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Old 20-11-2020, 16:42   #1
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Creating polars for an older cruising boat

So, I am getting serious about weather routing at last, and starting to develop my own polars before I head out cruising.

After owning this boat for 8 years I have a general sense of how she sails in various winds, but I am hampered by the fact that most of my sailing has been done on a bit of water that is shallow with really bad chop and a freakishly sharp wave action.

I am always amazed when I look at photos from when I bought the boat home. The speeds shown on the instruments in the background of the photos are so much higher than I get sailing around my local waters. But those photos were taken in Bass Strait with at least 30 meters under the keel.


I wondered if anyone had created their own polars for a heavy old cruising boat like mine. She's a Swanson 42. She has a LOT of beam, canoe stern and a cutaway forefoot keel. I've got her down to about 15 tons now, plus she has a feathering prop, so I am thinking something like a Tayana 42 or some of the Pacific Seacraft variants might have similar sailing performance. Perhaps even a HC 38?

If I could look at some polars created for similar boats I think I could extrapolate them to fill in the gaps that have been created by the fact that any time the wind gets over 15 knots here, the sea state turns to crap and my boat stops dead in the water. Most of the time I can go faster in 12 knots of wind than 17.

Can anyone point me to some suitable data?

Matt
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Old 20-11-2020, 19:31   #2
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Re: Creating polars for an older cruising boat

https://www.seapilot.com/features/download-polar-files/


They have a Tanyana 42 in there among many others...
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Old 20-11-2020, 21:44   #3
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Re: Creating polars for an older cruising boat

Quote:
Originally Posted by Spot View Post
https://www.seapilot.com/features/download-polar-files/


They have a Tanyana 42 in there among many others...
Thank you, I had tried that site but was struggling to download any of the files. In the end your post encourage me to try a new approach.

Downloading a zipped file of all of them worked, and that's helpful because I can trawl them looking for boats with similar numbers to my sub-15 knot wind figures.
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Old 21-11-2020, 04:12   #4
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Re: Creating polars for an older cruising boat

Yeah, it took me a couple tries to figure out that the one zip file works.
Do you have a way to combine and plot the data? I do not, I can sort of plot it in CAD but it is not scripted so it takes a while.
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Old 21-11-2020, 04:21   #5
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Re: Creating polars for an older cruising boat

You can plot it for example in weather routing software like qtVlm and I think also in OpenCpn with the correct plugin.
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Old 21-11-2020, 04:48   #6
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Re: Creating polars for an older cruising boat

Remember, the polars are also sail specific! Most polars are made for newish sails and they go south from there with ageing sails.
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Old 21-11-2020, 05:57   #7
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Re: Creating polars for an older cruising boat

To make your own polars, you can do it quite simply in Microsoft XL and it automatically updates as you make changes. Don't look at the numbers I've put in, I've just applied flat rate formulas to wind speed...... That way I can do 39kn in 30kn of breeze. �� Create a spreadsheet and then go to insert, charts, radar, then select the style you want and its done.
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Old 21-11-2020, 07:49   #8
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Re: Creating polars for an older cruising boat

In reality, cruising speeds are always slower than the polars indicate, as they are made for optimum conditions, full crew and so on.

For my weather routing, I use polars from a slower boat with a similar setup. This means I can match or exceed the weather routing predictions.

Also, I tend to adjust the polar diagram for wave action, and also night sailing, which normally is slower. All easy to do in qtVlm, which in my opinion is a nice weather routing software, considering the price (free).
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Old 21-11-2020, 08:15   #9
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Re: Creating polars for an older cruising boat

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Originally Posted by Dave_S View Post
To make your own polars, you can do it quite simply in Microsoft XL and it automatically updates as you make changes. Don't look at the numbers I've put in, I've just applied flat rate formulas to wind speed...... That way I can do 39kn in 30kn of breeze. �� Create a spreadsheet and then go to insert, charts, radar, then select the style you want and its done.
Thanks for the example pic, I got Open Office to do something similar. The radial angles are not 100% chart to data and I guessed at dead downwind being 75% of speed at 150 degrees TWA but it is starting to look like a pattern with very little typing on my part

I will have to look at qtVlm and OpenCPN. I though I put something in OpenCPN to let the Shipdriver plugin be close to my 6.4m boat while sailing. But it is also fun to motor in Shipdriver at 100 knots
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Old 21-11-2020, 10:12   #10
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Re: Creating polars for an older cruising boat

Hi Matt,

We have a PSC 40 and I have fooled around a bit with iPolar from the Apple store. It seems to produce ok diagrams and data sets but I haven't done anything with it to a large extent. I think you should take a look at it if you have not seen it before.

In using it sporadically, the only thing I noted was that the predicted speeds seemed close, but a little higher than what we get when we are out sailing - but I attribute that to our casual sailing style.

I did pass this onto Dockhead at one point, but I have no idea if he ever used it.

Ron
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Old 21-11-2020, 10:46   #11
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Re: Creating polars for an older cruising boat

Can someone explain Polars?
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Old 21-11-2020, 10:55   #12
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Re: Creating polars for an older cruising boat

Quote:
Originally Posted by Spot View Post
https://www.seapilot.com/features/download-polar-files/


They have a Tanyana 42 in there among many others...
Anyone know where to find polar data for a Hans Christian 41T? It's not included on the SeaPilot site and I have not been able to find it elsewhere.
(Please forgive me GILow for using your thread)
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Old 21-11-2020, 11:08   #13
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Re: Creating polars for an older cruising boat

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Originally Posted by peterbeale View Post
Can someone explain Polars?
Graphics presentation of the boat speed to all points of true wind and various wind speeds.
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Old 21-11-2020, 11:17   #14
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Re: Creating polars for an older cruising boat

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Anyone know where to find polar data for a Hans Christian 41T? It's not included on the SeaPilot site and I have not been able to find it elsewhere.
(Please forgive me GILow for using your thread)
Upon sufficient effort on my part, mea culpa; the comparisons on the SeaPilot site should be adequate.
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Old 21-11-2020, 12:15   #15
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Creating polars for an older cruising boat

OK, this thread is nice, but just to be clear, I’m looking for people’s self-created polars if I can.

The theoretical stuff online is ok, but as noted, don’t always reflect real-world boat speeds.

When I say “polars”, I’m not talking about the pretty pictures, I’m talking about the data set that creates them. And, as explained, I cannot create that data set here in my current home waters due to the significant wave action.

For now I am able to make a reasonable assessment from the Taman 42 polars on the sea pilot site by extrapolating the differences on my sub 15 knot wind speed knowledge of this boat up into the higher wind speeds. I got some credible numbers using this approach. Probably good enough for passage planning software.

For the record, I’m using fastseas which will create the pretty pictures for you if that’s what you are after.
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