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ENGINE SPEED: What is Optimum
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ContinuousWave Whaler Moderated Discussion Areas
ContinuousWave: Whaler Performance
ENGINE SPEED: What is Optimum
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| Author | Topic: ENGINE SPEED: What is Optimum |
| jimh |
posted 12-21-2005 12:18 AM ET (US)  
[I started this separate thread because this topic kept getting introduced into other dicussions. It seems like a good discussion, worthy of its own thread.] What is the optimum engine speed? |
| Sal DiMercurio |
posted 12-20-2005 09:25 PM ET (US)
Again, optimum fuel economy between 3000 & 3500 rpms. as I insisted. Where did the 4500 optimum come in from rev-25? Sal |
| jimh |
posted 12-20-2005 11:32 PM ET (US)
Sal--What is "rev-25"? You have lost me completely. |
| Revenge 25 |
posted 12-21-2005 12:01 AM ET (US)
Sal, The following information, which I provided before, comes directly from Yamaha's Performance Bulletins on their website: The Yamaha 250 OX66 EFI pushing a 25' Cobia obtained optimum mpg at 4000 rpm. The Yamaha 250hp OX66 EFI test result I cited was on a 25' Cobia w/ cuddy cabin, weight as tested: 5,310 lbs. It is Yamaha Performance Bulletin #017-COB-Z. The Yamaha HPDI 250hp pushing a 25' Parker obtained optimum mpg at 4000 rpm. The Yamaha HPDI 250hp test result above was on a 25' Parker w/ a pilothouse, weight as tested: 7,559 lbs. It is Yamaha Performance Bulletin #02-172-PKR-B. The two Yamaha four-stroke 250hp pushing a 32' Century, obtained their optimum mpg at 4500 rpm. The Yamaha 250hp Four Strokes got their worst mpg at 3000 rpm. The Yamaha Four Stroke 250hp test result above was on a 32' Century (dual engines), weight as tested: 10,800
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| jimh |
posted 12-21-2005 12:33 AM ET (US)
From what I can understand: Sal's point: Optimum fuel economy always occurs between 3,000 and 3,500-ROM Revenge-25's point: Here is data which shows that is not true. My point: I think in older two-stroke motors there is a tendency for the fuel economy of the motor to be quite poor at lower speeds. The motor needs to run about two-thirds throttle to really get into a region where it has good fuel economy, at least from the point of not throwing a lot of unburned fuel out the exhaust port. New four-stroke motors don't have this handicap. They can be about equally efficient over most of their RPM range. All boats will tend to be most fuel efficient at a the speed at which they just come onto hydroplane. For many Boston Whaler hulls this will be around 25-MPH. If you combine these three trends you can make combinations of boat, motor, and propeller which will produce optimum fuel economy at all sorts of different engine speeds. But feel free to discuss it further. |
| bsmotril |
posted 12-21-2005 09:06 AM ET (US)
I don't think you can look at RPM as beeing the only factor isolated from the situation in which the motor is working. The load the engine is seeing at that rpm will also be a factor. The metric for measuring that would be manifold pressure. The point on the rpm curve where the manifold pressure is highest will be the point(s) of best econonmy. And, that rpm point of highest pressure will vary for that same motor installed on two different size and weight hulls. I think this is also why you sometimes see better economy on a pair of twins versus a single. The twins move that data point to a lower point in the RPM curve while at the same time their ability to run a higher (than a single) pistched prop gives more boatspeed at that RPM point than would a single. Take a single 250hp outboard on the back of a big heavy boat like a Conquest 23. That motor will need to turn about 3500 rpm to get the boat on plane. Old magazine articles I have show fuel burn of about 2.5 mpg at 4000 rpm for max economy. At 3000 rpm, that motor would be laboring heavily and the boat plowing bow high. Engine load would be high trying to climb the hump, and I bet the fuel burn there would be 2 mpg or less. Put a pair of 135hp twins on that same boat, and it will plane at 2800 rpm. At 3200 rpm, the motors are well over the hump, and almost loafing along at an easy cruise. I know from experience those motors will burning 3mpg for best cruise. A fuel flow gauge is nice to have, and will certainly give you good solid data. But, a captain with a lot of experience running outboards comes to sense that rpm point where the motors are happiest, and not running with high load. That is the point where you'll find the best economy. And even then, that point will vary day to day depending on the passenger load, sea state, and number of fish in the cooler (if you're lucky). BillS |
| boatdryver |
posted 12-21-2005 09:42 AM ET (US)
In answer to the lead in question, "what is optimum engine speed", most of the discussion seems to revolve around the best rpm range to achieve the best nautical mpg. It seems that the difference between 3.0 and 3.3 mpg doesn't translate into a lot of extra range or cost savings in comparison to all the other expenses in boat ownership and use for most non commercial operators.. Is there any hard data as to the magnitude of longer engine life that can be achieved in outboards by keeping rpm's down ? |
| rtk |
posted 12-21-2005 10:05 AM ET (US)
My best fuel economy is at the 3200-3700 rpm range. 25-30 mph, 10 to 12 gph. Around 2.5-2.7 mpg at that rpm and speed range. I can run on plane at a minimum rpm of 3000-3200. When the motor is run at higher rpm ranges, fuel economy is a pretty consistent 2 mpg up to about 4800-5000 rpms. After that, it drops to 1.5 to 1.75 mpg. 1997 21 Outrage, 2003 Mercury 250 EFI two stroke. Wide open throttle gallons per hour is around 28-29 gph, top speed is around 48 mph. These fuel economy figures are when the boat is run in relatively calm waters and winds. When the wind and seas kick up I have seen my fuel economy drop by as much as 50%. Rich |
| davej14 |
posted 12-21-2005 10:09 AM ET (US)
In the boats I have owned there always seems to be a particular speed that just "feels" the best. Of course conditions dictate the limits, but in my current craft 3300-3400 rpms is just right. |
| fourdfish |
posted 12-21-2005 10:38 AM ET (US)
I don't think hard data exists that the number of rpms of regular outboard engine use determines the life of an engine but plenty of data exists in the racing industry.The manufactures also know but won't tell! Car engine data exists that shows that higher average rpms shortens the life of the engine. It also shows that lower rmps give better gas milage. Hence, overdrive in transmissions. It is ovious that a great deal of difference exists between boaters as to how many hours and miles they put on the engine. Since it is not unusual for me to put over 50 miles per trip and 8hrs salmon fishing at a time/4 days a week in season, I do in fact care about gas milage. My fuel bill is higher than many. I find the nice cruiseing rpms range around 3200rpms when the water permits. |
| Tom W Clark |
posted 12-21-2005 12:29 PM ET (US)
The motors on my Revenge 25 clearly get far better fuel mileage at engine speeds in excess of 3500 RPM. In fact, I get the best fuel mileage between 4000 and 4500 RPM. It would be ridiculous to insist, as a general statement, that "optimum fuel economy [is] between 3000 & 3500 rpms" |
| fourdfish |
posted 12-21-2005 01:06 PM ET (US)
I agree with Tom as every boat and engine combo are different! |
| The Judge |
posted 12-21-2005 02:01 PM ET (US)
Without a flowscan you are all just guessing. All boats have different "Happy spots" with different engines. Not sure if optimum MPG is achieved but I run where my engine seems happy. On my 19' revenge with the 140 zuki she liked 39-4100 rpm. my 20' Hydra-sports likes 37-3900. My 17' with the 70 zuki she likes 35-3800 but seems happy all the way up to redline. I think this is due to the EFI which takes away the "kicking in of 4 barrel" sound which you get on 2 strokes. I go by boating mag and try to find a comparable boat, weight, engine, top speed, etc and then I have a closer idea as to optimum MPG and GPH figures. This however is difficult with classic whalers because no new hull is gonna be like a 70's/80's Whaler as far as weight and shallow deadrise. Most new 25' boats have about 1000lbs on a classic and all seem to run 24 degree v's which takes away the efficiency at midrange that our Whalers are so famous for. |
| Peter |
posted 12-21-2005 02:32 PM ET (US)
Per my Navman, the 225 Fichts on my 27 seem to be most efficient around 3750 to 4250 RPM. On my 18 Outrage with the 150 Johnson, the sweet spot seem to fall around 3000 to 3300 RPM. The sweet spot on the Revenge 22 with the Yamaha 225 was around 3400 to 3900 RPM. All Boats have a most efficient cruise speed. It doesn't matter what size motor is pushing them, the efficient cruise speed for the hull is the same. If you undersize the motor, operation at the efficient cruising speed will require more flywheel turns. If you oversize the motor, operation at the efficient cruising speed will require fewer flywheel turns. Ideally, one should pick a motor size that will allow the motor to run quietly at the boat's most efficient cruising speed. |
| Tom W Clark |
posted 12-21-2005 02:53 PM ET (US)
Nick, I have a Floscan. I am not guessing. I know. My motors get their best fuel mileage between 4000 and 4500 RPM. Peter, My boat is powered at its absolute maximum. I do not get the best mileage at relatively high RPM becuase my boat is underpowered and the motors have to work harder to get it to its best cruise speed. I maintain that it is my motors that are more fuel efficient at higher RPM, not the boat. |
| Peter |
posted 12-21-2005 03:15 PM ET (US)
I don't know what to tell you Tom other than your boat is operating West of the Mississippi where a different set of rules seem to apply. |
| Sal DiMercurio |
posted 12-21-2005 03:58 PM ET (US)
In most all test it's concluded that optimum fuel economy is obtained just a hair above when the boat flops over on plane. If thats true & it takes your engine [ s ] to twist 4500 rpms to just get on plane, I would say somethings not right on that application. This is an interesting subject so don't anyone loose their cool & get miffed,...much is hard data that can be debated because it dosen't pertain to all applications & some is opinion. For instance the optimum rpm for the twin Yamaha 4 stroke 250 hp on a 32' century is 4500 where one single engine of that particular vintage on a different 20' boat is gonna be a different rpm. I really don't know of any of the 6 cylinder carbed 2 strokes of yesteryear that got better fuel economy above 4000 Most got 1.5 - 2 mpg at best in their comfort zone & once they went above 4000 - 4500 rpms they needed fire hoses for fuel lines Sal |
| RJG |
posted 12-21-2005 06:38 PM ET (US)
My 225 Yamaha Excell gets best fuel ecomomy pushing my 1988 22 Revenge WT WD at 4400 RPM per my Navman 3500. I do not have a fuel flow meter for my 1988 18 Outrage with a 150 Yamaha but going by sound alone it does best at 4100 RPM. At that speed you can barely hear it above the wind noise. |
| Tom W Clark |
posted 12-21-2005 07:27 PM ET (US)
Sal, "In most all test it's concluded that optimum fuel economy is obtained just a hair above when the boat flops over on plane." You know, I always thought that too, but now I don't think so. Can you cite any examples of test results that show this? My boat will plane comfortably at 22 MPH but gets its best fuel mileage at 35 MPH. I can remember reading OMC outboard motor owner's manual where they counsel you to find the best fuel economy at minimum planing speed and I do think there is that "sense" one has when operating a boat, but the evidence is against it. I can remember being very surprised to read performance data from Boston Whaler showing the old Nauset getting better fuel economy at higher engine speeds, and this with an old Evinrude from the 1960's! Peter, I guess my point is that the idea that there is a rule-of-thumb for the speed at which one gets the best fuel economy with an outboard powered boat seems to be bogus. I don't think it necessarily has to do with the type of outboard technology used. I don't think it has a lot to do with the type of boat or the level to which it is powered relative to its maximum horsepower rating. I think different boat and motor combination are different. Unless somebody shows me a pattern with qualified data that supports a generalized statement of "the best fuel economy is always in the range of xxxx RPM", I'm not going to believe it. |
| bigjohn1 |
posted 12-21-2005 07:54 PM ET (US)
I spend 99% of my time at 7-10knots trolling speed. On my 170 MONTAUK, I currently get between 4.1-4.5 mpg (depending on sea state) with a Merc 115efi and 17P 4-blade prop. This is at about 2,400-2,600 rpm's. Am I to assume that going down to a lower prop and bringing my trolling rpm's UP may well INCREASE my fuel economy? I wonder if the expense of a new 16P prop would be justified by substantially better fuel economy. I troll around 200 hours per year. Just thinking outloud.... |
| Revenge 25 |
posted 12-22-2005 01:13 AM ET (US)
Based upon the information I have reviewed on optimal fuel economy, I have concluded: 1. There is no rule-of-thumb for the speed at which one gets the best fuel economy with an outboard. I do not see consistent evidence that planning speed is optimal. Numerous test show speeds above planning as optimal. 2. There is no rule-of-thumb for the rpm at which all boats gets the best fuel economy with an outboard. The optimum rpm seems to vary significantly. 3. Putting the maximum rated horsepower on your boat may or may not provide the optimum mpg at a given cruise speed or overall. However, there obviously exists a point where a boat is "underpowered" and as a result obtains mpg under the optimal amount possible with a larger outboard. 4. Very litttle difference exists in mpg between Two Strokes and Four Strokes at their optimal cruising speeds. However, the data does show a pattern where Four Strokes get much better mileage at idle. |
| Sal DiMercurio |
posted 12-22-2005 11:00 AM ET (US)
Revenge25, 4 stroke engines get much better mpg at idle ?????? Show me a 4 stroke 200 hp that takes 3 hours & 20 minutes to burn 1 single gallon of fuel at salmon troll speed & than I will believe it. My 2000 Evinrude DFI 200 hp gets that milage. I've seen documentation that the e-tech [ same hp ] gets better than mine. All the hype about the 4 strokes getting better fuel economy & cleaner emissions is a bunch of bull puky. The DFI 2 strokes are getting better fuel economy & putting out less emissions than the 4 strokes. Sal |
| fourdfish |
posted 12-22-2005 01:09 PM ET (US)
Sal my E-TEC does get that trolling! sweet! |
| jimh |
posted 12-22-2005 02:45 PM ET (US)
Here is a plot of the fuel consumption (gallons per hour) of an E-TEC 225-HP motor as a function of engine speed (revolutions per minute). The data is taken from six different boat tests. It shows a general trend. It looks like the slope of the curve is somewhat constant until you get to about 4,000-RPM. Above 4,000-RPM the fuel consumption tends to increase at a higher rate. From this data you could infer that the optimum speed for this motor is around 4,000-RPM or lower. http://continuouswave.com/whaler/reference/graphics/ eTec225GPHvRPMLegend.png |
| Revenge 25 |
posted 12-23-2005 12:30 AM ET (US)
I should have said Four Strokes get better mileage than Carbed and EFI Two Strokes at idle. The ETEC vs Yamaha Four Stroke is close. The 2005 E-TECs gets 7.5 MPG at 1,000 rpm The 2005 E-TEC gets 4.4 MPG at 1500 rpm |
| Revenge 25 |
posted 12-23-2005 12:39 AM ET (US)
Sal, According to BRP the E-TEC burns about 0.61 GPH at 1,000 rpm. I do not understand how you are able to get 3 hours and 20 minutes on 1 gallon at salmon troll speed. |
| jimh |
posted 12-23-2005 09:11 AM ET (US)
Here is a plot of the fuel consumption (gallons per hour) of a VERADO 225-HP motor as a function of engine speed (revolutions per minute). The data is taken from three different boat tests. It shows a general trend. It looks like the slope of the curve is somewhat constant until you get to about 4,000-RPM. Above 4,000-RPM the fuel consumption tends to increase at a higher rate. From this data you could infer that the optimum speed for this motor is around 4,000-RPM or lower. http://continuouswave.com/whaler/reference/graphics/verado255GPHvRPM.png |
| fourdfish |
posted 12-23-2005 10:01 AM ET (US)
Revenge 25-- That engine idles at a smooth 500rpms. At 600 rpms my boat goes 2.1mph which is about the best trolling speed for salmon here in the Great Lakes. By the way at 500rpms you can barely hear the engine from the helm. |
| Revenge 25 |
posted 12-23-2005 11:07 AM ET (US)
Fourdfish, If your trying to make me jealous, it worked. The E-TEC is a great motor. I agree with Sal that a lot of hype surrounds the Four Strokes. When you look at the performance reports they do not match the hype. |
| elaelap |
posted 12-23-2005 02:17 PM ET (US)
My classic Outrage 18 with a Yamaha 115 four stroke EFI feels best--finds her sweet spot in semi-smooth seas--at between 3400-3800 rpm. I have no way of knowing whether this is her most economical cruising range, but the boat just feels right at those turns and I suspect she's doing very well on fuel use...she glides along easily, tracks smoothly and straight as an arrow, handles effortlessly, and just sounds right. I'm going maybe 22-24/25 mph at those rpm, which eats up my longest non-stop runs (25-30 miles) fast enough, and on those nice calm summer days I almost don't want the trip to be over and the fishing to begin. Tony |
| fourdfish |
posted 12-23-2005 02:57 PM ET (US)
Revenge 25- Actually, I was just trying say that Sal is right on when he said he only burns .3gph at about 500rpms or so! I go way to fast at 700rpms. Been experimenting with props this year to get best combo! |
| Sal DiMercurio |
posted 12-23-2005 08:52 PM ET (US)
Revenge25, my salmon troll speed is 2.5 .... 2.8 mph which is dead idle at only 600 rpms., not 1000 rpms. Hopefull "Seahorse" will chim in as he's the master tech on these engines & he will confirm that the Evinrude FICHT 200 hp gets 3 hours & twenty minutes on 1 gallon of fuel at idle troll & it is my under standing the e-tech gets the same at only .03 gallons per hour [ 600 rpms, not 1000 ] which equates to over 3 hours per gallon with 1/10 of a gallon left after 3 hours of troll. My 200 DFI Evinrude gets better troll milage than my 15 hp 2 stroke carbed Johnson at 2.5 - 2.8 mph. Even at 800 rpm it still gets 3 hours on a gallon. Sal |
| Sal DiMercurio |
posted 12-23-2005 08:57 PM ET (US)
I should have added that my milage could possibly be different on a deeper "V" hull as it would take more revs to move a deeper "V" hull to the speeds I get at lessor rpms. Sal |
| fourdfish |
posted 12-23-2005 09:19 PM ET (US)
Sal- it sounds like you troll a little faster for salmon there than we do here on the Great Lakes. We rarely troll over 2.3mph and I can troll from 550rpms to maybe 650 rpms. My engine starts and runs at 450rpms in neutral-500rpms in gear. |
| Revenge 25 |
posted 12-23-2005 10:38 PM ET (US)
Sal, I did not realize you meant 500 rpm. At that speed your numbers are reasonable. I troll for Salmon off Morro Bay at 4-6 mph using crocodile lures. |
| Sal DiMercurio |
posted 12-23-2005 10:40 PM ET (US)
Here on the west coast ocean [ saltwater ocean compared to fresh water lakes ] people troll anywhere from 1.8 to 4.00 mph. Fresh water salmon fishing on the west coast is done much slower than the ocean & than there's people who troll the fresh water at up to 5 mph & do very well. It's all about presentation & the secret is to put the lure or bait right in their mouth instead of expecting the fish to swim 10 ft for it. Sal |
| Sal DiMercurio |
posted 12-23-2005 10:48 PM ET (US)
Yep Doug, 4 - 6 mph on your 25 ft boatis gonna pull more fuel from the tank than 2.5 mph on mine will thats for sure. Plus your boat is 5' longer & much deeper than mine so it's not a fair compairison. Sal |
| andygere |
posted 12-24-2005 01:45 AM ET (US)
Here is the data I collected this spring running my Outrage 22 Cuddy with a carburated 2.4 liter Mercury 200 2-stroke turning a 19 pitch High Five prop. The boat was loaded with 60 gallons of fuel, 2 adults and 2 kids. This was a flat water run, with little wind, running against a mild current. The best fuel economy was between 4000 and 4400 rpms.
This data shows a similar sweet spot to that of Tom Clark's Revenge, which is not all that surprising since he is running a pair of carburated Mercurys. Any discussion of the optimum engine speed must also consider the prop that the engine is spinning. Over or under propped boats are going to shift the optimum rpm band up or down, since engine speed is ultimately related to prop pitch. |
| fourdfish |
posted 12-24-2005 12:02 PM ET (US)
Andy-- Let me ask you! When you troll for salmon, Do you use the big motor? What rpms do you usually troll at? What speed do you usually troll at? Do you know approx what the gph is during trolling? |
| andygere |
posted 12-24-2005 01:55 PM ET (US)
I never troll on the big motor. It typically burns around 2 gph at idle, and pushes the boat faster than the 2-2.5 mph trolling speed that I prefer. It also produces a lot more smoke at idle speeds than the little kicker does at its trolling rpms. |
| Sal DiMercurio |
posted 12-24-2005 03:30 PM ET (US)
Keep in mind Andy's engine uses a carburetor and the new DFI engines should--I'm not sure about Mercury--burn less than 0.5-GPH. As I stated, mine burns LESS than 0.3-GPH.--Sal |
| swist |
posted 12-25-2005 08:03 AM ET (US)
Minimum planing speed would seem to be optimum for a lot of applications, but what speed is this really? Most planing boats I have driven require a certain amount of power to get on plane, let's say for argument it's 4,000-RPM, but then you can reduce your throttle and the boat will still stay on plane over a certain range (on my last boat you could get down to 3,400-RPM or so). But is was my (admittedly unscientific) feeling that the boat was certainly not performing optimally in that range even thought the engine speed was lower. It had the feel it was lugging a bit, and of course unless you were on flat water, the slightest wake or other disturbance could throw it off plane. I'n not sure just a pure fuel flow measurement can represent this situation. |
| elaelap |
posted 12-26-2005 10:54 AM ET (US)
I know what you mean, swist. If I slowly increase my speed from a dead stop or from trolling speeds, it takes maybe 35-3600 rpm to get my OR 18 up and over that big wave I'm pushing in front of her. The second she's finally over the top and planing, I can reduce turns down to 3000-3100 and stay on plane if I want, but I don't often do this because my boat doesn't feel like she's moving along effortlessly until she's running a couple of hundred rpm faster. When I goose her from a dead stop or slow speeds, she just immediately (3-4 seconds) hops onto plane by the time she's twisting 3000-3100 rpm, probably because she hasn't had time to build up a big bow wave to overcome, and then I ease back from full throttle to stop accelerating at that sweet spot between 34-3800. I always (when I'm paying attention, that is) trim the motor full in when I'm about to plane, and then ease it out a ways, depending on wind and waves, when I'm at cruising speeds. Wish I were out there right now doing it instead of sitting in my room looking out at the rain and writing about it ;-) Tony |
| andygere |
posted 12-26-2005 02:07 PM ET (US)
When the boat is "just up on plane" there is more wetted surface, therefore more drag, than when it's running a bit faster and more of the boat and the gearcase are out of the water. I think that is why fuel economy improves at speeds above "just planing". |
| Perry |
posted 12-27-2005 11:13 PM ET (US)
The sweet spot on my 190 Nantucket with Honda BF135 is around 3500 RPM. At that RPM, boat speed is 25 MPH, Navman 3100 shows 3.5 GPH which equates to 7.1 MPG. At 4000 RPM, speed is 30 MPH and fuel economy drops to 5.5 MPG. |
| jimh |
posted 12-28-2005 12:01 AM ET (US)
Fuel economy of 7-MPG is fantastic! Numbers like that would sell a lot of boats. The factory test reports show about 5.3-MPG as the best 190 OUTRAGE fuel economy (with a 135 Verado). |
| Perry |
posted 12-28-2005 02:06 AM ET (US)
Yea Jim, I was also impressed with the 7+ MPG but the new Honda BF135/150 is known for its excellent fuel economy at cruising speed. At 4500 RPM the MPG is a little over 5 then it drops to 3.7 MPG at 5000 RPM and 3.1 MPG at WOT which I believe is pretty much inline with the Optimax of similar HP. All MPG figures were computed by calculator using speed displayed from GPS and GPH readings from Navman because I am having trouble getting my GPS and Navman to talk to each other. |
| Tollyfamily |
posted 12-28-2005 12:56 PM ET (US)
This is great info! I see another thread where people are talking about their Navman units, how about some more info on your best mpg rpm. Dan |
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