Saturday, May 21, 2016

Weather Discussion Day 3

If looking at the map above, the red markers are position reports Bill has sent.  The yellow circles are the eddies Bill referred to in his Day 2 post (the one on the left has an image of the RTOFS  Gulf Stream simulation if you go to the interactive map).  The right most Green marker is Bermuda and the two lines are the Rhumb-line from Florida to Bermuda and my estimation of the route Expedition is suggesting (based on Bill's description).  Now for a bit more discussion on weather routing:

A bit late in the day, but wanted to explain a little bit about what Bill and Ken are weighing in terms of deciding how best to head towards Bermuda.

First a little bit about sailing performance.  Most boats, and VofJ in particular is fastest when reaching, i.e. the wind roughly coming across the side of the vessel 90 degrees to the direction you are heading.  Because we are a relatively shallow draft cruising boat, and because we have a permanently rigged Reacher that sits about 1' in front of the jib, we don't go upwind very well.  True wind upwind angle is something like 50-55 degrees off the wind.  A decent race boat might be closer to 40 degrees and the old IACC america's cup boats were closer to 35 degrees with such narrow beams, and deep efficient keels.  Similarly, because we don't carry a spinnaker (big colorful lightweight downwind sail), we don't go dead downwind all that well either.  We fixed that greatly when we added a whisker pole after our first Bermuda race so we can pole out the reacher and go dead down-wind pretty well, but we don't like to run that at night due ot reduced maneuverability and it is a lot of work to set up.  Where VofJ goes REALLY well is with a true wind angle of about 120 degrees from the bow.  Because of the speed of the boat going forward, the apparent wind will be something like only 80 degrees, but this is where she really flies and the polar data I described earlier that we feed into Expedition tells the software this information in very detailed form.

The computer knows that it is worth extra distance to keep the wind on the beam and not go either too close to the wind, or too far away from it.  Add to that the ocean currents that spin off the gulf stream and the computer software is suggesting that the boat make a dogleg towards Bermuda, heading further north now, and then bending back east when the wind is expected to change to a south-westerly direction.  This theoretically allows the boat to sail a bit further off the wind right now when going upwind which will help it go faster now and then later when the wind changes to the SW, it will keep the wind more on the beam than the stern which is again faster.

The "danger" of that course that Ken is warning about is that if the wind goes more southerly than south-westerly and you actually go further north than Bermuda, then you have to keep going upwind the whole way which would be slower and you would have added all that extra distance as well.

The software simulations are only as good as the data you feed into them and weather forecasts are just that, a forecast, not a certainty, so Ken is suggesting staying a bit prudent by following the favorable current and  Easterly wind north now, but not so far that if the wind doesn't swing all the way to the SW, you aren't forced to beat the whole way to Bermuda.

Generally on a race course you want to be on the "inside" of a wind shift.  i.e. if the wind is going to shift towards the right (Clockwise), then you want to go right at the beginning, so later you can come from that side and be "lifted" to your destination.  Ocean racing and passage making is a bit different as the distances are further and you aren't just going upwind and downwind, so in this case is looks like it might be better to go left early as the shift to the right looks to be almost 180 degrees, so you can end up ahead by sailing a little futher off the wind now, and then a little closer to the wind when it comes around from behind you....anyway, that is the idea....only time will tell.


1 comment:

Jo said...

Gram... you never cease to amaze me with your breadth of knowledge. Whether you are an airline stew or a captain you manage to wear each hat well...Love you MOM