SOG or STW for course to steer

[Richard5]

Groundspeed is what you want. Now if you really want to get into it you figure your correction for current and wind angle. But I suspect you are now in the realm of tan and cosine and +/- adjustments as you home to the destination. Stay low to the wind as long as possible. Point high at the end of the passage. Reason being 1 hour pointing high Vs 1 hour pointing low is beating close to the wind yields less COG in same time.

[goboatingnow]

we had nearly 1000 posts in two threads , several months ago on computing CTS. knock yourself out with the search facility

dave

[muttnik]

Originally Posted by goboatingnow

we had nearly 1000 posts in two threads , several months ago on computing CTS. knock yourself out with the search facility

My punt, which we'll need the OP to confirm or deny, is that this isn't a "how do I determine my course to steer?" question (which I am sure has been debated many times before) but "I have a method to calculate course to steer which requires boat speed as an input: is "Boat speed" in this context STW or SOG?". I am arguing that if the OP has learned from RYA/British navigation books (as may be likely from location), the input to the CTS calculation that would be explained there is STW: "boat speed" in those texts is generally synonymous with log speed. That methodology was taught before people could just read SOG off their GPS displays. Now that's a lot of assumptions on my part which means I could be utterly wrong about the actual question being asked: Hopefully mthomson can clarify what the rest of the formula looks like to confirm or deny which "speed" is the missing piece.

[boatman61]

If its a classroom question he'll have been given a fixed speed to calculate from... and fixed tidal/wind data... and a chart to plot it on...at least that's what I was given in exercises 30 odd years ago...
If that is the case he should do what the instructor taught... they don't like independent thinkers...

[Dockhead]

There have been several correct answers.

The only reason to calculate a Course to Steer is to get efficiently across a moving body of water. As was discussed in extreme detail earlier in the year on another thread, the fastest way from Point A to Point B across a moving body of water is a straight line -- through the water. A straight line through the water is a constant heading, NOT constant COG.

Therefore, when solving this task you are working with Heading and Speed Through the Water, definitely not COG or SOG. COG and SOG will vary greatly during a successful passage across moving water. The Heading will not vary -- because you are sailing a straight line through the water, which is the most efficient path.

If you look in the archives, you will find an exhaustive discussion of this interesting problem. You will see mathematical proofs that the most efficient path across the English Channel, for example -- an excellent example of the kind of passage for which you must have a Course To Steer calculation -- is an "S" curve as seen from the ground perspective -- an "S" curve over ground, but a straight line through the moving water.

If you attempt to sail a straight line over the ground in the English Channel -- that is, hold a constant COG -- you might never get there. You will be fighting the tide first one way, then the other way, and you will add lots of miles to your passage. Your boat sails through the water, after all.

[Dockhead]

Originally Posted by Richard5

Groundspeed is what you want. Now if you really want to get into it you figure your correction for current and wind angle. But I suspect you are now in the realm of tan and cosine and +/- adjustments as you home to the destination. Stay low to the wind as long as possible. Point high at the end of the passage. Reason being 1 hour pointing high Vs 1 hour pointing low is beating close to the wind yields less COG [SOG?] in same time.

Don't confuse SOG with VMG to waypoint. These will deviate if the water is moving. SOG doesn't necessarily tell you anything useful at all when the water is moving. It is definitely not any kind of substitute for STW data -- they are very different things.

If the water moves in only one direction at a constant speed, then you don't need to calculate a CTS. You can simply adjust your heading until COG equals bearing to waypoint and Bob's your uncle.

It is only when the water is moving in different ways that you need a CTS. The whole purpose of a CTS calculation is to give you an efficient constant heading -- again, not constant COG -- across a body of water which moves in different directions and/or speeds during your passage.

[mtompson]

Many thanks everyone! Muttnik, your right about me looking @ RYA CTS, it was because it was often not clearly defined, & with ( as I mentioned ) so many different ideas around, that I needed to ask. I don't actually have any RYA lessons or questions to hand, so I'm not sure if they nail this down in their actual official material?
I do agree, for my needs STW seems right.