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Are Horses Getting Smaller?

I am thinking of returning to canal boating after sailing for 35 years.
Looking at boats for share and sale I see a vast improvement in heating, lighting and comfort levels.
I am interested in why engines have grown so much. My last boat a 35ft Piper had a 10hp engine and we used on the canals and ventured onto the River Trent. I believe it takes about 6hp to propel a 50ft narrowboat at 4mph so why are 35hp and above now the norm?
What are the downsides and fuel costs of using these engines that are seldom on little more than tick over?

Asked by: Jack Thompson  | 1.34pm, Monday 3 January

WW says:

You are quite right that the power needed to push a narrowboat is so low, however, there are a number of reasons that higher power engines are used.
Firstly, there are the losses provided by the gearbox, alternator, waterpump, etc, plus the inherent inability of the propellor to deliver the power supplied to the water- so only a small amount of the generated energy is required. Consider that, 30 years ago, a 35amp alternator was standard (max power around 400W) and now alternators totalling 200amp (2400W) can be fitted, and that is where some of the power is going! Maybe less than 20% of the engine power will ever end up actually as useful propulsion power.
Also, most boaters don't want their engine running very fast at canal speed and flat out on rivers, so choose an engine that delivers canal speed around 1300 to 1500rpm. I agree that specifying too large an engine can lead to problems, such as glazing of cylinders and oil sludging.
There is another way of looking at this- pushing a boat is more related to the torque output of the engine, than the direct horsepower. Torque for a diesel engine is most closely related to engine capacity. While many modern engines produce more hp per litre capacity, the torque remains fairly static.
So, a 10hp engine from the 1970's(say, a SABB 10GG) would have a cubic capacity of about 0.8litre. A modern diesel engine of that size would produce abour 20hp, but with a similar level of torque (or even less, possibly)
My 60ft narrowboat is powered by a 35hp, 1.8litre engine, and this gives a reasonable relaxed cruising speed on the canals
(1300rpm or so) and isn't too stressed on tidal rivers (1800 to 2000 rpm).
Fuel costs can be more, with some of the bigger engines, but running mid-range in their RPM range is more economical than engines running close to their maximum rating; around 30% extra fuel consumption in the last 15% of the rev range is normal.
If you think about older engines, such as air cooled listers, they arequite large capacity engines, but produce modest horsepower, at quite low engine speeds, but the mass of the engine, combined with the cyl;inder capacity, gives high levels of torque to push the boat. However, they are, in theory, less fuel efficient, if measured in terms of hp, but not when pushing a narrowboat!
incidentally, a 35hp engine uses around 1 to 1.5 litres of fuel an hour, on normal cruising, give or take a bit!
Hope that helps- if not, just shout!

Mark Langley  | 2.04PM, Monday 3 January

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