SAILING TIPS Mike Huston


and the direction the masthead fly will indicate. If a boat is not moving, true and


Davis 3120


Using Your Masthead Fly Apparent vs True Wind


One of the first things I do when


I get to the docks is look up at the top of someone’s mast to see which way the wind is coming from. It is almost instinctive for me – I don’t think to do this, it is just a habit. What I am looking at is the masthead fly – the wind vane which indicates wind direction, that virtually every sailboat has at the top of her mast. The masthead fly, or spar fly, can be an extremely valuable tool in many situations. In this month’s article we will examine many of these uses. Before we dive into its uses, let’s


look at the masthead fly itself. Most consist of an arrow that swivels, so that it always points toward the wind, like a wind vane on a barn. Most have two additional rods with small squares at the end of them. These rods are normally adjusted to line up with the arrow when the boat is sailing close- hauled. It is really a simple device – but very useful. The first use that comes to mind


is when we are preparing to leave the dock. It is a good idea to look up at the masthead fly to see where the wind is coming from. This will tell us what the wind is going to do to the boat once the


Figure 1


Wind produced by bicycle speed.


mooring lines are cast off. And, for that matter, it will help tell us which lines to cast off first. The same is true when returning to the dock – a quick glance at the masthead fly when nearing the slip will provide needed information about the impending docking. The next usage that comes to mind


is when we raise the mainsail and need to have the boat headed into the wind. Here the helmsperson uses the masthead to steer – keeping the arrow pointing directly toward the bow. This brings the wind straight down the boat and allows the main to go up without catching a batten in the lazy jacks, etc. On boats with furling mains this is not as necessary, but it is still helpful to have the boat headed close to the wind.


Before we get into masthead fly


usage while sailing, there is one very important concept we need to discuss – true wind versus apparent wind. True wind is the direction the wind is blowing relative to fixed objects. It is the wind direction someone standing on a dock would sense. Apparent wind direction is where someone standing on a moving boat would sense wind to be coming from;


Wind produced by boat moving forward, boat speed.


apparent are equal – for example, a docked boat’s masthead fly will indicate the true wind direction. But once a boat starts to move the two can quickly become different. Let me demonstrate: pretend you are riding a bike and there is no wind. If you start to ride you will feel wind (caused by your movement) coming from directly in front of you. Now pretend there is wind coming from your left side at 10 MPH. If you start to ride at 10 MPH your motion will generate wind from the front. The net effect of both winds will be an ‘apparent’ wind coming from 10:30 on the clock and it will be ‘blowing’ at 14.1 MPH. Figure 1 shows some examples.


The apparent wind direction is indicated by the dashed arrows, wind produced by vessel/bicycle speeds are the solid arrows pointing straight down at vessel (bicycle) and the true winds (actual wind direction) are the solid arrows pointing at vessel/bicycle in these examples. Note that wind speeds are proportional to length of the arrows and apparent winds will be stronger than the true winds when going to weather and slower than true wind when sailing downwind. We can explain apparent wind using the vector math most of us were taught in middleschool. Hint: to calculate the exact apparent wind, put the true wind and vessel speed vectors head to tail to find the apparent wind vector (see the downwind example where I left


Wind produced by boat moving forward, boat speed.


True wind


Actual wind (true) wind blowing on bicycle.


48° NORTH, MARCH 2011 PAGE 34 direction on boat. Sailing to Windward


Boat speed vector, used for calculating apparent wind speed.


Wind (apparent) felt on bicycle.


Apparent wind on boat.


Apparent wind on boat.


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