The background to model testing

There can be no doubt that Froude’s first task had to be to show that tank testing of models worked. There was only one way and that was to test a model of an existing ship, predict its resistance to motion, and then go out and measure the resistance of the full-sized ship.

 

It all seems very simple but one must consider the situation in the mid 19^th century. We are accustomed to seeing photographs of vessels of all sorts under way. The photographs are taken from the air and, even if they are not taken with the study of the waves and the wake in mind, the surface effects are there for anyone to study. Froude would have had to look at bow waves and wakes as they were being made and never from a useful observation point. Nevertheless it seems that Froude over the years had recognised that the resistance to motion of a ship was caused partly by friction between the hull and the water that created a wake in which energy was stored and then lost in random eddies and partly by making waves that very obviously carried energy away from the ship.[1] [2]

 

Figure 11-2 is a photograph taken from an advertisement and shows a racing sculling boat on the move. The use of oars means that, with the exception of the circular waves on the right, the effects that we see on the surface are entirely due to the action of the hull on the water and not confused by the presence of a propeller. We can see the wave pattern coming from the bow and another wave being generated at the stern and we can see the confused wake caused by friction between the hull and the water. In the extract above Froude has commented on all these things. I think that one can have some sympathy with the people who were trying to think about the problem of ship resistance in the 19^th century.

 

I have wondered why Froude was so confident about his plans. He was not a young man so he had had time to gather a great deal of information and to experiment to clear the way for his major experimental programme. One criticism of the testing of models was the effect of surface tension that was obviously a factor for a small model but equally obviously not a factor for a ship. Froude had cleared this out of the way by experimenting to find the minimum length of a model for the effect of surface tension to be small enough to be ignored. He decided that this length is about 6 feet (2 metres) and all his models were longer than this and in some cases much longer at 18 feet. These days the length might be 20+ feet but towing tanks are somewhat larger. But there is another factor that I think that Froude would have known about and it would have given him some confidence. Steam engines in those days were slow speed machines that were made to quite high standards[3] and the behaviour of steam in the cylinders was very predictable. This meant that the power given by the steam to the pistons could be calculated quite accurately as the indicated horsepower. The mechanical efficiency of these engines was known within ordinary engineering limits so the power going to the propeller could be calculated and finally the propeller efficiency was virtually fixed if the propeller was made to reasonable standards. The net effect is that the power required to drive a given ship when it was under way could be calculated with some confidence. A graph could be drawn of the power needed to drive the ship versus speed.

When the ss Great Britain was recovered from the Falkland Islands a model of its hull was tested to assess its design in relation to modern designs. Graph 11-1 gives that graph redrawn from Corlett’s book in order to restore the origin and the proper proportions to the x axis. The red lines show the power required to drive the ship at the service speed of 12 knots. The power required would have been 900 horsepower. At 810 horse power the speed would have been 11.6 knots and at 990 horse power 12.3 knots. So a 10% difference in the power only changed the speed by 3+%.

 

In engineering terms these are acceptable figures. So Froude did not need to make predictions that were highly accurate in order to determine the power needed to drive a ship. Nevertheless he went on to make his measurements to the best of his considerable ability and the outcome was fundamental to the subsequent evolution of the science of fluid flow.