Thanks, Fred and Ward for the good info!
Maybe it is indeed about time to start rebuilding our own system at the end of the 2004 season, since: (a) both the old fuel starvation (too little gravity drop, too narrow feed hose, too small a filter) and engine vibration (touching engine mounts plus worn-out cutless bearing) problems have now been solved; (b) the need for daily battery charging at anchor has been reduced (more solar power, LED lighting and improved refrigeration system); and (c) although I am pretty sure that my 10-year old high output alternator has a 6500 RPM maximum speed limit (according to the manual and the fact that it will soon need it's third set of new bearings) the newer high output alternators do indeed max out much higher (Balmars at approx. 10,000 RPM according to the WM catalog). This leaves the basic question what the optimum operating speed is for our Yanmar marine diesel engines. There is no good reason to disbelieve Yanmar's recommendations for optimum mechanical output performance in the 3000-3600 RPM range (depending on load conditions). In my laboratory we have been running a small Kubota diesel engine for soot emission studies as a function of fuel composition and driving patterns (i.e. differences in rpm and load). From a thermodynamic and mechanical output point of view most smaller diesel engines perform most efficiently in that RPM range. What is very doubtful, however, is whether these relatively high RPM conditions are also optimal for achieving maximum engine lifetime, conserving fuel and producing minimum total soot and other emissions. Also, we all know only too well that -- with our poorly soundproofed and ventilated diesel engine compartments -- both engine noise heat and diesel smell generation can become big factors, especially in poor weather when it becomes necessary to close the companionway.... Since none of us is trying to win any powerboat races, the low RPM domain that Terry, Bill and I as owners of early 90s, fuel-starved Yanmars appear to have been condemned to may not be so bad after all; as long as propellor pitch (i.e. "propellor load") and alternator gear ratios are properly adjusted to minimize coking (carbon deposits) within the cylinders and exhaust system and maximize battery charging efficiency at these lower RPMs. I leave you with one more thought: the latest generation of medium speed (1500-2000 rpm) marine diesel engines is being sold with a 20,000 - 24,000 engine hour revision recommendation....!!! Yes, I know that these are bigger, higher-powered engines that have been optimized for maximum performance at these speeds. However, can anyone name a mechanical rotary device that achieves longer lifetimes and MTBFs at higher speeds? Finally, if you were Yanmar, would you rather have your clients buy a new engine every 3,000-5,000 hours or every 10,000-15,000 hours........???? Season's Greetings Flying Dutchman
