4-blades vs 3-blades
4-blades vs 3-blades
Testing 19 props in a day – part 1
By Thomas P.

It is a fact that the propeller of our engine is one of the major factors that affect greatly the performance of our boat. Therefore it is necessary to be able to choose the right prop for the combination of our boat and engine.

First of all we should have in mind that we can not have everything in the world of the sea. For something extra that we will gain in a field, something else will be lost in another one.

It is impossible, for example, to aim the highest speed and expect to ride very well in the rough sea or have great accelerations, at the same time. We have to choose between the two goals, as well as among many other things.

Unfortunately, there is no ideal propeller which will make everything perfect. Each propeller is designed to perform well under specific conditions.

The gear ratio of our engine, the design and the length of the hull, the usual load on board, the mounting height of the engine and the existence or not of a bracket are some of the key factors that directly affect the selection of the appropriate propeller.

Therefore, the first thing that we have to do before starting looking for the right propeller is to wonder the exact usage of our boat.

We would prefer a different propeller if our main goal is to achieve maximum speed, another one if we are mainly interested in great acceleration or good performance in tough sea, another one if our goal is the low consumption and the highest possible cruising speeds at medium rpm. And of course, we would choose another prop if our goal is to transfer loads or pull skiers or if we are interested in the trolling speed.

But whatever usage our boat is intended for, the propeller which will finally be chosen must allow our engine to reach its maximum speed at WOT (wide open throttle).

This is an inviolable rule that should never be forgotten because only this way we will ensure the proper operating of our engine.

So having always in our mind the purpose for which our boat is intended and the fact that our engine should reach its maximum rpm then we are ready to choose the right propeller.

Load your boat with the usual loads and go out to the sea. Select a day of calmness and start your tests. Push the throttle lever forward until the end and then start to trim up the engine. At some point you will hear the familiar sound coming from the propeller when the air is sucked into its blades. This means that if you trim up more, thrust is not produced, and you will realise it by looking at the tachometer. You will see that from this point onwards no matter how much you trim up the speed will not be increased while at the same time engine's rpm rapidly goes up, and the boat slows down. This is the result of air being pulled into propeller blades and therefore blades lose their grip on the water.
So trim down a little lower, to the point that the prop's ventilation stops. Look at the speed on tachometer at that time and record the engine's rpm. If the engine runs within the specified rpm range at full throttle then you do not have to worry about anything and the propeller you wear matches just fine in your engine-boat-loads combination.

What should be remembered?

  • Τhe same propeller can’t give us both high speed and maximum power. If we will get more thrust, we will lose final speed. If we will get top speed we will lose thrust.
  • Either we will we ride better in bad weather or faster with calmness.
  • In a propeller, diameter and pitch work hand by hand. We never have to look for the right pitch separately, but always in relation to the diameter. In general we can say that the most effective relationship of diameter-pitch is the following:
    • Large diameter with a short pitch and
    • Small diameter with a long pitch
  • The best combination Diameter-Pitch related to the length of the hull, our usual loads as well as the hull’s design, will give us the right balance of thrust and speed.

General rules

  • Every inch of the pitch ‘counts’ 150-200 rpm for the engine at WOT, an inverse relationship. Increasing the pitch, the load on the engine increases, since the propeller "grips" a larger volume of water, thereby the engine’s rpm reduces. The reverse happens when we decrease the pitch. When we increase the pitch by 1 inch, we should expect a reduction in a range of 150-200 rpm to our engine. If we go from a 21΄΄ pitch propeller to a 19" pitch propeller then we expect an increase of 300-400 rpm to our engine. Therefore, when our engine operates below the proper rpm we will fit a propeller with less pitch and vice versa.
  • Only two inches in pitch are enough to cause tremendously different results over the performance of our boat.
  • The pitch defines how quickly we will move forward when the propeller is rotated (acceleration).
  • The pitch is ultimately responsible for the final speed of the boat.
  • A propeller with a long pitch will travel us further in a rotation, than a propeller with a short pitch.
  • A propeller with a short pitch will push better an overloaded boat and will make us have better handling in closed manoeuvrings in confined spaces.
  • When we increase the pitch, we get higher top speed and higher mid-range speeds, but we will have less acceleration, and we will get on plane later.

The opposite happens when we decrease he pitch.

...keep Ribbing!

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