Derating 16v92s for longer service life

[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.