Scaling Outboard Engine Declared Mid-WOT SHP to Maximum SHP

Discussion in 'Outboards' started by SimonR, Jan 10, 2019.

  1. SimonR
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    SimonR New Member

    In the absence of outboard engine Shaft Horse Power (SHP) manufacturer data, with the exception of the declared mid-WOT value, scaling to the maximum WOT value has been performed on the basis of:
    SHP proportional to Δ x V^3
    where Δ is boat's loaded displacement & V is boat's velocity.

    For the same boat, the following relationship should be approximately true in the WOT range:
    (SHPmaxw / SHPmidw ) = (Vmaxw / Vmidw) ^3
    where maxw refers to values at maximum WOT and midw refers to the mid-WOT range values.

    Thus:
    SHPmaxw = SHPmidw (Vmaxw / Vmidw) ^3

    The 1983, International Council of Marine Industry Associations' (ICOMIA) Standard 28-83, "Power measurements and declarations for marine propulsion engines and propulsion systems", states:
    · Paragraph 3.1.1. In the case of ungoverned engines, the declared speed shall be the mid-point of the full throttle speed range recommended by the manufacturer for propeller selection.
    · Paragraph 3.3.1. Power shall be declared as Propeller Shaft Power at the propeller shaft of engines sold with complete propulsion units……
    · Paragraph 6.2. It is recommended to choose the full throttle engine speed range mentioned in 3.1.1 in such a way that the highest power within this range does not exceed the declared power by more than 6%……
    · Paragraph 7. Manufacturing tolerance. The corrected power at rated speed of any individual marine propulsion engine or propulsion system must not deviate more than ±10%……

    The measured performance data for three similar small, light, monohull planing boats all powered by Honda BF60A outboard engines, with a WOT range from 5000 to 6000 RPM, were considered. The table below provides the measured RPM vs. boat velocity for each planing craft, the declared mid-range SHP at 5500 RPM (European BF60a's are short-changed by about a HP), the potential maximum power at the ICOMIA 6% & 10% levels and the estimated maximum SHP at 6000 RPM scaled from the measured boat velocities at 5500 & 6000 RPM. Clearly, for an individual engine at the high end of the 10% manufacturing tolerance, the maximum power would be greater than the recommended 6% value of the declared power. A Honda representative has stated there can be "significant" variation in the mid-WOT power between engines of the same model but the declared rating should be considered a minimum value.

    The table shows that in each case the predicted SHP at the maximum of the WOT range is significantly greater than the maximum SHP required by ICOMIA Standard 28-83. Maximum SHP values of 75-86 HP for a nominal 60HP engine do not appear credible.

    Any insights into these results would be very much appreciated!

    upload_2019-1-10_20-59-55.png
     
  2. DCockey
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    DCockey Senior Member

    What motivated this study? Is this a student project?

    In the table each boat, except perhaps at the the highest speed, was not traveling with the throttle fully advanced which is also known as "wide open throttle" or WOT, and the engine was not producing the maximum power is is capable of at the stated speed.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    It might be useful that the object of the exercise be explained.
     
  4. DCockey
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    DCockey Senior Member

    Two sets of test data from the Honda outboard website. The data for these boats which SimonR posted above is obviously from these tests.
    SeaArk 1872 SS test data: https://cdn.powerequipment.honda.com/marine/pdf/props/SeaArk 1872 SS - BF60 Tiller.pdf
    Starfire 1600 SC test data https://cdn.powerequipment.honda.com/marine/pdf/props/Starcraft Starfire 1600 SC - BF60.pdf
    Agree. My guess is this is some sort of school/university project or exercise. We will see if SimonR responds.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    As noted above....a question (objective) would be nice.
    Can see one thus far...
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    It has long been obvious that the sticker HP number on outboards is only a rough guide. There have been occasions when makers have had 140hp 4 cyl engines and 150 hp sixes in the line-up, and there was vastly more than that nominal 7 % or so difference between them.
     
  7. DCockey
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    DCockey Senior Member

    Any data to support this claim? Are you comparing peak horsepower at the prop or something else?
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Some 2 stroke engines would wind out to higher revs because of porting, carb jet, timing advance, tuned exhausts etc, on basically the same block, than more conservatively equipped engines in the same line, the differences were not noticeable at "sensible" operating rpm, only at WOT, although the nominal HP might be 50% greater. I know fairly recently Suzuki had a 140 hp 4 stroke engine as well as a 150hp, and it was clearly the impression of users that the difference was chalk and cheese, in the bigger block 150 being much stronger.
     
  9. SimonR
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    SimonR New Member

    I wish I was young enough to be a student again!

    The original motivation was to understand why I was getting a lower top speed (by 4 knots) when I replaced my original BF60A (176 engine hours) & propeller with a brand new identical set-up, installed exactly the same on the transom and with the same boat weight distribution. Simple Crouch & Wyman speed formulae calculations failed to reveal anything except, as many people testify and to the vexation of physics purists, the Crouch ^1/2 did better than Wyman's ^1/3. I then carried out a series of Savitsky type calculations to compare with sea trial measurements and GoPro action camera recordings of the boat's dynamic performance over the whole speed range. The calculations overpredicted the SHP at maximum WOT, with values similar to the scaled ones in the table due to the V^2 drag components that are then multiplied V/550 to derive the EHP. I recognise my Savitsky model is probably overpredicting drag at the high Volume Froude Numbers and it does not correctly account for the dry chine and water breaking off the transom that I observe in the GoPro recordings. However, I learnt a lot about planing boats from this exercise!

    Going back to my basic issue from the engine point of view, I was surprised by two aspects. Firstly, the same model Honda outboard is not identical the world over for technical, licensing and marketing reasons. Differences are relatively small but, just like sea trial environmental conditions, can be important when comparing measurements. My biggest surprise was the large manufacturing tolerances allowed by ICOMIA Standard 28-83, to which I understand all major manufacturers subscribe. Two identically branded outboards could theoretically be 20% different at their mid-WOT power. This compromises attempts at accurate scaling, whether using speed formulae, V^3 as I have done or the aero propeller "Propeller Load Equation" using RPM^3.

    Unfortunately, Honda do not in general provide any specification uncertainties or power curves for their outboards. However, a few believable 4-stroke power curves and dynamometer measurements exist on the internet (e.g.OUTRAGE 18: Honda BF 150: I've Decided - Moderated Discussion Areas http://continuouswave.com/ubb/Forum4/HTML/005011.html). A fairly linear RPM vs. HP plot for the Honda BF150 shows the power is about 160 HP at 6000 RPM. Dynamometer measurements at 6000 RPM are quoted at around 163 HP. Assuming the RPM-HP curve is similar, this would suggest for the BF60 a power at 6000 RPM of around 65 HP, which is close to the ICOMIA 10% maximum value.

    The reason for putting my scaling table on the forum was to find out where I was being stupid. As I see it, this must be true or else my Honda engine is producing around 20 HP at the top end more than its branded power (be useful if someone had a dynamometer measurement!) or there is a fundamental shortcoming in the scaling assumptions.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    If the motor isn't fully run-in, you might expect less revs. And are you 100% that the same holes in the mounting bracket were used ? Even one hole lowered will slow them considerably.
     
    Last edited: Jan 11, 2019
  11. DCockey
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    DCockey Senior Member

    SimonR appears to have made a fundamental mistake. He appears to have equated the "Shaft Horse Power" (SHP) required to turn the propeller shaft of a boat to propel the boat at a given boat speed with the maximum (WOT) power available at the shaft of an engine at the corresponding engine speed. The power at the shaft of the engine will match the SHP (if the boat is being propelled at constant speed) BUT the power at the shaft may be less than the maximum (WOT) power at that engine speed. Except at the maximum speed of the boat the power at the shaft of the engine will always (except in very unusual situations) be less than maximum (WOT) power as shown on a "power vs engine speed" curve.

    The assumption that the estimated SHP at a boat speed less than the maximum speed represents the maximum (WOT) power of the engine at the associated engine speed is simply incorrect.

    It is not possible to calculate the maximum (WOT) power curve of an engine from boat speed vs engine speed data.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    SimonR

    It is starting to make sense now...and DCockey appears to have hit the nail on the head.

    1. Do you understand there is a difference between EHP and SHP?

    2. What "scaling" aspect are you referring:
    3. Because you then add:
    Thus where is the scaling if the vessel and its condition is the same?


    Thus is your question as simple as......."..i replaced my outboard with another and the speed is different - why?..."
    Is that it? As the rest is just noise and adds nothing.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    There are a number of possible reasons for a discrepancy, as I mentioned earlier, the motor still a little "tight" is a possibility, but there are many others, motor height not exactly the same, trim setting for your speed test, propeller differences, fuel different, different on-water conditions, (headwinds, e.g) any or several of a number of things, though 4 knots is a fair bit. You may even have had a speed-over-ground differential due to tides and currents. I would revisit it after putting a few hours on the engine, you may find it gains a couple of hundred revs when run-in properly.
     

  14. gonzo
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    gonzo Senior Member

    Outboard motors are, as you correctly posted the regulation, required to be rated at shaft HP. The exercise would only make sense if you compare your results to that. Otherwise, the proper way to get power is with a dynamometer.
     
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