Frank, there is a clear cut procedure for evaluating the prop size and engine performance. The soot could be indicative of a problem, one of which is an over sized prop. Constant generation of soot can lead to coking of the exhaust, which in turn increases back pressure and other problems. If you can't borrow a handheld tach, you might want to hire a marine diesel mechanic for the task. Also, you can get a handheld laser tach off of Ebay for $25 plus shipping.
As it has been explained to me by my mechanic, you need to establish the no load max rpm by briefly revving to max rpm in neutral and putting a tach on it. After reducing to idle, run the vessel at full throttle in gear until the boat speed maximizes. If the loaded max rpm is within 10% of the unloaded max rpm, the prop is properly sized. Then your cruisng rpms should be 80% of the loaded max rpms. The max unloaded rpm can also be taken from the engine manual. Having established the cruising rpms is much less helpful without installing a tach for reference when you are cruising.
In case you didn't know, the standard tach for the Universal diesel is about $145. Installing a tach is relatively uncomplicated except for calibration where once again a handheld tach is needed. Operating without a tach its hard to determine proper cruising speed since it will vary with conditions (waves, wind) that you will have to interpret with your throttle setting. Using boat speed to set cruising rpms, you will need to somewhat discount the expected boat speed such as when going into a strong wind or seas, but how much ?
Our first boat had a smoking problem which a mechanic determined was from an oversized prop. When we checked, it indeed had an oversized Martec folding prop. The previous owner came up with the original Michigan sail prop and all was well.
I've never heard that running too slow is a problem. On the other hand, running the diesel to fast will wear it out faster. For our M25XP I think the manual recommends 2400 or 2600 for cruising with a max unloaded rpm of 3200. We cruise at 2400 at 6 knots.
Frank Langer said:
Without having a tach on our Ericson 30+, how can one tell what the best/most appropriate speed is? I have been told that running too slow leads to carbon in the diesel (and doesn't get us to the destination), but too high RPMs could place too much stress on the engine. Any advice for the Universal 18 h.p.? At this point I let it warm up 10 minutes, then bring to full throttle and back off about 10% or so as the cruising speed at about 6.0 - 6.2 knots. I did end up with a bit of black soot on the transom.
Thanks,
Frank.