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Board » General Discussion » Minimum constant Boat Speed.

Posted by
JB


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2013-03-07 08:54:34
2014-04-07 11:32:29 - Minimum constant Boat Speed.
For the moment it’s not possible to fix Waypoints in the SOL navigation system as also is not possible to “reduce” sail area in order to obtain a particular Boat Speed (“BS”), but with a little work I think it would be possible to impose a minimum constant Boat Speed (“mcBS”) during a period of time you previously define.

It would be the same as reducing/trimming the sail area (some reefs on the mainsail plus a smaller genoa, jib or gennaker, whatever) to have the boat underpowered and moving under a constant BS.

Knowing the exact distance between turning points, with a constant Boat Speed (“BS”) is equal to know the exact ETA’s on every waypoint you need to change the boat Course Over Ground (“COG”) as Time of the leg is obtained dividing de Distance to be made by the Speed in that run, that’s simple kinematics.

I’ll give a simple example to illustrate better my wish for the new SOL navigation utility.
The race is the Race 2 on the SSANZ B&G Simrad Series 2013 where you have to round the Little Barrier Island by night - see picture in the attachment.
Coming from South and the wind blowing from the 100º quadrant, you’ll have to make at least #2 changes in COG to round the island back to South.
Some of the sailors will have to make some DC’s (“Delayed Commands”) to perform that task.

In this case, the main problem to make the correct DC’s is to perform the exact calculation of the (will be) Boat Speeds (BS’s) after changing the boat COG in the turning points, knowing the distances between them. If you over calculate your BS you’ll end up going aground. If, by the contrary, you underestimate it, you’ll waste a lot of precious miles going way from the exact points of turning (the picture illustrates precisely booth cases with the same boat tracking).

Looking to the Boat Polar (“BPo”) you can estimate that during the period of rounding the island your minimal BS would be 6 knt (let’s say, dead downwind) and the maximum 10 kn. If you don’t want to take risks on using several estimated Boat Speeds in your calculations, the solution will be to use a unique mcBS for calculating the “ETA’s” in the turning points and impose it in the navigation to perform.

But, if for instance, your imposed mcBS was above the minimum permitted by the BPo during that period of turns, the mcBS would be that precise minimum and so, corrected during the crossing for all efects.

I imagine the utility of that tool on a race like the Turkey one, with two (dire) straits in a row.
Sail Fair.
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