Morgan Jane
Active Member
If one was to buy any one particular boat that offered 4 diesel engine options ranging from 350hp to 500hp, which engine option would get the best fuel mileage?
Diesel Few Miileage Hypothetical Question
- Thread starter Morgan Jane
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markrinker
New Member
How fast do you want to spend? 
The slower you are willing to cruise, the less horsepower required, and the more economical the fuel burn rate will be - to a point that their is not sufficient HP to put the boat on plane (planing hulls) where it is most efficient.
SeaRay engineers know this. So, the lowest power rating available from the factory for that hull can assumed to be the most fuel efficient...and the slowest.
Again - how fast do you want to spend?
The slower you are willing to cruise, the less horsepower required, and the more economical the fuel burn rate will be - to a point that their is not sufficient HP to put the boat on plane (planing hulls) where it is most efficient.
SeaRay engineers know this. So, the lowest power rating available from the factory for that hull can assumed to be the most fuel efficient...and the slowest.
Again - how fast do you want to spend?
Morgan Jane
Active Member
Again - how fast do you want to spend? [/QUOTE]
Doesn't really matter to me, just looking to get the best fuel mileage? Whatever HP or speed that takes for any particulare boat is what I am looking for.
So the lowest HP engine would get the best fuel mileage and based on what you are saying, and going the slowest you can go without coming off plane will raise the fuel mileage even more?
Makes sense. Thanks.
[/QUOTE]Doesn't really matter to me, just looking to get the best fuel mileage? Whatever HP or speed that takes for any particulare boat is what I am looking for.
So the lowest HP engine would get the best fuel mileage and based on what you are saying, and going the slowest you can go without coming off plane will raise the fuel mileage even more?
Makes sense. Thanks.
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Alex F
Well-Known Member
- Nov 14, 2006
- 9,166
- Boat Info
- 2005 420DB with AB 11 DLX Tender, Raymarine Electronics (2x12" MFDs) with Vesper AIS
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- Cummins 450Cs, 9KW Onan Generator, 40HP Yamaha for tender.
I agree that for the most part it all comes down to the speed you're looking to cruise at. This brings up another question, are you more concerned about the GPH or MPG? As you may know, there's a big difference between the two. Some people look at the GPH and happy to see a low number when on plane. However, it doesn't mean they're getting the best fuel efficiency. IMO, planing hulls have two sweet spots:
1-hull speed. I'll use my boat as an example. My hull speed is about 7.4kts. When traveling at slow speed I keep no more than 1000RPMs. I found this to be my sweet spot for hull speed, b/c this is what it takes to get the boat to around 7.4kts. If I add RPMs, I loose the efficiency, b/c I will gain very little speed and more RPMs will just use more fuel. So, my hull speed sweet spot is about 7.4kts @1000rpms burning 4GPH total (both sides).
2-planing cruise speed. This is (as you very well may know) is what most of us after since we all have planing hulls. I've noticed that the faster I go the more efficient I'm, b/c the boat come out and rides on top of the water. My sweet spot is 2300rpms which gives me around 23kts in normal conditions. Depending on the current and the wind, obviously my speed could be a little more or less. If I go slower than 2300RPMs I'll start digging bigger hull and even though my GPH drops, my MPG will drop as well, which means I'm loosing the efficiency. A good example is when I drop 300RPM and keep it at 2000RPMs I loose about 6kts. As you can see, from efficiency stand point going from point A to point B that's a big lose. There's small margin where you can gain 2kts for each 100RPMs. I found that my boat's margin is from 2000 to 2400RPMs, to WOT 2600RPMs. The key part here's not to "overwork" your engines. I think that even if I'm still efficient at 2400RPMs and can get additional 2kts, it's way too aggressive for my engines to cruise all day at 2400RPMs. So, with advice from Cummins mechanic I stick with 2300RPM.
Hope this helps a little.
1-hull speed. I'll use my boat as an example. My hull speed is about 7.4kts. When traveling at slow speed I keep no more than 1000RPMs. I found this to be my sweet spot for hull speed, b/c this is what it takes to get the boat to around 7.4kts. If I add RPMs, I loose the efficiency, b/c I will gain very little speed and more RPMs will just use more fuel. So, my hull speed sweet spot is about 7.4kts @1000rpms burning 4GPH total (both sides).
2-planing cruise speed. This is (as you very well may know) is what most of us after since we all have planing hulls. I've noticed that the faster I go the more efficient I'm, b/c the boat come out and rides on top of the water. My sweet spot is 2300rpms which gives me around 23kts in normal conditions. Depending on the current and the wind, obviously my speed could be a little more or less. If I go slower than 2300RPMs I'll start digging bigger hull and even though my GPH drops, my MPG will drop as well, which means I'm loosing the efficiency. A good example is when I drop 300RPM and keep it at 2000RPMs I loose about 6kts. As you can see, from efficiency stand point going from point A to point B that's a big lose. There's small margin where you can gain 2kts for each 100RPMs. I found that my boat's margin is from 2000 to 2400RPMs, to WOT 2600RPMs. The key part here's not to "overwork" your engines. I think that even if I'm still efficient at 2400RPMs and can get additional 2kts, it's way too aggressive for my engines to cruise all day at 2400RPMs. So, with advice from Cummins mechanic I stick with 2300RPM.
Hope this helps a little.
Gofirstclass
Well-Known Member
A boat's most efficient speed, as Alex noted above, is its hull speed. To calculate that, you need to know the length of your boat's water line. The formula for calculating hull speed is:
,
or, 1.34 times the square root of the length of the waterline. Obviously the longer your hull the higher your hull speed.
For most diesels, there is a sweet spot that comes at around 80% of the WOT rpm speed. My WOT rpm speed is 2150. 80% of that is 1720 rpm's. At that rpm speed I'm running around 22kts with 1/2 fuel and water load.
On a somewhat related note, if you only idle diesel engines they tend to gunk up and won't run as efficientlly. Diesels need to be heated up to around 175-190 degrees periodically. The "rule" I heard was that you should run your diesels at 80% throttle about 20% of the time. This allows them to burn off the "gunk" that tends to accumulate from running them at low rpm ranges. They will last longer and need less maintenance if you follow that rule.
or, 1.34 times the square root of the length of the waterline. Obviously the longer your hull the higher your hull speed.
For most diesels, there is a sweet spot that comes at around 80% of the WOT rpm speed. My WOT rpm speed is 2150. 80% of that is 1720 rpm's. At that rpm speed I'm running around 22kts with 1/2 fuel and water load.
On a somewhat related note, if you only idle diesel engines they tend to gunk up and won't run as efficientlly. Diesels need to be heated up to around 175-190 degrees periodically. The "rule" I heard was that you should run your diesels at 80% throttle about 20% of the time. This allows them to burn off the "gunk" that tends to accumulate from running them at low rpm ranges. They will last longer and need less maintenance if you follow that rule.
Alex F
Well-Known Member
- Nov 14, 2006
- 9,166
- Boat Info
- 2005 420DB with AB 11 DLX Tender, Raymarine Electronics (2x12" MFDs) with Vesper AIS
- Engines
- Cummins 450Cs, 9KW Onan Generator, 40HP Yamaha for tender.
I've been using 80/20 rule for years in all kinds of applications/projects. It's interesting how the same rule also made it's way to boat's efficiency. Currently, our schedule doesn't allow me (or shall I say that to cover more distance and see more places we choose not to) to travel 80% of the time at the hull speed. But, it's good to know the rule for the future.
Morgan Jane
Active Member
"The slower you are willing to cruise, the less horsepower required" This is agreed.
But this is really not what I was going after. My point is this. From a conservation of energy standpoint it takes so much horsepower for a particular boat to go a particular speed. No matter what engine size or rating is in a boat, the engine is only making the amount of horsepower that is required for the particular throttle position. Example, if the hull planing speed is 10 knots, then it is going to take a specific amount of horsepower to make the boat move 10 knots. Lets say that amount is 250HP. If you had a 400 HP engine in one boat and a 500 HP engine in an identical boat and both boats were doing 10 knots, both engines would only be making 250 HP. Obviously at full throttle the 500 HP boat would be going faster because it is putting out 500 HP. But let's say at max throttle using the 400 HP engine the boat will go 20 knots. If you put the 500 HP engine in the same boat and go 20 knots, would the 500 HP rated engine use more or less GPH than the 400 HP engine. Thats the question. I can see it boat ways. The 500 Hp engine has a higher capacity so it would appear that is working easier but maybe because it is not at max HP it is less effiecient? any opinions?
But this is really not what I was going after. My point is this. From a conservation of energy standpoint it takes so much horsepower for a particular boat to go a particular speed. No matter what engine size or rating is in a boat, the engine is only making the amount of horsepower that is required for the particular throttle position. Example, if the hull planing speed is 10 knots, then it is going to take a specific amount of horsepower to make the boat move 10 knots. Lets say that amount is 250HP. If you had a 400 HP engine in one boat and a 500 HP engine in an identical boat and both boats were doing 10 knots, both engines would only be making 250 HP. Obviously at full throttle the 500 HP boat would be going faster because it is putting out 500 HP. But let's say at max throttle using the 400 HP engine the boat will go 20 knots. If you put the 500 HP engine in the same boat and go 20 knots, would the 500 HP rated engine use more or less GPH than the 400 HP engine. Thats the question. I can see it boat ways. The 500 Hp engine has a higher capacity so it would appear that is working easier but maybe because it is not at max HP it is less effiecient? any opinions?
mainstream
New Member
- Jul 18, 2011
- 543
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- 2009 Sea Ray 350
Raymarine c120 classic
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- 8.1 Horizons
v drives
I'm not sure that the senireo you use is completely correct. With more horse power comes more ft lbs of torque which means you can drive bigger wheels I believe torque is what we need more then the hp. So all things being equal I would wonder a higher hp with more torque would be running less rpms to achieve the same hull speed as a 400 hp with less torque. But it is an interestingi question I'd be interested in Franks resPonce. I know gearing plays a big part to. Just my .02
Morgan Jane
Active Member
Sort of. Think of it this way. HP is a result of Torgue being applied over an amount of time or in our case RPM. the "minute" being the key phrase. You could have two engines running and both putting out 400HP each. If one was operating at 1000 rpm's and the other at 2000 rpm's, the 1000 rpm engine would have to be putting out twice as much torque as the 2000 rpm engine in order to be able to supply the 400 HP required. But they would both be creating the same "energy" or HP. It comes down to which motor is more efficient I would imagine. Is a engine rated at 400HP operating at max rpm more or less efficient than a 500HP rated engine that is only operating at an rpm level producing 400HP? Hopefully an efficiency expert will chime in.
mainstream
New Member
- Jul 18, 2011
- 543
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- 2009 Sea Ray 350
Raymarine c120 classic
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- 8.1 Horizons
v drives
There is a good example on boat test the 450 and 470 one with 380 cummins and Zeus the other with 480 zeus at 2300 rpm the 450 with the 380s gets 21.7 mph at .81 mpg and the 470 with 480 cummins spinning 2300 rpms gets 20.3 mph with .82 mpg so larger engines with same rpm seam to be more efficient it would appear
Morgan Jane
Active Member
Thanks Mainstream, that is what I was looking for.
I took a look at the data sheets for the Cummins QSB 5.9 family which comes in a variety of displacements/HP and seem to be frequently offered as scalable power options in new boats. At 2000 RPMs, the ft-lbs of torque delivered are higher the larger the displacement as expected, ranging from 582 ft-lbs for the QSB-225 up to 936 ft-lbs in the QSB-480. The GPH for each engine is about the same around 6.1 GPH. I then compared the torque and RPMs at the same HP level using the QSB-425 as the baseline. It delivers 719 ft-lbs at 3065 RPMs WOT, 420HP, 23.1 GPH. The QSB-480 delivers 420HP at 2400 RPMs with 918 ft-lbs of torque burning 9.3GPH. I found it interesting that the peak torque for these engines is at about 2/3 of the max RPM, for the 480 it's at 2200 RPM delivering 942 ft-lbs; the 425 max torque is at 2000 RPMs delivering 908 ft-lbs. I imagine this defines the best and most efficient cruise speed; for the 425 that's certainly true in my experience.
IanBat
New Member
- Nov 19, 2011
- 2,297
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- Pro Tournament 36' 1999
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- Twin Cummins 5.9L 370hp B Series
I actually find your start of this post quite interesting, as I gather all you have really done here is started a conversation for the sake of it.
There has been some really good answers here and all are pretty on the ball
In your question, you have not provided what the boat is what the options are. What is the hull? What is the weight of it? What are the 3 engine options? Also very importantly are they the same drives ?
If one option is shafts and the other is Zeus or the Volvo option, fuel economy will be vastly different
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There has been some really good answers here and all are pretty on the ball
In your question, you have not provided what the boat is what the options are. What is the hull? What is the weight of it? What are the 3 engine options? Also very importantly are they the same drives ?
If one option is shafts and the other is Zeus or the Volvo option, fuel economy will be vastly different
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Morgan Jane
Active Member
Absolutely, different drives and hulls would just add to the variables. My questions is if the only variable is the engine HP, and the boat was offered from the factory let say with 3 different diesel options, which HP engine would get the best GPH at one given speed? Say 20 knots.
Didn't start a thread for the sake of conversation. I have my 48 Sundancer for sale. I plan replacing it in a few years with a late 90's 40 or 480 Sedan bridge. My main concern is GPH because I plan on doing some traveling in it. Not going to be in a hurry. Traveling 20 knots will be fine. What is the best diesel engine for this purpose. High HP or Low HP? Thanks for the response.
Didn't start a thread for the sake of conversation. I have my 48 Sundancer for sale. I plan replacing it in a few years with a late 90's 40 or 480 Sedan bridge. My main concern is GPH because I plan on doing some traveling in it. Not going to be in a hurry. Traveling 20 knots will be fine. What is the best diesel engine for this purpose. High HP or Low HP? Thanks for the response.
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Alex F
Well-Known Member
- Nov 14, 2006
- 9,166
- Boat Info
- 2005 420DB with AB 11 DLX Tender, Raymarine Electronics (2x12" MFDs) with Vesper AIS
- Engines
- Cummins 450Cs, 9KW Onan Generator, 40HP Yamaha for tender.
We have quite few 480DB owners with QSM11 and they're very happy with those engines. I think Gary would be one of your best sources as he owns one for almost 10 years.
Just in general, comparing different diesel options for the boats I was looking at (early 2000s 400DBs were offered with 3 different diesel engines options), I've learned that CATs produce more torque and require lower RPMs. This indicated that they use slightly less fuel. So, if you're looking to go slower, perhaps CATs application would be better in regards to fuel economy. However, I'd be curious to learn the side by side comparison in terms of MPG, b/c as I've mentioned earlier going slower doesn't always mean you're saving fuel.
Just in general, comparing different diesel options for the boats I was looking at (early 2000s 400DBs were offered with 3 different diesel engines options), I've learned that CATs produce more torque and require lower RPMs. This indicated that they use slightly less fuel. So, if you're looking to go slower, perhaps CATs application would be better in regards to fuel economy. However, I'd be curious to learn the side by side comparison in terms of MPG, b/c as I've mentioned earlier going slower doesn't always mean you're saving fuel.
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- Oct 6, 2006
- 12,155
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- 1996 450DA
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- 3116 Caterpillars
I'm reading along here and wonder why you are not considering torque as a key variable in the discussion. Total hp is almost meaningless unless torque is considered because of the wide variations in engine design. For example, Cummins now offers the QSC in HP ratings very similar to the QSM's, the catch is you have to run the QSC at ±3200 rpm to get equal hp. Guess which engine is going to be more efficient......a QSC running at 3200 rpm or a QSM at 1900? HP is about the same, but the torque is vastly different.
Carrying that logic further, what about Yanmar light weight, high rpm engines.......running them at 3200 rpm or a comparable hp Caterpillar running at 1900? They both may be 300 hp engines, but Yanmar will bun about 2X the fuel.
There are so many engine related variables....torque, fuel delivery system design, weight, etc that trying to devine efficiency from hp ratings only is just fodder for endless discussion with no reliable conclusion.
But, there would be an even more important variable if I were shopping for a sedan bridge.......engine dependability, reliability and the risk of a catastrophic failure. If I were considering a 480DB with QSM's (an extremely reliable engine) or a 480DB with 3196 Cats (with a known well documented and extremely high failure rate due to aftercooler problems) I could give a rat's a$$ if the 3196 were more fuel efficient................
Carrying that logic further, what about Yanmar light weight, high rpm engines.......running them at 3200 rpm or a comparable hp Caterpillar running at 1900? They both may be 300 hp engines, but Yanmar will bun about 2X the fuel.
There are so many engine related variables....torque, fuel delivery system design, weight, etc that trying to devine efficiency from hp ratings only is just fodder for endless discussion with no reliable conclusion.
But, there would be an even more important variable if I were shopping for a sedan bridge.......engine dependability, reliability and the risk of a catastrophic failure. If I were considering a 480DB with QSM's (an extremely reliable engine) or a 480DB with 3196 Cats (with a known well documented and extremely high failure rate due to aftercooler problems) I could give a rat's a$$ if the 3196 were more fuel efficient................
Alex F
Well-Known Member
- Nov 14, 2006
- 9,166
- Boat Info
- 2005 420DB with AB 11 DLX Tender, Raymarine Electronics (2x12" MFDs) with Vesper AIS
- Engines
- Cummins 450Cs, 9KW Onan Generator, 40HP Yamaha for tender.
I couldn't agree more with you, Frank. When shopping for a new boat I always try to look at what I call "the overall package". A good example could be also a serviceability. If an engine A burns 2GPH less than engine B, but when you have to call for service and pay a lot more b/c a reputable shop is hours and hours away, the fuel savings might go down the drain very fast. The example you've mentioned is no brainer....
Now that we know which boat model Morgan is after it's much easier to comment and continue research.
searay40dad
Active Member
- Oct 23, 2006
- 1,161
- Boat Info
- 2001 480 Sedan Bridge w/ Novurania RIB
- Engines
- 660 hp CAT 3196's
hey, c'mon, you're killing my resale! CAT said i was golden (knock on wood)...they wouldnt lie to me would they
Alex F
Well-Known Member
- Nov 14, 2006
- 9,166
- Boat Info
- 2005 420DB with AB 11 DLX Tender, Raymarine Electronics (2x12" MFDs) with Vesper AIS
- Engines
- Cummins 450Cs, 9KW Onan Generator, 40HP Yamaha for tender.
hey, c'mon, you're killing my resale! CAT said i was golden (knock on wood)...they wouldnt lie to me would they
Use your performance number as a selling point...:grin:
A slightly a different twist. If I was a shopping for this boat I would use this as a key phrase "...with a known well documented and extremely high failure rate...". We're not dealing with mom and pop shop, so I would assume that by now CATs have developed some sort of a solution to the problem prevention.
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Morgan Jane
Active Member
"Carrying that logic further, what about Yanmar light weight, high rpm engines.......running them at 3200 rpm or a comparable hp Caterpillar running at 1900? They both may be 300 hp engines, but Yanmar will bun about 2X the fuel" Frank Webster.
Perfect, this is what I have been trying to figure out. So what Frank is saying is that if you have two engines that are producing the same HP, the one doing it at a lower RPM is the one that will use less fuel. There are many other factors to take into account when sizing an engine for a particular boat but I wanted to start with the simple question first.
Thanks Frank and everyone else for chiming in.
Perfect, this is what I have been trying to figure out. So what Frank is saying is that if you have two engines that are producing the same HP, the one doing it at a lower RPM is the one that will use less fuel. There are many other factors to take into account when sizing an engine for a particular boat but I wanted to start with the simple question first.
Thanks Frank and everyone else for chiming in.