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.
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.