1840.] On Low Steam Power to Vessels, instead of Sails. 229 



not only overcome more than a proportional resistance, but also do it 

 in half the time, the actual power of engine employed for 10 knots 

 speed must in fact be about G times the power of that required for 5 

 knots or about 10 times, what is necessary for 4 knots, though it will only 

 be in action half and two-fifths the tiuie respectively, iu accomplishing 

 the same distance. 



Hence if a vessel of 400 tons requii-ed at 10 knots an hour an engine 

 of 180 horse power, and 50 tons of coal per 1000 miles, she would at 5 

 knots an hour requh-e an engine of 30 horse power, and 8^ tons of coal 

 for that distance; and at 4 knots, an engine of 18 horse power and 5 tons 

 of coal for the same. This has been proved by experiments in many 

 ways, to be about the proportionate power required for the speed. 



And with respect to the effect of the size of the vessel, it is also well 

 known that the resistance to a large vessel is less in proportion than to a 

 small one, that is, if a vessel for instance of 1600 tons, required an 

 engine of 400 horse power for any given speed, or one horse power 

 to every 4 tons, a vessel of ^ that size, or 200 tons, would re- 

 quire double that proportion, or 100 horse power, being 1 horse power to 

 every 2 tons ; the cause of this is obvious, that the body of water 

 to be removed by any vessel is in proportion to the breadth and 

 depth multiplied together, but her capacity is in proportion to the 

 breadth, depth and length multiplied together. Thus a vessel of double 

 dimensions will displace four times as much water in moving, and conse- 

 quently meet with four times the resistance, but her capacity will be 

 increased 8 fold, so that while she carries 8 times as much cargo, she 

 only meets with 4 times as much resistance, and consequently requires 

 in proportion to her tonnage, only half as much power. It is on this 

 ground, that large vessels are found to answer better as steamers than 

 small ones, and it was because this principle was forgotten, that it 

 was so confidently asserted, that in the present state of the knowledge 

 of steam navigation, a vessel could not carry fuel enough to cross the 

 Atlantic, the calculations being made from small vessels without allow- 

 ing for this consequence of an increased size. 



Hence we find that in steam navigation, the same vessel requires for 

 a double speed more than double the quantity of fuel, and about six 

 times as powerful an engine to go a certain distance. The same vessel to 

 go for a certain time at a double speed, requires about six times the 

 quantity of fuel, and six times the power of engine, but she will go in 

 that time double the distance. For a given speed, a vessel of eight times 

 the tonnage as another, will only require four times as much fuel, and 

 four times as great a power of engine. 



