By Thomas P.

As we have seen in the previous article entitled «Engine’s mounting heigh», one of the most important factors affecting a lot the boat’s performance is the engine’s mounting height on its transom.

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The engine’s mounting height depends on many factors, the most important of which are:

  • the type of the boat’s hull
  • the propeller which we will match our engine
  • the propeller’s distance from the bottom of the transom
  • the use our rib is intended for

In this article we will see how much the engine’s placement at one hole lower can affect the boat’s performance.

Can two centimeters lower change the overall behaviour of our boat?

So we tried a Technohull 999 on the transom of which two Verado 300hp were mounted. The engines were initially placed on the highest possible position, i.e. at the 5th hole. After the tests, we noticed our measurements in the above position and then we set the engines by one hole lower going again for the new test.The comparative results of our measurements were particularly impressive:

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Engine’s mounting height on 5th hole pdf

 

Engine’s mounting height on 4th hole pdf

By setting the engines one hole lower we noticed:

  • an impressive increase of cruise speed at low and medium rpm. Specifically, at 3500 rpm we took 4 knots more and at 4000 rpm 5.5 knots more
  • an impressive reduction of slip numbers which numbers were reduced by 50% at 3500 rpm while at 4000 rpm were reduced by about 70%
  • a reduction of fuel consumption at the whole rpm range
  • a reduction of the acceleration time up to 30 knots by about 1.5 seconds
  • shorter time to get on plane and lower rpm at which the rib remained on plane
  • a reduction of the final speed by 1.4 knots

As we will see in the above detailed tables, during our tests, there is a slight difference in the load which is however not capable to alter mainly the results of our measurements.

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Translating these data we see that the engine’s lowering by one hole improved the overall boat’s behaviour significantly with the only exception the reduction of the final speed by 1.4 knots:

  • the rib got on plane faster and stayed on plane at lower rpm which is very important when travelling in bad weather.
  • throttle response was more noticeable and more immediate giving us more flexibility in boat’s handling which is particularly necessary in rough sea
  • we are able to travel faster keeping at the same time our engines working at low rpm
  • the optimum range increased
  • we have greater control of the bow, due to the ability of trimming our engines higher, thus very important when following seas start to get really big.

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We can always have a general rule in our mind which applies to any boat-engine combination:
When we mount our engine too high we should expect high slip numbers at low and medium rpm, which indeed tend to increase as much as the loads increase.

It is clear that the «Higher is Better» applies only when our only goal is to increase our top speed.

In our case, however, the rib is used for pleasure, carrying much more load than our test. Therefore, it is worth sacrificing a little from our top speed and mounting the engines lower by one hole so as to maximize the performance and behaviour of the rib at low and medium rpm.

We could say that the most main factor that will determine the mounting height of the engine or engines on the transom of our boat is the use is intended for!

...keep Ribbing!