Would changing to smaller injectors save fuel?

[Genesis]

I wouldn't touch the engines.

If you want to run at a somewhat-higher RPM, de-pitch the props. That will accomplish unloading the engines while leaving the reserve there, and its cheaper to put the props back than it is to put the injectors back.

[Brian Degulis]

Actually if you adjust the governer to a lower RPM you would actually require a larger prop not because the HP is reduced but because the prop RPM is reduced. Remember if you double the prop RPM you must quadrouple the HP to drive it. Of course if you don't change the RPM and reduce the HP then the prop must be smaller.

On de rating to a lower RPM you not only get the efiecency of a larger prop you also run the engine at a lower RPM for the same cruise so you get some savings there remember an engine uses fuel even when it's just running itself like idle neutral.

The other gain is that de rating or lowering max RPM puts the engine closer to rated load thru the entire RPM range. Running at or near rated load gets better efiecency than being grossely under loaded as is the case with most of our boats at displacement speed.

This whole idea is very misunderstood but if you look at the prop and engine power curves you can see how it works. My last boat was a 55' trawler with 671 naturals rated 240 HP @ 2300 RPM I derated to 200HP @ 1800 RPM. The original prop size was 27X24 if I remember corectly? And the derated engine called for a 30x22. When I did that my savings in fuel @ a 9 knot cruise was just under 2 GPH. My cruise RPM went from 1800 to 1450 so it was quiter and hapier at the lower cruise RPM. Of course the downside was I lost the use of 40HP per engine. That didn't matter as it was HP I couldn't or wouldn't use anyway.

It's basically customizing and engine for your intended use.

Brian

[Boatsb]

If fuel = energy than the lower amount of fuel would = less energy. Just running at the lower RPMs will save fuel and even though there may be a very small savings in fuel use at a slow speed I don't believe it will offset the cost of the injectors in the life of the engine. Add to that the etra costs of re tuning, Maintaining a non stock engine and I see a loss. If it can be de tuned to another version that was avaialable itmay be worth it but based on pure physics I see not savings coming that way.

[Boss Lady]

If you intend to only run the boat within certain parameters, then really look at the most efficient set of props for your intended rpm range and power production within that rpm range. To do this right, you will need to install the smaller injectors and retune, then install larger diameter or more aggressive pitch props to fully load the engine at the new WOT rpm and cruise rpm. This will be the same as installing smaller engines if you derate the HP of the engines. You will get better mileage, but it is mainly from the more efficient props. To be honest you could skip the smaller injectors and just reset the govenors for the lower rpm and install wheels that will fully load the engines at the HP/rpm. Start by looking at the HP/prop load curves for the 8v71Ns that you have. To the best of my memory the curves look something like 120 HP available at 1400 rpm and prop loading is only 80 hp, so you could pick up a couple knots of speed but would never reach the old WOT. This would be the cheapest route to go, and you would only be out the cost of the wheels, and if you don't like the result then just sell them and put the old ones back on.

[Pete]

So far in this discussion, the engine cooling system has not been mentioned. The amount of cooling sea water going through the engine is directly proportional to the engine RPM. More horsepower at a lower RPM equals more heat with no additional cooling water. Detroits do not have excess cooling capacity built in so this is another factor to consider. Engine loading on the propeller curve is what keeps everything cool at cruising speed. Loading at maximum available horsepower at any given RPM will give the same marginal cooling results seen when running at WOT on the propeller curve.

Pete

[Boss Lady]

Good point Pete. However on these Naturals, the power produced at 1400 rpm is 1/3 the rated power, so even if fully loaded you cannot overheat them. I guess the amount of raw water being pumped should be verified just to make sure, but I do not see a problem IMHO.

[REBrueckner]

I'd really like to understand the correct answer to the original question better.

Here is what I think I understand:

(a) Original question: Fuel consumption at the same cruise/hull speed will not be reduced by 21% just because the injectors are that much smaller.

(b)To maintain prior cruise speed with existing props, cruise RPM would have to be increased if smaller injectors are installed.

(c) Larger props are more efficient than smaller.


Brian's post about his trawler (#12) is very interesting because he ended up swinging larger props at lower RPM. Was cruise/hull speed maintained. I can't explain this;it sounds like magic! What does "detuning" mean here?

I'd like to hear more since in general just using smaller injectors should require higher RPM to achieve the old cruising/hull speed with even the same size props, let alone larger ones.

[Genesis]

Using smaller injectors with no other changes will do very little to change fuel consumption. You will still need the same RPM and you will still produce the same horsepower - the injector helix will just be further advanced.

If you make other major adjustments (e.g. less pitch in the wheels, etc) you may be able to get the BSFC curve into a more advantageous range, which WILL improve fuel consumption for distance travelled. To know, you'd have to have a BSFC map .vs. output and RPM - I don't have 'em for that engine.

[Eventide]

Great feedback and great ideas, guys. Thank you very much. I think I love what Brian did with his former trawler, de-rating his 6-71s and changing out his props. Doing that he improved his fuel consumption by about 2 gallons per hour, which is significant over a long distance. Thank you Brian!

If there was some way to do that with my 8-71Ns, or maybe even reduce fuel consumption a little more since they are larger engines, it would be great. I would also be happy with lower RPMs, like Brian, and with maybe going a little slower, perhaps 1.1 times hull speed over long distances, or about 8 nmph.

In other words, I'm very flexible about how I get there if there is some way to materially reduce fuel consumption. I have so much invested in this boat in terms of systems, effort and love that I want to do whatever it takes to keep her until I start drooling, at which time I want to be pushed over the stern into the Gulfstream to be part of the food chain so I won't have to go into a nursing home and into that long, tortured decline toward the black abyss (but that's a whole 'nother topic, I visit the nursing home every day and have for the last 15 months and it is truly a living nightmare).

Again, thanks for all your thoughts.

Of course, per Karl's post, this might all be for naught if we did a really hard cost/benefit analysis, but that is for another day. Right now I am getting a terrific education and we're all just brainstorming.

[Brian Degulis]

I think I would re set the governers for a lower max RPM I went from 2300 to 1800 because 1800 is the work boat continious duty RPM. But you could go even lower just remember the lower you go the larger the prop has to be (you need the hull clearance) and the more you reduce the usable HP of the engine.

I wouldn't go to the 55 injectors the improvement from that would proubably not be measurable.

Brian