Four Requirements Your Propeller Should Meet

By Thomas P. e-ribbing.com

After endless hours of testing, measurements and observations I can safely say that, among other things, the propeller we will finally choose for our boat must meet the following 4 preconditions at the same time:

1. Match the Prop’s Characteristics to the Engine and Hull

This is one of the most critical requirements that our propeller has to meet. It has to match the hull and the engine at the same time. Otherwise, either the boat’s performance will be poor or the good functioning of our engine will be undermined.

To make it clearer, our propeller must match our engine’s horsepower in relation to its gear ratio and at the same time with the type, length and width of our hull. Additionally, we cannot install just any propeller on any engine. Nor can any propeller push any hull.

There are restrictions defined by the horsepower and gear ratio of each engine as well as by our own hull.

For example, in a low-horsepower engine with a relatively thin propshaft, we cannot install a large propeller, because the load on the propshaft and the gear will be enormous.

On the other hand, each hull requires a certain amount of thrust i.e. a minimum of propeller’s diameter to run efficiently. If our propeller’s diameter is smaller than the minimum required, then, even if we have the best propeller in the world, the performance will be disappointing and the slip values will be high.

2. The Mounting Height of the Engine.

Nowadays, there are many well-designed propellers and if they are properly used they can give us unique performance. Each propeller is designed to run at specific engine mounting heights. Some propellers perform better when running deeply in the water and others when running near the water’s surface.

So if we have a propeller that is designed to run near the surface and our engine is positioned low on the transom, we won’t get the maximum performance from that propeller.

Apart from the propeller design, however, there are some basic principles that define the engine's mounting height in relation to the propeller we want to match. Generally speaking, we should prefer higher engine positions for propellers with a larger diameter or larger overall blade area compared to smaller diameter propellers or propellers with smaller blade area.

3. Match the Use for Which it is Intended.

There is no propeller for any use, no matter how great the design. Each propeller is designed to perform better under certain circumstances and in specific applications. Some propellers are designed to achieve high top speeds, others to get the boat on plane faster, others to achieve high stern or bow lift, others for better fuel economy and others to perform better at low and medium rpm.

But there is no propeller that can do it all together at the same time. So we have to choose our propeller based on our primary goal.

For example: if our main goal is a general use of pleasure or travel, which is the vast majority of most cases, using a propeller with the largest possible diameter and short pitch is the most efficient combination. Of course, in this case we will not get the higher possible top speed, but we will gain tremendous benefits at low and medium rpm, a range in which our engine operates most of time.

If our goal is to achieve the highest-possible final speed, a smaller diameter and longer pitch propeller is the key to success.

Generally, propellers with large diameter and short pitch have great efficiency until almost 40 knots. After this speed, the large diameter and volume of the gearbox lead to excessive drag.

4. The Engine Must Reach, at Least, the Middle of the Maximum RPM Range Recommended by its Manufacturer.

If our propeller does not allow the engine to reach the maximum rpm range, or if our engine overcomes it, the engine’s good operation is in danger, with a great risk of damage.

We must keep in mind that when our engine does not reach its maximum rpm at WOT, it does not operate properly even at low and medium rpm.

Even if one of the above preconditions is not met by the propeller, there is no doubt that either the engine will not operate properly or the boat’s performance will be suffering. However, if the propeller fulfills all of the mentioned requirements at the same time then a great set-up will have been achieved!

Article courtesy of e-ribbing.com.