Measuring your leeway for better sailing

by Grant Headifen on 26 Jun 2010 Leeway is just one of those things that is a law of the universe, just like gravity. Still with gravity – the advantage is that it’s highly predictable. And so then is leeway.

You will already know that leeway is the sideways slip motion of our sailboat down wind from the pressure of wind against our boat and sails, and always has to be taken into consideration.

Airplanes suffer from the same issue. When flying in a cross wind, the plane crabs (slide slips) downwind. The course becomes different from the heading.

Explaining leeway in detail:
Not accounting for leeway will have you sailing (or flying) in a fairly unnoticeable arc to get to your destination, or worse miss it altogether. To represent an example with a destination to the north and a westerly crosswind, here’s what happens; you aim for the mark at 000, your boat slips sideways to the west. Now your destination is at 359 but you don’t really notice it. After a few minutes (or hours, or days, depending on how far away it is) your destination is at 358 still in noticed. Still later your heading is 355 then 350 etc. All because you keep aiming at the destination but you’re being pushed to the east by the wind. Your course over ground becomes an arc and is the long way around.

The prudent sailor will account for the leeway and sail a constant heading depending on their known leeway of say 350 for the example above. The sideways slip motion will deliver them to the destination in a straight and shortest line.

Now that we’re in the electronic age, navigators on a sophisticated racing boat will plug in the destination to the gps. The autopilot which is cross talking to the gps takes care of the rest. The gps analyses the cross track (the boat’s distance away from a straight line to the mark) and feeds back to the autopilot the proper heading to minimize this in real time. Thus resulting in a straight course to the mark.

The problem with this is that you cannot turn the autopilot off and then on again for any reason (eg avoiding an obstacle), because the autopilot, when turned on again, will turn too quickly to get you back on the track.

Anyway, once you know your own boat's leeway, you can account for it in advance, without electronic assistance.

Leeway is particularly more prevalent when you are sailing on a close haul or close reach and can be as much as 20 degrees depending on the wind conditions, water conditions, your sailboat design, your apparent angle with the wind and how your sails are set.

When, for instance, trying to negotiate a narrow channel with wind and current against you the leeway can be even greater, and the cruising sailor always has to be prepared to react quickly to counteract it when the situation demands.

Here’s a few general tips to follow to minimise your leeway:

Over sheeted sails cause more sideways force and thus sideways slip (leeway).

Fly the telltales diligently.

Aim for a position to windward of any destination you're headed for.

The more you are sailing on an upwind course, the more the degrees upwind you should aim.

The higher the wind speed, the higher above the mark you should aim.

In general, on a close haul, allow 10-15 degrees. Adjust this less if the wind is light, more if the wind is strong. Reduce this amount linearly as you bare away from the wind.

Make sure your boat is trimmed with slight weather helm, as described in last week's tip.

When doing serious navigation we absolutely must account for leeway and an excellent understanding of how your boat performs leeway wise is essential and how to solve for it once you know it.

So, using a gps and a nice steady windy day, you can do a simple determination of your sailboat’s typical leeway.



Calculating your 'typical' leeway:

(1) Begin sailing on an angle slightly off a close haul direction and with a recognizable land marker dead ahead.
(2) Measure your speed
(3) Douse the sails and begin motoring at the same speed in exactly the same direction.
(4) Take note of your gps course.
(5) Deploy the sails and turn off the engine.
(6) Continue to aim for the same point on land.
(7) Now read out your gps course.
(8) The difference in course angles will be your leeway
(9) Repeat for different points of sail
(10) Repeat for the opposite tack.
(11) Repeat on different days with different wind strengths

Note that:

(a) As long as the current is light, this method is relatively immune from current because you have normalized it out by performing the motoring task. In an area of heavy current, you will also need to test this and allow for it.

(b) this method will not account for the leeway due to the hull of your boat presented to the wind.

If you think you can benefit from some online sailing lessons from the comfort of your computer seat, check out the excellent www.nauticed.org!Nauticed site.
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Letter from Reader:


Sender: Terry Cain

Message:
The article by Grant Headifen 'Measuring your leeway for better sailing' contains a factual error relating to the analogy with aircraft tracking in an air mass that is moving. In fact, it would have been best not mentioned in such an article. There is only one context in which deliberate sideslipping is commonly used and I will come to that below.
Leeway is well understood and I applaud his attempt to analyse same, however he has made a statement which shows an absence of understanding of how aircraft operate in tjhe air mass.
First let's deal with the special instance of deliberate sideslips to maintain track:
* When a light aircraft is landing in a crosswind which has a speed component approaching the published limit for that particular aircraft, it is common practice to carry-out the final approach using a heading which offsets the drift caused by the crosswind, this is observed as crabbing or sideways appearance to the ground observer (the aircraft knows only the air mass in which it is proceeding --- cares nothing for the approaching ground, and is flown fully co-ordinated and without sideslip or yaw /skid). When the ground is close -- (the transition height to another compensating technique depends on the particular configuration) -- the pilot exits the 'Crab and Kick' approach and the kick on the rudder straightens the machine so that it now points in the direction of the landing strip centreline. The pilot then may elect to lower the 'into-wind'wing at some angle to effect a small sideslip during the flare and hold-off to maintain the centreline track. This slip toward the t
he resolved crosswind component is quite small and of relatively brief duration until the wheels contact the surface.

* Apart from this, and a deliberate sideslip to help reduce altitude in unusual situations such as flap failure, the way an aircraft proceeds through the air mass is in the main as though wind does not exist.
The machine is flown at maximum efficiency in a properly co-ordinated fashion without slip or skid (Yaw).

* In climb and cruise and descent phases of flight, the heading flown is chosen so that the resultant track across the ground is the desired track.
There is no slip or skid involved as these would markedly reduce efficiency due to increased drag on the airframe.
The heading used ot offset drift is often called WCA or wind correction angle, although as far as the aircrafts performance is concerned, wind does not exist in these phases of flight unless there is a transition from one air mass regime to another as the flight proceeds.