V-drive
A V-drive is a power transmission system for boats that consists of two gearboxes, two drive shafts, and a propeller.
Whereas the conventional arrangement sites the engine with its gearbox aft, driving the propeller shaft directly, in a "V-drive" layout, the engine is reversed, to have the gearbox in front. This primary gearbox typically drives a short shaft forward to a transfer gearbox which reverses the transmission to the main tailshaft which is directed rearwards the propeller, at a "V" angle to the short shaft.
A V-drive system variation is for the tailshaft to drive a saildrive propeller, mounted on a skeg below the hull. This is common on Lagoon catamarans.Image:Saildrive-2.jpg|thumb|A Volvo saildrive
A variation of the two-shaft V-Drive layout is the "close-coupled V-drive" whereby the engine is still mounted "back-to-front", but the main gearbox incorporates an output flange that has already been reversed. This system obviates the need for any short primary shaft.
Counter-intuitively, a V-drive system will not necessarily mean that the engine is sited further rearward; the whole engine/transmission may be sited forward than in a conventional arrangement.
V-drive rationales
V-drives are typically fitted to sport motorboats or cruising yachts and catamarans. The pros and cons tend to be different for each type.For sportboats, a V-drive may be an alternative to an inboard/outboard drive. Here the V-drive will allow the engine to be moved further forwards and the propeller will be under the boat. This can give a flatter ride, greater choice of propellers, and a safer stern access for swimmers.
For cruising yachts, the rationale of the V-drive installation may include:
- the propeller can be mounted further forward in deeper "quieter water".
- the engine can be mounted in a more space-efficient position.
- the position of the hull's centre of gravity may be optimised,