cruising rpm

[curiouskb]

Have a pair of Volvo D6 435 getting 2450 rpm at WOT. To have a cruising speed of 20 knots for my planning hull I need to rev at 3150 to 3200 rpm. Just wonder if it is advisable to run at this engine speed for cruising and if not what rpm should it be? Thanks for the advice.

 

[capsco]

Do you mean 3450rpm @ wot?

 

[Anders_P42]

Rated speed for these engines 3500rpm, they should make around 3540rpm WOT with clean bum. Crusing rpm is 10% off rated speed assuming it makes full rpm, or 3150rpm. If your engine is not making full revs, adjust crusing revs down by revs lost off top. So I'd go with 3050rpm cruise rpm.

I check my WOT speed once a month and adjust cruise rpms accordingly on my D6-370. I lost 150rpm off top before she was scrubbed after six months.

Anders

 

[Nigelpickin]

Have a pair of Volvo D6 435 getting 2450 rpm at WOT. To have a cruising speed of 20 knots for my planning hull I need to rev at 3150 to 3200 rpm. Just wonder if it is advisable to run at this engine speed for cruising and if not what rpm should it be? Thanks for the advice.

I have these on my 500s. Anders figures spot on.
I think you need a lift and scrub...what boat are you in?

Regards

 

[Deleted User YDKXO]

Crusing rpm is 10% off rated speed

AFAIK it's not rated speed but actual speed. So if your engines are only making 3400rpm flat out due to hull fouling or whatever, then the cruising speed is 90% of 3400rpm, not 90% of 3500rpm. Also, it's not 90% for all manufacturers. Many Cat engines, for example, have a max cruising rpm of 80% of WOT speed

 

[Anders_P42]

AFAIK it's not rated speed but actual speed. So if your engines are only making 3400rpm flat out due to hull fouling or whatever, then the cruising speed is 90% of 3400rpm, not 90% of 3500rpm. Also, it's not 90% for all manufacturers. Many Cat engines, for example, have a max cruising rpm of 80% of WOT speed

I think I clarified that in second sentence of my post? I did read CAT engines have different ratings (not that I own CAT engines), they talk about load factors of 30% for E-rated engines. I'm not sure how your warranty would stand if you cruised at 80% load everywhere? I know time spent at idle and manoeuvring around marinas will help reduce overall load, but 30% sounds quite low.

 

[Deleted User YDKXO]

I think I clarified that in second sentence of my post? I did read CAT engines have different ratings (not that I own CAT engines), they talk about load factors of 30% for E-rated engines. I'm not sure how your warranty would stand if you cruised at 80% load everywhere? I know time spent at idle and manoeuvring around marinas will help reduce overall load, but 30% sounds quite low.

Sorry, I didn't read your last sentence like that. I've never heard of an engine with 30% max load factor. Doesn't seem like a very good engine!!

 

[Anders_P42]

Sorry, I didn't read your last sentence like that. I've never heard of an engine with 30% max load factor. Doesn't seem like a very good engine!!

This is taken from bottom of C12 Acert spec sheet 715 mhp (705 bhp)

E Rating (High Performance)
% Load Factor: up to 30
% Time at Rated RPM: up to 8
Typical Time at Full Load: 1/2 hours 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.

If you use more load-factor than 30%, I bet engine life is reduced from their quoted 8500 hours.

 

[Deleted User YDKXO]

This is taken from bottom of C12 Acert spec sheet 715 mhp (705 bhp)

E Rating (High Performance)
% Load Factor: up to 30
% Time at Rated RPM: up to 8
Typical Time at Full Load: 1/2 hours 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.

If you use more load-factor than 30%, I bet engine life is reduced from their quoted 8500 hours.

Somebody flame me if I'm wrong but IMHO the % load factor is an overall figure in terms of total fuel burned over the life of the engine because total fuel burned is a measure of how much work the engine has done over it's life. So if the engine burns say 100l/hr at max rpm, then over the whole of it's life, it's average burn should not exceed 30l/hr. As you say, the engine will spend a lot of it's time at low rpm which should bring down the average fuel burn considerably. 30% still seems quite a low figure to me so, as you say, maybe it is related to quoted engine life as well.
Then within that overall parameter of 30% you can operate the engine at full load for 8% of the time which is equivalent to 1/2hr out of 6 hrs

 

[Anders_P42]

Somebody flame me if I'm wrong but IMHO the % load factor is an overall figure in terms of total fuel burned over the life of the engine because total fuel burned is a measure of how much work the engine has done over it's life. So if the engine burns say 100l/hr at max rpm, then over the whole of it's life, it's average burn should not exceed 30l/hr. As you say, the engine will spend a lot of it's time at low rpm which should bring down the average fuel burn considerably. 30% still seems quite a low figure to me so, as you say, maybe it is related to quoted engine life as well.
Then within that overall parameter of 30% you can operate the engine at full load for 8% of the time which is equivalent to 1/2hr out of 6 hrs

That's my understanding of it, but if you 'cruise' everywhere at 80% load won't it take a lot of hours at idle or 10-15% load to bring overall load to 30% or below?

 

[Deleted User YDKXO]

That's my understanding of it, but if you 'cruise' everywhere at 80% load won't it take a lot of hours at idle or 10-15% load to bring overall load to 30% or below?

Yes that's my feeling too. However I had the previous version of the C12 (3196) in my last boat and 80% load was equivalent to quite a high speed, in fact around 26kts on that boat. The reality was that I cruised mostly at around 20-22kts which was equivalent to about 60% load so probably, considering slow speed operation in and around ports/marinas and that some of the time, we were cruising at displacement speed anyway, I'm guessing our overall load factor might have been around 30%

I also guess that it is related to engine life too (where does that 8500hrs figure come from?). No leisure planing boat is ever going to get anywhere near 8500hrs in its lifetime so if the average load factor was say, 50%, that might be equivalent to an estimated life of, say, 4000hrs which is still well in excess of what most leisure planing boats will ever do. Note that in Cat's spec, they state typical hours/year at 250-1000 which is way above what we pleasure boaters ever do

 

[Anders_P42]

Yes that's my feeling too. However I had the previous version of the C12 (3196) in my last boat and 80% load was equivalent to quite a high speed, in fact around 26kts on that boat. The reality was that I cruised mostly at around 20-22kts which was equivalent to about 60% load so probably, considering slow speed operation in and around ports/marinas and that some of the time, we were cruising at displacement speed anyway, I'm guessing our overall load factor might have been around 30%

I also guess that it is related to engine life too (where does that 8500hrs figure come from?). No leisure planing boat is ever going to get anywhere near 8500hrs in its lifetime so if the average load factor was say, 50%, that might be equivalent to an estimated life of, say, 4000hrs which is still well in excess of what most leisure planing boats will ever do. Note that in Cat's spec, they state typical hours/year at 250-1000 which is way above what we pleasure boaters ever do

True if you're cruising at less than 80% it will help overall load factor. I got the 8500 hours from maintenance schedule, it recommends full re-build at this many hours or 378,540l fuel burned, which works out at 44.5l per hour per engine average over the 8500 hours.

On your boat, can you access total fuel burned through CAT engine displays? The CAT spec shows WOT / full load fuel burn of 138l per engine per hour. If you take your total fuel burned and divide by your engine hours you'll know average fuel burn per hour. Then a case of working out what percentage this is of 138lph to get load factor (I think).

 

[Deleted User YDKXO]

True if you're cruising at less than 80% it will help overall load factor. I got the 8500 hours from maintenance schedule, it recommends full re-build at this many hours or 378,540l fuel burned, which works out at 44.5l per hour per engine average over the 8500 hours.

That figures then because the max fuel burn is 138 l/hr and 44.5 is equivalent to 32%

On your boat, can you access total fuel burned through CAT engine displays? The CAT spec shows WOT / full load fuel burn of 138l per engine per hour. If you take your total fuel burned and divide by your engine hours you'll know average fuel burn per hour. Then a case of working out what percentage this is of 138lph to get load factor (I think).

No, as I said, my boat had the Cat 3196 model which was the previous generation of the C12. The display only showed instantaneous fuel burn in l/hr and instantaneous load factor. There wasn't any data recording. However, I believe that the latest Cat displays can display total and average fuel burn. jfm of this parish might be able to confirm that as his boat has the latest C18 engines

 

[Anders_P42]

That figures then because the max fuel burn is 138 l/hr and 44.5 is equivalent to 32%


No, as I said, my boat had the Cat 3196 model which was the previous generation of the C12. The display only showed instantaneous fuel burn in l/hr and instantaneous load factor. There wasn't any data recording. However, I believe that the latest Cat displays can display total and average fuel burn. jfm of this parish might be able to confirm that as his boat has the latest C18 engines

Yes 32% makes sense.

Apologies Mike I had you confused with HenryF, who owns Princess with CAT C12 Acerts.

Anders

 

[Deleted User YDKXO]

Apologies Mike I had you confused with HenryF, who owns Princess with CAT C12 Acerts.

Well I'm honoured although I'm not sure that henryf would feel the same way!

 

[MapisM]

No, as I said, my boat had the Cat 3196 model which was the previous generation of the C12. The display only showed instantaneous fuel burn in l/hr and instantaneous load factor. There wasn't any data recording. However, I believe that the latest Cat displays can display total and average fuel burn. jfm of this parish might be able to confirm that as his boat has the latest C18 engines

LOL, I believe the first thing he would confirm is that he has a dozen more cylinders and 28 (!) more liters in his e/r overall, being Match powered by C32 rather than C18 engines...
Anyway, I'm pretty sure that the historical numbers are indeed recorded in any electronically controlled Cat engine, including the 3196.
IIRC, there's some trick (i.e. button combination) for showing them in the display, but I'm not positive about that.
Anyway, Cat engineers can surely read all that with their diagnostic tool - also because that could affect the warranty, incidentally.

In fact, the Cat rating system is somewhat complicated, because it's not only based on the load factor - whose meaning is not obvious btw, but your explanation in post #9 was spot on. The only other thing worth mentioning is that the half hour out of 6 is not meant just as an example of the 8% "load cycle", as they call it, but rather as a sort of limit, though how strict is anyone's guess.
In fact, from a mathematical standpoint, since the 8% load cycle and the 30% load factor are meant as averages on the whole engine life, you could as well use your E-rated engines for a whole season building up 100 hours always at WOT, arguing that you had in mind to use them at idle for the rest of their life.
But it's easy to imagine how happily Cat would rebuild your blown engines under warranty, after connecting the diagnostic and realizing the type of use the engines went through...

 

[curiouskb]

I have these on my 500s. Anders figures spot on.
I think you need a lift and scrub...what boat are you in?

Regards

Sorry the WOT rpm should read as 3450 and not 2450, my mistake. That is about 50 rpm less than the the rated max rpm. My boat is a Princess P43 with clean bottom, 3/4 fuel and water load with 6 persons on board plus a light weigh aluminium AB RIB.
Just wonder if I should cruise at 90 or 80% of WOT engine speed. Cruising at 3,000 rpm the boat can only make 19 plus knots with slight seas and it got 26 plus knots WOT. Thanks for all the feedbacks.

 

[Portofino]

I,ve googled Prinny P43 -your 425 looks underpowered fraid to say ! There s a 370 option .
Just to put a comparison in - its got near identical dimensions to my Itama 14-55 m x 4-18 M although yours dry is 14-thousends some hundred Kg ,mines 17 thousends -some hundred Kg .you have 3000 kg advantage over me .
But and it's a big but Ihave 700 hp MAN s ( twice the torque value -never mind Hp) With suitable gearing + prop pitch
From1000 rpm ( 20knots and planning ) I have to ask crew to sit down while I accelerate up to 1500 which is 26 knots .
1500 to 1800( 32 knots cruise and 80 % load factor ) will also knock em off there feet if they are not seated .or push you into your seat
Wot at 2150 equates to approx 38 knots .I have no real intension of ever going there -After the sea trial
I tend to "cruise ". @ 26 1500 prm -from memory the display is 60 % load .
Remember you have a 3000kg advantage and my liquids are 500 lL water and up to 2400 L fuel in three seperated tanks .
Fouled prop hardly well so far I have not noticed any difference .

 

[curiouskb]

I,ve googled Prinny P43 -your 425 looks underpowered fraid to say ! There s a 370 option .
Just to put a comparison in - its got near identical dimensions to my Itama 14-55 m x 4-18 M although yours dry is 14-thousends some hundred Kg ,mines 17 thousends -some hundred Kg .you have 3000 kg advantage over me .
But and it's a big but Ihave 700 hp MAN s ( twice the torque value -never mind Hp) With suitable gearing + prop pitch
From1000 rpm ( 20knots and planning ) I have to ask crew to sit down while I accelerate up to 1500 which is 26 knots .
1500 to 1800( 32 knots cruise and 80 % load factor ) will also knock em off there feet if they are not seated .or push you into your seat
Wot at 2150 equates to approx 38 knots .I have no real intension of ever going there -After the sea trial
I tend to "cruise ". @ 26 1500 prm -from memory the display is 60 % load .
Remember you have a 3000kg advantage and my liquids are 500 lL water and up to 2400 L fuel in three seperated tanks .
Fouled prop hardly well so far I have not noticed any difference .

What a power house you have there. Speed is not too important to me and I am happy to cruise at 20 knots. Just want to know I am not over stressing the engines cruising the VP with 90% WOT rpm.

 

[Nigelpickin]

So we tend to 'potter around' at around 2800 revs and that equates to between 18 and 21 knots and a 5ish litre per mile fuel burn. Wide open at 3.4k and we achieve the brochure speed, 28 - 31 knots, burn rate goes up to 7 LPM.

Of course this is balanced by our time at slower speeds, half of our time is spent at 7knots or less at 1k rpm.

I think you are fine at the 80 to 90 percent of max and that these engines are designed to sit at those numbers.
Hope you are enjoying the boat