Hi all,
My last boat had Perkins 6.354MGT engines, and my current boat has Perkins 6.354M engines. 185hp in the first case, 135 hp or so in the second case. I've spent a lot of time working with them.
My experience is that these are very straight-forward, old-school diesel engines. They need air and fuel and run on compression. And, if things go wrong or get out of balance, looking at how they start and at the smoke can tell you a lot.
If the fuel is good, the supply is unrestricted, the filters are good, the injector pump is good, and the injectors are good, the fuel gets a nice spray pattern and can burn completely. If, in isolation, something goes wrong with this, one thing that can happen is that the fuel doesn't get a nice spray pattern, gets fewer larger globs, and doesn't burn completely. This presents as lingering white smoke that smells like diesel and often times a slick on the water.
On the turbo models, especially the MGT, the pump over supplies fuel at idle and lower RPMs, which results in some white smoke and possibly slicking at idle -- but the engine catches up at relatively low cruising RPMs and this goes away. This is just a case of the pump and injector combination not having a curve that can align perfectly with the engine, so it was tuned for cruise not idle. So, if the only problem is at idle, it can safely be ignored on turbo models.
If the fuel filters get clogged (or there is some other constriction, or fuel is running low and not supplying well), this can present as being hard to start and/or white smoke. White smoke is possible because if things get really clogged, fuel supply is restricted and the injector pump can't create enough pressure for a good injector spray pattern. Another thing that can happen is that RPMs can surge. The restriction slows the flow of fuel so the governor opens up wider. Fuel rushes in, maybe past a by-pass, and the RPS surge to something appropriate for the governor that is open too much, and the governor closes down a bunch and RPMs fall. Wash rinse repeat.
If everything else is as it should be, but the air is restricted, the fuel will burn incompletely, because there is good fuel spray and enough compression to burn it - but not enough oxygen for the chemical reaction. So, the burning produces less energy and different chemicals as exhaust. The exhaust is filled with carbon and therefore has more dark grey and black. This can happen with a clogged air filter, but that is rare (Perkins don't always have air filters and engine rooms are relatively clean places). That can happen with Turbos if they aren't as efficient as they should be and aren't pushing as much air in as they should, leaving less oxygen than is needed for the fuel. It can also happen during brief periods when the load or throttle are initially advanced, as the turbo, which is driven by exhaust gases from before advance, needs time to "catch up" and develop the boost to push in enough air. It can also happen when the load is too high, e.g. by over propping or dirty bottoms. In this case the throttle is advanced supplying more fuel, but the load on the engine prevents it from getting higher RPMs. So, there is more fuel from the advanced throttle, but not enough RPMs to get enough air to burn it. In other words, the low RPMs from the load lead to a relative surplus of fuel and relative shortage of oxygen.
If an engine has low compression, this can show up as excessive white smoke, black smoke, or more likely both -- grey smoke. Basically, if compression is bad, fuel could be left unburnt or be burt badly.
If injector timing is off, the engine may be hard to start or produce white smoke, or both. Basically, injecting fuel too late or too early in the cycle will lead to it not all burning. On the 6.354, the timing is most commonly adjusted at the injector pump, but it can also be adjusted at the auxiliary gear and the auxiliary gear shaft.
Blue in the smoke is, as one would expect, motor oil working its way in, and burning.
Valve timing being off can lead to white smoke, especially if they are too tight. It chances the timing of the compression cycle, leading to fuel not being burned.
White smoke that doesn't linger and doesn't smell like diesel is likely coolant or raw water, e.g. a bad cooler or head gasket.
With no load, the max RPM is controlled by the governor setting on the injector pump.
So, if we take a naturally aspirated 6.354 engine and start it in moderate weather, we should see some smoke at the slip. They aren't precisely dialed in engines, ever. In a naturally aspirated engine, it is likely to be grey in color, which is a little white and a little black mixed. On a turboed engine, there is likely to be more smoke and it is likely to be more white in character -- more unburned fuel to the same incompletely burned fuel.
At cruise, this smoke should totally go away. In part, the engines should run cleaner. But, in part, the small amount of smoke is just spread out over distance instead of staying in one place. If one sees smoke at the low end of cruise, something is up. The color can help to diagnose it.
If the boat has enough more horsepower than it really needs, e.g. the 6.354MGTs in my old boat, as the boat gets past cruise range, it starts pushing into and climbing up the bow wave more and more. This load limits RPMs. The smoke starts to get darker and darker as one opens the throttles more and more. The boat can go faster, but runs dirtier and dirtier as it runs less and less efficiently.
The point at which this happens can come sooner if a load is imposed by a dirty bottom or over propping. But, the tell-tale for overloading is that smoke that was absent, white, or light grey becomes dark grey or black and much more prominent.
So, in my mind, one wants to get the boat cleaned really well and take the boat out and up to WOT. At this point, one should see some grey or black in the smoke. If not, the boat may be underpropped, underpowered, or have the governor set wrong. Underpowered isn't a problem. The governor set wrong can be adjusted by turning the screw at the slip. And, underpropped by a little isn't a big deal.
The next thing that one wants to do is to back down the throttle until the smoke, or at least the blackness/greyness, clears up completely, or as close to completely as it will. This will probably be about 80-85% of WOT RPMs. Now you've found your cruise speed.
If one has to lower RPMs much more than that, then the boat might be over propped. It could also be an older engines, be lower on compression, and just like to run at lower RPMs. But, even in this case, reducing prop pitch might let it run cleaner at higher RPMs.
Also, the entire underway part of this discussion assumes the transmission ratio is right for the intended use. Obviously, if that isn't matched, the boat can end up in the wrong RPM band for the load.
The boat that I own now had the same owners for many, many years. They cruised it very extensively at about 8kn. This is apparently what the prior owners had told them was its cruise speed. It is a little surprising since its hull speed is right about there, so one might assume a faster cruise speed. The next owner, who had the boat for about 3 years, wanted more speed out of it. He had the engines rebuilt and the props refurbished and dynamically balanced, etc. It cruised for him at -- about 8 kn. I bought the boat, figured it might run faster if only I wished it to, gave it a good tune up and took it out. You know where it wanted to cruise for me? 8.3kn.
Why is that an 8.3kn boat, not a 9.3kn boat? I don't know. Maybe I'll figure it out and understand. Or, maybe I'll figure it out and tweak it. But, probably not. The history is that it has been an 8kn boat since it was new. Most likely there is something about the hull shape or rudder drag or whatever that I don't understand. What I can say is this --
it runs nice and fuel efficient at 8kn, which is apparently what it was designed to do.
So, what I'm suggesting here is that:
1) I agree 100% with the suggestions to get a phototach to check RPM and use GPS or some other tool to check speed, as the speed instrument and tach gauge can easily be off. Good data is essential.
2) I wouldn't expect a Perkins 6.354 of any kind to reach it WOT (idle) speed underway. I've always seen the engines to have lower WOT RPMs under load than at idle. And, I've never seen black smoke to indicate this is a problem.
3) If true, that RPM does seem high to me and that speed does seem low to me, so I'm wondering about /under/ propping (not over propping or a dirty bottom).
4) There has been no discussion about the smoke or RPM stability here. And, that is really important data, especially the smoke color and volume at the slip, underway, at WOT, etc.
5) See #1. It is worth reading twice.
Cheers!
-Greg