It is very well known that every change in propeller’s pitch has a direct effect on the performance of our boat. It is also known that each inch of the pitch «costs» 150-200 rpm for our engine at WOT, a relationship inversely proportional.
As the pitch increases, the load on the engine increases too, since the propeller «grips» a larger amount of water, leading this way to the reduction of its rpm. The reverse happens when we decrease the pitch.
To determine the effect of the propeller’s pitch on the boat's performance as accurately as possible, we tested three exactly same propellers, the only difference of which is the size of their pitch, on a Suzuki 300hp which was mounted on a 930 Marvel rib.
The tests were made with the 3-blade propellers of Suzuki, the pitch’s size of which was 18.5΄΄, 20΄΄ and 21.5΄΄, at the same time, in the same sea area and of course with the same loads.
Looking at the above tables we see that as the pitch of the propeller increases:
- the cruise speed at 4000 and 4500 rpm increases by 2 knots per each 1.5 inch of pitch. With the 18.5΄΄ pitch propeller our speed is 26 knots at 4000 rpm, with the 20΄΄ pitch propeller our speed is 28 knots while with the 21.5΄΄ one our speed is 30 knots at the same rpm
- the engine’s rpm reduces at WOT
- the acceleration gets slower and thus the throttle response reduces
- the slippage increases at the whole rpm range
- the time to plane gets longer and the boat stays on plane at higher speeds and more rpm.
So we see that the change in propeller’s pitch significantly affects the time to plane, the acceleration, the cruise speed, the slip, the fuel consumption and of course the engine’s rpm.
The shorter the pitch is, the faster we will move forward (acceleration) and the easier the engine will raise its rpm.
The more the prop’s pitch reduces the lower the speed in which the boat stays on plane gets, a fact that is particularly preferable when we travel in rough seas and the boat should be held on plane at the possible lower speed.
The more the prop’s pitch reduces the better the behavior of the boat gets concerning the grip, the acceleration, the throttle response, the ability to carry heavy loads and the handling particularly in rough seas.
The shorter the pitch is the easier the propeller will push an overloaded boat or the easier the boat will go up in a big wave.
The speed is directly related to the pitch of the propeller.
The longer the pitch, the higher the cruise speed at midrange rpm.
But the same doesn’t happen with our top speed because from a certain point onwards, i.e. when the pitch of the propeller is too long, the engine does not have the required power to turn the propeller, it can not raise its rpm and the final speed is reduced.
The same propeller can not achieve both the highest top speed and the maximum acceleration. We have to choose which of them we prefer more depending on the use of our boat.
As it has been pointed out many times, the most appropriate pitch is the one that matches to the use of the boat and also enables our engine to reach at least the middle of the WOT rpm range recommended by the manufacturer.
In the above tests, for example, we have to notice that the boat is substantially empty of loads. Therefore it would be better not to prefer the 20΄΄ pitch propeller because when we have the usual loads the engine’s rpm will drop too low – a dangerous condition for the engine- and the performance will be very poor.
Therefore, we should prefer the 18.5΄΄ pitch propeller so as to have the ability to load the boat as much as we want and maintain at the same time our engine’s rpm within the WOT range having also very good performance.