Boat engines and underwater scooters
This category holds outboard engines, petrol and electric, and alongside them underwater scooters. On the combustion side it comes down to two-stroke against four-stroke, and new engines are, because of the emission limits, almost all four-strokes. The telltale stream is the only thing you see of the cooling while under way, and propeller pitch keeps the revs inside the prescribed range. On scooters you read runtime per charge and the depth limit instead of horsepower.
Boat engines and underwater scooters
This category holds propulsion, that is outboard engines running on petrol and on electricity, and alongside them underwater scooters for divers and snorkellers. The boats the engines hang on sit in the neighbouring category, here it is the propulsion itself that matters. An outboard hangs entirely off the stern and lifts off, so at the end of the season you carry it indoors even if the boat stays outside. An underwater scooter, by contrast, is hand-held, tows the swimmer behind it and has nothing in common with a boat but the water.
The first question on a combustion engine reads two-stroke or four-stroke. A two-stroke is lighter and simpler, but it lubricates itself with oil mixed into the fuel, which therefore ends up partly in the water and in the air. It is precisely because of the exhaust that the European recreational craft directive sets separate limits for two-stroke petrol engines, four-stroke petrol engines and diesels, and an ordinary carburetted two-stroke does not meet them. New engines are therefore almost all four-strokes, while two-strokes turn up mainly second-hand, and there you have to reckon with mixing the fuel.
The second thing is the cooling and the telltale stream. Most outboards are cooled by drawn-in water that a rubber impeller drives through the pump under the powerhead. The stream that comes out of the side is not decoration but the only part of the whole circuit you see while under way; if it weakens or stops, the engine has to be shut down at once. The impeller is a wear part changed roughly once a year or after a few hundred hours, and running dry damages it very quickly.
The third is propeller pitch, that is the second number in its designation. It is given in inches and means how far the propeller would screw itself forward in one turn through a solid material. The engine maker prescribes a rev range at wide-open throttle, and the propeller is the lever you hit it with, because each extra inch of pitch drops the revs by roughly a hundred and fifty to two hundred. Outside that range you either leave power on the table or over-rev the engine, and neither pays off.
The last group is underwater scooters, and there you read entirely different numbers. Leisure models run at about three to six kilometres an hour, and one charge usually lasts thirty to sixty minutes. The depth limit counts here as much as the runtime, because ordinary models stop around forty to fifty metres, and deeper than that water gets into them.
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