Key aspects of repowering shaft-drive boats

For anyone considering their re-power options for a shaft driven vessel, there are some surprising, yet critical elements of the exercise that should be taken into account.

Owners of these craft will automatically think about such obvious factors as horsepower, fuel economy, weight, size and of course, cost. However, there will inevitably be several vital engineering considerations to be thought through as well.

These latter elements of shaft drive re-powers require specialist knowledge and skills, such as those long associated with respected local marine and industrial engineering company C.E. & A Co. in Royal Park.

We asked Sandra Matthews of C.E. & A. Co. to outline and explain the key engineering features involved and were surprised by what we learned.

For example, just as few motorists understand the workings of their car’s gearbox, so too do very few boat owners fully realise just how complex is the role of a humble propeller.

It is essential to know whether or not the existing propeller will be of the right size and direction of rotation. As well, a bigger or stronger propeller shaft and perhaps a more robust rudder might be required.

More horsepower may mean that the existing shaft is no longer up to the task. If not, a shaft of stronger material or larger diameter will be called for. If the shaft diameter does increase, that may also involve a new stern-tube and/or skeg bearings, a new coupling and a new shaft seal. The propeller, stern-tube and skeg might need to be bored out to take the new shaft and bearings. Extra horsepower could also mean a change of propeller.

The faster the shaft spins the smaller the propeller diameter. Fitting a gearbox with a higher ratio may require a larger diameter propeller. If so, for ideal water-flow conditions there should be a clearance of around 15% of the propeller’s diameter between it and the hull. If that can’t be achieved, C.E. & A. Co. could most likely engineer a diameter/pitch compromise to try and produce similar propeller performance.

Sometimes, replacing the engine and gearbox can result in the direction of rotation changing. Unfortunately a propeller can’t be just turned around. Running the gearbox in reverse all the time to go forward is never a good idea, although some gearboxes run in either direction, so it is worth checking beforehand.

Any increase in boat speed will most likely have a direct impact on the rudder and rudder stock calculations, making it necessary to ensure that the boat’s steering will be able to handle any new loads generated by the rudder.

To get the full benefit of an expensive re-power, boat owners, particularly those whose craft are under survey, should make themselves aware of gearbox ratio options and recommended propeller sizes as integral parts of the overall package.
For further information and advice on these rather technical aspects of modern boating, contact Sandra or Phillip Matthews of C.E. & A. Co. Pty. Ltd. on Schenker Drive in Royal Park, on 8240 0777 or by email at