Would changing to smaller injectors save fuel?

[REBrueckner]

Am I mistaken, or is Genesis suggesting running an engine at a somewhat higher RPM (via reduced prop pitch for example) while Brian is recommending running at a lower RPM? Can both be right?

Also, just what does resetting the governor for a lower maximum RPM accomplish? I can understand that it limits maximum RPM to a lower level, but if, as Brian did, he made his props larger, why bother..the larger props will reduce the maximum RPM...Or does such a governor lmit also affect operation at lower RPM levels?

Also, Brian, when you gained 2 GPH with your changes, about what percent improvement was that? And did that come essentially from using larger props or was there some efficiency gained from the engine by running at lower RPM as well?
Thanks....

[q240z]

What I'm about to say might be considered sacrilege on this site, but as the sometimes-resident wooden Chris Craft Connie owner, sacrilege is kinda my middle name around here.

In the ships' logs covering the 2004 Nordhavn Atlantic Rally, vessels reported turning their Lugger mains at 1000-1200rpm @ 5kts, yielding 2.5nmpg or so for several thousand miles. Granted, those are trollers with quite different hull forms compared to our drop-dead sexy yachts, but the message wasn't lost on me. I'm going to be running at 11-1200 for the next few weeks to see what happens to fuel consumption. Oil consumption may also be worth tracking, since it seems to rise with higher RPMs in my 40-year old 6-71s. If I see good results, the props will come off for tweaking to take advantage of that RPM range without underloading the engines.

[Genesis]

Its all a matter of the BSFC curve.

You have to get that to know where you want to be. There's no way around this - you can guess but without that all you're doing is guessing (trial and error), and if you're changing injectors or props, the "guesses" are kinda expensive. Make one or two bad ones and it'll be a looooong time before you save enough fuel to pay for them!

[Brian Degulis]

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Am I mistaken, or is Genesis suggesting running an engine at a somewhat higher RPM (via reduced prop pitch for example) while Brian is recommending running at a lower RPM? Can both be right?

Well WE REPORT YOU DECIDE!!!! LOL

I don't think Genesis is right. The idea behind lowering RPM and going to a larger prop at displacement speeds is just a way to alter an engines performance to be at or near best in that range. At displacement speeds larger props are more efiecent. Also the engine is closer to being properly loaded at a lower RPM both improve efiecency. The downside is you reduce the engines usable HP.

You don't absouloutly have to re set the governer but if you don't and you push the throttle up to high you will overload the engine. I would always re set the governer to prevent that from happening it's very easy to do.

On my old boat I went from 1800 RPM @9.5 knots burning 10GPH to 1450 RPM @ 9.5 knots burning 8.2 GHP This was total burn for both engines

Brian

[REBrueckner]

Whatever Brian did, he sure accomplished some fuel savings...about 18%!! that's substantial.

[REBrueckner]

q240z: What's the luggers rated RPM? Likely pretty low....
Cruising in the 1100 RPM range with your 71 naturals will likely be ok for the engines. I tried that with my 8V71TI's and they tended to smoke more when initially speeding up to blow them out. Whereas it might have taken three or four minutes for all exhaust smoke to clear, after cruising at about 1400 or 1500 RPM for even longer periods, when speeding up smoke is lighter and clear in under a minute...so I returned to 1400/1500 RPM cruising.

I found cruising at about 8 or 9 knots instead of about 11 knots awfully boring on long runs. Ok on short ones. But it was good for reading in open waters...with the autopilot steering I could catch a few paragraphs at a time before resuming lookout duties...

[q240z]

REBrueckner, here's what Nordhavn says about their various propulsion machinery:

Nordhavn 47': Lugger 173 hp (turbo) diesel engine...A ZF 3.96-to-1 transmission and 2 ¼” Aquamet shaft drives a 4-blade, 34” prop slowly, quietly and efficiently.

Nordhavn 50':LUGGER L-6108A / 250 hp @ 2400 RPM & ZF Transmission - Model IRM280PL with a 3:1 reduction gear

Nordhavn 55': John Deere #6081AFM w/ M3 rating @ 330hp @2100rpm & ZF #ZFW320 4:1 reduction Transmission

Nordhavn 57': The Lugger L-6l25A 325 hp engine (continuous duty rating @2100rpm ) has been found to be an ideal choice when coupled with a 3-to-l reduction gear. This gear reduction allows the propeller to drive the vessel at full displacement speed at moderately slow rpms, minimizing prop noise and vibration.


My 52' Connie runs two 270hp Detroits through 2:1 Allison gears, with WOT@2350. At 1100rpm I'm turning between 7-9kts depending on the tide--performance that's not out of line with the Nordies, given my semi-planing hull and two screws turning.

I find that pace relaxing some days and tedious on others. If memory serves, boredom is one of the big challenges of passagemaking. Now, if everybody used wooden boats they'd have plenty of stuff to keep them busy.

[Boss Lady]

large props are more efficient, just about any prop calculator will show that. There is a reason that the new boats have gone to large props and deeper gears. Adding prop tunnels allows them to keep drafts at manageable depths with the larger wheels. If you have a propeller that is 20% more efficient at a given rpm then you would see some fuel savings. Our biggest obstacle to efficiency is our fixed pitch props, they only load the engine at one rpm. We go from way under loaded, to loaded, to way overloaded as the rpm goes up from idle to cruise to max speed. This is why you have constant load rpm ratings and short duration max rpm ratings. Our boats were not propped to run at very low rpms, so the engines are under loaded and will not create enough heat to operate at these low rpms with causing problems like wet stacking, glazing the cylinders, etc. You can change this by putting a more aggresive pitch prop or larger diameter one on, but you will give up higher speeds since the engines will not turn up, once they are over loaded.

If you don't care about going fast, then put the largest wheels you can fit with appropriate gear reduction, or just reduce the governor to make sure that you cannot severely over load the engines.