George asked: > I had often read that one should avoid running a diesel engine unloaded > (i.e. without the prop turning) as this will result in glazing of the > bores. This is usually mentioned relative to the idea of running the > engine for electrical needs.
snipped central bit: > Anyway, suppose one had an alternator on their engine capable of > producing 3.75 kilowatts, and a way of using that (washer, vacuum, > etc.) > Then since 1 hp is roughly .75 kw, such an arrangement would appear to > use 5 hp, just the same as normal cruising, and so would be no more > harmful to the engine. > > Tell me where I've gone wrong. A little knowledge is a dangerous > thing. This is a complex subject and I don't think that anyone can give a definitive answer in terms of 'use of an x hp engine at less than y hp for z time will result in bore glazing'. Firstly, you are correct that designing a boat with a huge engine, well in excess of your real needs, will tend to exacerbate the problem because you are running it so lightly loaded. So, stick to an engine as small as you can reasonably get away with.....but remember to allow for those emergency situations where you find yourself punching a tide, maybe. There's only one engine worse than a bore- glazed one and that's an engine that's overheated, or worse still blown up, because you've been forced to flog it hard for hours on end. And, of course, this will almost certainly happen on a tidal waterway where it is less than convenient (master of understatement) to be left with no power ;-))) Secondly, modern engines are more likely to be affected due to the fact that they use highly sophisticated lubricants with lots of additives. If they were running on older grades of lubricants you would probably not get the problem, however the warranty conditions will preclude oil grades below a certain level. So use the lowest grade of oil that the manufacturer will allow (proabably API CF or CG, or even higher by now, CF being a lower spec than CG) Now to the practicalities. Modern boat engines tend to be marinised from respected industrial engine manufacturers, such as Kubota, Isuzu, etc. These industrial engines are designed to withstand a lot of abuse in their normal industrial lives. Anyone that thinks that an industrial engine in a plant and machinery application gets lots of TLC is sadly misguided ;-)) I have found, with my Beta BV3300 (Kubota based 75hp) that generating electricity via a Dometic (Electrolux) Travelpower hasn't been a problem, but I do switch on as many loads as I can (Two immersions, washing machine etc) to give the engine something to work against. Also, with the Travelpower you need the engine revs to be set slightly higher than tickover to ensure that you can generate a decent output so this helps to minimise problems. If I were speccing another engine I'm not sure that I'd worry too much about bore glazing. It's a commonly quoted problem but, perhaps, has become slightly exaggerated and tended to become an urban myth. I'm not saying that it doesn't exist in the case of abused engines, but it doesn't in my experience raise its head as often as its reputation may suggest. In the case of a wide beam boat used on wide waterways I suspect that one would use sufficient engine power to keep the engine in reasonable nick. Roger
