Outboard Engine Mounting Height: Does Less Than an Inch Make a Difference?
By Thomas P. — e-Ribbing.com
One of the most important factors affecting the performance of a rigid hulled inflatable boat (RIB) is the mounting height of the outboard engine(s). The mounting height depends on many factors, the most important of which are the type of hull, the propeller and its distance from the bottom of the transom and the boat’s intended use.
In this article we will see how much influence moving the outboard down one hole in the mounting bracket has on a boat’s performance.
Can .79” (2 cm) Change the Behavior of a Boat?
We had a 33’10” (10.3 m) long Technohull 999 RIB with two 300-hp Mercury Verados mounted in the highest position possible, the fifth hole on the bracket. We recorded a full test with the engines at this position. Then we came in, lowered them by one hole and headed back out for another round of testing with the same personnel and fuel levels.
By Setting the Engines One Hole Lower we Noticed:
- A speed increase at low and medium rpm. At 3500 rpm we recorded a boost of 4 knots and at 4000 rpm we saw an additional 5.5 knots.
- Slip numbers were reduced by 50% at 3500 rpm and by 70% at 4000 rpm.
- Reduced fuel consumption through the power curve.
- Quicker acceleration time up to 30 knots by about 1.5 seconds
- Shorter time to plane and lower rpm at which the RIB remained on plane
- Top speed was reduced by 1.4 knots
- Throttle response was stronger and more immediate. This will give an operator more flexibility in the boat’s handling in a wider range of conditions.
- Optimum range increased
- Greater control of the bow because we could trim the engines higher, which will come into play when we run in following seas.
As is shown in the performance charts, there was a slight difference in the load.
