TnaD Winter Issue Navigation Quiz Answers!!

[NiallD]

Please post your answers for TnaD Nav quiz !

[smurphy]

325 degrees?

[DaveWalsh]

Let everyone have a go, and then let anyone who is right or nearly right or even in the ballpark be prepared to explain their methodology. No prizes for lucky answers. A stewards\' enquiry every time.
Can those who have given their answer publicly explain their methodology privately? Or must they be prepared to go the distance in full view?
The problem with torture is its really only fun for those dishing it out!
DWalsh

[DaveWalsh]

There are some remarkable absentees from this exercise. What has happened to all the level 5s, the old ones, the new ones, this week\'s wannabees ? Whoever set the first quiz surely must roll up their sleeves? I note from a recent TND article that GPS was used by one group making this crossing this summer and I hope that isn\'t because they can\'t do without? Anyone pretentious enough to use wooden paddles can\'t not set their stall out on an occasion like this! How can anyone, male or female, royalty or common, blonde or not, paddle around Ireland or across the Irish Sea and not have a go at this little gem of a puzzle. Come on people.
Courage!
On behalf of the entire membership of ISKA, I promise we won\'t laugh at your efforts.
You know you can trust me.
DWalsh

[DaveWalsh]

Right, here goes.

There were only 2 answers given to this quiz, one public and one private, giving answers 325 and 330 respectively. 325 wins as being closest, so congratulations to Sarah Murphy who will hear from the National Lottery as to her prize in due course.
The information needed to answer the question properly was all there. Both candidates realised that a falling tide at Doolin represented an ebbing tide, so that the magnetic bearing to the target 310 had to be added to to achieve a “ferry angle” to nudge up into the South flowing current. As the kayaks headed out across the sound, they would be dragged back down onto a correct course. That is usually enough in itself to achieve a safe passage. If you get that much right, you should be safe, and would have to really work at getting the ferry angle wrong to get dangerous again.
To calculate the ferry angle for a straight crossing of a channel as here, if there is sufficient information available as here, the formula is simple :

Current Speed divided by Boat Speed multiplied by 60.

Both candidates applied the formula correctly. Nevertheless they both got the wrong answer, because they inputted wrong boat speeds and wrong current speeds. Sarah gave 1 knot for current and 4 knots for the boat – one quarter of 60 = 15. 310 + 15 = 325. ANother gave 1 knot for current and 3 knots for the boat – one third of 60 = 20. 310 + 20 = 330. In fact they both got the current speed off and they both got the boat speed off, so little wonder they both got the resulting ferry angle off.

As to boat speed, I have never seen a kayak cruise at 4 knots across the surface of the water for any sustained period (other than down a serious wind). Conditions were calm, and we were not told this party sprinted, merely that an experienced party set out confidently. On the other hand 3 knots understates their likely speed. A party comprising intermediates being led does 3 knots. It is a given that a managed party can sprint at 4 knots, cruise at 3, and dawdle at 2. A party such as we are told we have in this quiz, in these conditions over this distance, would travel at about 20% above those figures, say 3.5 or 3.6 knots. But all that has only a minor impact on the end result compared to the other error they both factored identically into the equation (or rather perhaps, didn’t, as theirs was a sin of omission!).

They both used 1 knot as the current speed. That was totally wrong. 1 knot was given as the maximum spring rate for the sound between Doolin and the island. What that means is that during a spring tide, the fastest current speed that will be achieved by the current is 1 knot. Now both candidates spotted the current was south going, but neither of them addressed the issue of what stage of the 6 hour tidal cycle they were dealing with, and neither addressed the issue of whether it was springs or neaps. Now it happens the paddlers were setting out at about the beginning of the maximum flow rate for the ebb that afternoon, so limited harm was done there, but it was a neap tide, so the flow was significantly less than per their calculations.

Take flow rates within the 6 hour cycle first. There are a couple of formulas, the “rule of thirds” and the “10, 50, 90” rule. Both say much the same thing. The rule of thirds says that in the first hour the tide will flow at one third the max rate, two thirds in the second hour, and three thirds in the third, etc. The other rule gives 10% max flow in the first hour, 50% in the second and 90% in the third, and is considered the more accurate but a little unwieldy, perhaps too fussy for kayak accuracy requirements. The parameters therefore of what we had that day last November were that the flow was 90% or 100% of whatever it was going to do that day.

Second, take neap tides. There are 3 formulas. Neaps flow at half spring rates, neaps flow at two thirds spring rates, and you calculate back from the proportion of the tidal range (rise and fall) on the day compared with the range in a spring tide, again the more accurate but perhaps a bit fussy for our needs.

In combination, whether you go two thirds of 100% or half of 90%, the parameters of the south going flow that afternoon were from 0.45 to 0.66 knots, so the ferry angle parameters are 7.5 degrees to 11 degrees. Even allowing for a slower boat speed the parameters don’t go out beyond 13 degrees. My own preference is to choose somewhere between half and two thirds, on whatever figure which when rounded up or rounded down gives an even end result. Compasses are notched every 10 degrees and with my eyesight the result should always be a multiple of 10. Thus the theoretically acceptable answers lie between 317 and 323 degrees, and in practice, 320 is right.

In summary, both candidates did well in sussing that the flow was south going, and both applied the ferry angle formula correctly. Both though overlooked discounting for the neap tide or for the 10/50/90, so both overcooked the ferry angle. Fortunately one of them also overstated boat speed and probably thus saved a life-boat call-out, and so wins the prize. Well done everyone and a big clap for Sarah Murphy.

DWalsh