The Craig-Dorwald Engine 
A Twelve-Cylinder 150 b.h.p. Racing Marine Gasoline Motor
By the English Correspondent of the Scientific American
The development of gasoline-motor-propelled boat racing has resulted in an increased demand for greater horse-power and accelerated speed, similar to that which attended the development of cars by the organization of automobile races. The outcome is that the gasoline marine motor is being rapidly improved and rendered efficient and reliable for the severe demands which it has to fulfill. One of the latest of these marine engines of this type is the Craig-Dorwald motor, which has recently been completed by the Putney Motor Works of Putney, London, to the order of a gentleman who intends to participate in such racing contests. This particular engine resembles the standard Craig-Dorwald gasoline motor, which possesses several novel and interesting features, though an important deviation from the standard practice is made by placing the opposite cylinders at an angle.
The engine consists of twelve cylinders cast in pairs. They are ranged in two rows of six each, and are placed diagonally on the crank chamber at an angle of 90 degrees to one another. The advantage of this design is that the opposite cylinders work against one another, resulting in a greater development of power and the utilization of a greater proportion of the developed energy than is possible with vertically-placed cylinders.
Each pair of cylinders is cast in one piece with no joints. This is a conspicuous feature of this type of motor, as the absence of joints to pack prevents any leakage of compression through blowing joints. Extra large water spaces are provided, especially round the valves, so that all possibility of the engine running hot is reduced to a minimum.
The crank chamber is cast out of aluminium, with three large inspection doors placed on the top, and another at the end secured by three nuts. Easy access to the crankshaft is thus assured, while if necessary the bottom of the crank chamber can be quickly withdrawn, being cast in two pieces for this purpose. By the removal of the end plate, the cam shaft actuating the inlet and exhaust valves can be lifted completely out. Both the inlet and exhaust valves are mechanically operated, and, what is another advantage, are interchangeable. The heads of the valves are made of closely-grained cast iron with nickel-steel stems. The latter is forced into the head while it is red hot and riveted over. By the removal of a single nut, any valve can be removed.
The camshaft is made of nickel-steel, and is turned out of a solid single forging. In order to avoid torsion strains, it is of large section. As the special requirements of this engine involve the reduction of weight to the utmost extent, the shaft is hollow, while the webs are also fluted. Furthermore, the shaft is tapered, the greatest diameter 2 3/4 inches being at the flywheel end, where the greatest strain is experienced, including the overcoming of the inertia of the flywheel, while at the opposite end the diameter is reduced to 2 inches. The crankshaft, which only weighs 93 pounds, is supported upon four substantial 5-inch phosphor-bronze bearings. Four connecting rods work on each web of the crankshaft. The main feature of the Craig-Dorwald motors is retained in this engine, i.e., the crankshaft is so set that it is in advance of the axis of each cylinder. The advantage derived from this arrangement is that shock and vibration are reduced to almost the minimum. Greater power is furthermore derived from each explosion of the charge within the cylinder, for the piston has a direct, practically vertical thrust downward on the driving stroke; while on the return stroke, when the speed of the piston is at its greatest, it has an appreciably easier passage through the cylinder. A considerable mechanical advantage is thereby gained at the end of the compression stroke of the piston.
The pistons also are of substantial construction, and are 4 7/8 inches diameter by 5 inches bore. Heavy connecting rods with equally heavy long bearings are provided. in order to effect an economy of weight, the connecting rods, like the crankshaft, are hollow. This again enables greater efficiency in the lubrication of the piston to be attained. When the connecting rod plunges into the oil at the bottom of the crank chamber, the upward stroke sucks the oil up through the hollow connecting rod, and discharges it at the top over the gudgeon pins. The pistons are machined inside and out are perfectly balanced, and each is provided with four piston rings. Each of the latter has a novel locking device, which prevents rings so working round that the other four slots may come in line, in which event loss of compression would possibly result.
The arrangement of the gudgeon pins is also ingenious. Instead of projecting through the side and ending flush with the external surface of the piston, lugs for carrying the ends of the gudgeon pins are fitted to each end. These project through the top surfaces of the pistons, and are secured by bolts. The advantage is that by releasing these bolts the piston can be removed entirely from the connecting rod for easy examination. Furthermore, it is impossible for any scoring of the cylinder wells to occur from this source. The gudgeon pins are of large diameter, with the center of same near the top of the piston. Side thrust is therefore obviated, and it is also rendered possible to use a short piston and a long connecting rod.
The pistons are placed at an angle of 120 degrees, and no two cylinders are fired simultaneously. The latter are fired in the following sequence: No. 1 is fired first, then its fellow 1a opposite fires, followed by 3 and 3a, and then by 5 and 5a. Firing then recommences from the front, starting with 2 and 2a, then 4 and 4a, followed finally by 6 and 6a. Thus there is a rapid consecutive firing cycle of each od the twelve cylinders, and as each cylinder works against its opposite fellow, the full energy of the developed power is obtained.
The gasoline vapor is supplied from three vaporizers, one of which is placed between each opposite pair of cylinders, thereby supplying four cylinders in all. By this arrangement no cylinder is starved of its requisite quota of explosive mixture. Craig-Dorwald automatic vaporizers are employed, the feature of which is that the necessary quantity of air which the engine desires to render the mixture of the maximum explosive nature, is obtained through an automatic air valve. The harder the engine sucks, the more air is admitted through this valve. Consequently, the mixture of air and gasoline vapor is always automatically adjusted. The extent of the charge admitted into the cylinder is also automatically regulated by means of a long longitudinal piston working on a spring. When the engine is running dead slow and requires only a small charge, this piston by the action spring, is caused to almost close the inlet passage. Great economy in the consumption of fuel is thereby attained; the possibility of overheating the engine is reduced, and furthermore, the engine is governed by the automatic regulation of the throttle, thereby dispensing with the centrifugal governors.
The exhaust gasses from the cylinders are conveyed through short, large-diameter gun-metal pipes to a cylindrical exhaust box placed longitudinally between the two rows of cylinders. Sloping backward from the center of this silencer is a short funnel. which carries all the exhaust fumes into the atmosphere without any inconvenience to the passengers within the boat.
The valve cams are so arranged that by the simple movement of a lever the motor can be stopped quickly and reversed at will. The pinion driving the 2 to 1 gearing are placed within the front of the crank chamber, but are immediately accessible by the removal of the front end plate.
Ignition is by accumulators and coil. A large twelve-wipe contact distributor of the Craig-Dorwald type is fitted on the 2 and 1 shaft at the front end of the engine, and two single trembler coils are used.
The engine is self-starting. A device is provided for pumping an explosive charge into each cylinder. To start the engine, it is only necessary to switch on the current. The engine is highly flexible in running, and from 100 revolutions can be accelerated to 900 and even 1,000 revolutions per minute, at which speed it develops 155 B.H.P.
The motor is very compact. It measures 4 feet 3 inches in length, by 2 feet 8 inches wide and 2 feet in height. The total weight of the motor complete including the flywheel is only 950 pounds. The most prominent feature of the engine is its simplicity and instant accessibility at all parts, two important qualifications for an engine designed for racing purposes, since in the event of a trifling breakdown it is essential that the engine should be so constructed that one can easily and quickly open the crank chamber or the pistons in a minimum of time.
The engine is to be installed in a 40-foot boat. The hull is being made of cedar, with a thickness varying from 3/16 inch at the thwarts, gradually increasing to 5/16 and 5/8 inch at the bottom. The engine will be placed somewhat forward of amidships, and the developed power of the engine will be transmitted, through a special reducing gear and a propeller shaft about 20 feet in length, to a single propeller 2 feet 2 inches in diameter.
(Transcribed from the Scientific American Supplement No. 1517, Jan. 28, 1905, p. 24309. )
[Thanks to Greg Calkins for help in preparing this page]