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Yanmar Pleasure Boat Marine Engine Help
CAUTION An installation where the gearbox has not been specified by Yanmar in the engine brochure should have a Torsional Vibration Analysis (TVA) calculation carried out on the entire rotating assembly, from the engine to the propeller inclusive.
Failing to ascertain whether the drive train components are compatible or not may result in an engine, gearbox or drive component failure. THIS IS NOT WARRANTY!
Glossary: Transmission, also known as; gearbox, clutch, reversing clutch.
TRANSMISSION MODELS and OIL INFORMATION
CAUTION: When measuring the transmission oil, sit the dipstick on the threads, do not screw it in.
The most efficient propeller is installed parallel to the waterline of the vessel. The steeper the propeller shaft angle the less efficient the transmission of power into the water. Yanmar provide the following types of transmission to optimise the installation.
- Bobtail. No gearbox! (Code letter 'M', i.e. 4LHM-STE) The flywheel is bolted directly to a water jet unit or remote mounted gearbox via a
'CENTAFLEX ®' type coupling.
- Parallel gearbox. (No code letter, i.e. 3GM30)The output shaft is parallel to the engine crankshaft shaft but lower. Often called a ' drop ' box as the output shaft is lower than the input shaft.
Pro's: The thrust is concentric with the propeller shaft therefore easier to align and usually has less vibration.
Con's: The propeller shaft is at an angle to the waterline, depending on prop size and clearance from the structures.
- Downangle gearbox. (Code letter 'B', i.e. 4JH3-BE) The output shaft comes out of the back of the gearbox at an angle of 8 degrees. Available from over 30 horsepower and up.
Pro's: The engine can be mounted lower in the engineroom, parallel to the waterline and therefore saves space.
Con's: The propeller shaft angle is still 8 degrees from the waterline and when the thrust is applied to the back of the gearbox the engine tilts forward, throwing out the alignment. A propeller shaft thrust bearing can be fitted to eliminate this. Note, it is not so important on engines under 60 horsepower.
- Saildrive gearbox. (Code letter 'C', i.e. 4JH2-CE) The propeller shaft is parallel to the waterline and protrudes, via a leg, below the hull.
Pro's: Easiest to install (by a couple of days)as a separate prop shaft and strut
assembly, raw water inlet are not needed. Now available for all models up to
75 horsepower. Power is applied parallel to the waterline so is more efficient.
Weight is concentrated around the engine so the balance for GFRB's (Go Fast
Racing Boats) can be better placed.
Con's: Initial cost is higher (although for new builds this is offset by not
having to buy a prop shaft and strut assy,gland/seal etc), maintenance cost is
higher. Drag while sailing is marginally higher.
In the picture above you can see the fibreglass engine bed. It is cut with a jigsaw to fit the shape of the hull and then fibreglassed into place.
- 'V' drive. (Code letter 'V', i.e. 3GM30V) A 'V' drive allows the engine to be installed over the top of the propeller shaft.
Pro's: Space optimizing.
Con's: More expensive. Installation, shaft maintenance and alignment are more
difficult. Not available in all markets.
- 'Z' drive. (Code letter 'Z', i.e. 6LP-STZP) 'Mercruiser'
Bravo series (Preferably "X Drive"- now standard) sterndrive is packaged with
4LHA and 6LP series engines. The prop is parallel to the waterline and the
angle adjustable via the trim assembly. (Note, dont try and fit a conventional
petrol 'Z' drive to a diesel engine unless you enjoy breaking down at sea.
The maintenance cost is excessive!)
Pro's: Space sacing compact package. Parallel steerable thrust. Inboard/outboard
with attendant advantages, e.g. boat is easily trailerable and beachable.
Con's: High initial cost, high maintenance cost, added complexity.
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