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 apart from the preformance?
One of the major factors that affect the engine’s life is the correct propping.
Once we find the type of propeller that finally matches our boat-engine combination, the next step is to choose the appropriate pitch which will allow the engine to reach its maximum rated rpm.
If we choose a propeller of too short pitch (underpropping) our engine will over-rev, which will have direct impact on the valves and crankshaft operation. However, we face the same risk if we choose a propeller of too long pitch which will not allow the engine to operate at its intended limit of rpm at WOT and can finally cause major damage.
So, we put our engine at risk either by having too short or too long pitch. Therefore, choosing the appropriate pitch is of great importance.
We must select a pitch that would allow the engine to run at its maximum rpm when we are not heavily loaded. In this way we will be able to load our boat as much as we like, while our engine will continue to run at the intended limit of rpm at WOT.
For example, if the recommended rpm range of our engine is 5700-6300 rpm, the pitch we are going to choose must allow the engine to reach 6300 rpm when we are not heavily loaded. So 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 and of course, under no circumstances, will its rpm drop below 5700.
It is true, however, that many boat owners prefer to use longer pitch propeller than the one which is really proper, in order to achieve higher cruise speeds and possibly lower fuel consumption, knowing that their engine does not reach its maximum rpm range.
They even think that in this way, by traveling at lower rpm than they would 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?
Lets suppose we have an 300hp Suzuki engine the WOT range of which is 5700-6300 rpm. With a 18.5΄΄ pitch propeller the engine reaches at 6100 rpm, while with a 21.5΄΄ pitch propeller the engine reaches at 5600 rpm.
With the 18.5΄΄ pitch propeller we can travel at 4500 rpm with 30 knots. But with the 21.5΄΄ pitch propeller we can travel in the same speed at 4000 rpm.
So this is the «trap» many boat owners fall in and they finally 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». This is simply because, while we have lower rpm, the throttle is more open, the engine is overloaded and this can cause severe damage.
The pitch of the propeller virtually bears the role of the transmission in a car.
Having a propeller with very long pitch, as in our example above, it is like starting our car in the third gear or running to an uphill road in the fifth gear. Then the throttle is more pressed down, it takes much longer to accelerate and the engine is really strained at low speeds.
The same happens in our boat as well. Having a propeller with very long pitch, we will need more time to get on plane and more time to accelerate, so we will further be pushing the throttle lever forward.
Our engine then is highly strained at low and medium rpm because a tremendous load is applied on the crankshaft and the pistons.
So we need to understand that when our engine fails to reach the maximum rpm at WOT, it does not operate properly at low or medium rpm. On the contrary, when our engine is able to reach its intended rpm range it will work effortlessly at the whole rpm range.
So, as far as the choice of propeller is concerned, the most important thing is that the propeller must allow our engine to 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΄΄ pitch propeller compared to the 4500 rpm with the 18.5΄΄ pitch propeller.
A proper propeller means a longer engine life span.