Derating 16v92s for longer service life

[petohazy]

Wondering if it is feasible to significantly extend the service life of 1350 hp 16v92s, by derating them say down to 1000 hp (using smaller props with less pitch). This would put the hp to cu in ratio from .92 down to .68, which per conventional wisdom should predict a longer service life. Any opinions on how many hours I could expect to gain, over the typical 2500 hours between overhauls? I also like the idea of slowing down a bit at 1800 rpm cruise speed. According to the boatdiesel prop calculator, at 1800 rpm, my cruise should go from 22kts using 82 gph (.27 nmpg) to 19kts using 60 gph (.32 nmpg). Max speed should drop from 30 kts to 26 kts, which is plenty for me on a 58 convertible.

[TopHattandTails]

Have you, by chance, calculated what your "big engine loaf" cruise rpm is? Conventional cruise rpm is 90% of wot. With big engines I have found that there is a "rated" cruise (say 2,050 rpm) and a big engine loaf (say 1,825) that actually is the sweet spot and produces a slightly less speed (2 knots or so less), but produces significantly better efficiency (say from 160gph to 132gph on a 16v2000 I run). Might be a similar trade off for your engines that would get you close without swapping injectors and blades.... the "loaf" rpm is actually much more efficient with speed and distance concerning a gph calculation (without going off plane or turbo).

[Fanfare]

You can pretty much extend the lives of any engine by just slowing down a bit. This effectively derates it while preserving all the options built into it by the engineers and marine architects who designed your boat. These guys put a lot of thought into all the parameters to come up with the optimal compromise of performance and economy for the particular vessel as built. Everything on a boat is a compromise. Length, width, draft, weight, horsepower, etc. Using esoteric materials (kevlar, carbon fiber) can save weight but really runs the cost up. The newer electronically controlled common-rail diesels give better mileage, but at a cost of greatly increased complexity. We don't know what the longevity of these more efficient new engines will be, particularly when compared to our old reliable Detroits, which seem to soldier on year after year. The new engines are so expensive that moving up to one just seems not to be cost effective--we will never get our money back based on improved fuel economy. While we here on this forum, as amateurs, can debate prop size and injector ratings, we have to realize that in most cases we cannot improve much, if at all, on the experts--the boat's designers.

In general, we get our best mileage just a tad below our boat's hull speed, determined by the formula 1.34 x (square root of the waterline) of your vessel. This is the formula for a displacement hull. In choosing to go slowly we must be sure our engines are working hard enough to maintain a certain minimum temperature, usually around 175-180 degrees so the rings don't carbon up and permanently loose efficiency. Many Hatteras yachts are planing hulls and with more power will climb onto their bow wave and go faster, but at the cost of vastly increased fuel usage. Even with this there is usually a "sweet spot," usually a bit above planing speed, but not yet at full throttle which is the best compromise between speed and fuel consumption at the higher speed of being on plane. While it is fun to debate these factors, it seems that these are the best two ways to ensure economy and long life for our engines--slow down a bit, or slow down a lot.

[saltshaker]

Wondering if it is feasible to significantly extend the service life of 1350 hp 16v92s, by derating them say down to 1000 hp (using smaller props with less pitch). This would put the hp to cu in ratio from .92 down to .68, which per conventional wisdom should predict a longer service life. Any opinions on how many hours I could expect to gain, over the typical 2500 hours between overhauls? I also like the idea of slowing down a bit at 1800 rpm cruise speed. According to the boatdiesel prop calculator, at 1800 rpm, my cruise should go from 22kts using 82 gph (.27 nmpg) to 19kts using 60 gph (.32 nmpg). Max speed should drop from 30 kts to 26 kts, which is plenty for me on a 58 convertible.

Is this a Hatteras 58C? If so I would like to talk to you about performance and fuel burn. I am interested in a 58C but don't have any first hand knowledge.

[REBrueckner]

If you fool around with props, just cut the pitch not the diameter to retain best prop efficiency.

But as noted above, all that does is lighten the prop load a bit, reduce top speed without exceeding rated RPM, and is about the same as running the engines at a slower cruise RPM. I agree with Fanfare, if you want to extend the life of an engine run it in the 1500 to 1900 RPM range when rated RPM is 2300. Don't run 2000 RPM or above under normal circumstances for extended periods. Backing off some RPM for normal cruise is normally the easiest and most direct way to increase fuel economy....

Industrial detroits that are often run at 1800 RPM on say electrical generators go easily 10,000 hours...often a lot longer.

Modest cruise RPM, frequent use, proper oil change intervals, attention to operating temperature, and clean fuel will extended the operating life of an engine as much or more than any tinkering....

[ThirdHatt]

I spend alot of time on a 16V92TA powered boat that is actively cruised. The 16V92's are great, reliable and powerful beasts. These particular engines were originally rated at 1450hp but were detuned by the current owner when he bought the boat in 1999 down to 1,000hp to increase longevity. After being detuned, the boat likes to run at about 1700rpm and burn 55-60gph. They lasted about 4600hrs and were beginning to show signs of wear and oil consumption was creeping up so they were majored again just this past summer. With this example, the answer to your original question is that you can reasonably expect to add 2,000+ hrs to the life expectancy of those engines if properly detuned to 1000hp, never spend any real time above 2000rpms and good maintenance.

GOOD LUCK!

[captddis]

Is this a Hatteras 58C? If so I would like to talk to you about performance and fuel burn. I am interested in a 58C but don't have any first hand knowledge.

I delivered two 58C's in 1990. Both had 16v92's , one open one enclosed. Fun boat to run and awesome acceleration. 28 to 29 kts@2000 138GPH.
They were breaking the 3.5 in shafts but I believe they resolved that issue.
The EB was an aluminum enclosure and there were some issues with breaking windows on the EB. Not sure how that was resolved.

[REBrueckner]

Thirdhatt: great post....

Any idea how many hours were put on the 1450 HP variety before they were detuned? In other words, was much of that 4600 hrs time put on them was when they ran at high HP and maybe high RPM??

I see no reason why such engines run at 1700 RPM and used frequently would not last significantly beyond 4600 hours....yet the marine environment is tough...

[saltshaker]

I delivered two 58C's in 1990. Both had 16v92's , one open one enclosed. Fun boat to run and awesome acceleration. 28 to 29 kts@2000 138GPH.
They were breaking the 3.5 in shafts but I believe they resolved that issue.
The EB was an aluminum enclosure and there were some issues with breaking windows on the EB. Not sure how that was resolved.

Thanks Dave. I'm looking at a '90 open bridge boat. Performance is much slower than what you saw on the new boats. She only runs around 23kt@2000 and 27kts WOT. Survey report showed 20kts @1800 and 27kts@2310. I think the port engine is tired and the difference in performance from new would support that. She is a beast but with beautiful lines.

[ThirdHatt]

Thirdhatt: great post....

Any idea how many hours were put on the 1450 HP variety before they were detuned? In other words, was much of that 4600 hrs time put on them was when they ran at high HP and maybe high RPM??

I see no reason why such engines run at 1700 RPM and used frequently would not last significantly beyond 4600 hours....yet the marine environment is tough...

The detuning was done at the same time as the complete teardowns in 1999. They would have lasted longer, it's just that one of the blowers had a bearing issue and when it came apart some metal shavings made it down into a few cylinders requiring a partial teardown. The owner had recently finished a complete refit of the entire boat (excluding the engine room) so he chose to take the opportunity to strip both mains back down to the blocks and rebuild properly. The Northern Lights gensets (installed new in 2002) are being rebuilt now. They made it to 12k hrs each which I thought was a little low for Luggers.