On Sat, Feb 23, 2008 at 9:55 PM, George Pearson <[EMAIL PROTECTED]> wrote: > I had often read that one should avoid running a diesel engine unloaded <snip> > However, it seems to me that narrowboat engines are usually only > lightly loaded. In the old days, only one horse (one horsepower) was > needed to propel the boat, though nowadays we prefer a little quicker > get up and go. I read somewhere recently that on a normal canal run, > only about 5hp are needed. However, boats may have 20-45 hp engines > installed. Clearly such engines are not heavily loaded, a so perhaps > bore glazing is to be expected with ordinary usage?
Boatman used to recon that one horse on the ground was worth 10(?) in the water. I also suspect this was HP not BHP (so the BHP would be even greater). Although I do always imaging 10 horses swimming in front of the boat when I think of this! :-) It does all depend on how fast you want to go, and while a horse would pull a fully loaded boat at around 3mph, a 9HP bolinder would push the boat faster (where there was the depth). Anyone just staying on the canals can get away with much less power, but as you've already said, River/Tideway running requires more. My SR3 feels underpowered on the tideway, while on Kismet (42BHP) it feels much safer. > The wikipedia page on Diesel Generators says, with respect to bore > glazing, that "Ideally diesel engines should run at least around 60-75% > of their maximum rated load, and at around 75% of their maximum speed." > (An interesting sidelight to this page is that diesel gennies that are > lightly loaded also can get bore glazing). A friend of mine worked for a generator company, and if the generators were run under load it would invalidate the warranty. Many building sites suffer from this problem and will leave lights (and other equipment) on during the day for example to give the engine something to do. > My understanding is that the extra engine power is needed for quick > stopping and river cruising, so engines should not be smaller. > However, I remember that some automobile company had an engine that > would turn off some cylinders when less power was needed. I don't know > enough about diesel engines to know if that would help deal with the > problem - if there in fact is one. I think there are any diesel engines that do this, only petrol ones. Diesels work best when running hot. Still I would think it would be interesting to give it a try. > Of course, maybe it takes so long to develop significant bore glazing > that most boatowners would never see it. While generators are running much longer times, bore glazing can start to effect them after only a couple of months. > 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. Here is a bit of sales blurb from a genset producer website: ************* All IntelliGen systems can operate for extended periods of time at low loads. Under normal circumstances a diesel engine is required to operate at a minimum of 60% of the designed output for extended periods. (i.e. a 10kW/e generator is not designed to operate below 6 kW/e.) Extended operation at low loads can cause engine bore glazing, excessive oil consumption and reduced engine life. The IntelliGen variable speed power systems are designed to always operate in excess of 60% of the engine's power rating throughout the 1200 to 3000rpm range to avoid any such issues. ************* It seems to suggest that under low load conditions, running the engine slower will help. However I also think there is a limit to this, as engines work better when hot. Keeping some revs over tickover to keep heat in the engine will be required. Running a propeller when free running is much lighter work than when moored or towing, but I still think that unless you are going everywhere at tickover (as some do) long lock free sections would be fine for most boats. It's once you are in lots of locks does low load running really start to kick in and I really notice the difference in heat produced from my SR3 when in heavily locked sections (i.e. its running light). > Tell me where I've gone wrong. A little knowledge is a dangerous > thing. > > - george > And a bit more knowledge is even more dangerous! :-) I think the secret is to run your engine with as heavy load as you can if you do need to run it. Otherwise invest in more batteries and use an inverter and large altenater (with a charging controller) to keep up engine load instead of wasting huge amounts of desiel warming up the canal even more (the energy put into the canal as movement will eventually end up as heat (even the noise of the water swirling around!)). Cheers, Mike nb. Entropy -- Michael Askin http://shoestring_DOT_zapto_DOT_org/
