Marine Diesel Engine Ratings and Limits

HelloBoat

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Hi,

I am having a few questions regarding engines, ideal usage and limitations and was hoping there were some folks on here with the experience and willing to explain. Currently I am in the process of buying a boat and want to make sure I know the limitations beforehand.

Almost all boats I come across have engines with an E Rating which seem to be the standard for pleasure craft. For this example let's go with the Caterpillar C12 ACERT 715hp but I doubt Volvo etc is any different.

Manufacturer reports a top speed of 30kn with a cruising speed of 25kn.

So first I look up the engine specs:
Source: http://www.bmgs.com.tr/doc/GucSistemleri/Marin/Marine-Engines/C12/C12-ACERT-715-MHP-spec.pdf

Code:
...
C12 ACERT® MARINE PROPULSION
715 mhp (705bhp) 526 bkW
Displacement........................................11.95 L (729.23 in3)
Rated Speed................................................................2300
Refill Capacity
Cooling System.....................................45.0 L (11.9 gal)
Lube Oil System..................................... 28.0 L (7.4 gal)
Oil Change Interval..................................................250 hrs
...

And then I look up the engine rating and it states:

Code:
E Rating (High Performance)
% Load Factor: up to 30
% Time at Rated RPM: up to 8
Typical Time at Full Load: 1/2 hour out of 6
Typical Hour/Year: 250 to 1000
Typical Applications: For vessels operating at rated load and rated speed up to 8% of the time (up to 30% load factor). Typical applications could include but are not limited to vessels such as pleasure craft, harbor patrol boats, harbor master boats, some fishing or patrol boats. Typical operation ranges from 250 to 1000 hours per year.

So now I am trying to figure out what to make of this.

1) I am trying to find out if I understand this correctly. I have seen an example that used fuel usage to calculate the load factor. So for this engine, it would use 138L per hour at full speed. So if you go on an 8 hour cruise and burn a total of 400L of fuel, you can calculate the load factor as 400L fuel burned / (138L per hour x 8 hours) = 36%. That would mean that for this engine to reach the maximum load of 30% you can burn 41L per hour which according to the Maximum Power Data table is about 1100 rpm. This is almost idle speed and makes absolutely no sense to me. Any points on this and how I should interpret the term load factor is very much appreciated.

2) Then onto Time at Rated RPM; so if I understand correctly you are allowed to go up to 2300 rpm, which would be full speed, every 30 minutes per 6 hours. I assume anything above that would cause excessive wear? And after that 30 minutes, what would be an acceptable engine speed to continue, how much rpm do you go down?

3) The engines are supposedly designed for 250 to 1000 hours per year. Now let's say you use them 1500 hours in a year, is that an absolute no-go or is this doable if you shorten your oil change intervals and calculate that some additional work would likely need to be done.

4) Is it normal to change oil that frequently? Every 250 hours is not particularly a lot of time. I would have expected modern engines to be more durable to be honest. Can't see a diesel generator or a truck changing oil that often to be honest.

5) I have read that if you run low rpm on diesel engines you may have incomplete combustion and get exhaust deposits. I have seen statements that describe this as running below 20%, is that a fair number?

6) And lastly, is the rule of thumb that cruising speed is generally 200rpm less than the maximum engine speed?

Sorry for all the questions, just trying to gain some additional knowledge here :)
 

scottie

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I think that you are comparing oranges and apples confused with straight bananas by mixing up all these above

Most engine manufacturers recognise that that not all their engines will be used in the same way and will give an indication of how they expect them to be operated
For most cases relevant to this forum is is light duty pleasure for planing craft or a more conservative version for semi or displacement craft and these are what you want to look out for.
Between the builder and engine manufacturer the agreed power rating should be established and the boat fitted with suitably sized propellers and only signed off after a sea trial and an agreed short term and cruising power stated.
 

Portofino

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Ok I,ll try and make sense of it all

Don,t mix up truck / earth moving plant with leisure marine .
The leisure marine planing versions are highly stressed ,output wise .
This is because they will never be doing 100000 Km between overhauls or even 250 hrs / year .

Sure a commercial variant in a tug boat may do 1000 hrs in a year buts ir 1/2 the Hp .
The road Vs of the same block are about 1/2 to 1/3 rds of the power as are the commercial marine variants .

There’s no gear box in a boat the strain on the engine is from the kg,s ( functionality of drag in planning boat ) and the prop pitch .

As a rule of thumb I subscribe to to the 80 % of load all day running cruise setting .
Fortunately with the MAN electronic screen we can see load so it’s pretty easy for me to set that .

The rpm and speed vary depending on prop hygiene/ sterngear cleanliness and Kg,s - excess weight .
We have done so far this year since service ( May ) 52 hrs and I reckon there’s another 15 @ least planned before the end of the season (Med based )

With that back ground
Trying to answer your points in the same order .
1- idle is 600 rpm and 1st click about 18 % load .
As you say about 30 % @ 1100 rpm ish .
For me D speed is 8/900 rpm just below the bow wave effect =. 9 knots
Used to pootle about , in busy areas .

At this point I think I need to bring Exhaust Gas Temps in .
With diesels because of the high pressures needed to burn none volatile oil the rings are set to expand and seal at far higher temps than say a petrol engine .
Below theses temps say @1100 rpm for 5 days cruise they will not seal and crap , gets below the rings and wears the bores by oil contamination of abrasive substances.

Now you see the regular oil change Fq coming into play .
It’s not the lubricants ability that’s gone in say 250 hrs or what ever it’s the build up of abrasives due to low EGT,s .
In your truck with a gearbox ,up hill and down dale the EGTs are always up .

2- they say around 600 degrees or perhaps a wee bit higher is about as far as you want to go with EGTs for a sustained run .
Too high to long and the exhaust valve start to mushroom and loose the best fit .This is slow insidious thing , not a bang or blow up .
Power is lost the engine is prematurely aged etc .

I’ve seen 625 @ WOT hence the 30 min warning etc .


The American boat engine forums with MAN suggest sustained no more than 80 % load -
You can,t just deduct a arbitrary 200 or 10% - without load load gauges it’s guess work - as said depends on sterngear hygiene and kg,s of the boat as well as obvious bottom fouling if applicable.

Eg real example my WOT is 2150ish ? and run at 1850 max normally 1780.
Here the EGTs are typically 560/580 ish below 600
The 80 % load rpm comes down as the season progresses due to fouling as said .

3- yup - I think my oil changes are every 200 hrs or annually.
Which I do on the annual bases even though only 60/70 hrs to remove the abrasive stuff that gets past the rings from sub optimum EGT rpm s like marina manoeuvre etc .

4 - see above ^^^^ and google “ Agglomeration “
5 -
Agglomerated soot clusters are commonly called sludge. Second, only to high temperatures, sludge does more damage to a diesel engine than anything else.
Without oil, a diesel engine runs steel on steel. Pistons rub on cylinders and valve hinges grind. In a matter of minutes, the engine will overheat and seize up. With any luck, the damage will not be permanent.
But, sludge literally, sands down the interior components of a diesel engine. The engine will not typically overheat or seize up immediately. But, the damage created by sludge can be far worse, by excess sub optimal EGT running .

6 - see 2 ^^^ .

In the absence of load or EGT gauges - your guessing basically.


Don,t know what boat / engine combo you are considering but given a choose imho you want the most powerful for the least Kgs to push around.

Regarding general engine longevity here an extract from an America marine engine expert

Back to stress again ,the application with regards the kg,s they push around .

“Marine Diesel life

Diesel engines are capable of having a long life when the power to displacement ratio is low. But when they start jacking up the power, beyond what the manufacturer originally intended, that benefit disappears. There is a very simple formula you can apply to estimate service life: simply multiply the cubic inch displacement of the engine times one. The result is the maximum amount of horse power you can have and still expect a reasonable service life. A 6V92 engine is 552 CID; at powers greater than 550, these engines don't last. At 450, they'll go 10 - 15 years easily.
We recently surveyed a yacht with a pair of 8V71N (naturally aspirated) Detroit Diesels rated at 325 hp that hadn't been overhauled since new 1981. Now, an 8V71 has a 568 cubic inch displacement; the fact that these engines have a 0.56:1 power/displacement ratio explains why they could run so long.
Conversely, divide the CID by the horse power, and the greater the result UNDER the factor of one (1), the longer engine life you can expect. If you have an engine with an 0.70 CID/HP ratio, then you can expect 3000 hours engine life. Otherwise, you are a victim of the myth.”
 
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HelloBoat

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Thanks so much for your pointers, very helpful!

1- idle is 600 rpm and 1st click about 18 % load .
As you say about 30 % @ 1100 rpm ish .
For me D speed is 8/900 rpm just below the bow wave effect =. 9 knots
Used to pootle about , in busy areas .

I am not saying 30% is 1100rpm, the example I used here is a formula that I found on the internet, I am wondering if it is accurate / usable.
You state you try to stay around 80% load rpm, that makes sense to me.
The rating for this marine CAT engine however states in the manual; up to a load factor of 30%. I am trying to figure out what that number means, and more specifically what limit that does impose.

I am trying to make sense of all the data before I make a decision on which boat to buy, mostly which engine to choose as most used boats come with various engines.
I anticipate lots of cruising (500+ engine hours) a year so i want to make an educated decision.
Obviously I don't want an under powered boat, however the engine specs I am comparing show major differences in fuel usage (like 138 vs 203L/u).
Which right now is what I am trying to figure out. I guess I will open a new topic about the fuel usage and real life experience on that matter.

Thanks a bunch for your insight Portofino!
 
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Anders_P42

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Bear in mind you don't start a boat an open up throttles to 80% load. You start up, untie ropes, leave marina, warm up, cruise to your destination, slow down, anchor or enter new marina.

My single D6-370 uses circa 50-60lph depending on state of fouling. If I average total fuel burned over the 260 hours on engine then I average 21lph. The engine burns 74lph at full load according to spec, so my average is about 28%. I do pay attention to cruise rpm, if the engine maxes out at 3200 rpm (should be 3500), then I drop 10% off 3200rpm and cruise at 2800/2900rpm.

My cruising ground is short one hour trips, if your cruise at 80% load is three hours then your average fuel burn will be higher.
 

Portofino

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So 5.5 L = 335 cid , so your 370 Hp is over the arbitrary 1-1 ratio .
D6 do 330 and 310 better and possibly some lower commercial variants .

Not saying this formula is gospel it’s just banded about by marine engine folks in the states , and a lot of engines theses days are more / less on the 1/1 ratio - interesting that’s all
 

Eren

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5) I have read that if you run low rpm on diesel engines you may have incomplete combustion and get exhaust deposits. I have seen statements that describe this as running below 20%, is that a fair number?

This is a point that I also inquire a lot as I cruise excessively at low rpms due to speed restrictions in my cruising ground. I may not provide references from other sources as Portofino does but, my usage in the last 7 years is a live data. My boat spends more than 80% of its working hours around 30% loads. I make oil analysis every year and we don"t see any mechanical wear traces at the analysis results. This data was in the last 7 years and 0 to 750 hours of my boat’s engines (CAT C9s, which are not even that late design actually). The main issue is about how old is the design of the engine. The newer engines (especially common rails) calculate the exact amount and mix of fuel and provide close to perfect combustion at all rpms. This is confirmed by the MAN chief engineer that I personally brought up the issue at the MAN stand at the boat show (my current boat has MANs). However I will not disagree that 80% load cruising is still healthier for the engine. If conditions permit, go for that but, there is not much need to be afraid of low rpm usage, if you have a contemporary engine.
 

HelloBoat

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Bear in mind you don't start a boat an open up throttles to 80% load. You start up, untie ropes, leave marina, warm up, cruise to your destination, slow down, anchor or enter new marina.

My single D6-370 uses circa 50-60lph depending on state of fouling. If I average total fuel burned over the 260 hours on engine then I average 21lph. The engine burns 74lph at full load according to spec, so my average is about 28%. I do pay attention to cruise rpm, if the engine maxes out at 3200 rpm (should be 3500), then I drop 10% off 3200rpm and cruise at 2800/2900rpm.

My cruising ground is short one hour trips, if your cruise at 80% load is three hours then your average fuel burn will be higher.

Right and this is exactly what I am wondering about but I am not 100% sure how I should word my question correctly.

Almost all marine engines for pleasure yachts I have seen so far (MAN, CAT, VP) are rated more or less the same at 30% load factor
So with your 28% you are right up there, however if you only use 21 Lph on average on 260 hours (which is quite a lot) while at full load your engine would use 74Lph, I am guessing you are cruising at quite a low speed then?

Let me try to put it differently;

The boat I am looking to buy has two of these engines I quoted, so two CAT C12's of 705hp, rated at 2300 rpm etc. Cruise speed of the boat is a reported 25kn with a top speed of 30kn.

According to the specs, each engine would consume 138 liters per hour, so the two of them would consume 276 liters per hour.
I have found sea trails for this boat with fuel reports that state around 23.5 kn of speed at 1750rpm (which is 76% of rated rpm), both engines consume 140 liters per hour combined.

Let's say I want to make a cruise of 1600 nautical miles. At 23.5kn this would take around 68 hours. Given 12 hours a day that would be a cruise of around 6 days. This is highly optimistic of course but just an example for the numbers.

So I would burn 68 x 140L = 9520 liters
While at top speed I would have burnt (276 x 68) 18768 liters
So then back to the formula; 9520 liters burned / 18768 liters at full speed = 50%

This 50% is much higher then the rated 30%, while I am running at cruise speed and not doing anything that should be avoided according to the specs. So would this be an issue? If you look at the numbers it would.

So I am wondering if this would be a problem or not. Is that method I used to come up with the 50% even accurate? I found it on a forum with little explanation but it was the only thing i could find on this matter. So now I am sitting here trying to come up with a decision if this boat/engines would be sufficient, should I go bigger etc etc.
Maybe I made it too complicated in my head, I don't know how else to describe it sorry :)
 

HelloBoat

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This is a point that I also inquire a lot as I cruise excessively at low rpms due to speed restrictions in my cruising ground. I may not provide references from other sources as Portofino does but, my usage in the last 7 years is a live data. My boat spends more than 80% of its working hours around 30% loads. I make oil analysis every year and we don"t see any mechanical wear traces at the analysis results. This data was in the last 7 years and 0 to 750 hours of my boat’s engines (CAT C9s, which are not even that late design actually). The main issue is about how old is the design of the engine. The newer engines (especially common rails) calculate the exact amount and mix of fuel and provide close to perfect combustion at all rpms. This is confirmed by the MAN chief engineer that I personally brought up the issue at the MAN stand at the boat show (my current boat has MANs). However I will not disagree that 80% load cruising is still healthier for the engine. If conditions permit, go for that but, there is not much need to be afraid of low rpm usage, if you have a contemporary engine.

Thanks, that makes sense.
 

Anders_P42

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Right and this is exactly what I am wondering about but I am not 100% sure how I should word my question correctly.

Almost all marine engines for pleasure yachts I have seen so far (MAN, CAT, VP) are rated more or less the same at 30% load factor
So with your 28% you are right up there, however if you only use 21 Lph on average on 260 hours (which is quite a lot) while at full load your engine would use 74Lph, I am guessing you are cruising at quite a low speed then?

Let me try to put it differently;

The boat I am looking to buy has two of these engines I quoted, so two CAT C12's of 705hp, rated at 2300 rpm etc. Cruise speed of the boat is a reported 25kn with a top speed of 30kn.

According to the specs, each engine would consume 138 liters per hour, so the two of them would consume 276 liters per hour.
I have found sea trails for this boat with fuel reports that state around 23.5 kn of speed at 1750rpm (which is 76% of rated rpm), both engines consume 140 liters per hour combined.

Let's say I want to make a cruise of 1600 nautical miles. At 23.5kn this would take around 68 hours. Given 12 hours a day that would be a cruise of around 6 days. This is highly optimistic of course but just an example for the numbers.

So I would burn 68 x 140L = 9520 liters
While at top speed I would have burnt (276 x 68) 18768 liters
So then back to the formula; 9520 liters burned / 18768 liters at full speed = 50%

This 50% is much higher then the rated 30%, while I am running at cruise speed and not doing anything that should be avoided according to the specs. So would this be an issue? If you look at the numbers it would.

So I am wondering if this would be a problem or not. Is that method I used to come up with the 50% even accurate? I found it on a forum with little explanation but it was the only thing i could find on this matter. So now I am sitting here trying to come up with a decision if this boat/engines would be sufficient, should I go bigger etc etc.
Maybe I made it too complicated in my head, I don't know how else to describe it sorry :)

At 3000rpm I cruise at 20 knots with clean hull burning 50-60 lph depending on state of hull.

When you look at CAT specs, use the prop demand fuel data. At 1800rpm, the C12 Acert will use 60.7 lph per engine (lets assume clean hull). That is 48% load. If you're cruising for long periods your average load will be higher than my example because you won't be idling around an anchorage etc.

It doesn't mean your engine will self destruct if you operate at higher than 30%, but the engine will be wearing out quicker as your total fuel burned will be higher. CAT list maintenance by hours, time lapsed or fuel burned.
 

Assassin

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Engines are rated differently for different applications and it is the application which is the key here.

Car diesel engine is rated at 100 BHP and in service it will spend as little as 5% at full throttle so would be defined as light duty.
Fit the same basic engine to a commercial water pump and it will spend most of its time on a high throttle/load so it would be derated (usually electronically) to around 75 BHP and be classed as a medium duty engine as its throttle loading is much higher for much longer.
Fit the same engine to a generator and it will run at its maximum rated speed with a continual high loading for its entire working life so it would be derated to 50 BHP due to its speed and loading.

This is in its very basic and simplistic form.

However, with such engines you get a maximum continual running speed which gives your maximum cruising speed for your boat, and then a maximum speed which gives you your top speed, but you also work to a cycle dictated by the engine manufacturer and this is the amount of time you can run the engine at its maximum speed and the cycle is the time in any given period you can run the engine at its maximum speed, so lest simplify this.

If your cycle is four hours (duty cycle) and your maximum run time at maximum speed is 10 minutes it simply means you can run the engine at its maximum speed for 10 minutes in any four hour period.
In one day you have 24 hours, this means that on a long cruise of 24 hours you have six duty cycles as 4 X 6 = 24, so on a continual cruise of 24 hours you can run the engine at its maximum speed of 10 minutes six times during this 24 hour period before dropping back to cruising speed.
 

Portofino

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See my post #4
1780 rpm with a P hull gives 90 L / h ,for each side .
More / less the same 6 cylinder block size as the C12 / D12 .
The illustration from the OP in post #1 is from a D hull with a higher prop curve demand exponent of 3
So the maths at cruise needs redoing from 70 to a more realistic 90 @ around 1800 ish to reflect a different prop demand curve of a P hull .
As you can see 1900 rpm burns about 100L / h per side .

Interesting to know what rpm those CAT s run at to push the prospective hull along at 23.5 knots ?

There are slight variations in “ load “ definition too if you google it.
Like % of fuel used / % of max fuel available.
This relates to opening times % of time open / time available of the injectors .

Assuming ultra clean straight back in after annual lift as the starting point ?
If in the real world with cruising stores ( excess kg,s the boat carries more drag as it sits lower ) plus seasonal hull growth = drag ,
And crucially prop fouling the load increases for a given rpm .

So while the screen shot shows 1780rpm 90L / h and 77 % load then it could equally show with full tanks dirty props 1780 rpm but 100 L/h and over 90% load for a 4 knot reduction in speed .

Those examples were actually done in Sept last year with the annual lift done in May ,but the sterngear is regularly cleaned throughout the season easy to snorkel / dive in the warm Med .

In a year or season as illustrated above by some one when you add in all the tick overs , marina / anchor manoeuvre etc it drags down the time spent @ cruise load average.

Sure a solid 6 day run for 12 hrs to cover a 1500 m Del trip is gonna push the ave up .Thats ok once in while for a E rated engine .
But if that’s your proposed modus operandi,eg a Rhine river cruiser then you need the lower rated variant not the liesure E .

As long as it’s imho around 80% load ,you can use % of fuel burn compared to rated WOT in the abscence of proper load guages you should be ok .
My Del tip was 2 days solid 10/12 hrs with the engines sat on 80 % load .

Re ERENs point about modern engines calculated exact fuel squirt relative to the air mass at low rpm thus minimising the potentially harmful,abrasive by products of lower combustion temps / pressures , yes I can see that but and it’s a big but it does not get round the ring seal argument running at low temps / pressures .
Again glance back at the 2nd imgur link in post #4 and see the EGT s drop
Anywhere above 500 ideally 578/580 is the best temp inside the chamber of a Diesel.
You will still get potential abrasive stuff made @ low rpms but not as much as before CR actuately measures the squirt .
What is produced and gets past the loose rings at 300 degrees is not gonna bother the new owner of a E rated engine .
like EREN example ( if I understood the example ? ) 750 hrs in 7 years mostly sub optimal EGT ,but CR MAN s?

I think this ( EGT / combustion pressures ) is the difference between engines lasting a few thousand hours or 10 ,s of thousands of hours before new pistons / rings / liners .

So really we are in the territory of 3/4000 hrs or 10/20000 hrs I guess ?
Or three camps
1- excess EGT s over 600 far to long = overloading. Rebuild around 1000hrs or sooner depending on the severity of the overloading ?

2- low ave EGTs - anywhere under 400degrees going to a few thousand hrs

3- optimal EGTs as much time as poss @ 550/600 going to tens of thousands hrs .

For E rated
So even 100 hrs / year is that’s 2000 hrs in 20/y so if an owner keeps the boat that long say at year 18 ,1800 hrs a big bill mentally this could be just added on bear in mind the normal marine aging of everything that seawater comes into contact ,coolers , Pumps opening up the engine for new liners / pistons - may as well look at the shells etc feels ball park .

Obviously in the real world there are very seldom owners taking a 12 L lump from new to 20 years ,boats change owners and the maintenance issue is deferred to the current owner .

Problem is as the 4th 5 th owner of a nicely presented and maintained boat of your dreams you don,t want to cop the rebuild bill .

How ever the last owner just picks up the parcel in a pass the parcel kinda way wondering how the previous owners treated it hoping dispite all the due diligence ( oil analysis etc ) it’s not gonna need cracking open in his ownership period .

I pick my boat up as the 3 rd owner ,a 2004 engined 12.8 L with 515 hrs 4 years ago .Its got 860 to date and my MAN engineer concurs run it at 1700 to 1850 rpm it should not need cracking open .Its not a CR ,it’s the modal just before but fully electronic so assume fairly accurate fuel squirts but not as measured hence the better economy of the successive CR variant - ERENs point.
Assume annual oil change .
 
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Portofino

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I very rarely WOT mine .
The only time is just after the annual haul out / maintenance to see the temps really .
Then I did it last week .
Reason
In May the sea temps are below 20 degrees and the engines run* at a steady 86 even at WOT no change .
How ever interestingly as the season and presume seawater flush with an added 60 hrs to date progresses with regular use the temps reduce to 85/84 .
This is dispite a sea water temp rise to recently 28 .
Even at WOT .
So I assume it salt pickles a bit layed up over the winter .This then somewhat self flushes out of the coolers with use in the summer .

Anyhow the max speed quoted in boat tests is a bit of a red herring .
What you need to know is the optimal cruise speed for that set up .
Ideally from the engine manufacturers POV .

As said in a previous thread ( the one about the bulk of the new SS hat 52 and Prinys of 50 ft ) I mate of mine has recently got a Porto 47 on D6 IPS but 3200 rpm only gives 22 knots .These are 5.5 L lumps
His fuel burn is about the same as mine @ 22 knots .We have been cruising in Co you see .

Buts here’s the rub
He’s highly loaded feels 3400 is pushing it ,he can acces fuel burn and it rockets above 3200
While ,lam lightly loaded hence similar fuel burn with 12.8 L lumps turning at 1450 rpm .
Both boats more less the identical dims .


So here we are picking up on application .
Strangely it’s got near 900 hrs on a 09 hull build .
No prizes for working out which ( current useage ) will need cracking open 1st .

So bigger engines in the same hull size worked less hard are the way to go if you have a choice .

* tick over they drop to 79/80
 
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