Being Seen and Heard

Having gone to the effort of doing our short range radio operator’s training back in May, now was the time to put our skills into use. Unfortunately that meant that Eric had to spend several frustrating hours cobbling together the wiring and an aerial. Although we had asked for it to be fitted during the build, we were told that narrowboaters wouldn’t need such extravengances. It was one of the items we mistakenly compromised on knowing we could add it later. Braidbar boats were not designed to have items retro fitted. Luckily wiring is something that Eric understands and his temporary fixes still tick all the safety boxes and some. And oh boy, were we glad we made the effort. Not only were we able to contact the lock keepers easily to ascertain if we were able “pen down”. But we were also able to hear what other boats were doing and plan accordingly. And ok, all this isn’t essential for the Trent and we could have made our contact by mobile phone, but for us, having that  reliability and extra source of information has made river cruising less stressful and more interesting. There’s a bit of a misconception that VHF radios are just for calling for help in an emergency, they’re not. They are an easy way to communicate, and once we’d got over our initial nervousness at following the protocols we quickly relaxed when we realised everyone spoke the same language, not everyone followed the prescribed protocols and most of all, no one minded.

River cruising is different to canal cruising. There are sandbanks and shallows lurking beneath the surface which mean the cruising channel can meander from bank to bank. The outer curve is usually deeper than the inner curve because of the natural erosion. The Boating Association publishes the Trent Cruising guides, an absolute must, for safe navigation. The cruising line is overlaid in red and useful information is highlighted.  River locks are operated by lock keepers and known as pens. So we ask to “pen” up or down. These locks are huge, hold a whole marinas worth of boats. This was Stoke Lock and besides us on the left, there were 2 wide beams 2 cruisers and another 2 narrowboats.

Having sorted our radio out. Eric also had to connect an appropriate navigation light. Boats travelling on tidal waters, including tidal sections of rivers, boats need to be seen in all conditions to avoid collision. The COLREG rules state that for a boat our length means having a mast headlight visable for 3 nautical miles, red port and green starboard side lights and white stern light visable for 2 nautical miles. (Disclaimer, there are more regulations and criteria than I have listed here) The logic being that although we might not choose to travel at night, tide times might make it necessary to be on the water at dawn, dusk or in poor conditions. A lot of narrowboaters simply choose not to venture into this territory, we’ve decided to explore as much of the system as practical so we’ve done our best to rig up a temporary mast. One day it will be sturdier and taller, but as we know we’re only going to be doing the very first section of tidal water between Cromwell and Torksey in very calm conditions it will surfice for now. Added to which, the river has so many bends chances are we can’t be seen in full daylight beyond half a mile so we’re happy.

Where the power goes – Part 2

In my last post I looked at the power usage on the canal.  This got me thinking about the data I collected while we were on the river Avon near Evesham last July.  It’s clear we used a lot less power to travel at the same speed.  This surprised me because we were travelling upstream against the flow.

I measured the flow on the river by timing flotsam floating down stream.  It was about ½ a mile per hour which is not a lot, but on the canals that much increase in speed requires a lot of extra power, so for us to use less power was initially a surprise.

Here is a chart that shows the difference between the two sets of measurements.  I have included a corrected line to show the effect of allowing for the river’s flow, which makes the result even more startling.

Chart showing the power required to cruise a Narrowboat on a |River ccompared with on a Canal
Power required for Canal vs River cruising

The difference is so stark I was left pondering.

A boat travelling forwards pushes the water out of the way, both to the sides and underneath.  This can be seen as a bow wave.  On a river there is width and depth to accommodate this.  On a narrower shallower canal this water creates more of a barrier because there is less space around the boat for the water to go.  The gap between the base of a narrowboat and the canal bed is sometimes only a few inches.  Whereas it can be a lot deeper on a river.

The effect was very obvious on the Macclesfield canal where the boat almost stops as it struggles to go under some of the very narrow shallow bridges.  It in effect acts as a piston pushing water through the bridge and raising the water level in front of the boat requiring a lot of energy.

As we go faster the size of this bowwave bulge increases, and the water rises further, so requiring more energy to lift it and push it out of the way.  The effect of this for the canal can be seen on the chart by how steeply the power demand goes up above 3 mph.  There is quite a noticeable “knee” at 2.5-3mph, which is why trying to travel faster than 3 mph increases the power and hence the fuel required out of proportion to the increase in speed.

Rivers are much wider and deep and so allow the water to flow round the boat easily instead of creating a bowwave bulge in front, and so requires a lot less energy.  The lack of what I call the piston effect, is seen on the power curve for the river because it is much smoother and lacks the knee where the power starts to rise rapidly.

The usual rule of thumb is that bottom effects can be ignored when the water depth is more than 10 times the draft of the boat.  So for us that is in water that is about 6m deep.  The same rule of 10 applies to river or canal width.  So on canals we are hit by both bottom and bank effects that we can ignore for on the Thames in London.

This is comforting because I was worried that going all electric propulsion might mean we were under powered on rivers, and a few naysayers told me as much.  We need so much less power to travel in deep water, that I am confident the boat is not underpowered.  The Avon is not as deep as the Thames in London, the river Tent, or the Ribble Estuary, all of which we hope to crusie.