High-speed v. Low-speed Engines.

Progress, Volume II, Issue I, 1 November 1906, Page 28

High-speed v. Low-speed Engines.

To th° Editor. Sir, — I wnte to you on the above subject feeling sure that there are many people whom }oui paper will reach who are interested in it There is an immense amount of loss entailed by purchaseis of difteient kinds of machinery simplj because they are inadequately infoimecl and accept without lebeivc every woid m the catalogue

of an engine which has taken their fancy. It is, of course, an understood thing that experience is the best teacher, but it charges the highest wages. A marine engine is usually classed as high speed when its revolutions exceed 600 r.p.jn., and as a low speed when below that amount, although the power of the engine has a good deal to do with this For instance, an engine developing S blip, and running at, say, 550 r.p.m. would be a low-speed engine, although at the top of the low-speed rating ; while an engine of 50 b.h.p. would be a decidedly high-speed engine if running at that speed, i.e. s^o r.p.m., provided it was constructed with no more than, say, four cylinders, otherwise the engine might be made up of 10 5-hp engines coupled up; in that case it would be within the low-speed limit. This is largely gauged however, by the piston speed per minute. Take an engine of 500 r p.m. and 1 5" stroke : this engine's piston will travel at 500 x 1.5" == 7500" per minute = 625 ft. per minute , this is high speed, especially for a heavy engine. Then again, take an engine at 500 r p.m. and stroke 7" = 3<;oo inches per minute = 291.8 ft. per minute, which is a fairly low speed This matter, however can be gauged at a glance at the makers catalogue by the revolutions and the weight. Now, what is the result when the different engines are put in commission ? This, like the former, depends as to results, largely on conditions. Take an engine of 20 h.p. and 800 r.p.m., and fit it into a light boat of narrow beam and little draught and the speed will be good because the small craft can move through the water and make way for the quick-revolving propeller, giving it plenty of clear unbroken water in which to do effectual work. Again, let us fit this engine in a serviceable sea-boat of say 50 ft. in length and to ft. beam and the result will be very poor — practically the same as putting a race-horse into a dray. The consequence is the boat cannot get away from the light quick-movmg propeller and does little more than churn water, as the boat would be only moving at approximately from 6 to 7 miles per hour, and this only in calm water. Now, let us fit a2O h.p. low-speed engine of, say, 3215 r.p.m. with propeller, say, 30 in. diameter, into the light narrow-beam boat as before described, and it will sink it. This would be the table reversed — the draught-horse m the racing sulky. But let us fit this engine in the 50 ft. serviceable sea-boat 10 ft. beam, and we will find that on the first revolution of the propeller the boat will literally jump from its moorings, and instead of the miserable 6 or 7 miles an hour, we will have approximately 10 miles an hour. This speed will also be fairly well maintained in rough water, but why ? Because of the slowrevolving propeller in which the amount of slip is reduced to a minimum. The propeller getting hold of the water does satisfactory service, owing to its large diameter and slow movement during which it does not get a chance to churn the water ; this being a great deterrent to the successful propulsion of a boat, through the water. " But,' says the novice, "we can increase the diameter of our high-speed propeller and so make it more efficient " This has been tried many times before and is a useless performance. The power of the high-speed engine is in its number of r.p.m., and if that is reduced to any appreciable extent, the power of the motor is thereby reduced and the engine is also working beyond its normal strain. In other words, if you want an efficient and serviceable propeller for a useful and serviceable boat you must buy a low-speed engine. Then we come to durability • the unmechanical eye has only got to look to some of these light high-speed engines, to see that they are not designed for long-life service, and that their up-keep must, of necessity, be greater than an engine of the same weight moving at half the speed , the internal explosions of these engines also plays havoc with their cylinders and piston rings, to which (he aforesaid applies. In conclusion, the value of an engine is its first cost, plus its efficiency, plus its economy, plus its up-keep, plus its length of life, .which is its durability The purchaser of an engine, if he wants- to make a good bargain, must fully consider these items of which the second one, 1 c., efficiency, is an easy first — I am, etc , San Francisco, 1/9/06 H C CHRISTIAN.