By Thomas P.

Many of us, at the chase of the ideal propeller, have tried quite many of them looking for the ideal one for our rib-engine combination. Through these tests we have noticed that each propeller has different effects on the rib behaviour.

Each propeller, among other things, lifts more or less the stern out of the water, a fact that we can see relatively easily either by looking at the wake our stern is making when underway or by observing the overall rib’s ride attitude. Definitely a 5-blade propeller lifts the stern much more than a 3-blade one of the same diameter οr, as we use to say, a 5-blade propeller is a more stern lift propeller than a 3-blade one of the same diameter.

If we accept the above mentioned, then we come to the conclusion that each propeller can «carry» the stern of a rib for a certain vertical distance. In other words each propeller can achieve a certain height of stern lift depending on its characteristics, the speed of the rib and our loads.

It is obvious that the propeller's stern lift reaches its maximum value when the tips of propeller blades run out of the water surface.

The question is whether using jackplate can control and determine the amount of propeller's stern lift, and hence the degree of the boat’s stern raise, depending on the circumstances.

The answer is affirmative, since with the jackplate we adjust how deep in the water our propeller will run and so we absolutely control its distance from the water surface. As much as we shorten this distance (i.e. as closer to the surface the propeller runs), the more the propeller’s stern lift is limited.

Therefore, the engine’s mounting height has a direct effect on the amount of propeller's stern lift.
The higher the jackplate, the more reduced the propeller's stern lift.

So if we have our engine very high mounted on the transom we should not expect great hull’s lift from our propeller because it already runs very close to the surface.

That's why we often hear the expression: lowering the jackplate, the boat is raising higher on the water.

The lower the jackplate is, the more the boat comes out of the water.

Therefore, it is obvious, that by using the jackplate we can control the propeller's stern lift. In other words the use of the jackplate can adjust the amount of hull’s wet surfaces depending on the conditions and our goals.

Αυτό This is very important because if we succeed in adjusting the percentage of our hull’s wet surfaces relatively with our loads and speed then we will be able to have the best setup in each condition and get the maximum performance from our rib.

The adjustment of the propeller's stern lift, which is proportional to speed and inversely proportional to our loads, must be absolutely accurate so as to maximize the rib’s performance in each condition.

• Regarding the WOT, the ideal is to raise the jackplate so much high so as to reduce the propeller's stern lift and maintain it at that critical point where only the last hull sections are in contact with the water, achieving the higher possible speed, and, at the same time very good stability.

But, if we have the jackplate higher than the above critical point, then we minimize too much the propeller's stern lift, increasing the wet surfaces of the hull, resulting in increase of the hull’s drag and lowering our final speed.

Instead, if we have the jackplate lower than the above critical point, then the propeller's stern lift is greatly increased and the entire hull comes out of the water, especially when we have few loads. So, without the necessary hull’s support surfaces, very high instability is presented.

All the above mentioned are highly visible to ribs rigged with racing orientation which have the fewest possible loads.

• Regarding the low and medium rpm range, the ideal is to lower the jackplate so as to increase the propeller's stern lift which leads to the raise of the hull more out of the water, the reduce of friction and slippage, the increase cruising speed and fuel efficiency.
...keep Ribbing!

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