A few months ago one of the outboard motors on our 6.2 metre rubber duck had a piston failure. A lean mixture caused the engine failure and we ended up with a hole in a piston. Being a triple cylinder motor it required a complete rebuild and the cost estimates to do this were extremely high. The engine was a little long in the tooth which meant there were a host of other items that would need replacement if the motor was dismantled, such as mountings, so it was not an option to repair.
The boat is in my opinion over-powered. It is equipped with two 90 horsepower two stroke engines, so I looked at the options of either replacing the broken engine with a used engine or replacing both with a smaller pair. The question of two stroke versus four stroke reared its ugly head and this also needed some consideration. I decided to call the manufacturer, Gemini Marine, and ask them what the boat was ideally designed for. I discovered more than I had bargained for.
The boat was originally designed for sea trials for a tender for SA Navy transporter boats. For the sea trials about ten boats identical to ours were made. Each one was fitted with two 60 horsepower two stroke engines, loaded with 14 soldiers and 50 kilograms of gear and and weaponry, and achieved a top speed of 35 knots. The hull and deck is stronger than the usual design strength requirement, and it has a different hull design in that the deadrise is optimal for rough seas. Deadrise is the angle of the bottom of the boat measured from a horizontal line at the level of the keel. The larger the deadrise angle, the more V-shaped the hull is. Boats with smaller deadrise have a much flatter bottom. It’s not a constant angle in most boats. Often a boat is more V-shaped at the bow and gets flatter at the back. The deadrise is significant because it determines whether the hull is to plane or cut through waves.
As the boat is now it is tail heavy with the two 90 horsepower engines, but the performance is outstanding. Empty it will do 43 knots at 5500 rpm, but in all honesty it is more airborne than on the water and almost impossible to control at that speed. When we do replace the motors at some point we will definitely go for 60 horsepower engines.
We did not get the boat with high speed trips in mind, and its primary use is a platform for our students as a calm, no pressure introduction to boat dives. For this it serves us well.
