Some thoughts on the discussion above and Perkins 4108
If you really want to get into the mysteries of propping a boat, you could go to www.boatdiesel.com
and look at all the discussion and articles on propping from our powerboat friends. If nothing else, reading about the discussions of fuel
burns in the range of 30-40 gals per hour makes me feel lucky.
However, my 4108 in the 20,000 # displacement Gulfstar
37 is deliberately overpropped (3 blade 15 diameter x 12 pitch) so as to hold down the rpm and noise
needed during normal operation - at 1850 rpm and a clean bottom I get between six to six and 1/2 knots, burn less than 3/4 gal hr, and the noise
is minimal...which is why I sail, but sometimes you need to get somewhere so you motorsail. My Perkins works great when you have 5-7 knots of wind
but need to get somewhere...about 1200-1500 rpm while sailing makes a huge difference - run the engine up to 2000 rpm about once an hour for a few minutes keeps the carbon and cylinder glaze from building up and burns little fuel.
I know it is this way, and I am happy with it. Theoretically, a boat should be propped so it will run the majority of the time at about a 60-75% load factor for a diesel engine to optomize engine life and diesel burned to power produced ratio. In practice, I like the quiet and having owned other boats, found that running the diesel at high speed made more noise than I wanted to deal with long-term.
I can run the engine up to about 2800 rpm propped this way, and the boat will move accordingly, but it will slowly but surely overheat if run this way for more than 15 minutes or so.
So if one was offshore
they would prop more conservatively since they might need high power levels for storms or such for long amounts of time, but for my sort of sailing - mostly daysailing or overnighters in good weather
, this works fine.
A Perkins, because of its design, when run at high rpm and high power load factors, will probably not have the same life as most of us are expecting from our relatively low power
factor applications in a sailboat. More power=shorter life..its all a function of the amount of fuel consumed during the operation of the engine. The 50 hp rating quoted was generally only available for machinery or automotive type applications, where the 50 hp is needed for only short periods of time, not continously as in a marine
application. Most writers on the Perkins agree that 25-30 hp continuous is about what you should expect from a Perkins 4.108
for long life.
I had a 4 cylinder 1967 perkins installed in an agricultural application that literally ran all summer for years pumping water
...rebuilt it once in 1978 or so, not because it needed it, but because I couldn't believe after running all those hours it didn't need it. I sold the farm the other day the pump is located on, the new owner changed the oil
, put fresh diesel in the tank( it had not been run for 10 years or so) and fired it right up - after bleeding it of course, as any perkins owner can tell you is a PITA. I can't imaging a Yanmar
or any other modern diesel doing the same thing after sitting for so many years.
If my 4.108 dies, I'm putting in a rebuilt one - assuming parts
are available in the future, it is a great sailboat engine if cared for properly.