SPEED OF VESSEL:

Background:

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 ) term and are thus a 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 engine reduction installation.

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 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.

We supply all our units targeting maximum rated engine rpm as the adjustable user pitch allows for simple pitch adjustment if required.

Power Curves:

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. Small changes in propeller rpm thus absorb larger changes in power.

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.

Kiwiprop optimise for cruise speed - not maximum rpm to target 99 % real world users. Symmetric blade foils performance is much closer to ogival at lower shaft rpm typical of cruise speeds.

In the real world virtually no one ever runs at maximum engine rpm for a variety of reasons typically noise, fuel consumption and engine life.

There is thus no point in comparing the cruise performance of any two propellers without recognising that this can only be done in the context of similar max rpm being achieved.