Problem with wind!!

[martinwoolwich]

As a near novice in boating there are some terms that I want to understand. Can someone give me advice on where I can get both a theoretic and a practical understanding of;

Apparent Wind Speed and Direction vs True Wind Speed and Direction. Velocity made good (VMG), and damping levels (when applied to electronic wind instrumentation).

I have read the manuals from cover to cover but the manufacturer, understandably, makes the assumption that the user already has a grasp of the basic concepts - which unfortunately with me is certainly not the case.

Regards

Martin

 

[SeanMcKeown]

Try This Is Sailing by Richard Creagh Osborne. It's mainly about dinghies - but the dynamics are probably more important than for cruisers.

 

[Twister_Ken]

Apparent vs true wind

You are in a boat at anchor, so that you are stationary in relation to the ground. The wind speed and direction you experience is called the true wind. However, lift the anchor and begin to move in relation to the ground, and you create a 'personal ' wind by virtue of your movement (imagine riding a bike on a totally windless day. If you cycle at 15mph you will experience a 'personal' headwind of exactly 15 mph).

The combination of the true wind and your personal wind is what is referred to as apparent wind. Examples: Imagine the wind is blowing from due West at 10 kts. You are at anchor and you experience 10 kts from the West. You lift the anchor, and motor to the West at a boat speed of 5 kts. You will now experience a true wind speed of 10kts westerly and a personal wind speed created by your movement of 5kts Westerly. Your apparent windspeed is therefore 10ktsW + 5ktsW = 15kts from the West. You decide to turn around and head due East at the same speed. Your apparent windspeed is now 10ktsW-5ktsE = 5ktsW. Rather than feeling a nice breeze in your face as you motored westwards, you will now feel a very gentle breeze on the back of your neck. You realise that the pub closing time is fast approaching and you advance the throttles so you are now doing 20kts boatspeed to the East. Your apparent windspeed is now 10ktsW (true)-20ktsE (personal) giving an apparent windspeed of 10ktsW-20ktsE = 10ktsE.

The wind speed instruments on the boat always measure the apparent wind but some can also calculate true wind. If you want to work out true wind from the apparent wind in your head, then dead upwind or dead downwind sums are easy to do. But when you start heading across the wind, it becomes more difficult and you'd probably have to use vector diagrams or trigonometry to do the sums on paper.

A wind instrument, if linked to a log measuring boat speed and a compass measuring heading (or a GPS measuring true speed and heading relative to the ground and unaffected by tide as a log and compass are) can resolve the problem of determining true wind instantly using low-level computing.

VMG - velocity made good - is the speed at which you are approaching a geographic position. If we are back in our motorboat doing ten knots with no tide or current and heading directly towards a mark, then our VMG to that mark is 10kts. If there are 2kts of tide directly against us then the VMG will be 10-2 = 8. Again, that's easy to work out. But if we are in a sailing boat, tacking towards a mark, in a cross tide, steering to wind shifts then the sums become complicated and to resolve them you'll need to draw vector diagrams. Resolving VMG using log and compass is easlily done by in-instrument computing, but is more accurately achieved by a GPS which will also show you XTE (cross track error) or how far off the ideal heading you are.

Damping refers to averaging of readings over a time period. The wind is never steady, but always has micro fluctuations of speed and direction, as well as less frequent macro fluctuations. If wind instruments recorded every micro movement, then they would hardly ever have numbers on screen long enough for you to read them. So the instrument is damped to average readings over a time period - say 5 seconds. Micro fluctuations are thus averaged out, while macro changes are visible.

 

[peterb]

VMG

VMG can have two different meanings.

The original meaning (before the widespread use of GPS, Decca, etc) was Velocity Made Good to Windward. This was useful, for instance, when steering a few degrees nearer to the wind causes the boat to slow down; does the gain from sailing closer beat the loss from slowing down?

More recently the term has been taken over by a number of GPS sets to mean the rate at which you are closing a waypoint.

Generally if your reading of VMG is coming from a wind instrument with an input from a water log but without a GPS input, then the meaning is likely to be the original one. If you have a GPS input and a water log, then either meaning might be implied. You can tell which by setting a downwind waypoint; if you get a negative VMG when you are heading for the waypoint then you are using the original meaning, but if the VMG is positive then it is the more recent GPS meaning.

 

[ToMo]

The following is an American site but don't let that put you off, I found it very helpful many times.
http://www.sailingusa.info/

TôMö