Sailing Polar Performance
07/07/15 12:34 Filed in: Offshore sailing
By Paul Shard, Copyright 2015. All rights reserved.
We have a lovely sunny day with light winds of about 6 knots on the beam. We are sailing at just over 5 knots. Is this fast enough? According to our polar performance data it’s not as quick as predicted. What is polar performance data? If you’re wondering if it relates to the “north pole” and what this has to do with sailing then read on :-)
Polar Sailing Data
For any given point of sail your boat could potentially make a maximum speed with the sails set perfectly in specific conditions. It is useful to have this information and we can use it to compare boats, plan passages and more. To do so we need to organize lots of data for various conditions and points of sail for our specific boat.
This can be quantified by making a table of the true wind angle, true wind speed and achievable boat speed. It is commonly described for a specific wind angle like this…
90 degrees - Wind speed 6 knots - Boat speed 6.5 knots, Wind 8 - boat 6.8, wind 10 - boat 7.0, wind 12 - boat 7.1 etc
So a table might look like this showing winds from 4 knots up to 30 knots along the top row and wind angle in degrees down the left side…
Polar performance can also be graphed in a polar diagram like the one below out of the brochure for our Southerly 49.
Theoretically with True wind 6 knots at 90 degrees we will make 6.4 however we make just 5.3. I think that's quite good for such light wind but it's a knot slower than we might in theory. Routing software can only suggest a good route if it can estimate where you might get to with your boat's actual achievable performance.
How to make polar performance data work for me!
Polar diagrams are calculated in a theoretical world with a clean bottom with no drag (no prop too) flat seas and perfect sails set perfectly. Our polar diagram includes us using a asymmetric spinnaker on a pole - an option we did not purchase so we will never achieve numbers so good when in those conditions. Also we will set sails and reef to suit our conditions and short-handed crew.
Routing programs usually allow us to set a "factor" to downgrade this to take into account the difference between the the perfect world and our real conditions. So setting this to 80% may get us closer to the mark. Our 6.7 knots we should be making will be reduced to match our actual performance of 5.3.
Then the program gives us a reasonable estimate that we might expect 8 more days on passage before we arrive at Flores. We are not racing after all and I had always wondered what this famously light wind passage would be like.
Out here in our high pressure world of endless sapphire seas, flying fish and tropical breezes we will sail gently on and enjoy this world that we as sailors are so lucky to experience!
Polar Performance Sailing Suggestions
- Try pre-made tables if available. You can modify them to suit your achieved performance using them as a starting point.
- If no performance data is available you might find a table for another model that is similar to your boat.
- Record your sailing performance as you are out in a variety of conditions - perhaps you can improve the tables by downgrading your pre-made table to reflect your cruising reality (older sails, conservative decisions, to much gear stowed on deck etc)
- Examine “close hauled”. This was one situation where tables seem inaccurate for us. We cannot point as well as theory and tables suggest. We have 4-6 knots on the nose so want to be close on the wind. I tweak all I can but just get 27 degrees apparent. Pretty good I think but nowhere close to what our table predicts. We should make 19 degrees AWA (apparent wind angle) which is 32 true. If we use this with the routing software it will have making unrealistic headway.
- Garbage in, garbage out. The best routing solution from a weather router to software like Predictwind or others will only be able to suggest a good route if they know how fast you will sail in different conditions and headings. With proper polar data these routing services are a great idea (routing Blog to follow!)
- Polar performance data from the boat manufacturer is quite likely to be somewhat optimistic. They are trying to sell boats and show just how well a model could perform IF everything was perfect. This is not how we will sail shorthanded on the ocean with the boat loaded for a 3 week crossing…
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