30 percent mileage improvement. However, the gap between two-stroke and four-stroke outboard fuel economy is beginning to narrow. Two-stroke outboard motor manufacturers have recently introduced new technologies that help to improve two-stroke fuel economy. LPG outboards In 2012, Lehr inc. introduced some small (<5 hp) outboards based on modified Chinese petrol engines to run on propane gas. Tohatsu currently also produces propane powered models, all rated 5 hp. Conversion of larger outboards to run on Liquified petroleum gas is considered unusual and exotic although some hobbyists continue to experiment. Outboard motor selection It is important to select a motor that is a good match for the hull in terms of power and shaft length. Power requirements Overpowering is a dangerous condition that can lead to the transom accelerating past the rest of the vessel and underpowering often results in a boat that is incapable of performing in the role for which it was designed. Boats built in the US have a Coast Guard Rating Plate, which specifies the maximum recommended horsepower for the hull. A motor with less than 75% of the maximum will most likely result in unsatisfactory performance. In the united kingdom, boats will have a CE plate on the transom which specifies max HP, shaft length, max engine weight and no of max persons/luggage Shaft length Outboard motor shaft lengths are standardized to fit 15-inch, 20-inch and 25-inch transoms. If the shaft is too long it will extend farther into the water than necessary creating drag, which will impair performance and fuel economy. If the shaft is too short, the motor will be prone to ventilation. Even worse, if the water intake ports on the lower unit are not sufficiently submerged, engine overheating is likely, which can result in severe damage. General dimensions Different outboard engine brands require different transom dimensions and sizes, that affects performance and trim. Operational issues Motor mounting height Motor height on the transom is an important factor in achieving optimal performance. The motor should be as high as possible without ventilating or loss of water pressure. This minimizes the effect of hydrodynamic drag while underway, allowing for greater speed. Generally, the antiventilation plate should be about the same height as, or up to two inches higher than, the keel, with the motor in neutral trim. Trim Trim is the angle of the motor in relation to the hull, as illustrated below. The