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  Does an Engine Need to Run at Its Rated Full Throttle Engine Speed

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Author Topic:   Does an Engine Need to Run at Its Rated Full Throttle Engine Speed
Graybay posted 05-21-2015 09:09 AM ET (US)   Profile for Graybay   Send Email to Graybay  
Last year I re-powered my Boston Whaler SUPER SPORT 15 with a Yamaha F50. Using a high quality stainless propeller I already had from another engine the [Yamaha F50] will run 5,100 rpm at WOT. The dealer said that is a little low but really did not justify the cost of buying a new prop to gain 300 to 400-RPM at the top end.

I never run at WOT. The conditions I boat in and my personal preferences usually have me cruising at 3200 to 4000-RPM. The boat rides great in that range. I probably put 150 to 200-hours per year and never have cause to run WOT. Is this [not running an engine at full throttle] a negative for the life of the engine? I understand not wanting the propeller to take the engine over the red line range, but what are the needs when running mostly in the mid-RPM ranges?

contender posted 05-21-2015 09:17 AM ET (US)     Profile for contender  Send Email to contender     
The piston goes up and down, the faster and the longer it goes produces the most wear. I think a lot of it is how you take care of it and at the same time the luck of the draw. You can have an engine with 5000 hours and it does not give you one bit of trouble, Or I have seen outboards with rods that come apart in the 1st 100 hours. Like I said luck of the draw. And I do run my engine full from time to time. It is a 1985 140 looper Evinrude.
Whalrman posted 05-21-2015 09:28 AM ET (US)     Profile for Whalrman  Send Email to Whalrman     
Yes, you need to [choose a] propeller [for] your engine to reach OPTIMAL rpm for the life of the engine. You are lugging the engine, it's like starting out in third gear on your vehicle. Things won't last long doing that. Fuel consumption is another consideration, by lugging your engine uses more fuel. Is the Yamaha F50 at the correct height? It helps to run at WOT from time to time to help clear out the engine so it will run trouble free, whether two- or four-stroke-power-cycle.
jcdawg83 posted 05-21-2015 09:40 AM ET (US)     Profile for jcdawg83    
I don't think it's necessary to run at WOT. As long as you run enough at cruising speed--sounds like you do--you should get the same benefits as WOT. I would think the worst thing would be to run an engine at or near idle speed all the time and then run at WOT. My boating sounds about like yours. I spend a fair amount of time near idle speed and cruise from 3200 to 4000-RPM. I rarely run my engine at WOT. Of course, new engines with fuel injection are probably not as sensitive to build-up as the old carburetor engines.
Marko888 posted 05-21-2015 10:08 AM ET (US)     Profile for Marko888    
[Choose a propeller that allows] the engine to be able to reach the recommended WOT throttle rpm range as specified by the manufacturer. This is not just about the WOT speed. It is more about gearing the engine correctly for the boat load it is pushing. Your boat is overgeared and working harder then necessary, which is causing additional stress and increased engine wear.

Propping for optimum WOT range will enable the package to get on plane easier, will make your midrange cruising even better, and will contribute to
getting maximum life out of that new F50.

Jeff posted 05-21-2015 02:03 PM ET (US)     Profile for Jeff  Send Email to Jeff     
With today's lubricants and quality manufacturing, the argument of significant wear damage does not carry as much weight in my book. Through my years I have seen repeated examples of worked engines outlasting and outperforming pampered ones. I know of many guys who are running track cars like BMW, Porsche, Honda, and alike vehicles that have 100,000 to 250,00 miles on the original motors. They take those cars out, and ring-out every last RPM they can; the cars do just fine. Those same people truly care for the engines and maintain as they should be and that is what goes into helping their longevity...

Outside of the track I have witnessed in multiple dyno-tuning-days where two exact same vehicles with the exact same engine tuning, simple bolt on modifications and nearly the same mileage put down ten to 20-percent different horsepower outputs at the wheels. Every time the car that the owner babied and never let the engine see all the RPM range was on the lower side. Why? Because those engines never gets a change to really heat up and burn off the carbon and other contaminates that build up, the internal parts never get a chance to really wear in and all other kinds of theories of course....

But, I have always subscribed to the theory that it is in your engine's best interests for it to always see all of the RPM ranges and to see the higher side of those ranges to give the engine a work out and to burn off all the bad crud every once and a while. Be it, boat motors, cars, motorcycles, atv's etc...all of my engines see redline a fair bit and in all of my years, I have only had one mechanical failure with an engine and that was in a Yamaha designed and built 2ZZ Ge "Toyota" motor. The failure happened at 8000 rpm on a motor with 140,000 miles. A valve spring retainer broke and dropped the valve into the cylinder which is a very uncommon type of failure. After the motor was out and broken down, the rest of the internals showed minimal signs of wear and tear.

jcdawg83 posted 05-21-2015 02:46 PM ET (US)     Profile for jcdawg83    
Jeff, I agree that engines that are run generally last longer and have fewer [repairs] than engines that are babied and run less. An auto mechanic friend of mine told me years ago when I was looking at buying a used truck with high miles; "miles don't hurt a car, neglect does". I think the same is true for outboards.

I think as long as the engine is run enough at a high enough rpm range to allow the carbon and other residue to be blown or burned out, the engine will last a long time. I'm not an engineer or a mechanic, but I would think as long as an engine sees rpms in the 3000+ range for a reasonable period of time each time the engine is run, that would be adequate.

About 20 years ago, a man I know bought a boat from a guide at Hilton Head. The boat was a two year old 23' center console with a 200 hp Yamaha engine with about 2000 hours on the motor. The guy I know got a great deal on the boat, with the expectation that he would need to repower within a couple of years. He ran the boat for about 5 years with no problems at all and sold it to someone else with about 2700 hours on the engine. That engine was never babied at all. My friend ran the poo out of it, halfway hoping it would fail.

EJO posted 05-21-2015 03:23 PM ET (US)     Profile for EJO  Send Email to EJO     
[Endorses] the comments to not baby an engine use but also not to neglect an engine. Get the right prop, run the boat at WOT once and a while (couple of times a season) and enjoy the better cruising with more efficiency at that 3,000 rpm range, you might find out you can even run it at a lower rpm getting as good a ride as you do now with the "wrong" prop. You got the correct tires on your car, don't you?
As with a car you must run it through its gears at maximum rpm and as you don't have 1st, 2nd, 3rd gear and so on you can only run your boat at WOT to get max rpm.
Several of my sedan (4 cykinder) cars ran up to & past 500,000 miles before somebody took the cars out through an accident. (pedal to the metal is not bad no maintenance is bad)
Marko888 posted 05-21-2015 06:13 PM ET (US)     Profile for Marko888    
I disagree with some of the points made. Lugging an engine is lugging an engine, and never a good thing long term. More heat, more friction, more wear--everything will break or wear out eventually, some sooner than orhers.
jimh posted 05-22-2015 06:52 AM ET (US)     Profile for jimh  Send Email to jimh     
The initial posting really has two questions, more or less mingled into one, and various responders have choosen one of those questions to answer.

The first question: how should a propeller size be selected?

The propeller should be selected so the engine can reach a full-throttle engine speed that is in the range recommended by the manufacturer. Some manufacturers may further define an optimum range of engine speed that is a smaller range. Some boaters believe that selecting a propeller so the full-throttle engine speed is in the upper half or upper third or upper quarter of the engine manufacturer's recommended range is best. These recommendations are mostly supported by anecdotal observations.

The power output of a four-stroke-power-cycle outboard engine like a Yamaha F50 is likely to have a curve of horsepower as a function of engine speed in which the curve rises steadily with increasing RPM. On that basis, to get the most horsepower from your Yamaha F50 you would have to let the engine accelerate to the top of its recommended full-throttle speed range.

The second question: how much time at full-throttle must be spent for a Yamaha F50 to avoid any negative effects?

There is no requirement that the engine be run at full-throttle engine speed for any minimum amount of time. Modern engine which can record the history of their engine speeds over time will verify that for the typical boater the amount of time the engine runs at maximum engine speed is very low, perhaps one-percent of the total engine time or less. Outboard engines typically spend the greatest portion of their operating time running at idle or near-idle speeds. The next highest percentage of time at a speed range will usually be around two-thirds to three-quarter of the rated maximum engine speed.

In the specific case of your Yamaha F50 reaching 5,100-RPM at full throttle, you must compare that with the recommended engine speed range from Yamaha. Perhaps you can tell us what is the recommended full-throttle engine speed range by looking at the owner's manual. The owner's manual usually gives that information. Without knowing what the recommended engine speed range at full-throttle might be for your Yamaha F50, it is hard to assess where 5,100-RPM lands in that range.

Operating your engine for 200-hours per year is higher than average, and should not have any harmful effects, as long as you comply with the recommended maintenance procedures and use good quality fuel.

tedious posted 05-22-2015 07:34 AM ET (US)     Profile for tedious  Send Email to tedious     
Hi Gray. As the owner of the same boat with an F70, I can tell you that you are not compromising durability, but you can improve your performance and safety, especially within your stated use model, by going to a lower pitch propeller.

You are within the manufacturer's recommended WOT range of 5000 to 6000 RPM, so no worries about durability.

Seems like you mostly like to cruise, rather than go all out. Same here, especially as I mostly boat on the ocean, and that's where all 15 owners know the challenge is to keep the boat on the water. Sometimes the minimum planning speed you can hold is still awfully bouncy. And that's where a lower pitch prop will help. You'll be able to hold a plane at a lower speed than you can today, and hold that speed in a wider range of conditions. That is, you will be less sensitive to dropping off a plane, when running at minimum planning speed.

Also, if you need more power and flexibility when running slowly (this could happen when heavily loaded, or rough conditions) the lower pitch will provide it. That's a safety issue, and not something I would recommend compromising on.

To me, it would be a no-brainer. Get another prop with about 4" less pitch, and sell the old one on eBay, craigslist, or here. Great time of year to do that. An excellent new propeller such as a Stiletto costs little more than $200. You spent many thousands on a nice new motor, and you're looking to keep it for a long time. Why not spend just a bit more and optimize its performance over that expected long lifetime?

Tim

jimh posted 05-22-2015 11:21 AM ET (US)     Profile for jimh  Send Email to jimh     
If there is a Rule of Thumb for how much change in engine speed will be produced by a change in propeller pitch, I would refer to the rule that says:

"Every two-inch increase in propeller pitch will decrease engine speed by 450-RPM, and vice versa."

This rule is cited or mentioned or given in many articles, such as:

http://www.formulapropeller.com/propellers.html
http://www.vicprop.com/propeller101.htm
http://www.fiberglassics.com/boating-faq-s/about-props
http://www.hhprop.com/how-boat-props-work.php
http://continuouswave.com/ubb/Forum4/HTML/007128.html

and in at least 100 more articles in the archives of CONTINUOUSWAVE.

I tested the Rule of Thumb and reported my findings in

http://continuouswave.com/ubb/Forum4/HTML/003906.html

On the basis of the Rule of Thumb, if the propeller pitch were decreased by four inches the full-throttle engine speed would tend to increase by 800 to 900-RPM. Applying that to the F50 running at 5,100-RPM at full throttle with its present propeller, the F50 would tend to run about 6,000-RPM at full throttle with a propeller of four-inch less pitch.

jimh posted 06-11-2015 01:59 PM ET (US)     Profile for jimh  Send Email to jimh     
Regarding the effect of a propeller on the engine speed, there is more to consider than just the pitch. The number of blades, the size of the blades, and the shape of the blades are also factors.

Several years ago I wrote an interesting article an the topic of propeller power curves. See

http://continuouswave.com/whaler/reference/propellerPowerCurve.html

Some graphical presentations I created in the article show rather clearly how the propeller power curve affects engine speed. See the plots in the addendum. In particular, there are two plots labeled "Several Propeller Power Curves."

In those plots, compare the red curve (for a SST 15 propeller) and the blue curve (for a MIRAGE 17) propeller. The pitch difference is 2-inches. Compare where the propeller power curves cross the engine power curve (green line): the two curves cross about 400-RPM to 500-RPM apart. This is a good affirmation of the "Rule of Thumb" mentioned above.

Now consider a third curve in those plots, the (violet line) REVOLUTION4 17. This is a four-blade propeller with very big blades and 17-pitch. We see the distance along the RPM axis for its crossing of the engine power curve is spaced more than 500-RPM. The spacing is closer to 700-RPM. This is a good affirmation that other factors like number of blades and size of blades affect engine speed, too.

The Rule of Thumb works best if the two propellers being compared are from the same manufacturer and in the same family style. If you want to compare propellers from different manufacturers or from different styles, the simple Rule of Thumb may need more elasticity to cover all outcomes.

In any case, I recommend reading my article on Propeller Power Curves. It will help you understand how an engine speed is controlled by the load of the propeller.

I am aware that many readers come here to just get a very simple and deterministic recommendation, such as "buy such-and-such brand, model, and pitch" propeller, but, if you are interested in understanding more about propellers, you might find the propeller power curve article interesting.

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