Yanmar model SD20 Saildrives are fitted to all Yanmar engines under 30 hp when a Saildrive option is selected with a Yanmar engine.

It is very important to understand the quite unique nature of this Saildrive in that it alone in the industry has what is described as a " dog clutch " to engage Ahead and Reverse. The nature of this " clutch " - which is not really a clutch in the conventional sense  - is that while it provides a very simple economical and robust mechanism that cannot ever slip - the trade-off is that engagement is instant once the dogs on the driving shaft engage with their matching dogs on the driven shaft. No other manufacturer of Saildrives uses this type of clutch arrangement.

In this situation - the idle rpm setting of the engine becomes very important to the overall durability of the drive train.

At  1000 engine rpm with this type of clutch - the output shaft goes to engine rpm speeds  in  ~ 1 millisecond - a huge acceleration. While the propeller shaft speed is reduced by the 2.64:1 of the reduction ratio in the SD20 - it still undergoes the lesser change in rotational speed in the same millisecond time frame. Lower engine rpm therefore reduce the levels of acceleration and forces involved.

It is critical that the engine idle rpm of the engine are maintained within Yanmar specification of 850 rpm.

Higher engine rpm leads to higher than designed loads with the near instant engagement of the reverse rollers with the blade root contact pads on Kiwiprop blades which will lead to higher than expected wear rates on these surfaces. Once damaged by these extreme forces the continued smooth operation of reverse engagement may be compromised.

All other Yanmar Saildrives - SD30, SD31, SD40, SD50 and now SD60's have cone or traditional clutch pack type clutches that slip during engagement so providing much lower forces during engagement of both Ahead and Astern functions. As the clutch slips - the propeller accelerates relatively slowly up to operating speed. Idle speeds remain important but do not have the same critical nature as they do with the SD20 model.

All other Saildrive manufacturers use this more conservative design approach utilising traditional cone and clutch pack type clutches that slip during engagement. The numerous problems that many encounter with the shearing of the rubber bushes between the boss and the blade assembly of their propellers are quite likely to be linked to the above design issues and the subsequent very high torque shock loads imposed on any propeller mounted on an SD20 Saildrive.

In addition for all other drive trains including Saildrives -  it is critical that idle speeds are not lower than expected.

This can allow the unit to generate reverse thrust before full reverse engagement leaving the blades at a ~ 45º angle and overloading the engine so that reverse thrust is reduced and the engine will not be abe to accelerate to higher rpm.