Engine RPM at Wide Open Throttle (WOT)

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This topic contains 15 replies, has 4 voices, and was last updated by  John Reynolds 2 months, 3 weeks ago.

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  • #8760

    John Reynolds
    Participant

    I should’ve listened to Ken Fickett (more on this further down the page).

    In late 2011, after we had been cruising aboard Easy (now Balena) for about a year, a reputable diesel mechanic (who shall remain unnamed) advised me during a sea trial that our boat was over-propped because it would not make rated RPM at WOT. Thus, the engine was being operated in an overload condition, which would result in higher maintenance costs and shorter engine life. Needless to say this got my attention.

    Easy has Yanmar 4JH4E engines rated at 54 HP @ 3,000 RPM. Maximum RPM is 3,200, limited by the mechanical fuel injection system.

    In neutral, each engine would make about 3,200 RPM. In gear at WOT, each engine would make about 2,625 RPM, about 88% of rated RPM.

    I reached out to other experts to get some additional input. I called the technical groups at Mastry, the engine supplier, and at Mack Boring, a supplier of Yanmar engines and parts. Both groups gave me essentially the same information: If the engine cannot get up to rated speed at WOT, then the engine is being operated in an overload condition, etc.

    Then I called the boat builder, Ken Fickett. Ken explained that yes, our boats are over-propped, which in turn makes them very fuel efficient. I relayed to Ken what the diesel experts told me. Ken’s reply was that, if this was a planning hull, then their input would be correct. But, since ours is a full displacement hull, the engines are not being operated in an overloaded condition. I told Ken that I was thinking about taking some pitch out of the props to allow the engines to make rated speed at WOT. He advised against it.

    I should have listened to Ken.

    Instead, I listened to the diesel guys. After doing some research, I decided to change the propeller pitch from 13” to 11”. Here’s what happened.

    When backing out of the slip after the prop change, the first thing I noticed was an obvious difference in the way the boat responded to throttle input. After putting the engines in gear, a significant amount of throttle was required to get the boat moving. During the sea trail with props set to 11” pitch, the engines made about 3,050 RPM at WOT, which was right on according to the specifications.

    After completing the work at Holland Marine on the St. Johns River, we traveled down the ICW to Key West in January, and retuned the following May. To get the boat up to hull speed we cruised at about 2,400 RPM, compared to 2,050 RPM with the props set to 13” pitch.

    After returning in May, I concluded that I had made an expensive mistake in changing the props. All that I had accomplished was to burn about 26% more fuel, increase the wear and tear on the engines, and generate more noise and vibration while cruising.

    Admitting my mistake, I had the props re-pitched to 13” and balanced at General Propeller.

    The following table summarizes the performance of the boat:
    Avg. Run Time Average Average Average
    Hours MPG MPH GPH
    13″ Pitch (As-Built) 7.6 3.26 7.1 2.3
    11″ Pitch 5.8 2.42 7.1 2.9
    13″ Pitch (New) 7.3 3.18 7.0 2.2
    Difference
    11″ to 13″ (New) 31% -2% -26%

    The average run times in the above table are based upon a significant amount of data recorded on trips from Baltimore to Jacksonville and return, and from Jacksonville to Key West and return. The average MPH is the speed over ground, computed by dividing the miles made good during the days run, divided by the run time. Easy is equipped with FloScan fuel flow monitors/digital tachometers, thus I have high confidence in the above fuel consumption numbers.

    So, is Easy over-propped? Yes, and it’s a good thing. Are the engines overloaded? The answer is: It depends. More to come in the next post.

  • #8761

    Thanks for the data, John. We have had similar conversations with surveyors and others about being over-propped. It’s great to have empirical data that supports the design decisions that Ken and Lou made.

    I have never researched the subject, but if I were to look for a spare set of propellers, does anyone know if our 3-blade, bronze 24-by-13 inch counter-rotating propellers are readily available?

    Dick Hermann
    N37 Avocet

  • #8762

    John Reynolds
    Participant

    The propeller manufacturer, Ellis in Jacksonville FL, is out of business. As far as I know, there are no Ellis 24 x 13 props available. I believe Ken Fickett used ACME machined props for his last N boat build.

  • #8763

    John Reynolds
    Participant

    Engine RPM at Wide Open Throttle (WOT) – Part 2

    In the prior post an explanation of expensive experiment was provided in which I had the prop pitch changed from 13” to 11” to achieve rated engine RPM at wide open throttle (WOT). After experiencing the negative effects on fuel economy, I had props re-pitched to the original specifications of 13”.

    After doing some additional research, some insight into the mechanics of the engine, propeller, overload, and WOT was gained.

    The fuel injection pump on the Yanmar engine contains a mechanical speed governor. The throttle lever sets the speed input for the governor. The governor adjusts the fuel supply to the engine to develop the horsepower that is required to achieve the set speed, based upon the applied load to the engine, i.e. the propeller. If a higher speed is set, i.e. the throttle lever is advanced, the governor increases the fuel supply to the engine, increasing the power output of the engine, until the engine reaches the set speed.

    If the load on the engine is such that the engine cannot develop sufficient power to achieve the set speed, then the engine is in an overload condition. Black smoke from the exhaust is a typical indicator of engine overload.

    On Easy (now Balena), the unloaded engine speed is limited by the governor to 3,200 RPM. Thus, at WOT, the engine “wants” to operate at 3,200 RPM. However, the propeller load on the engine limits the engine speed to about 2,670 RPM. Thus, by definition, the engine is overloaded. But, it’s not overloaded by very much.

    Please refer to the chart in the attached .pdf file.

    The purple line is the power curve for the Yanmar 4JH4E engine. Note that from 2,600 RPM (52 HP) to 3,000 RPM (54 HP) the curve is nearly flat.
    The black line is shaft RPM, equal to Engine RPM divided by the transmission gear ratio of 2.63.
    The red and green lines are the propeller power curves for 13” and 11” pitch, respectively.

    To generate the power curves, the following equation was used:
    HP=C ×〖RPM〗^X, Where C is a constant, X is a number between 2.2 and 3.0
    A value of 2.7 was used for X, as recommended in various propeller handbooks. The value for C is calculated at the WOT point, where the propeller power curve intersects the engine power curve.

    At the WOT operating point the engine is producing about 52 HP, or about 96% of maximum. Excess fuel supplied at this point is minimal, as evidenced by no, or little, black smoke from the exhaust. Although the engine is technically overloaded, we don’t normally operate at this setting for any extended period of time thus the overload condition is of no consequence.

    At the normal cruise setting, the engine makes about 29 HP; thus the boat requires about 29 HP to achieve hull speed at 2,150 RPM. At this RPM and load, the engine is capable of producing about 48 HP, with a corresponding reserve of 19 HP. Due to the large power reserve, this is definitely not an overload condition.

    Also note in the figure the HP at which the 13” propeller curve intersects the 75 HP line, about 3,050 RPM. The GH/N 47 boats have the same 24” x 13″ propeller and the same 2.63 transmission ratio. Most 47 boats are fitted with 4JH-TE engines, rated at 75 HP at 3,200 RPM. Thus, they should be able to spin the prop at most about 3,050 RPM, also technically an overload condition.

    A valid question is: At what RPM should the engine be operated while cruising to achieve maximum engine life? There doesn’t seem to be a clear cut answer to this question. I could not find a specific recommendation from Yanmar for this engine series. For the 1,2, or 3 cylinder GM series I found that the engine should be operated at 100%, 3,600 RPM, for no more than 5% of the time, and operated at 3,400 RPM or less for 90% of the operating time.

    In a well-known Propeller Handbook I found: “Specific fuel consumption is usually lowest at around 70 percent of rated RPM, and torque at this RPM is still fairly high. For this reason, the most economical and efficient speed of operation of many engines (particularly light. high-speed engines) is around 70 percent of the top rated RPM, and it is usually wise to choose an engine powerful enough to push the boat at cruising speed at this reduced RPM.”

    70% of the 4JH4E rated RPM is 2,100, which is the cruising range for Great Harbors.

    The same text states that the recommended operating range for light marine diesels should be 80-85% of maximum RPM, or 2,400- 2,550 RPM for the 4JH series.

    In the absence of a clear cut recommendation from Yanmar, my input is to cruise in the 2,000-2,200 RPM range, while going to WOT for 30 minutes at least once a day. In addition, I always followed the recommendation in the Yanmar operations manual for the shutting down the engine. The following is a direct cut/paste from the 4JH series operating manual.This procedure is applicable to all 4JH series, including the turbo 75 and 110 HP models.

    SHUTTING DOWN THE ENGINE
    NOTICE: Avoid engine damage. Do not stop engine abruptly during operation. Yanmar recommends that when shutting the engine down, allow the engine to run, without load, for 5 minutes. This will allow the engine components that operate at high temperatures, such as the exhaust system, to cool slightly before the engine itself is shut down.
    1. Reduce engine speed to low idle and put remote control handle in NEUTRAL.
    2. Accelerate from low speed to high speed and repeat five times. This will clean out the carbon from the cylinders and the fuel injection nozzles.
    3. Allow engine to run at low speed (approximately 1000 rpm) without load for 5 minutes.
    4. With the key in the ON position, push and hold the stop button. After the engine has stopped, turn the key switch to OFF.

    I usually performed the procedure at least twice a week, and would typically see some black stuff come out of the exhaust if I hadn’t run the procedure for a week or so.

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  • #8765

    John Reynolds
    Participant

    The table in the first post got messed up. It’s attached FYI.

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  • #8770

    Henry Dennig
    Participant

    Thank you for the post John

  • #8779

    John Reynolds
    Participant

    You are welcome Henry. I updated the chart for the GH/N 47 boats, based upon an assumed cruise at 2300 RPM. I also added the engine power curve for the 4JH4-TE 75 HP turbo engine.

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  • #8781

    Henry Dennig
    Participant

    John,
    What about a GH47, with a 4JH3-HTE (100hp) engine? WOT I think is like 3800, and at survey, we got 3400. The surveyor advised me that our loss of 400 rpms was a result of over propping. Ken advised me that it was a “good thing”.
    Henry

    • #8791

      John Reynolds
      Participant

      Henry, I found the power curve for your engine. The data sheet is too big to post. I’ll email it to you. I built a chart for your engine and propeller. The propeller curve was adjusted to pass through the engine power curve at 3,400 RPM.

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  • #8782

    John, I’m a visual learner so seeing the power curves along with the corrected table really made this concept come alive. Well done!

    Dick
    N37 Avocet

    PS, the “notify me of follow-up replies via email appears not to be working. I had to log in and browse to find these updates. I’ll check with Andy …

    • #8789

      Norm Miller
      Participant

      All
      Our GH47 has the 75Hp 4Jh3TE engines and we typically cruise at 2400-2600RPM and that move us at about 7-8MPH(no current). As for racing the engine to clean the carbon…I just cannot get myself to do that. The manual says to but it scares me. Do others do this with frequency?

      Norm

      • #8790

        John Reynolds
        Participant

        I have done it, i.e. ran the engines to WOT, and back to idle, while in neutral for five cycles, many times. There is a mechanical governor in the fuel injection system that limits, or should limit, the RPM to the specified value, e.g. 3,200 RPM for a 4JH4E engine. The governor needs to be periodically tested to verify that it works, in case the load on the engine is suddenly removed, e.g. the shaft key shears, the propeller falls off, etc. Yanmar recommends this procedure, as part of normal engine maintenance. Yanmar also recommends many other procedures, e.g. changing oil at specified intervals, changing fuel filters, adjusting valves, etc. Although none of these would seem to be as “radical” as running the engine at WOT in neutral, I always performed all of Yanmar’s recommended services.

        Have you ever heard the term “Blow the carbon out”, as it relates to automobiles, particularly when we were using leaded gas? It’s the same idea with a diesel. Years ago I had a 1980 International Scout with a 6 cylinder turbo-charged Nissan diesel engine, 198 cid (3,246 cc), 101 hp @ 3,800 rpm. Most of the time I drove it around town in a rather benign manner, always shifting at “normal” RPMs. Every now and then, the car behind would behave in such a manner as to cause annoyance. I would down-shift into second gear and floor it, letting it wind all the way up. The car would disappear in a big cloud of black smoke. But, it would only do it once. If I floored it again the same day, or within a couple of days, the exhaust would be clear. It would take about a week for the carbon to build up again.

        The diesels in our boats are driven most of the time just like I used to drive the Scout. Every now and then, you need to “Blow the carbon out”, per Yanmar’s recommendation.

        The five minute idle time before shutdown is very important, particularly with turbo engines. It take a few minutes for the turbo to get back to idle. Shutting the engine off stops the oil flow to the turbo, which is not good if the turbo is still wound up to high RPM, e.g. > 20,000 RPM

  • #8783

    Henry Dennig
    Participant

    Dick…fyi, I did not get the notify either

  • #8784

    John Reynolds
    Participant

    Henry: I could not find an engine power curve for the 4JH3-HTE engine. Just looking at the propeller power curve, attached, at the higher RPMs, it looks like it would cross the 100 HP curve at about 3,375 RPM. Thus, your engines and tachometers appear to be right-on. Just a guess, but I think your engine runs out of combustion air above 3,400 RPM, such that the power curve is nearly flat at higher RPMs, just like the other Yanmar 4 cylinder engines. At what RPM do you cruise?

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  • #8786

    Henry Dennig
    Participant

    John,
    We usually are running around 2600rpm, which usually has us about 8.5mph.
    Henry

  • #8788

    John Reynolds
    Participant

    Henry, What model transmission do you have? What is the ahead/astern gear ratio?

    John

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