What is a speed polar is?
A speed fleece... Let's agree right away, it has nothing to do with warm clothing to go fast.
For sailboats, the polar is a graphical representation, expressing the speed of a boat as a function of the direction and strength of the wind. In general, this graph is in the form of a semi-circular diagram, as the results are considered identical for both amures.
In other words, if you know the wind strength and wind angle, the polar will tell you how fast your boat is supposed to go... practical is pas??
What is the purpose of a polaire??
In general, a polar speed will allow navigation scenarios to be established. This is called routing. By knowing your average speeds according to wind and sea, you can estimate the sailing time.
If you have routing software, you can even compare scenarios to help you make decisions about safety or performance.
Finally, a polar speed measured on board will make it possible to check if the current speed corresponds to the usual measurements. Algae or pollution trapped in the keel could explain a slowdown.
What is a speed polar made of?
The speed of the sailboat is represented as a function of the wind angle, from 0 to 180 degrees.
The polar is plotted for several wind forces, generally from 4 knots to 25 knots.
Sometimes, it is designed for two sets of sails, one adapted to the downwind speeds (spinnaker plus mainsail) and the other upwind (mainsail plus genoa)
How to read a speed polar?
The semi-circle indicates the boat's speeds in knots (usually top left).
The angles to the wind are represented by the rays that break down the semi-circle from 0° (facing the wind) to 180° (downwind).
The speed of the boat is given by the curve. If there are several curves on the same diagram, each represents the speeds according to a given real wind force.
Here is a concrete example: the polar speed curve of a 9 m long lambda cruising sailboat in 15 knots of wind.
- We observe that upwind, sailing at 50° to the real wind, the target speed is 6kt.
- We will also notice that it decreases very quickly as it approaches the wind.
- 6kt is also the speed obtained in full downwind (180°).
- We also note that the maximum speed that the sailboat can reach is 8 knots between 120 and 140°.
And if there was any doubt, the boat we have "in front of us" is not hovering downwind, because the curve is quite linear.
How to obtain a speed polar?
Depending on the sailor's experience and the knowledge of his sailboat, we can write the polar.
It's as simple as it is tedious: we note in a table the boat's target speeds according to its angle to the wind and its strength.
Fortunately, routing software such as Adrena allows you to record your navigation performance to automatically create your boat's polar.
The other solution is to start from the theoretical results obtained with VPP (Velocity prediction program) programs.
These softwares offer to calculate the theoretical speeds of the hulls, it is a development tool for architects.
It is also an analysis tool for some gauges such as OSIRIS, the French gauge which seeks to group together in the same race categories, sailing yachts with similar performances. If your sailboat is less than 20 years old, by browsing the web you will probably find its theoretical polar.
So measurements or calculations?
Actual speed measurements are not necessarily available. So, many sailors refer to VPPs. But to tell you the truth, the only polar that matters is that of your boat with your crew in your region and with your sails.
If you put Loïc Peyron at the helm of your refitted boat, equipped with new sails, it is not impossible that the results obtained may present some variations with your mid-season surveys.
From a general point of view, the polar is to be observed in its entirety. She will answer some questions such as: is it a boat at constant speed or is it capable of gliding? What is the average speed to reach England through? What wind force is needed to advance at 5 knots?
But let us remain wary of interpretations. Even if your polar is made up of measurements taken on board your boat, many factors can influence the results, starting with poorly calibrated sensors or sea state. If it is a VPP, it should be remembered that the figures obtained are those of a software program that does not set foot on the deck to adjust your sails and never takes the helm when you are tired
