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.

[REBrueckner]

"The gas temperature in a given (single) combustion event is determined by the LOAD, regardless of rpm."

absolutely correct.

I posted the same concept from a slightly different but equivalent perspective: "Combustion temperatures relate to HP developed, not RPM.."

And if you look at any DD prop load versus HP curve you can see the equivalency: At a given RPM, HP developed on one curve maps exactly to a prop load on the other. For a given size prop they are locked together.

You can also understand both concepts, Load versus HP, this way: Say you run your engines at near idle neutral in the slip, lets say 700 RPM; those engines have virtually no load, are developing little HP as a result and burning little fuel...put those engines in gear and they should slow down from the increased load turning the propellers ....now push the throttles forward to resume 700 RPM...., the rack is advanced allowing more fuel to be pumped by injectors developing more HP and as a result more heat by products....your engines warm up faster....

[REBrueckner]

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

yes, four bladed. It was 1.91:1 reduction also. That's what the props were stamped, but who knows if they were ever changed??

Not knowing what engines nor boat you have I have little to contribute but a bigger heavier boat than mine, for example, with the same engines might require smaller props to get to rated 2300 RPM and HP....

My YF was originally clocked by HATT when new at 22.3 knots WOT. That seems about right, over 30 years later, because she did just over twenty knots at 2150 RPM when I owned her so I was not developing quite the rated HP.

[rsmith]

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

yes, four bladed. It was 1.91:1 reduction also. That's what the props were stamped, but who knows if they were ever changed??

Not knowing what engines nor boat you have I have little to contribute but a bigger heavier boat than mine, for example, with the same engines might require smaller props to get to rated 2300 RPM and HP....

My YF was originally clocked by HATT when new at 22.3 knots WOT. That seems about right, over 30 years later, because she did just over twenty knots at 2150 RPM when I owned her so I was not developing quite the rated HP.
I'd like to see a GPS screen shot of that because a 46c (lighter and less superstructure windage)with the same engines wont do anywhere near that.
and my 50c with 12/71's struggles to make 19@2100. Where did they do the speed tests off the edge of Niagra Falls?
Maybe Ohana Dave is right "the older I get the faster I was"

[REBrueckner]

rsmith....can't answer for other boats performance, but there are several early '70's YF owners on the forums...at least one with 8V71TI's....maybe they'll post their experience.

Your 50C with 12V71's matches the performance I have heard, but my 48ft is way lighter especially the engines....and I guess narrower,too...

My old Hatteras brochure about 1971 shows 36,000lbs for my 48YF and 47,00 lbs for the 53C...

I know my boat actually weights about 44,000 lbs so I suspect that 53C weight is way too light... anyway, 47/36 is 31% more weight for the 53C plus another 1ft 5" beam (10% more) and 5" extra depth....that doesn't come free....despite greater length...

It's those characteristics that made me decide to NOT buy a 50C many years ago...that and the sheer size of the the 12V71's...

[REBrueckner]

On the original issue: derating any DD engine is exactly what is done for workboats and crew boats. Such engines have lower RPM governor settings, that is lower top RPM, and smaller injectors and lower corresponding HP ratings with continous duty specifications....

All that is fine, but running an engine at lower RPM in a recreational boat accomplishes virtually the same thing at no up front cost.

[Boatnut]

On the original issue: derating any DD engine is exactly what is done for workboats and crew boats. Such engines have lower RPM governor settings, that ias lower top RPM, and smaller injectors and lower corresponding HP ratings with continous duty specifications....

All that is fine, but running an engine at lower RPM in a recreational boat accomplishes virtually the same thing at no up front cost.

Then why all this discussion on derating engines when all you have to do is pull back on the throttle. Something doesn't make sense here.

[krush]

Then why all this discussion on derating engines when all you have to do is pull back on the throttle. Something doesn't make sense here.


Go find the post I made with the prop HP curve vs the engine HP curve. non-linear vs linear.

[madhatter1]

Why not just under prop a little and keep the RPM's a little lower? I may be missing something but this seems like a simple way to take a load off the engine without detuning. We all watch our RPM's anyway so overspeed should not be an issue. Just thinkin.

[Genesis]

Yep - just underprop it a bit.

The governor will prevent overspeed, and you will have effectively reduced load.

This is the easiest way to do what you're looking to accomplish.

[krush]

Why would you underprop if you want longer life, just reduce how far you push the throttle.

Now if you want better efficiency at slower speed, you can increase pitch or prop size to load the engine properly at lower RPM--but you must reduce max RPM on the gov (high idle) to prevent overloading.

[rsmith]

You might want to go with this option if your serious about derating your power. You can always add more oars later

[saltshaker]

Why would you underprop if you want longer life, just reduce how far you push the throttle.

Now if you want better efficiency at slower speed, you can increase pitch or prop size to load the engine properly at lower RPM--but you must reduce max RPM on the gov (high idle) to prevent overloading.

100% correct. If you reduce pitch or prop size you loose the ability to run faster if you want or need to. Pulling back on the throttles will reduce load and not alter the boats performance.

Increasing pitch or prop size will certainly help with slower speed cruising. The biggest problem here is the size and cost of the wheels this boat already has. The 58C with 16V92's swing 40x48x3 5 blade Rolla wheels which cost about 35-40K! You will never make up the savings that a pair of new props will cost. You might be able to sell the Rollas and buy a cheaper set of 4 blades but who knows what that will do to the ride and performance of that boat. There is no economical way to push 92,000lbs through the water.

[krush]

I think I needa go into the prop making business. Modern CNC machines can whip amazing stuff out fast.

[Capndan]

I bought Brian's 61' which is swinging the bigger props. The primary reason it was done was for engine longevity. If commercial boats with the same engines go 10,000 to 12,000 hours between over haul and pleasure boat builders get 2000 hours who has it right? A 85,000 pound boat was not designed to be a speed demon yet builders and owners try to squeeze every last knot out of them at the cost of longevity. If your happy at 10 to 12 knots then why not try to get your engines best efficency at those speeds. Pulling the throttles back works but the idea with the bigger props is to increase load and keep the engines in a ideal world temperature range with the turbos doing their job.
The commercial guys have proven it for years. Pleasure boat builders have never had engine longevity as a design criteria. They just had to get the owner through warrenty and then it wasn't their problem! The selling point has always been speed. They never posted engine life expectancy in their brochures but they always had the performance reports.

[rsmith]

All these ex-spurt longwinded posts for a guy that kinda sorta maybe going to buy a boat someday?

[petohazy]

Why not just under prop a little and keep the RPM's a little lower? I may be missing something but this seems like a simple way to take a load off the engine without detuning. We all watch our RPM's anyway so overspeed should not be an issue. Just thinkin.

I can relate to that. Out of all the input I've received here (BTW, greatly appreciated)I think underpitching the current props a couple inches (which is in effect detunng) is the best reasonable, inexpensive modification. Would still be able to come within a couple knots of the original set up's top speed, while not loading the engines at .92 hp to cube ration. Doing that and pulling back the rpm to around 1700 at cruise should translate to a longer service life. Pitching up is all well and good if all you want to do is travel at hull speed. Good for a trawler, but not a planing sportfish.

[petohazy]

All these ex-spurt longwinded posts for a guy that kinda sorta maybe going to buy a boat someday?


Kinda sorta made an offer.

[saltshaker]

Kinda sorta made an offer.
Good luck! keep us posted to how the deal progresses.

[GJH]

As I recall, Brian also de-rated his engines along with making the prop change.

I have to admit, I am not following the logic of de-pitching the props; can anyone explain that thinking in plain English? If the objective is to go slow, why further de-load the engines? I am easily confused. I am a "slow boater" and have fooled around with various prop configurations, and long story short, the best solution seemed to be keeping everything OEM and running the engines up to 1800-2000 for ten minutes or so at the end of the day. I have 8v92TI's, or literally one half of a 16v92. Anyway I profess to be anything but an "expert" on this subject, but after 8000nm on this boat, I know what I like.

By the way rsmith. does someone come over to your place every morning and p-- in your Rice Crispies or what? I guess every forum needs a house curmudgeon.. so rock on!

[Boatsb]

I think the vessels and the engines were designed by someone who was pretty smart and experienced. I know we all bitch about why this and why that but in the end for a production boat of their period the were right on most everything. To think we can just change things to how we want them and call it an improvement is short sighted. Repowering with more modern engines and upgrading electrical and electronics is great but the balance of the whole vessel was well thought out.

Don't change what it can do. Change what you make it do. Running lower RPMs will not kill the boats and the engines will be OK if run harder once in a while. Break out the common sense. If you spend $15K to extend the engine live and get a few extra years from the engines was it worth the savings since that was $15K that could have been spent on the rebuild.

[krush]

Running a smaller wheel makes no sense to me when you can achieve the same by reducing RPM. By nature of the physics, lowering the RPM will "underload" the engine anyway.