average speed of 4.33kts (the boat doesn't travel along this vector; the actual path is a flat "v" below it).
Change the boat speed to 4kts, and Pythagoras gives us a 6.25nm vector for an average speed of 3.12kts - 72% of the previous.

On the NCL boat, first hour at 5kts with 3kt current, SMG is 4 kts; second hour with 2kt current, SMG is 4.58kts. The average of these speeds over 2 hours is 4.29 kts. At 4kts boat speed, the SMG in the first hour is 2.6kts and in the second hour is 3.5kts - the average overall is 3.1kts, which (surprise surprise) is 72% of the previous average.

Boat speed has an equivalent effect on the time it will take you to get to destination, regardless of whether you're NCL or CTS.

[jackdale]

When I need definitions I tend to turn to Bowditch

speed made good. The speed estimated by dividing the distance between
the last fix and an EP by the time between the fix and the EP. (On the current vector, the point where the CTS/S line intersects the CMG/SMG is a actually an EP based on an hours current.)

speed of advance. 1. The speed intended to be made good along the track.
2. The average speed in knots which must be maintained during a
passage to arrive at a destination at an appointed time.

speed over ground. The vessel’s actual speed, determined by dividing the
distance between successive fixes by the time between the fixes. (A GPS calculates SOG through a series of fixes.)

VMG is not SMG or SOG. Velocity Made Good, or “VMG”, is the measure of speed towards a waypoint. It is a GPS term, not generally used in paper charting. Racers often use VMG to determine which is the favoured tack on a beat.

Bowditch does not have a definition for VMG.

[Seaworthy Lass]

Originally Posted by Lodesman

I didn't realize there was such confusion over the term SMG. Sorry, I'm a traditionalist - to me it's the same as SOG. Starpath Online Classroom Glossary

A CTS boat may not be following a rhumbline, but it will follow a COG - the speed at which it follows the COG, otherwise known as SOG or SMG will be affected by a change in boat speed - I cannot imagine that it will not be affected to the same degree that a NCL boat would be affected by such a change in boat speed.

Lodesman, I just had a look at your link. The last paragraph reads:

"SMG is speed made good. A term used in the past, but without a unique definition according to modern conventions. It could mean speed over ground or velocity made good."

In previous threads Jackdale gave a method for calculating the SMG when using the RYA method for determining the CTS. In that instance he was referring to SMG as VMG, not SOG, so I have followed this convention.

Perhaps the term should be dropped if it can mean either SOG or VMG "according to modern convention". Highly confusing if you ask me!!!!