Expedition Navigation Software » Optimal Routes

tsenga · 7/07/2021 5:37 pm

We've been trying to get a better understanding of the routes Expedition has been giving us.

We're noticing that the Bsp that expedition gives in optimal routes, seems to be inconsistent with the set/drift/brg/distance output. I'm wondering if I'm misinterpreting the numbers or missing something.

ie:

From Brg & Distance, I have a ground track for the period
From Drift & Set, I can back out the contribution of tide for the period

The result should be water track, and course to steer for the period, which I can scale back down by the period, to give me Bsp.

However, this number seems to be different from the Bsp given.

((i also tried taking drift & set from this period and the next one and averaging.I do the above calculations by working out latitude and longitude components through trigonometry).

Am I missing something here?

Last edited by tsenga (7/07/2021 5:43 pm)

Nick · 7/07/2021 6:38 pm

One complication is that it is the instantaneous polar bsp at the point, whereas the time between points will be an integration over a range of speeds (and currents).

tsenga · 7/08/2021 4:19 am

Thanks, that is helpful.

And for tacking periods (dotted line), what does Expedition assume for Bsp? (upwind VMG? or extra distance due to tacking with upwind targets? or something else?)

Campbell_Field · 7/13/2021 1:49 am

tsenga wrote:
Thanks, that is helpful.

And for tacking periods (dotted line), what does Expedition assume for Bsp? (upwind VMG? or extra distance due to tacking with upwind targets? or something else?)

There is an article about this here: https://www.fieldyachting.com/question-and-answer/expedition/opt-route-incl-tacks-gybes-different-dash-sizes-mean

tsenga · 7/13/2021 5:14 am

Thanks Campbell.

So short answer is that upwind of target, VMG is used, with detail provided by your very excellent post.

Campbell_Field · 7/13/2021 5:15 am

tsenga wrote:
Thanks Campbell.

So short answer is that upwind of target, VMG is used, with detail provided by your very excellent post.

Glad you found it useful, and hopefully answered your question.

bwheeler · 3/06/2023 12:37 pm

Same question but for downwind. For a given route, Expedition may be showing a recommended TWA that is deeper than the boat's Target TWA.

For example - downwind Target TWA at say 17 kts TWS is 144, but the router is displaying 165 (with or without parentheses). Why is the router displaying deeper than polar targets?

Nick · 3/06/2023 12:52 pm

As above, it suggests tacking or gybing. The line will be dashed and in parentheses in the table.

Also see the Exp help.

You can tell Exp to prefer to tack at the time steps.

I often use the TG button to do both.

Last edited by Nick (3/06/2023 12:52 pm)

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» Optimal Routes - results analysis/consistency

7/07/2021 5:37 pm #1

Optimal Routes - results analysis/consistency

7/07/2021 6:38 pm #2

Re: Optimal Routes - results analysis/consistency

7/08/2021 4:19 am #3

Re: Optimal Routes - results analysis/consistency

7/13/2021 1:49 am #4

Re: Optimal Routes - results analysis/consistency

tsenga wrote:

7/13/2021 5:14 am #5

Re: Optimal Routes - results analysis/consistency

7/13/2021 5:15 am #6

Re: Optimal Routes - results analysis/consistency

tsenga wrote:

3/06/2023 12:37 pm #7

Re: Optimal Routes - results analysis/consistency

3/06/2023 12:52 pm #8

Re: Optimal Routes - results analysis/consistency

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