A polar situation !
|10 February 2012||Posted by Alex from Lagoon under Technical world|
Even if a cold wave is stricking the Europe, it is not the purpose of this article. Many of you seemed to appreciate the post about the VMG, so I will keep going with a series of technical posts which are rather easy to understand if explained simply and clearly. This article addresses the “polar diagram” or “speed polar”. It can be very useful and is as important as the VMG notion for cruising “efficiently”.
The polar diagram is a graphic representation of the boat speed at under a given wind angle. I propose hereunder to address the subject step by step, so as to understand each element that makes up a polar diagram.
On the first diagram you can see a half circle with a graduation in degrees: 0° at the top, 90° at the middle and 180° at the bottom. This graduation represents the true wind angle (TWA) of the boat course. Here, the course of the Lagoon 450 is at a 90° angle from the true wind direction. The true wind is the real wind that your windvane indicates when the boat is at a stop.
The boat can of course change her course so that I added a graduation on the second diagram. In simple words, the boat cannot follow a course with a TWA of 0° because she would be dead upwind (except if you are motoring, of course). If your course has a 180° TWA, it means that you are on a running course. This second diagram in fact reflects a notion we are all familiar with: a points of sailing graphic.
As a polar diagram is the visual expression of the boat’s TWA and her speed, the third diagram also represents the boat speed with half circles of different colors. The fourth diagram is the combination of both worlds: boat speed and TWA.
With this description of the different elements of a speed polar diagram, we can now look at and understand that of the Lagoon 380 below. You can see the same elements as those described above (boat speed and TWA) to which one last thing has been added: the True Wind Speed. Each blue curve represents a specific wind speed. On a beam reach (90° from TWA) with a 16 knots wind speed, the boat will sail at around 8 knots.
Speed polar diagrams can be very useful to fully understand the notion we talked about in the recent VMG post: “the compromise between course and speed”. You will easily see on the Lagoon 380 polar diagram that she will be faster at a 60° TWA than at a 50° TWA.
The speed polar diagram is a “picture” of the speed of your boat with a given TWA and a given wind speed. When I say “picture” I mean that you need to accept that it can only reflect specific weather and specific sailing conditions. On this Lagoon 380 the skipper had the whole mainsail and the genoa on until 20 knots, then one reef in the mainsail until 24 knots and 30% of the genoa furled with 26 knots. The acceleration that you can see between 110° and 120° from TWA is due to the use of the gennaker. Over 26 knot winds, the drift and the sea conditions have too much influence on the boat’s performance and this is why there are no data for stronger wind conditions.
A polar diagram does not represent the actual performance of your boat but more an indication of the boat’s potential performance. As you know lots of different factors can influence the speed of the boat and consequently modify the diagram, such as the load, the quality of the sails, sails trimming… If you are a Lagoon 380 owner it is hard to compare the data of your boat with this speed polar diagram because:
– if you have few equipments so a light boat you can beat the diagram;
– if your boat is fully equipped so heavy and has old sails, you will probably be slower;
– it is practically impossible that your data have been saved with the same conditions of this speed polar diagram.
If you want to have accurate data for your boat that can actually be useful for a navigation plan, you need to do it yourself by saving each data of True Wind Speed, True Wind Angle, boat speed, sails trimming… It will give you a real picture of your actual boat’s potential and will help you to sail faster to that fantastic destination you have so often dreamt of!