Using Polars

[Pitts]

HI
I'm trying out QTVLM and want to use a POLAR to assist the routing process. My boat is not listed in the Polars so I'm wondering if there is another more readily recognisable monohull yacht similar to mine that I can use from the polar list. If I describe my yacht and yours is similar can you let me know so I can check if there is a polar file that I can use?

My boat is a Swanson 38, built in Australia, 1985. It has a cutter rig/staysail and large yankee. The masthead rig has a fully battened main. I run an asymmetric spinnaker off the wind. The hull is canoe/double ended and weighs about 10 tonnes, with a 3/4 keel. 11.5 metres long with a beam of 3.4m.

If your vessel sounds anything like this can you let me know the design name/year and I'll check in QTVLM for a polar I can use.

Many thanks all.

Pitts

[Pizzazz]

If you just want a polar to play around in the app, one way to go is fastseas.com, where you enter your waterline and the closest angle to the wind you can sail to and it produces a generic polar.

To do better than that you need to creat your own polar. It helps to use a program like opencpn or Raytech to feed instrument data for wind speed, angle and boat speed then average it out over 10-15 min. It is not that hard and it will give you a truly useable polar.

[Pizzazz]

One more idea, download another great routing app, called Sailgrib WR for Android. It has one 400 polar files in it than you can easily extract.

[Pitts]

Thanks pizzazz- I’ll check this out.

[GordMay]

Carl's Sail Calculator v3.55.
You can use the Sail Calculator to look up sailboat dimensions in a large database of boats or enter your own boat's dimensions and compare it to the others. In addition, for any boat you can calculate a set of values that will help you measure how it will perform.

➥ Sail Calculator Pro v3.54 - 3200+ boats

[Pitts]

Thanks GordMay. That's a very useful piece of software and extensive database. Thanks for the link. Appreciate it.

[Suijin]

Honestly I would punch in any boat approximately similar to yours, like a Baba 40.

In my experience routing with polars gives you “directionally accurate” results that allows you to plan your trip on a macro level but which is of little use once you get underway. Plugging in any boat that is roughly the same speed as yours should give you good enough results.

One option is to look up your boat’s PHRF rating or equivalent and the choose a boat with a similar rating.

[Pitts]

Thanks Suijin.

[hellosailor]

Pitts-
There used to be an Excel spreadsheet online to build your own polars, you might hunt around for it. They really depend on things like how stable the hull shape is (if you put four crew on the rail or the boat wobbles as you walk across the deck, etc.) and how the hull shape performs in 3D as the boat normally heels over, or responds to following seas.
If you can find any other owners, or the builder, or a local racing association, they may be able to give you some leads.
Polars can give you some surprises, i.e. whether running dead downwind really is faster or slower than gybing off, whether you need to reef at 16 knots instead of 20...they're worth pursuing, as opposed to gathering a lot of data on your own.

[billknny]

A warning about all those free databases of polar performance files: They are worth exactly what you pay for them and maybe a good deal less than that if you use them to actually make decisions. In short, they are terrible.

They are all calculated with very simple (actually better described as very simplistic) velocity prediction programs. Short of very involved measurements and detailed calculations by a naval architect who actually knows something about the topic, the only way to USEFUL polar data is to measure it.