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Transmissions, whether crashbox, or shiftable, are only required if the drive system has no F-N-R gears integrated within it's design. Drives like the Volvo Aquamatic, Mercruiser Alpha & Bravo etc have such kit built into them, where as the old Mercruiser TRS, Sportsmaster racing drives etc, and most shaft/surface drives do not, so require a remotely installed gearbox to do the job of introducing Forward-Neutral and Reverse functionality.
The term 'crashbox' refers to a gearbox much like an outboard one in some ways, where dog gears are employed, which means selecting a gear whilst the engine is running will/could cause damage due to the high inertia of large stainless steel props & shafts. So one must stop the engine, shift gear, then restart the engine. A bit of a faff for manouvering.
That horrible ker-ker-ker-ker-klunk you hear when putting your outboard, or Alpha drive into gear, is the dog gears mating in the gearbox. One spinning at engine speed, one stationary and connected to your prop, being thrown together. Not nice, and usually lessened by snapping it quickly into gear rather than easing it gently. Every one of those 'klunks' you hear, represents another tiny chip of dog gear floating about in your gear oil and an even larger 'chistmas tree' of sludgy swarf growing on your lower unit's magnetic drain screw.
Typical racing crashboxes such as the classic Daytona, are immensly strong, have very few moving parts, and when in 'fwd' are basically an input shaft and an output shaft on the same axis, locked together by the huge dog clutch as a 1:1 unit, so virtually nothing to break.
Shiftable gearboxes, usually employing clutch plates, clutch bands, or cone clutches, allow one to shift smoothly between F-N-R with the motor running. Much more user friendly, but often not as ultimately robust/strong, hence the use of crash boxes such as the 'Daytona' in a lot of high power applications and racing.
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"I Agree with everything you say really!" - John Cooke to Jon Fuller - 26-01-2013
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