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.

 

[REBrueckner]

" 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."

sorry, but not a valid conclusion: here is why....

A "tired" engine can't rotate an original size prop at rated RPM.

If the port engine turns up proper WOT, I assume about 2300 RPM, then it IS developing full power. If she were NOT developing full power, she would NOT turn 2300 at WOT.....she might only be able to turn a prop load at say 2,000 RPM at WOT.....


It's possible other things are different from original set up .....like smaller injectors with a smaller prop....


As an example, I had a pair of 8V71TI's in my 1972 48 YF. One had a cylinder kit overhaul at my request when I bought the boat, the other did not. Yet both engines turned the same size 28" x 32" four bladed props at about 2150 RPM....the boat was slightly overpropped. Had one engine been "tired", the one without the overhaul for example, it could NOT have turned up the full RPM as the other engine.....That it DID turn up about the same RPM unambiguously indicates it was producing about the same HP are the refurburshed engine.

 

[captddis]

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.

The one with the open bridge was just backing into the slip at the factory when we arrived. Laird said they got 42mph on their radar gun that morning running up the Neuse River. We ran from Lauderdale to St Pete via Channel Five in 12 hrs. About 320NM.

 

[petohazy]

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!

Thanks for that real world info. I should clarify my original post. I am contemplating a 1990's 58 convertible and saw they come with either 12v92s or 16v92s. Both engines at rated power are pushing the hp/cube envelope. My thoughts were to go with a boat that has 16v92s, and detune to 1000 hp (about the same hp as the 12v92s, which are pushing a hp/cube ratio of .93) to lower the stress on the engines. Another positive aspect is being able to run at slow plane (better nmpg) with less possibility of wet stacking issues, because the engines will be turning higher rpms. Also, with the higher rpm at hull speeds, I would imagine the inherent inefficiencies of the detroit 2 strokes at low rpm would be improved to some extent.
Third Hatt, do you know what prop configuration worked for the owner to get down to 1000 hp? What cruise speed did 1700 rpm give; around 18 kts?

 

[ThirdHatt]

Thanks for that real world info. I should clarify my original post. I am contemplating a 1990's 58 convertible and saw they come with either 12v92s or 16v92s. Both engines at rated power are pushing the hp/cube envelope. My thoughts were to go with a boat that has 16v92s, and detune to 1000 hp (about the same hp as the 12v92s, which are pushing a hp/cube ratio of .93) to lower the stress on the engines. Another positive aspect is being able to run at slow plane (better nmpg) with less possibility of wet stacking issues, because the engines will be turning higher rpms. Also, with the higher rpm at hull speeds, I would imagine the inherent inefficiencies of the detroit 2 strokes at low rpm would be improved to some extent.
Third Hatt, do you know what prop configuration worked for the owner to get down to 1000 hp? What cruise speed did 1700 rpm give; around 18 kts?

My pleasure. I'm glad to be able to offer some good info about the engines. Unfortunately, the similarities between the boats end there because the boat that I am referring to is significantly larger so the speed will not be relative. The boat was originally designed as a 20kt boat but the owner cruises ar hull speed (~11kts) everywhere so the boat was detuned and repropped to provide that hull speed is achieved at the "big engine loaf" rpm, 1700rpm.

 

[REBrueckner]

"Another positive aspect is being able to run at slow plane (better nmpg) with less possibility of wet stacking issues, because the engines will be turning higher rpms."

Slower speeds often mean better fuel economy, but as written:Very unlikely...virtually impossible.

Combustion temperatures relate to HP developed, not RPM...Putting in smaller injectors and smaller props simply means you have to run at high RPM than previously to develop a given HP.
"slower plane" takes a certain HP and certain heat accompanies that....using smaller injectors and props won't change that much if at all...efficiency might drop (see below).

" Also, with the higher rpm at hull speeds, I would imagine the inherent inefficiencies of the detroit 2 strokes at low rpm would be improved to some extent."

Don't know what this means; have never heard of it. If you check DD two cycle prop load versus HP developed, via DD factory graphs, you'll find a nearly linear (flat) "curve",,, that is, almost a straight line....You'd have to explain why the curve is wrong and your perception correct. In other words, developing 800 HP takes about twice the fuel as does developing 400 HP.

Anyone with DD data can check my observations: Compare the fuel consumption of the 1,000 HP 16V92's against the 1400 HP 16V92's when, for example, each develops 800 HP... Fuel consumption should be virtually identical assuming both engines are turbo charged...

In fact, the logic proposed by detuning seems contrary to Brian Degulis posted real world test result on his 650HP 12V71TI's in which he INCREASED prop size to reduce cruise RPM....Brian's logic and test results make sense because prop efficiency can be increased with larger props, improving overall fuel economy:

"Props For Efiency And Engine Life When Running Big Engines Slow ecrease RPM..." (April 2006)
C:\Documents and Settings\Owner\My Documents\BOAT STUFF\Larger Props For Efiency And Engine Life When Running Big Engines Slow - Hatteras Owners Forum & Gallery.mht

 

[Boatsb]

"Another positive aspect is being able to run at slow plane (better nmpg) with less possibility of wet stacking issues, because the engines will be turning higher rpms."

Slower speeds often mean better fuel economy, but as written:Very unlikely...virtually impossible.

Combustion temperatures relate to HP developed, not RPM...Putting in smaller injectors and smaller props simply means you have to run at high RPM than previously to develop a given HP.
"slower plane" takes a certain HP and certain heat accompanies that....using smaller injectors and props won't change that much if at all...efficiency might drop (see below).

" Also, with the higher rpm at hull speeds, I would imagine the inherent inefficiencies of the detroit 2 strokes at low rpm would be improved to some extent."

Don't know what this means; have never heard of it. If you check DD two cycle prop load versus HP developed, via DD factory graphs, you'll find a nearly linear (flat) "curve",,, that is, almost a straight line....You'd have to explain why the curve is wrong and your perception correct. In other words, developing 800 HP takes about twice the fuel as does developing 400 HP.

Anyone with DD data can check my observations: Compare the fuel consumption of the 1,000 HP 16V92's against the 1400 HP 16V92's when, for example, each develops 800 HP... Fuel consumption should be virtually identical assuming both engines are turbo charged...

In fact, the logic proposed by detuning seems contrary to Brian Degulis posted real world test result on his 650HP 12V71TI's in which he INCREASED prop size to reduce cruise RPM....Brian's logic and test results make sense because prop efficiency can be increased with larger props, improving overall fuel economy:

"Props For Efiency And Engine Life When Running Big Engines Slow ecrease RPM..." (April 2006)
C:\Documents and Settings\Owner\My Documents\BOAT STUFF\Larger Props For Efiency And Engine Life When Running Big Engines Slow - Hatteras Owners Forum & Gallery.mht

It may be true for naturals but the increased air from the turbos and the smaller amount of fuel should burn a bit more efficient at the lower HP with higher RPM.


if it were my boat ( and it isn't ) I would just run the engines at lower RPM and bring it up every now and then to keep it from glazing. The cost of detuning the engines compared to just running slower should be enough of a reason to not do it. What would you save and how long until you made it back to even.

If it were in the rebuild stage it makes better sense to replace the parts you are already replacing with the lower HP versions as the cost / benefit makes more sense there.

 

[rsmith]

" 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."

sorry, but not a valid conclusion: here is why....

A "tired" engine can't rotate an original size prop at rated RPM.

If the port engine turns up proper WOT, I assume about 2300 RPM, then it IS developing full power. If she were NOT developing full power, she would NOT turn 2300 at WOT.....she might only be able to turn a prop load at say 2,000 RPM at WOT.....


It's possible other things are different from original set up .....like smaller injectors with a smaller prop....


As an example, I had a pair of 8V71TI's in my 1972 48 YF. One had a cylinder kit overhaul at my request when I bought the boat, the other did not. Yet both engines turned the same size 28" x 32" four bladed props at about 2150 RPM....the boat was slightly overpropped. Had one engine bean "tired", the one without the overhaul for example, it could NOT have turned up the full RPM as the other engine.....That it DID turn up about the same RPM unambiguously indicates it was producing about the same HP are the refurburshed engine.

28x32? what gears did you have 3:1 I run 28x 26 1.91:1 and get 19kt @2100

 

[Canuck Dennis]

So, starting and running my car uphill, I should use the highest gear possible, keeping the engine rpm up as high as possible while still trying to get max speed under the load.
I totally disagree with Brians' idea, I ran it past the service Mgr at DDA and he said...easiest way to lose warranty and wear...Have not seen a factory boat put out with those mods, why??

 

[luckydave215]

There seems to be a fundamental misunderstanding here in regards to combustion temperature, and what drives it.
The gas temperature in a given (single) combustion event is determined by the LOAD, regardless of rpm.
An engine run slow, but correctly loaded (Like Brian's), should develop sufficient combustion chamber temps and pressures to keep the rings happy, an engine run fast with a low (per individual combustion event) load will run its hot parts too cold and with low peak cylinder pressure which will contribute to ring sticking and cylinder wall glazing.
While I'm not in any way preaching that old wives tail that Detroits like to be run wide open all the time (I usually run mine at 1050 - 11 knots), de propping to get better fuel economy isn't the answer, pulling back the throttle is.
Running at the correct load for the given rpm is very important, making only 400 hp at the rpm for 800 isn't going to make your engines last longer, just the opposite.
If you want to make 400 hp, prop correctly to make that hp at the right rpm in the engine's rpm propeller curve.
In other words read and understand your manual, the engineers that designed our engines weren't idiots.

 

[rsmith]

I agree with Dave but I know a lot of guys who's old wives were real rats but I cant say they had tails.