Does Propeller Choice Affect Engine Longevity?
It is known that the choice of the propeller directly affects the engine’s performance. But how many of us know that the propeller affects the outboard’s longevity as well? One major factor that affects the engine’s life is the correct propping. Once we find the type of propeller that matches our boat-engine combination, the next step is to choose the appropriate pitch that will allow the engine to reach its maximum rated rpm.
If we choose a propeller with too little pitch (underpropping), our engine will over-rev, which will have a direct negative impact on the valves and crankshaft operation. However, we face the same risk if we choose a propeller with too much pitch that will not allow the engine to operate at its intended maximum rpm range at WOT and cause major damage.
We must select a pitch that will let the engine run at its maximum rpm when we are not heavily loaded. In this way we can load our boat as much as we like, while our engine will continue to run at the intended max rpm limit at WOT.
For example, if the recommended rpm range of our engine is 5700 rpm to 6300 rpm, the pitch we choose must allow the engine to reach 6300 rpm when we are not heavily loaded. We will have the option to load our boat with as much extra weight as we desire, while our engine will still reach about 6000 rpm. Under no circumstances, will its rpm drop below 5700.
Pitch Perfect
Many boat owners prefer to use a higher-pitch propeller than recommended to achieve higher cruise speeds and possibly lower fuel consumption, knowing that their engine does not reach its maximum rpm range. They also think that if they had mounted a shorter pitch propeller, their engine will be running under lower “stress” during the trip.
Indeed, when we select a longer pitch propeller than the one that enables our engine to reach at least the middle of the maximum rpm range, we are able to travel at lower rpm with the same cruise speed.
But is This Better for Our Engine?
Let’s suppose we have a 300-hp Suzuki engine with a WOT range of 5700 rpm to 6300 rpm. With an 18.5” (46.99 cm) pitch propeller the engine reaches at 6100 rpm, while with a 21.5” (54.61 cm) pitch propeller the engine reaches at 5600 rpm.
With the 18.5” (46.99 cm) pitch propeller, we can travel at 4500 rpm with 30 knots. But with the 21.5” (54.61 cm) pitch prop, we can travel in the same speed at 4000 rpm.
This is the “trap” many boat owners fall in. They choose the longer pitch propeller, considering that with this propeller they can travel at higher cruise speed while, at the same time, their engine will be running at 4000 rpm and thus under less “stress.”
But in this case, lower rpm does not mean that our engine operates under lower stress. We may have lower rpm, but the throttle is more open and the engine is overloaded. This can cause severe internal damage.
As far as the choice of propeller is concerned, the most important thing is that it must let our engine reach at least the middle of its maximum rpm range at WOT.
This is an inviolable rule if we want to guarantee a long life span for our engine. In our example, the engine will be strained at 4000 rpm with a 21.5” (54.61 cm) pitch propeller compared to the 4500 rpm with the 18.5” (46.99 cm) pitch propeller.
A proper propeller means a longer engine life.