Kiwiprops™

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Pitch Reverse

Some comment on the fact that the reverse pitch setting on a Kiwiprop unit is not adjustable - the units all go to maximum pitch in reverse irrespective of the pitch that has been set in ahead - without perhaps understanding that here are very good reasons for this.

The two largest selling sailing yacht auxiliary units in the world by far require a design that delivers higher pitch in astern than ahead and the reason is very simple.

Shaft installation Yanmar's all have a reverse ratio of 3.06:1 irrespective of the chosen ahead ratio which is normally 2.61:1- one of three available options.

Volvo's will typically have 2.37:1 in ahead and 2.72:1 in astern in their most popular configuration.

This is also one of the reasons that folding propellers very often deliver poor reverse performance on these popular motors as by using centrifugal force to open the blades, higher reverse reduction ratios will deliver lower shaft speeds - and thus reduce the centrifugal force required to open the blades against the reverse thrust acting on the blades of the propeller.

There are of course many other gearbox options on the market - many with the same or similar ratios in ahead as astern, typically ~ 2:1 and some where the reverse ratio is in fact much lower than the ahead ratio which will deliver higher shaft speeds in reverse.

The graph below taken from the Yanmar 3YM30 site shows how significantly the power requirement of a propeller ( Blue line ) rises with shaft rpm and is a much steeper curve than the power curve ( Red line ) of the engine. This indicates the power of the propeller is more sensitive to rpm than the engine. As the engine rpm increase by 50 % ( 2000 to 3000 ) the power required for the propeller goes from 4 kW to 12 kW or 300 % !

ym30c

By going to maximum pitch in reverse and emulating a fixed three bladed propeller - Kiwiprop units deliver exceptional reverse thrust.

Economics is always an ever present design constraint as buyers always rightly expect to receive optimal value for their purchase.

To build in a reverse pitch adjustment that is not required for the two largest selling brands on the market does not make a lot of sense - and it is for this reason that we have chosen to forgo the option of a reverse pitch adjustment.

In addition the vast majority of reverse function usage is with relatively low engine speed maneuvering, eg docking where maximum reverse thrust is the design objective at relatively low engine speeds.

While some will argue that unless the engine can achieve max rpm in reverse it will not be able to develop maximum power - that is true, but by studying the graph above the power output curve for a modern diesel is relatively flat and is not directly proportion to engine rpm.

A very large proportion of the engines maximum power is available at quite low engine rpm. The additional pitch of the Kiwiprop unit in reverse compensates for the difference in the power curves of the two graphs at lower prm.

The power output of this 29 hp engine at 3600 is still 25 hp at 2500 engine rpm.

Thus the great majority of the engines output is available at a fraction of maximum rpm and it is appropriate to utilize this given the design requirement for the majority of reverse usage.

We still believe that the design trade-off we have made in terms of economics and practicality by not offering user adjustable reverse pitch driven by actual usage considerations coupled with the actual power curve outputs of modern engines and propellers is optimal.