By Thomas P.

At that morning we loaded 19 propellers in the small cabin of the Shearwater 860 which was powered by a single 300hp Suzuki outboard engine and headed to Serifos Island. Our aim was to make an exhaustive test of various props and then to choose the right one.

testing propllers

We wanted to see whether the theory keeps up with the practice in the real sea world.

If you belong to the category of users who are interested in getting the maximum performance from the combination of engine-propeller-boat, then get ready with ... strong nerves and get “on plane” together!

Test conditions

  • All the propellers were tested in a Shearwater 8,60m to which a Suzuki DF 300 was mounted.
  • The tests took place in the same sea area, the same day, in the same sea state and wind and of course in the same direction from the bay Koutalas towards Livadi.
  • The engine's mounting height was such that the position of the anti-ventilation plate was six centimeters above the hull keel.
  • The trim was 35% from 2.000 up to 3.500 rpm in all tests, 50% from 4.000 up to 5.000 rpm and full trim from 5.000 rpm and more. The tests in acceleration started with trim 30% to reach a full trim.
  • Concerning the loads we were trying to be as identical as possible, at least for the propellers of the same design. All tests took place with a crew of 4 people and fuels from 160-420 liters.

In other words, we tried to be as objective as possible regarding the performance of each propeller. At this point it should be emphasized that the results of the measurements concern only the particular rib, with the particular engine, with the same loads, the specific conditions and in no way are they comparable, even with another Shearwater with the same engine. The only comparable thing is the performance of each propeller after each test under almost the same conditions.

As we have pointed out many times, the most important criterion for the selection of the most suitable propeller is based on the recommended operating range, at wide open throttle, for our engine. So, from the 19 propellers we singled out only 8 which allowed the engine to reach in the middle of its recommended operating range.

So according to our measurements, propellers that allow the engine run at the desirable levels are:

testing propellers

The right prop, then, is among these eight, and we just have to find it out...

We have to find the propeller that will give us a satisfactory performance at 90% of the use of our boat. In other words we will look for that propeller which will give us the best combination of: top speed, cruise speed at medium rpm, acceleration, getting on plane faster and fuel consumption at mid-range speed. Of course, these results should keep up with the good behaviour and handling of the boat as far as the quality of riding is concerned.

The first conclusions

As it has been emphasized in theory, the diameter and pitch are the fundamental characteristics of the propeller and the most important parameters which mostly affect the performance of the propeller and the boat.

There is no doubt that the remaining characteristics of a propeller -cup, rake, skew, number of blades, surface and shape of blades, blade thickness and so on-, affect its performance, but certainly to a lesser degree.

Comparing the propellers which have exactly the same design but differ only in their pitch, the general rule that says that every inch of pitch “counts” 150-200 rpm for the engine is confirmed.
Comparing a Revolution 19'' and a Revolution 21'', as well as a Mirage Plus 19'' and a Mirage Plus 21'' it can be clearly seen that the difference in rpm at WOT is 400 for the first and 300 in the second one.

Looking at the accelerations table we can see that we can not draw accurate comparative data for all propellers because of different loads.

testing propellers

But we can certainly lead to some conclusions.

First of all, it appears clearly that the 3-blade Solas 16''x19'', which was running with more loads from the first three ones, is definitely the fastest propeller at 0-20 knots. From the measurements is also seen that the propellers Solas 16''x19'', Revolution 19'' and Mirage Plus 19'' have the best acceleration to 20 knots, with a time of 12 seconds, with the specific loads. Of course, we should not blame the Solas 15 3/4x20'', which would probably been located in the top three had it not been for so many loads.

We can, however, safely compare the Revolution 19'' with the Revolution 21'', because they are two propellers with exactly the same design, which differ only in their pitch.
Comparing these two propellers, which "ran" with approximately the same loads, we'll see that at 0-20 knots the Revolution 19'' is clearly faster and more 'explosive' than the Revolution 21'' and is running faster at 30 knots.
Therefore, it is confirmed for once more the theory that the shorter the pitch is the greater the acceleration gets when of course all factors are stable and the compared propellers are of the same design and of course the same diameter.
If we take a further look, we shall see that after 20 knots the Revolution 21'' wins slowly the lost ground, to arrive finally at 40 knots, having the same time with the Revolution 19''.
In other words, the 19'' pitch propeller starts more dynamically and reaches faster to its limits, while the 21'' pitch propeller starts more "numb" because of higher slip numbers, but afterwards reaches at the same time at 40 knots.

Let's see now the data with the top speeds of candidate propellers:

testing propellers

Παρά τα διαφορετικά φορτία που σε κάποιες περιπτώσεις έχουν μεγάλη απόκλιση, μπορούμε να προχωρήσουμε στην αξιολόγησή μας.

Despite the different loads that in some cases have large deviation, we can proceed with our evaluation.

The Suzuki 15''x21'' comes first, however with doubts, since the Solas 16''x19'' ran with 200 litters more fuel and had only 1, 4 knots less top speed. So if we take account the loads to all propellers, rather Solas 16''x19 '' has the maximum speed at full throttle.
Beyond that the propellers Revolution and Mirage Plus are quite close to each other.

 

...keep Ribbing!