How Fast Am I Going?

For most landlubbers, most of the time, this is a question that is quite simple, and a variety methods will all give you essentially the same answer. When you are standing on, or rolling over, the ground, the ground is always your reference point, so measuring Speed Over Ground (SOG) is pretty much the only thing that makes sense. It is unusual that you would even think of something else.

Sailors have a much more complex relationship with the word “speed.” They certainly understand the concept of a “Speed Over Ground” (SOG), but until the widespread use of satellite navigation systems, SOG was only available intermittently, and rarely accurately. The information that was available was “Speed Through the Water” (STW). Sometimes, these numbers are the same, if there are no currents in the water, other times they can be different. Sometimes very different, if the current speed is more than a small fraction of the boat’s STW.

The modern sailor has both SOG and STW easily available. SOG from the GPS satellites, and STW from a number of mechanical or ultrasonic methods.

Sailing along the east coast of the USA we have to contend with one of the largest flows of water in the world. Parts of the Gulf Stream move over 2 cubic miles of ocean water every minute to the north and east. Surface speeds of three, four, even four and a half knots are common.

As the Gulf Stream moves along the East Coast of the USA it twists, turns and wiggles. The fastest water is not in the same place from week to week, or even day to day.

Four knots is a significant fraction of most sailboat’s speed, so no matter if you are headed north and looking for an extra boost, or headed south and trying to avoid bucking the strongest part of the current, knowing what the current flow is can be a really important navigation tool.

Given a Course Over Ground, a Speed Over Ground, a Heading, and a Speed Through the Water it has always been possible to calculate the speed and direction of the local current–if you were handy with vector arithmetic and trigonometry. Modern sailing instruments can, of course, do this in real time and present the data to the navigator as it happens both graphically and numerically.

To me, this data is worth a LOT while making a long distance ocean trip. If I only know the Speed Over Ground, or only the Speed Thru the Water, I feel very under informed about my situation. Not knowing how the water I am sailing in is moving is missing a key piece of the puzzle.

Playing the currents can make a big difference in how long a passage takes. As an example, last year while we were migrating north, we had stopped off the coast of South Carolina to fish. While we were there, a much larger, and theoretically much faster, sailboat passed us heading north. That was the last we expected to see of them…

We used what we knew about the expected behavior of the Gulf Stream, and what our instruments were telling us about the current speed and direction, to stay in the fastest currents we could find. Three days later, as we approached Cape Henry at the entrance to Chesapeake Bay, who comes up on us from behind, but our bigger, faster friend. They actually called us on the radio, “How do you get here so fast?” Karen was impolite enough to laugh…

This entry was posted in Underway and tagged , , , , , , , , , , , , , , , , , , , , , , , , , , . Bookmark the permalink.

3 Responses to How Fast Am I Going?

  1. bradford simms says:

    “The modern sailor has both SOG and STW easily available. SOG from the GPS satellites, and STW from a number of mechanical or ultrasonic methods.”

    Good information on the post, SOG is easy enough but STW is a little more trickey. What would you reccomend budget frendley way to go? Thanks Brad

    Like

    • Bill Kinney says:

      Bradford,

      I did just sort of skim over that… We haven’t found the perfect, budget friendly, answer to a reliable water speed transducer. Certainly any of the paddlewheel devices work… until they don’t.

      We installed an Airmar CS4500 ultrasonic device (NB: Not budget friendly @ $800) that worked very well for 3 years, and then died. Upsides: no moving parts, no regular cleaning needed, good data even at very low speed. Downsides: Initial cost, and (apparently!) lifespan.

      We are back to the old fashioned paddlewheel. Cheap and USUALLY good data. On the other hand, they are very susceptible to fouling especially while not moving, but even sailing they can pick up pieces of weed or other debris that give them grief. They must be kept clean. We haven’t found them to last all that long, but the cost is low enough it’s hard to complain about that.

      Airmar has a new ultrasonic unit that has more bells and whistles, but it is very expensive, and who know about it’s expected lifespan?

      There is always a length of cord, a wood block and a stopwatch!

      Kind of ironic that technology lets us use satellites to easily pinpoint our location to within inches, but we still have to fuss with water speed!

      Like

      • bradford simms says:

        Hi Bill,

        Thanks for getting back so fast. A tow behing device is very appealing for the infrequent cruser.Keep it in a bag and chuck it in as needed. I recall seing one for sale but can’t remember the details. How hard can it be 🙂

        Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s