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Old 24-11-2020, 20:25   #31
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Re: Creating polars for an older cruising boat

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Originally Posted by GILow View Post
Ilenart, sorry for the delayed reply, I missed your response.

I did not know about the granularity of the weather models. You've given me some homework, thank you.

Fastseas does have those summary pages buried in there. They are terrific.

There's such a price difference between the two. I wish Predictwind had a free trial or something so I could make a meaningful comparison of the two.

I'll see what my reading on forecast model granularity suggests. If it comes down to really only being an issue for racing boats wanting a cutting edge, then Fastseas will be good enough for me. But if there are significant safety implications I'd be prepared to pay more.

PW does the routing calculations on their own servers with the most precise model information (based on subscription level I assume) and only send the resulting information back to their client, so the data transfer is very small and the granularity is very high.

Does Fastseas do the same? If not, that might be a point of difference for offshore use.
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Old 24-11-2020, 23:24   #32
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Re: Creating polars for an older cruising boat

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PW does the routing calculations on their own servers with the most precise model information (based on subscription level I assume) and only send the resulting information back to their client, so the data transfer is very small and the granularity is very high.

Does Fastseas do the same? If not, that might be a point of difference for offshore use.

Not sure, another thing to consider. Tho I have no offshore connectivity at all, so the point may be moot.
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Old 25-11-2020, 00:25   #33
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Re: Creating polars for an older cruising boat

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Hi GILow,



had a quick look at Highseas. They sound pretty similar. Looks like FastSeas uses one model, GFS, which I assume has a 50km resolution? Predictwind uses GFS plus three other models. The two Predictwind models are PWG which uses the NCEP global initial conditions and PWE uses the European Center for Medium Range Weather Forecasts global initial conditions. Both have Resolutions of 1km/8km & 50km global available. It also uses ECMWF forecast from the European Center for Medium Range Weather Forecasts model. They have just announced two additional models (SPIRE & UKMO) which I have not looked at yet.



Both have routing and departure planning. Looks like the Predictwind forecast remote ability sounds a bit more sophisticated as it ties in with Iridium Go and can download GRIB files, which can be displayed on their Offshore program. I assume Fastseas remote response is an email route which you load into another program.



With Predictwind I found the summary page provided with routing and departure planning very handy, particularly the percentage upwind with each forecast. This provided an easy and quick way to compare different routes and departures. Does Fastseas have a summary page?



Predictwind also has a large number of Polars built in, which you can use. However when I tried the one for my boat (Hallberg-Rassy 40) I found it to optimistic and not matching my experience with ocean sailing. My guess is that it was calculated in flat water with brand new sails. My sails are from 2003 and I generally sail in the ocean with lots of waves. Therefore I have ignored the polars and input my own details (ie average speed in 15 knots whilst closehauled, reaching and running).



Found this article, which talks about Predictwind and Fastseas. Hope this helps.



https://www.cruisingworld.com/weathe...e-for-sailors/



Ilenart

In PW you can adjust the upwind and downwind polars in the range 20-200%, so you could still use your boatís polar file.
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Old 25-11-2020, 02:28   #34
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Re: Creating polars for an older cruising boat

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In PW you can adjust the upwind and downwind polars in the range 20-200%, so you could still use your boatís polar file.
I tried using the percentages and I still couldínt get meaningful results. I suspect they were using the polars off the HR website, which includes spinnakers, which I donít have.

https://www.hallberg-rassy.com/filea...eeddiagram.jpg

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Old 25-11-2020, 03:04   #35
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Re: Creating polars for an older cruising boat

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I tried using the percentages and I still couldínt get meaningful results. I suspect they were using the polars off the HR website, which includes spinnakers, which I donít have.



https://www.hallberg-rassy.com/filea...eeddiagram.jpg



Ilenart


Yes, the percentages option still assumes too much about the boatís behaviour on certain points of sail. For instance, the average cruiser adds a bimini, maybe some davits, lee cloths... suddenly the upwind numbers are all gone to pot.

My boat looks a like a bloomin square rigger when I am heading upwind.

And then, as you noted, do you have a spinnaker or maybe an asym?

Then, as has been noted, thereís the age of the sails. Bagged out sails are going to devastate the numbers forward of the beam.

No, I really think these have to be hand assembled for a cruising boat. The stuff from the factory is kinda academic, with or without the percentage adjuster. All it seems good for, to my mind, is compensating for your sailing style, and then thatís mostly for the higher winds speeds for most of us.
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Old 25-11-2020, 03:05   #36
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Re: Creating polars for an older cruising boat

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Originally Posted by fxykty View Post
In PW you can adjust the upwind and downwind polars in the range 20-200%, so you could still use your boatís polar file.


Yes, fastseas has the percentages option, but see my thoughts on that in my response to ilenart. Not certain I am correct, but I really donít think they are all that much use to us cruising types,
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Old 25-11-2020, 03:11   #37
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Re: Creating polars for an older cruising boat

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Not sure, another thing to consider. Tho I have no offshore connectivity at all, so the point may be moot.
In my recent testing with PW for the Souther Ocean I found the results pretty accurate out to three days. After that the four models started to diverge depending on how complicated the weather was. This agreed to when I was using PW on a Fremantle to Bali trip; all four models were generally consistent when the weather was just a simple big high.

However in the testing for the southern ocean (from Albany to Tasmania) I was running the models each day, replicating daily progress of about 150NM per day. Over the 10 day period the weather was a lot more complicated with lows forming, disapearing and combining. The 3 day forecasts did start to change. There was a large high going past and the models generally sent the boat further south to take account of the centre of the high moving lower. At one stage a Low was predicted to form right in front of the route in 5 days time and 3 models said to head for Melbourne and one model headed south below 50 degrees! (I had PW set to avoid +25knt winds). However the low ended up being delayed and all 4 models basically got the boat to 40-43 degrees south and headed straight east to the bottom of Tasmania.

GILow if you donít have offshore communication capability the routing software is only going to be good for about three days and you will miss all the changes that will occur after this period. My plan is to use Predict Wind with Iridium Go, this worked fine on the Fremantle / Bali trip. Have you looked into offshore communication? Otherwise the routing function may not be very useful. However the departure planning will still be useful.

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Old 25-11-2020, 13:38   #38
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Re: Creating polars for an older cruising boat

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Originally Posted by ilenart View Post
In my recent testing with PW for the Souther Ocean I found the results pretty accurate out to three days. After that the four models started to diverge depending on how complicated the weather was. This agreed to when I was using PW on a Fremantle to Bali trip; all four models were generally consistent when the weather was just a simple big high.

However in the testing for the southern ocean (from Albany to Tasmania) I was running the models each day, replicating daily progress of about 150NM per day. Over the 10 day period the weather was a lot more complicated with lows forming, disapearing and combining. The 3 day forecasts did start to change. There was a large high going past and the models generally sent the boat further south to take account of the centre of the high moving lower. At one stage a Low was predicted to form right in front of the route in 5 days time and 3 models said to head for Melbourne and one model headed south below 50 degrees! (I had PW set to avoid +25knt winds). However the low ended up being delayed and all 4 models basically got the boat to 40-43 degrees south and headed straight east to the bottom of Tasmania.

GILow if you donít have offshore communication capability the routing software is only going to be good for about three days and you will miss all the changes that will occur after this period. My plan is to use Predict Wind with Iridium Go, this worked fine on the Fremantle / Bali trip. Have you looked into offshore communication? Otherwise the routing function may not be very useful. However the departure planning will still be useful.

Ilenart
Thank you for that, a great breakdown and analysis.

My offshore comms SHOULD be ok, so the plan was to have someone on shore run the numbers for me each day and relay any changes over HF radio.

If I were planning to do more offshore sailing I'd look into some form of internet connectivity, but my plan is to be a coastal sailor, at least for the next few years. (This passage is a one-off and may yet turn into a coastal route depending on a few other factors.)

Maybe by the time I'm finished doing coastal there will be some more affordable options. Fingers crossed.
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Old 25-11-2020, 19:53   #39
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Re: Creating polars for an older cruising boat

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Thank you for that, a great breakdown and analysis.

My offshore comms SHOULD be ok, so the plan was to have someone on shore run the numbers for me each day and relay any changes over HF radio.

If I were planning to do more offshore sailing I'd look into some form of internet connectivity, but my plan is to be a coastal sailor, at least for the next few years. (This passage is a one-off and may yet turn into a coastal route depending on a few other factors.)

Maybe by the time I'm finished doing coastal there will be some more affordable options. Fingers crossed.
Yep, that sounds like a plan. I was also hoping that Iridium Go would be upgraded for the new satelites and / or other options would become available. Hence I have not yet purchased Iridium Go. Hereís hoping.

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Old 25-11-2020, 20:08   #40
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Re: Creating polars for an older cruising boat

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Yep, that sounds like a plan. I was also hoping that Iridium Go would be upgraded for the new satelites and / or other options would become available. Hence I have not yet purchased Iridium Go. Hereís hoping.



Ilenart

You can rent Iridium phones and possibly the Go for any offshore passages you may be planning in the near future.

At least for the next year or so, InReach and Iridium are the least (relatively) expensive options, if you donít already have SSB + Pactor.

In our case we inherited an Iridium 9505 phone as well as SSB on our boat. We use the SSB for voice comms but arenít bothering getting a Pactor and instead buy airtime for the phone. We use 4-10 minutes time per day, depending on how much weather watching we do.

Starlink and the other systems are still just vapour-ware for marine use. But it sure would be nice to have something new and better at the lower price points.
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Old 27-11-2020, 08:18   #41
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Re: Creating polars for an older cruising boat

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OK, this thread is nice, but just to be clear, Iím looking for peopleís self-created polars if I can.

The theoretical stuff online is ok, but as noted, donít always reflect real-world boat speeds.

When I say ďpolarsĒ, Iím not talking about the pretty pictures, Iím 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, Iím using fastseas which will create the pretty pictures for you if thatís what you are after.
I use fastseas also. Just give your best estimate. Like you said surface conditions can change a lot. Then as time goes by you can tweak your polar. It will never be entirely accurate and will only give you an approximation. What I do with my generalized polar is adjust the efficiency up or down according to your own observations re wave height and length or any other factors that will effect your speed. I usually run my calculations at 90%. And if I am particularly slow or fast I go back and tweak.

Hope that helps
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Old 27-11-2020, 12:25   #42
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Re: Creating polars for an older cruising boat

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The theoretical stuff online is ok, but as noted, donít always reflect real-world boat speeds.
Polars reflect real-world speeds only by coincidence. Many of them are created with real-world data for a particular boat, (but usually a lot of interpolation is used) but that data is valid for only one sea state, and only if that sea state is aligned with the boat in the same way as it was during the testing (waves do not always cooperate in lining up with the wind direction). Add in vagaries of sail age, sail trim, crew on board, loading and load distribution, optional equipment ruining the aerodynamics, bottom condition, etc, etc, etc, and the probability that a given boat sails to its polars is near 0.

You have already seen that with your boat, sailing in your typical conditions differently than it sailed on a passage (which is different that it will sail on another passage).

So if you can find the polar data for a similar boat (like a Tayana 42 pilot house) you are as close as you can reasonable get, until you start collecting your own data for your own boat, sailed as you sail her. In finding that similar boat, you would want a similar displacement, similar ballast ratio, similar waterline length, similar sail area/displacement ratio, and similar sail compliment, particularly regarding a symmetrical spinnaker or lack thereof.

The error likely in getting "pretty close" with data from a similar boat is far smaller than the error inherent in weather and sea state forecasting.
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Old 28-11-2020, 01:36   #43
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Re: Creating polars for an older cruising boat

I spent most of my life doing technical stuff on computers. I like technical stuff. The math for transport problems, which weather routing is, was and is beyond me. However, I cruised 2/3 around the world for 3.5 years, mostly with Predictwind. I characterize weather routing as an extremely precise rendering which is also extremely inaccurate. It is a fun tool for departure planning, but will not be very accurate on the water. Besides all the vagaries of getting polars that match your boat under different conditions, sail changes, reefing, SEA STATE, bottom condition, and a plethora of other factors combined with actual winds which are often 90-180 degrees and 20 kts different from predicted make a jumble of the math and those pretty segments, tacking, timing, etc. for a created route. Note that I said polars, plural, because they can be quite different especially based on sea state, wave direction, etc.

OK, I may be a luddite, but my techie side was quite disappointed with the accuracy of weather forecasts underlying weather routing to say nothing of only being based on one polar of many which may apply during a particular passage. It was not so bad in the trades around the equator, but downright dangerous to depend on in the 30's and 40's.

YMMV
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Old 28-11-2020, 08:13   #44
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Re: Creating polars for an older cruising boat

You need polars to go cruising? What was I missing in the tens of thousands of miles that I have cruised without them?
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Old 29-11-2020, 00:20   #45
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Re: Creating polars for an older cruising boat

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You need polars to go cruising? What was I missing in the tens of thousands of miles that I have cruised without them?
Possibly lots. Possibly not. Maybe you should try them.
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