Apart from the diameter and the pitch, also the number of the blades of a propeller, the designing interferences of the blades, as well as other variations such as the thickness of the blades, the weight of the propeller, the material are made of etc. have their own share of "responsibility" in performance of a propeller.
Let's see what the Rake is and which its role is.
The term rake refers to the angle of the blades in relation with the propeller's hub. Be careful though, it must not be confused with the pitch, since the angle of the blades in the rake concerns their inclination forwards or backwards in relation with the propeller's hub.
That means that the rake angle is measured on the propeller's hub and it is formed vertically on its hub with the straight line which connects the base of the blade with its tip. Consequently, when the rake angle is 0 degrees, then the blades are upright on the propeller's hub. The more backwards the blades lean, the higher the rake and bigger the angle that is formed accordingly.Rake is negative when the inclination of the blades is forward, i.e. when the blades face the gearbox, whereas it is positive when the inclination of the blades is backwards, i.e. opposite the gearbox.
In most outboard engines propellers have positive rake, i.e. the blades have a backward inclination between 5 and 15 degrees, whereas in some highly efficient propellers it can reach, or be over 30 degrees.
As we trim the engine upwards, the bow of the boat is raised above water, resistance is lowered and efficiency is increased. At this point exactly, the high rake of a propeller plays an important role.
It improves its performance because it keeps the water between the blades and the propeller's hub longer and in this way, the ventilation of the propeller is limited up to a great extend in high trim angles. Then, when our boat runs higher and it is near or out of the water surface, high rake helps the propeller to produce bigger impulse and helps to the elevation of the bow more effectively, having as a consequence the reduction of the hull wetted surfaces and the achievement of higher final speed.
But higher final speed is achieved for one more reason.
High rake gives us the possibility of mounting our engine higher, where the hydrodynamic resistance of the lower unit is smaller, since the last one is exposed less on the water.
In other words, propellers with high rake «work» and perform better near the surface of the water in comparison with propellers with low rake and this happens because they “bite” better in conditions which foster the ventilation of the propeller. Ventilation conditions also appear in steep turns as well as during our handling in heavy weather conditions. Therefore, in these situations the propeller rake also plays an important role.
In conclusion, a propeller with high rake preserves more water and directs it to the propeller's hub, allowing us consequently to mount our engine higher or trim up it more, achieving higher final speed.
How positive rake works
The more we trim the outboard engine upwards, the more our speed is increased. But, there is a crucial limit after which, if we continue trimming up, phenomena of ventilation appear on the propeller.
Beyond this crucial limit, our propeller starts “biting” more air than water and as a result, it is not able to produce thrust. The rpm of the engine start increasing rapidly (since the load on the propeller has greatly reduced because the density of the air is smaller than the density of the water) without increasing our speed though.
And at this exact point, the advantages a propeller with high rake offers to us are involved, which derive from the increased inclination of its blades backwards. Exactly because it takes advantage of the limited amount of water that it catches and sends towards the propeller’s hub, it allows the propeller to hold the water longer and it basically works in these conditions (high engine mounting height and increased trim angles) with more water, always in comparison with a propeller of the same characteristics but with lower rake.
Consequently, a propeller with high rake shifts the crucial limit where ventilation phenomena in higher trim angles appear, producing actually more thrust where other propellers cannot.
Moreover, since it takes better advantage of the limited amount of water, the bow functions better in trimming, rises more, having as a final result the increase of our final speed.
On the contrary, negative rake is used when we want to keep the bow low, finding implementation on boats which conduct special work.