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Engine life and the wrong propellers

The article linked to below was written by David Pascoe on his excellent Yachtsurvey.com site. The brief excerpt below should serve as a warning to those who try to get a few extra knots speed from their vessels “on the cheap”.

Naval architects design a vessel with a power plant and propulsion system in mind. They may have several different options being taken into consideration, but the general resistance, power, and thrust curves are all prominent in their choices. Now, when a boat owner tinkers with the outcome by, for instance, using a propeller with improper pitch, the overall mechanical soundness of the vessel becomes unstable. Engines wear faster, propellers might now be subject to cavitation, or you could make your passengers ill due to excessive vibration.

When you start talking about vessels that were originally designed to go fast, the differences magnify. Yes, a change might see a temporary increase in speed. Yet, an important thing to keep in mind is that even the best-maintained, well-designed vessels see increases in slip over time: starting with the day the vessel is commissioned.

When France Helices designs a propulsion system, we account for some of this degradation when we are making our proposal. However, once the basic construction characteristics of the vessel are changed, either by the yard or the owner, reliability goes out the window. It is not even related to the owner’s knowledge, per se. We are experts in designing propellers and drive systems. It is what we do. Even the most informed and educated boat owner is unlikely to possess that expertise (In much the same way that we don’t design navigation systems… Though we have staff who may be highly familiar with those systems, it is not our expertise.)

We encourage you to read David’s entire article before tinkering… Though written a while back, it is informative and useful information.

Propellering  We ran into three  cases, back-to-back, where the boat’s engines and propellering were not set up properly. Whether the owners had attempted to squeeze more speed from the engines by overtuning or playing around with propeller size, we’re not sure. But in all three cases the boats were found with oversize propellers that had a tremendously detrimental effect on the present condition of the engines. To make a long story short, the engines were overloaded and this overloading ultimately caused the demise of the engines prematurely. By prematurely, we’re talking less than 1,000 hours, and in one case under 700 hours.

Diesel engines are designed to operate at an optimal speed. In order to achieve that speed a boat’s propellers have to allow the engine to operate at that speed at wide open throttle. In these cases, the engines did not turn up to their rated RPM,  the result of the propellers being the wrong size. This caused the engines to run slower and become overloaded. Overloading causes incomplete combustion that, in turn, causes excessive carbon build up (among other problems) that leads to premature wear and ultimate failure.

Just to give you an idea of how acute this situation can be, one of our boats actually had two different types of propellers on it. Apparently the owner couldn’t afford to buy new ones, so he found two used props that were the same size, but had different blade configurations. This caused one of the two engines to become even more overloaded than the other. When the engine survey was completed, the aforementioned  engine had about 3 times the amount of wear as the other. It was ready for an overhaul.

Speed, speed, speed. Everyone wants to go faster. But let me tell you, folks, that there can be an horrendous price to be paid in the quest for a couple more knots. In all three cases, the propeller sizing had been changed from what the builder had originally supplied. This was not done because the boat was initially a poor performer, for all three were plenty fast. No, it was done in the quest to squeeze yet a few more knots of speed for its own sake.

Please read the whole article HERE: http://www.yachtsurvey.com/More_on_Performance_Diesels.htm

Read the original blog entry...

More Stories By John Miele

I am currently Sales and Marketing Manager, Asia / Pacific for France Helices. Created in 1977 by Paul BEZZI, FRANCE HELICES is today an international company. French marine propulsion leader, FRANCE HELICES is also one of the international leaders. Research and development are a very important part of FRANCE HELICES program, the CAD's department is equipped with the latest computer technology, using most up to date programs. Our engineers and technicians determine from the customers specifications and designs, the type of propulsion system to suit the boat's application. They can also advise on the ideal choice of power and optimum gearbox ratio to obtain the maximum thrust. There are many FRANCE HELICES' inovations and patents such as Surface Drive System (SDS) which enable boats to obtain very high speed with excellent handling capabilities and high quality Controllable Pitch Propeller systems (CPP) for both professional use and pleasure application. The constant research using cavitation tunnel testing, guarantees high efficient propeller blade shape for our customers. More than 20 000 propellers per year are manufactured by FRANCE HELICES workshops. In all sizes from small sailing boats to large fishing vessels to navy boats or mega yachts. The FRANCE HELICES workshops are equipped with modern foundries capable of casting propellers up to 3.5 tons in NiBrAl. They are also equipped with CNC milling machines and CNC lathe machines. FRANCE HELICES has four sites which cover the complete range of production. The development of FRANCE HELICES, supported by shareholders places our company as a leader in the international market and insures a constant development, worldwide, in order to be close to the end user.

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