One of the most important factors affecting very much the performance of our boat is the engine's mounting height on its transom. By the term "engine mounting height" we mean the distance of engine's Antiventilation plate from the keel and therefore how deeply submerged in the water the lower unit is. In this way we basically determine the depth to which our propeller runs.
But what is the proper engine mounting height for our boat?
The answer of course is not so simple. Let's take firstly some notes which will help us in our final choice:
- The higher the engine is mounted the greater the speed increases due to less exposure of its lower unit in the water. On the other hand, the thrust of the propeller is reduced.
- The lower the engine is mounted the greater the propeller thrust is, while our top speed decreases.
It is obvious that we can not have both higher top speed and maximum thrust at the same time. So there is no "ideal" mounting height. We must first decide the use our rib is intended for and afterwards to end up whether we mostly need the final speed or the thrust.
For the usual operation of our rib the mounting height of the engine should give us the best combination of speed and thrust. However, we must first be interested in the propeller thrust and then in the speed. Therefore we'd better avoid raising the engine very high.
In this way we will have greater accelerations, will get more quickly on plane and will have better throttle response at low and medium rpm.
All these are absolutely necessary when we are riding in rough weather while at the same time we will be able to control in a better way the bow lift of our rib because of the ability of achieving higher trim angles.
On the other hand, with very high mounting heights we will lose all the above and will only gain higher top speed.
The engine mounting height depends on the type of the hull, the existence or not of a bracket as well as on the prop which will match our engine. There are propellers designed to work better in deep water, propellers which perform better when running near the surface and propellers which perform only when their blades work half out of the water.
It becomes obvious that in order to choose the appropriate mounting height we must take into account all the above factors. And of course only with many tests in the sea can we reach the desired result.
So if you are interested in racing performance you should mount the engine as high as possible, match a propeller which is designed to run close to the surface and go out to the sea for test. But be aware that the boat's performance at low and medium rpm will be less efficient.
But if you are interested in having the best performance at low and medium rpm, getting your boat on plane faster, having greater accelerations, riding better in rough weather, having very good cruise speeds at lower rpm and loading your boat as much as you wish, then you should have to sacrifice a little of your final speed and lower the engine on your transom.
Consider that 90% of the operation of our ribs is at speeds of 25-35 knots and not at 60 or 70 knots. So, for gaining 2-3knots more by setting the engine higher, it may not be worth loosing all the other desirable benefits.