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Speed - Motoring


Measurement of boat speed through the water can only be done on a calm day with the average of two consecutive runs in opposite directions. Remember for any analysis of boat speed we are only concerned with speed through the water - not speed over the ground which is what GPS systems measure.

The reason data must be collected on a calm day is that windage drag is not proportional to speed and therefore you cannot average the two runs.

Formulae for Hull Speed:

Much good work was done many years ago on the theoretical speed of a vessel through the water. Basically this reflects the wave making ability of the hull.The formulae were reasonably accurate for similar vessels of the day which were all longer displacement launch type hulls.

The formulae contain no  Displacement, Power or Prismatic Coefficient ( Fatness of hull ) terms and are thus a very rough  guide only. Modern light weight yachts are a very different hull form from what was used at the time.

While the formulae correctly predict that longer waterline hulls will have a higher theoretical speed they do not provide a definitive speed that an owner can expect to achieve from a particular vessel with a given waterline length and displacement coupled to a particular engine reduction installation. While owners frequently refer to the theoretical hull speed of their vessel - it is a rather meaningless figure as it is calculated solely from waterline length.

For detailed Speed / Power / Displacement / Length Formulae refer  Dave Gerr - The Propeller Handbook

Google "The Propeller Handbook"  ( Copyright Material )  refer page 9 onwards where a very detailed analysis is given on this topic with specific formuale and graphs.

Warranty Issues:

New engines typically require that the installation reach max rated engine rpm as a condition of warranty. It is well known that by fitting a larger propeller ( Diameter and / or Pitch ) - boat speed at cruise will improve. This will always however be at the expense of achieving maximum rated rpm.

Increasing the pitch of the propeller will achieve the same effect with higher boat speed at cruise - again trading off the ability to achieve maximum rated rpm.

On older engines where warranty is no longer an issue users may prefer to increase boat speed at cruise rpm in the knowledge they will not be able to achieve maximum rated rpm. Under this scenario and particularly on turbocharged engines these installations must never be run at full throttle as they will invariably damage the engine generally due to higher temperatures and pressures than the engine has been designed for. Higher temperatures will increase the expansion of various components and may remove the design tolerances with serious consequences for engine life.

We with some exceptions such as very high speed Perkins 4.108's - we supply all our units targeting maximum rated engine rpm as the adjustable user pitch allows for simple pitch adjustment if required.

Cruise Speeds vs Max RPM Achieved:

The following graph from the Voile Magazine tests in 2009 shows by plotting speed at cruise rpm of 2200 on the Y-axis vs  the max rpm in descending order achieved by the engine on the X-axis - that there is a very strong correlation between lower max rpm and higher cruise speeds.  As we have stated previously - any comparision of propeller perfromance can only be made in the context of similar max rpm constraints.

New engines require that they reach a minimum max rpm to qualify for warranty. The Voile tests of the K3 unit under INDEPENDENT REPORT in the K3 section of these web pages show in red the four units that did not meet these criteria.

Kiwiprop Performance at Cruise RPM:

Users are always interested in the motoring speed thay can expect with a Kiwiprop along with the other  features available. The following information was taken directly from the Voile Magazine report at 2000,  2500  &  3000 rpm which covers virtually all cruise rpm regimes  that the vast majority of users would operate within.

The left most colums are simply the data as reported. The next three columns take the same data and rank it by ascending vessel speed at those particular column heading rpm settings. Red indicates the unit was outside the minimum rpm required as indicated in the Voile report.

What it shows is the Kiwiprop on this installation was very highly ranked at all three cruise speeds relative to all other units that met warranty criteria. There is no reason why users cannot expect similar relative performance from their own installation bearing in mind the constraints in place for this evaluation.  Boats that have more displacement, shorter waterline lengths, greater wetted area, steeper shaft angles, less power or higher shaft speeds will invariably have lower speeds than reported on this vessel.



Kiwiprop vs Fixed 3 Blade benchmark unit:

The following graph shows the performance of the Kiwiprop vs the fixed 3 bladed unit. The differences are very small as can be seen from the blue and black line which virtually overlap at all speeds. This is taken from the original Voile report which compared each unit in turn against the benchmark 3 bladed fixed unit.

Power Curve Shapes:

Below are drawn three curves representing typical power curves of an engine and a propeller optimised for max rpm performance.  A propeller optimised for cruise rpm is shown intersecting at lower rpm.  The power function of a propeller typically varies with the shaft rpm ^ 3. Thus small changes in propeller rpm thus absorb larger changes in power. Typically a 10% increment in rpm  will result in a 1.1 x 1.1 x 1.1 = ~ 1.33 or 33%  increment in power required, This illustrates why it is so crictial to use the correct reduction ratio and engine rpm to calculate the shaft rpm accurately which drives correct sizing.


It is thus possible to optimise a propeller for cruise speed or maximum speed ( or any other speed ) but you can only have one optimal solution - where the two curves cross. By optimising for max rpm as required under warranty - the difference between the higher power of the engine from the blue curve at cruise rpm and the power absorbed by the propeller at cruise on the red curve represents the additional power that can be applied at cruise rpm to increase vessel speed at cruise.

The trade-off will be the reduction in max rpm achieved and the consequent warranty issue implications.