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Choosing the right engine, or engines, for your new RIB is vital if the boat is to be used to its best potential. The most fundamental element has to be power output, along with the correct propping (in terms of propeller size and pitch) for your needs. What's needed is an engine that, combined with the boat's abilities to lift a load of divers on to the plane, will allow an efficient cruising pace without undue stress, with some extra boost left in hand for when it might be needed. Boatbuilders almost always offer a boat-and-engine package deal for which a choice of outboards, from one or two manufacturers, will be available. The builders can usually advise on the engine sizes and power outputs you should consider. Single minded An old chestnut is the single versus twin engines debate. Generally speaking, while twins offer the back-up of a second engine if one breaks down, a single engine works better. In other words, an 80hp unit will provide more "oomph" than twin 40s. Purchase cost, simplicity of installation, and economy in overall running costs tend also to favour the single engine. Go for a single engine, but maintain it as though - or as indeed - your life might depend on it. Making progress To give an idea of choices and costs, the following table lists a selection of main-make models of 40-150hp, the principal range considered for club-diving RIBs. Note that some makers produce many more engines than it has been possible to list. Unlike RIBs, the essential designs and construction methods of which have remained constant for years, if not decades, engine technology moves with the revs. One major development has been the ongoing rush into four-stroke outboard manufacture following an early lead taken by Honda. The other main area of progress has come in fuel-injection technology, which has boosted the efficiency of all engines, but especially two-strokes. There's electronic fuel injection (EFI) and an even more recent development, direct fuel injection (DFI). This injects fuel and air under pressure and ignition occurs with the exhaust ports shut. This means there is practically no wastage of petrol and emissions are reduced. The result of this has been a good head-to-head between improving two-stroke and four-stroke brands, some makers opting to put most effort into one type, others playing it down the middle and developing both. Two by four Like car engines, four-strokes have sump oil lubrication and fire on every other rotation of the crankshaft. A two-stroke requires a petrol-and-oil running mix and fires on every turn of the crankshaft. There are no inlet and exhaust valves, but ports in the cylinder walls. The crankcase and piston areas compress and expand to induct and expel gases. Two-stroke outboards are mechanically simpler and have been lighter. They can produce very high power-to-weight ratios, and are cheaper to buy and service. Four-strokes have appealed on the grounds of quietness, smoothness, economy and complaint-free idling. But the lines are moving closer. Four-stroke designs are becoming lighter, while the low-rev/idling performance of two-strokes has improved with the advent of the electronic injection systems. Between idling and full running, the petrol-to-oil ratio requirement of a two-stroke can vary by some 100:1 to 200:1. Fixed-ratio systems had to be a compromise and old two-strokes could oil up if idled for long periods - a typical need of divers. The new systems allow a variable petrol-to-oil mix, so the engine gets the ratio it needs across its rev range. An exception among these developments has been Yamaha which, while no doubt researching electronic systems, has stuck with carburettors on the grounds that they're mechanically simple and utterly reliable. Considerable cross-development and supply takes place between engine manufacturers. The Mercury and Mariner brands are co-sourced. So are Evinrude and Johnson. We are told there are development links between them and Yamaha, and that Suzuki produces some Evinrude and Johnson four-strokes. Tohatsu apparently supplies OMC. However, Honda, which is completely committed to four-stroke designs, maintains that it stands alone in development and production. Contacts Evinrude/Johnson Simpson Lawrence, Northampton 01604 497641 Honda UK, London Freefone 0800 378086 Mariner E P Barrus, Bicester 01869 363636 Mercury Sowester, Poole 01202 667700 Selva Marine Sales UK, Poole 01202 706454 Suzuki GB, Crawley 01293 518000 Tohatsu Marine, Lymington 01590 670787 Yamaha Motor (UK), Weybridge 01932 358000 |
 Mercury 40hp - sticks with easily maintained carburettors.  150hp Johnson - 2 stroke with advanced electronic injection  70hp Suzuki - shares 4 stoke qualities of smoothness and economy |
