When designing a vessel, the choice of the proper engine and propulsion system is the most critical aspect of vessel performance. Choose right, and you have a finished vessel that can complete its’ intended mission. Choose wrong, and you have a whole slew of performance problems and failed missions. The following list is comprised of situations MarineDiesel has encountered, normally after a problem already exists and we have been contacted to solve the issue:
1. WEIGHT: This is, perhaps, the most common, and critical, issue that we encounter. Naval architects and shipyards often have the habit of under-estimating the finished vessel weight when the vessel is being designed (VERY seldom is the actual vessel weight lighter than predicted). Predicting weight is not an exact science. There are many variables and situations that occur. Often, it is the vessel owner who demands changes to a design after it has been designed, and the yard builds what the customer wants. Note that on a 10m boat, a difference of 1,000 kg can have an impact of 3 or 4 knots (or more) of speed, so the question is not merely academic. As a manufacturer, we can sometimes adjust the engine ECU to compensate, but we are limited by the Laws of Physics by what we can do, in many cases.
2. Inadequate Ventilation: Proper engine ventilation is critical to heat dispersal and engine life. Due to space constraints, this aspect of vessel design is often overlooked. The design of louvers and vents also falls under this category.
3. Looking at Horsepower, rather than Torque: Sometimes, it is a simple mismatch. Most engine dealers are forthright, but sometimes, the pressure to make a sale leads to poor engine matches.
4. LCG: The LCG of the vessel is impacted by engine weight, but also hull design. On every vessel, there is a “sweet spot”: The location that is a precision location for LCG. Poor planning can result, with an impact on speed. This is also related to vessel trim and propulsion choice.
Under powering due to price: Performance costs money. We usually see this more in the recreational market. Vessel owners try to get performance from smaller, cheaper engines: This almost never delivers the desired performance. Additionally, expectations may not be a good match with reality.