The “Rake” of the propeller
How to choose the Right Propeller
4-blades vs 3-blades
Undoubtedly, three-blade propellers are those more commonly used in boats. Their performance is usually very good in all sectors, they normally achieve higher final speeds, they are available in a bigger variety of diameter-pitch sizes and their cost is lower.
However, throughout the past years, the usage of four-blade propellers has entered into a continuous struggle of improving the performance of our boat, and, of course, this was not coincidental. It is evidenced that four-blade propellers usually increase the engine's load, but the production of more and more powerful engines has opened the way for their use.
Apart from the evolution of the engines, boat manufacturers have also focused on the improvement of the hull design. Therefore, boats with reduced-friction hulls are increasingly constructed, which have actually decreased the load applied on the engine to a great extent, allowing for higher load tolerances. As a result, even lower-horsepower engines are able to accommodate the additional load of a four-blade propeller.
These are the two main reasons for the increasing usage of the four-blade propellers.
From theory to practice
Is everything we believe about the four-blade propellers true? And to what extent are they true in real conditions? Does the hull design affect the performance of the four-blade propellers? Can the design and the characteristics of a four-blade propeller itself play a significant role on its performance?
All the above factors certainly play a really important role, and that's why the effectiveness of the four-blade propellers greatly varies, while the performance of the very same four-blade propeller highly depends on the boat on which it is mounted.
We can only find the truth by performing tests in the sea, provided that we are objective and that we use the same weights and measures. As a matter of fact, the truth lies somewhere in between.
In our tests on a Shearwater 8.60 Rib with a Suzuki 300hp, we saw that:
- The Solas 16΄΄x19΄΄ three-blade propeller provided quicker acceleration than all the four-blade propellers we tested, despite the higher loads.
- The Suzuki 16΄΄diameter three-blade propellers provided quicker plane times and kept the boat on plane at lower revs compared to the four-blade propellers.
- The Solas and Suzuki 16΄΄ diameter three-blade propellers had much smaller slip at the widest range of revs compared to the four-blade propellers.
And these are only some indicative remarks which suggest that things are kind of more complicated...
Based on the above, we come into the conclusion that we cannot define behavioural rules when comparing three- with four-blade propellers and that such a generalized comparison is nonsense.
We can only compare a specific four-blade propeller with a specific three-blade propeller, on a specific boat, with specific loads and with a specific engine and then we can try to evaluate the difference in their performance.
Caution is needed! Our comparisons need to be based upon the same criteria and they must not be generalized.
The comparison between a three- and a four-blade propeller, as far as the effect of the additional blade is concerned, is meaningful only when all the other characteristics of the two comparable propellers are exactly the same. The two propellers need to have the same diameter, the same pitch, the same extent of the cup, the same rake degrees, the same surface area in each blade, the same blade shape, etc. Only in this way can we determine the effect of the addition of one more blade on the performance of the propeller.
Unfortunately, there are no such propellers. It would be really interesting if there were Revolution three-blade propellers with exactly the same design characteristics as the respective four-blade propellers. Only then would we be able to determine the difference the additional blade provides to the performance of the propeller and the effect of the additional blade on the revs for our engine. Unfortunately, each propeller is unique, with completely different design from any other. Therefore, things are getting more complicated and we need to be more careful in our comparisons. Otherwise, we would not be able to come up with a conclusion or even with an approximate estimation.
Common mistakes
Some people have mounted a three-blade propeller on their engine and they have actually achieved really satisfying performance, but having in mind that the four-blade propellers are the "perfect" propellers, they tend to try them out and they often get disappointed with its performance and the final result. (At this point, its worth mentioning that there is no perfect propeller. There are propellers that better suit our engine, our hull and our loads.)
Of course, the above test does not yield the expected results not because the four-blade propeller is not as good as it is deemed to be, but because the rest characteristics of its design do not match with the engine-boat combination.
What goes wrong? The four-blade propeller that these people tested had a different diameter, or a different cup or rake compared to that of the three-blade propeller the had on. So, because the three-blade propeller had a specific diameter, pitch, cup and rake, which, even accidentally, suited very well with their boat, if the four-blade propeller differed in any of these characteristics, it would reasonably not yield the expected results.
They did not try to keep the same characteristics with the very good three-blade propeller they had on, but they switched to a four-blade propeller, ignoring its individual characteristics.
Therefore, when we change a three-blade propeller with a four-blade propeller, we need to be really careful so as the characteristics of the two propellers to be as common as possible. This is very crucial in order to be able to make a good judge. It is not possible to compare the performance of our three-blade propeller, when the design of the four-blade propeller to be tested is completely different.
Some other people, on the other hand, completely ignoring the three-blade propellers design -no propeller is the same with any other- and without even thinking to try which three-blade propeller would be suitable for their boat, they directly switch to a four-blade propeller.
I personally believe that this is not right either, because if we don't know in advance which is the appropriate three-blade propeller for our boat and our usual loads, then we are not able to have a reference point as to the performance of our boat. So, how will we be able to know that we have achieved the maximum performance for our boat if we just put a four-blade propeller on? What will be the characteristics that will help us choose not the more suitable, but the four-blade propeller for testing? Besides, it is obvious that it would not be possible to test ten different four-blade propellers in order to choose...
Apart from the design of the propellers, the design of the boat's hull is of decisive significance for the propellers performance.
Going deeper into the individual characteristics
The choice of the propeller which will provide us with the best performance is not easy at all and no matter how much knowledge and experience we may have, only the sea will help us find the answer.
Let's move a little more deeper into the blades of a propeller!
It is highly likely that a well designed three-blade propeller suits our engine-boat combination so much, that it would be needless to put a four-blade propeller on. An additional advantage is the higher final speed the three-blade propellers usually provide.
Let's examine an example: Some three-blade propellers have a sharp cup, which runs along the whole length of trailing edge of the blade and extends out to the blade tip. This allows the propeller to get a better "bite" on the water. In this case we have better "grip", decreased propeller ventilation, less slip and quicker plane times.
Furthermore, such a propeller with a sharp cup works very well when the engine is trimmed very high, "catching" more water and leading us to higher speed. So, it is obvious that the behaviour of a three-blade propeller can dramatically change just by modifying its cup. Imagine how more effective a three-blade propeller would be if the rest of its characteristics are improved. Besides, it is understood that a propeller "runs" by combining all of its characteristics at the same time: diameter, pitch, cup, rake and so on. Each one of these characteristics has a different role.
We should probably consider the possibility that a propeller of such a design performs much better than a four-blade propeller with no cup at all and that, eventually, if we put the appropriate three-blade propeller on our boat, we may achieve such a performance that even the best four-blade propeller would be jealous of.
It is therefore understood, that in order to be able to compare a four-blade propeller with a three-blade one, we need to know their basic characteristics -at least the measurable ones-, which should be as similar as possible. Only in this way would a comparison be "merit" and only in this way will we be able to determine the difference that the additional blade offers, provided that the remaining characteristics of both the propellers are the same. Otherwise, our conclusions will be false.
In other words, it is not "fair" to compare a four-blade propeller with sharp cup with a three-blade propeller with no cup at all. In this case, we will not be able to understand if the "grip" of the four-blade propeller is owing to the additional blade or to its sharp cup. But if both propellers have the same cup and the four-blade propeller still has a better "grip", then we can be certain that the better "grip" is due to its additional blade. Only then can we prove that the four-blade propeller has an advantage over the three-blade propeller.
Imagine the case of the two compared propellers having several other differences in their design, apart from the number of their blades, such as different diameter, different cup and different rake. How would it be possible to judge if the difference in their performance is due to the additional blade or to the effect of their remaining characteristics?
This is how we can explain the fact that some four-blade propellers are faster than some three-blade propellers at the same pitch or how some three-blade propellers have a better "grip" than some four-blade propellers at the same pitch.
Four-blade propellers differences
- A four-blade propeller has bigger total blade surface area compared to that of a three-blade propeller of the same or smaller diameter. Practically, this means that in case we need higher thrust and we cannot go with a three-blade propeller of big diameter due to space limitations or due to the transmission ratio, then we end up to the use of a four-blade propeller.
- A four-blade propeller has bigger blade surface area compared to that of a three-blade propeller of the same diameter and pitch, it "catches" more water and therefore has a better "bite". A better "bite" means better "grips" and this explains the sense of higher propeller power we feel, which is really important in rough weather. However, we should consider that the bigger bite or the bigger holding power of a four-blade propeller may also be due to the fact that during each propeller rotation there are two, not only one, blades in the water at all times.
The bigger "bite" of the four-blade propeller -compared to that of a three-blade propeller of the same diameter, pitch and design- results in increased difficulty in rotating compared to the respective three-bladed propeller.
Therefore, the load applied on the engine is higher and, thus, when switching from a three-blade propeller to a four-blade propeller of the same design, we have to decrease the diameter by one inch or the pitch by two inches, in order to avoid significant alteration in the engine revs. In this way we compensate the additional load the heavier propeller and its bigger blade surface area bring about.
We can also raise the engine higher on the transom, which is feasible with a four-blade propeller, retaining the revs without having to decrease the diameter or the pitch.
When we have to compare two propellers of different design, things become more complicated. In this case, we have to account for how each propeller characteristic would affect the revs -if this is feasible-, so as to retain approximately the same revs for our engine when switching from a three-blade propeller to a four-blade propeller.
On the other hand, if we switch from a three-blade propeller to a four-blade propeller of approximately the same design and we decrease the pitch in order to apply the same load on our engine, this will result in a final speed reduction and even bigger grips (due to the pitch reduction). If we take into consideration, apart from the final speed reduction, the expected speed reduction due to the additional hydrodynamic resistance forces the additional blade presents at high speeds, then we can comprehend the additional final speed reduction when using a four-blade propeller.
So, it is obvious that if our main goal is to achieve a maximum final speed, then choosing a three-blade propeller may be the best option in most of the cases. And I mention that this is the best option in most of the cases, because there are cases where four-blade propellers can provide equal or even higher final speeds compared to the respective three-blade propellers.
This is either because of their very good design or because they run better when raising the engine higher, so when they run near the water surface, the additional blade helps them "catch" more water with reduced hydrodynamic resistance forces from the smaller surface of the lower unit which is exposed and "ploughs" the sea.
- A four-blade propeller, as we mentioned above, has a bigger "bite" than a three-blade propeller of the same design. But, does a bigger "bite" means smaller propeller ventilation trend after all? This is how we can explain why four-blade propellers have advantage regarding the ventilation effects: in sharp accelerations, in sharp turns, in extreme trim and lift positions.
- Four-blade propellers have a smoother and softer behaviour and they provide better handling and higher stability and balance with much less vibrations. Generally, the more the propeller blades, the less the vibrations and the smoother the propellers run. During each full rotation of a three-blade propeller, three blades make a full circle, while during each full rotation of a four-blade propeller, four blades make a full circle. In other words, a four-blade propeller makes more circles per minute, which results in the reduction of the vibrations and in their smoother behaviour.
- A four-blade propeller has bigger blade surface area, but this does not mean that the additional blade can substitute the role of the diameter. That is, a four-blade propeller does not behave in the same way as a three-blade propeller of a bigger diameter, even if we assume that they have exactly the same total blade surface area. They may be comparable to an extent, but we need to bear in mind that the water flow is completely different among the three blades compared to that among the four blades, which certainly affects the performance of the propeller.
All the above observations apply to all four-blade propellers, but they differ as to the extent of the performance they will provide, which directly depends on the special design characteristics of each propeller!
Finally, are four-blade propellers more effective than three-blade propellers?
Taking into account all the above mentioned, I believe that this question makes no sense and the only answer I could give would be "It depends"! Certainly, each propeller, three-bladed or four-bladed, is unique and it behaves as such!
Pros and cons
Let's have a look at some of the advantages that attributed to the four-blade propellers:
- They keep the boat on plane at a lower speed.
- They provide quicker plane times.
- They are faster at lower and mid-range revs.
- They run smoothly and softly with no vibrations, providing better boat handling.
- They provide higher stability and balance.
- They have better accelerations.
- They provide better holding power in rough conditions.
- Generally, they provide higher stern lift, helping in the acceleration of the hull, especially in the back-weighted boats.
- They present lower ventilation rate at the whole speed range, but also in sharp turns.
- They usually have smaller slip numbers.
Our first remark when studying the above advantages is that most of them are due to the bigger blade surface area of the four-blade propellers. This is why these advantages are approximately equal to the advantages of the three-blade propellers with big diameter, as was analyzed in a previous article where we examined the role of the diameter.
On the other hand, four-blade propellers:
- Add more load on the engine.
- Usually have lower final speeds.
- Are more expensive.
- Have less available sizes (as of diameter and pitch).
As we can see from the above, the four-blade propellers obviously excel over the three-blade propellers. At least, this is the general perception for the performance of the four-blade propellers.
