Tuesday, October 25, 2011

Calibrating Canal Planners using the stop watch you didn’t know you had.

Sometimes you don’t even know what you’ve got when you’ve got it.  I was thinking overnight about the probable inaccuracies in my CanalOmeter for the Bottom Road (see yesterday’s post) because on a good day you can zip through close flights of locks much faster than you expect. If only I had timed our ascent with a stop watch, I could check.  Suddenly I realised I had an answer.  Let me start from the beginning

I use data from Nick Atty’s brilliant Canalplan to calculate travelling times. Experience has shown me, and thousands of others, that it’s a pretty good guide to cruising times. I used to just take the distances and number of locks and do the rest of the calculations myself based on what speeds I expect to achieve and how long it takes to do locks locally.  However since Canalplan introduced the option to show cumulative times I’ve just used those.  Stupidly I forgot to adjust Canalplan’s settings to reflect the “local conditions”. 

Locking times can vary enormously.  Anything from 5 minutes (e.g. Northampton flight) to 35 minutes (River Nene, where you have to tie up, walk round the corner, and wind one of those bl***y wheels).

For narrow canals, Canalplan’s default settings are

Average speed 2.5 mph.  Time for individual lock 12 mins, -  for locks in flights 8 mins

2.5 mph might seem slow, but as an average I might accept it because of slowing down passing moored boats and any small delays.  The lock times don’t seem unreasonable either, but I recalled that going up Farmers Bridge flight we seemed to be doing a lock every 5 minutes or so.   If only I had timed the flight I could have compared actual times with Canalplan’s predictions.

Wait a minute. Eureka!  I do have the times.  Simples. Digital photos have a time stamp don’t they. In fact I have photos, hence times,  for the whole trip that day, from the first lock we entered to when we emerged at Farmers Bridge.

First lock (Minworth bottom lock) 07.34 (believe you me, we don’t often start that early)

P1050812 (1024x768)

Entering Farmers bridge bottom lock (after a short tea and sandwich break) and 6 miles and 14 locks already done (phew) 11.20



Coming up through Farmers bridge top lock 12.26

P1050862 (1024x768)

So from the first lock that’s 3h 46 mins to the bottom of Farmers Bridge locks and another 1hr 6 mins to the top.

Blimey! We seem to have done 13 locks in 66 minutes, so that’s about 5 mins per lock.

Now using Canalplan’s default settings we get 5hrs 26 mins for the first part plus another 2 hrs 26 mins to the top.  A total discrepancy for the whole day of precisely 3 hours. I could have had another couple of hours in bed!!

Is this Canaplan’s fault?  Are CanalOmeters a waste of time? Not a bit.  We should have done the settings first. We knew we had extra crew to set up locks in advance and there were very few other boats moving to delay us.  There were 26 locks in all that day, so overestimates soon pile up.  A minute ago I just redid the Canalplan route with more realistic settings and the answer came out within half an hour of the true figure.  Not bad for a long day’s work.

The lesson for you in using Canalplan and for me in making CanalOmeters is to think hard about the default settings before you start.    I may well re-do the Bottom Road CanalOmeter to reflect this.  My older ones for the southern GU which did use local knowledge of speeds and locks have proven to be pretty accurate.

1 comment:

David said...

Neil, This is getting like a Miss Marple case. I'm having to re-read it to check all the timings and who was where and when. I think it was Rick wot done it. Rainman.