Using Polar Charts
Nowadays most boat manufacturers publish polar diagrams. The Polar Diagram for the Saona 47 is shown here. They can often be difficult to read and do not give a lot of detailed information. A Polar diagram is read by looking at a given true wind speed and true wind angle but typically only performance numbers at 10,20 and 30kn true wind speed are provided.
Polars can be misleading as they are based on lightship, the definition of which will vary from manufacturer to manufacturer, but typically will mean no provisions of any sort on the boat, clean hulls, half water and half fuel.
Further some manufacturers calculate polars theoretically and others sail the boat after creating a prototype.
Usually polar diagrams are created with the boat operating just under main and genoa (jib).
It is important to understand how your boat performs at certain wind angles and strengths, with all of your sail wardrobe in place at certain times. Most owners of production catamarans understand that these boats do not have a great deal of pointing (sailing at a low angle to wind) ability, as slim hulls and daggerboards may allow. Typically a production cat will slide sideways (leeway) at low wind angles or alternatively the sails will stall, and the boat slow down. As a result when sailing upwind if you want to the fastest passage then tacking at optimal wind angles for speed will result in the best Velocity Made Good, which is the velocity in the direction from one waypoint to the next.
How do we then determine the best point of sail. This can be done by compiling a polar chart based on theoretical calculations as well as testing boat performance.
Fortunately, Robbert Verboon has done a lot of work in this regard here detailing the performance of his Saona 47, which to all intents and purposes should translate to the Tanna 47.
A few important points need to be made here.
Firstly, Robbert’s chart was compiled on a fully equipped boat, so it would not be reasonable to compare his polar data with that of a manufacturer at lightship.
Secondly, Robbert himself placed several caveats that should be applied when using the data.
- Current – when determining boat speed current must always be accounted for
- Waves – Robbert’s data was collected in flat waters. Wave action will slow the boat
- Wind shear – wind reading at the anomometer (top of mast) can be very different to to lower down
- Wind variation – stable wind is infrequent – often you have to ascertain a fair number over various wind speeds
- Calibration of instruments – not linear, different on each tack
- Sail wardrobe needs to be taken into account.
It was important to me that I update Robbert’s data for my boat and sail wardrobe so that I could use this data to calculate the best course to follow and which sails to use, as well as to determine whether I am getting the best out of the boat as compared to these metrics.
In addition to the mainsail and genoa I have the following sails.
Flying Genoa Flying Genoa 115 sq.m (typically called a Code 0)
• Triradial cut
• Nylon Challenge Fibermax 144 gr
• Mid Ghirt 55%
• Range of use from 0 to 20 true knts
• Angol of use from 50 to 110 AWA
• Anti-torsion cable 13 Ø
Gennaker A2 170 sq. m
• Triradial cut
• Nylon Challenge Fibermax 64 gr
• Range of use from 55° to 160° AWA
• Range of use from da 0 to 25 TWS
• Radial reinforcements
• UVcover (only white)Code Zero
To create my own polar chart I started with Robbert’s data.
My experience with two previous FPs was that attempting to sail at less than 40 degrees apparent meant that the sails stalled or that leeway was extreme. In any case sailing at tight wind angles will not result in maximum VMG. As a result I eliminated some of the numbers (orange).
I tend to motor if the boat is not moving at least 5knots SOG. This is an arbitrary decision and others will have their own number. As a result I designated those numbers in yellow as “no sail zones”.
In my experience having had code 0 sails in my two previous boats the best one can get is 50dAWA which also happens to be the specification from the sailmaker. This determined when I could fly the Code 0 (green). The wind speeds in question were determined by the calculated speed based on wind speed and 50dAWA. They are obviously theoretical maximums.
I imposed some limits on the Code 0 performance. Firstly, I limited the boat speed to 10Kn as typically at these wind speeds the sea state is increasing and 10+kn is uncomfortable, but theoretically in flat seas higher speeds are possible. Secondly, I throttled back the numbers if they exceeded 70% of wind speed, as based on my experience a production boat will rarely exceed this. Lastly, I restricted the Code 0 to 17Kn True winds. Although it is rated to 20Kn I have been caught too many times when wind speeds have increased suddenly and the sail is difficult to furl in.
Working out when to furl the code 0 and put up the A2 Gennaker was difficult. Whilst the A2 gennaker is rated only to 150d True, it can be rigged to work at all angles although it has to be jybed when necessary, which is a bit of a chore, but better than carrying a spinnaker or parasailor for now whilst in the med. The gennaker is rated from 55-160AWA which for my purposes I changed to 60-180AWA.
The performance chart is shown Tanna-Polar-updated