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ICOMIA 28-83

The International Council of Marine Industry Association's Standard 28-83 is often cited by manufacturers of outboard engines in the United States, but access to this document is difficult via on-line sources. Accordingly, a text version of this standard is reproduced below so that it might be available for reference by users of this website, where frequent discussion is held in our forum concerning outboard motors and their horsepower ratings.

This document is a best attempt to accurately reproduce the International Council of Marine Industry Associations' (ICOMIA) Standard 28-83, "Power measurements and declarations for marine propulsion engines and propulsion systems" in a text format. The source for this information was an on-line document which appeared on the ICOMIA website as a JPEG file, i.e., a scanned or rasterized version of the document presented as a graphic image instead of as readable text. Every effort has been made to make an accurate transcription from that rasterized image to HTML text, but there is no guarantee of suitability for any purpose of this document.

This document is presented for informational purposes only and is not to be relied upon as accurate or authorative. Users who require accurate and authorative copies of this specification should contact ICOMIA and arrange for purchase of such documents as necessary.

This information is provided to facilitate understanding of horsepower ratings by outboard engine manufacturers who make reference to the standard in their literature and employ the standard in rating their engine horsepower specifications.

(Begin transcription of the ICOMIA 28-83.)

Internation Council of Marine Industry Associations

Standard No. 28-83

Power measurements and declarations for
marine propulsion engines and propulsion systems

1 Scope and field of application

This Standard specifies the test requirements in addition to those given in ISO 3046 for determining the power--at a single point or as a power curve--of marine propulsion engines or systems for recreational and small commercial craft.

It also provides a means for documenting and controlling the declared (rated) power published by the manufacturer.

2 References

3 Terminology and Declarations


Declared (rated) speed is a crankshaft speed, specified as follows:

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.

3.1.2--In the case of governed engines, the declared speed shall be choosen by the manufacturer.


Corrected power shall be the full throttle power of an engine or propulsion system measure in an engine dynamometer laboratory as specified in this standard and corrected to the standard reference conditions specified in ISO 3046/1 using the correction methods specified in ISO 3046/2.


Declared (rated) power of an engine model or propulsion system shall be the full throttle power at the declared (rated) speed at the final output shaft of the engine or propulsion system as offered for sale by the manufacturer, based on corrected power of one or more engines or propulsion systems

3.3.1--Power shall be declared as Propeller Shaft Power at the propeller shaft of engines sold with complete propulsion units, and at the couple to the propeller shaft of engines sold with reduction or reversing gears.

3.3.2--Power shall be declared as Crankshaft Power at the engine output shaft of engines sold without reduction or reversing gears, stern drive or sail drive units. In such cases, the power declaration shall be accompanied by a statement that usable power will be reduced by gear losses.

4 Test procedures

4.1 Engine Installation

4.1.1--The test engine or propulsion system shall be representative of the maufacturer's production units and shall be fitted with the exhaust system and all auxiliary equipment delivered with the unit except the propeller. Auxiliary equipment shall be listed and described.

4.1.2--Carburettor wedges may be removed or added to maintain carburettors in a normal running angle if the engine is run in a horizontal position.

4.1.3--If it is necessary to utilize auxiliary equipment, such as an exhaust pipling, nor furnished with the propulsion system, the exhaust back pressure at rated speed shall be within 0.75 kPa of the maximum back pressure specified by the manufacturer at which the declared power can be achieved. If the exhaust system as delivered is not complete and no back pressure is specified by the manufacturer, the unit will be operated at 10±0.75 kPa back pressure measured at the exit of the engine exhaust manifold or turbocharger. If the exhaust system as delivered is complete, the laboratory exhaust system shall maintain exhaust pressure at the unit outlet within ±0.75 kPa of barometric pressure at the test bed.

4.1.4--If the engine air inlet is connected to a laboratory air system, the system shall supply air to the engine within ±0.75 kPa of barometric pressure at the test bed.

4.1.5--For liquid cooled engines, the temperatures of the coolant at the sea water inlet shall be maintained at 293±15K (20±15°C) except that for intercooled engines the temperature shall be maintained at 293±5K (20±5°C). The coolant supply pressure shall not exceed 50 kPa. The coolant outlet temperature shall be within the range specified by the manufacturer if a range is specified.

4.1.6--Fuel temperature at the inlet of the compression ignition fuel injection pump shall be controll to 313±3K (40±3°C).

4.2 Fuel and Lubricants

4.2.1--Fuels used shall conform to the manufacturer's specifications.

4.2.2--For spark ignition engines, record research (ISO 5164) and motor (ISO 5163) octane numbers and density (ISO 3675) or API gravity of liquid fuel.

4.2.3--For compression ignitions engines, record cetane numbers (ISO 5165), density (ISO 3675) or API gravity and heat content. Use ASTM D-975-2D or equivalent fuel if compatible with the engine.

4.2.4--Lubricating oil used shall conform with the manufacturer's recommendations. Record oil classification and viscosity (if applicable) of the lubricant.

4.3 Run-In

The engine shall be run-in according to the manufacturer's recommendations.

4.4 Test Conditions

4.4.1--Data shall be obtained under stabilised normal operating conditions with an adequate fresh air supply to the engine. Test conditions such as inlet air temperature shall be controlled as near to standard as possible (Paragraph 3.2) in order to minimize the magnitude of the correction factor. Adjustments shall be made before the test in accordance with the manufacturer's instructions. No changes or adjustments shall be made during the test, except as indicated in the test procedure.

4.4.2--The generator or alternator, hydraulic systems, and similar systems intended for intermittent operation, shall be operated under no-load conditions.

4.4.3--No data shall be taken until torque, speed, and engine temperature have been maintained within 1 % for at least two minutes.

4.4.4--Engine speed should be held as nearly constant as possible during a run or reading, and shall not deviate from the nominal speed by more than ±1 % or ±10r/min whichever is greater.

4.4.6--Wet exhaust systems shall be operated with normal water flow.

5 Data Acquisition

5.1 Accuracy

Test measrements and their degree of accuracy shall be as specified in ISO 3046/3.


Temperature of the inlet air to the engine (ambient air) shall be measured in a manner to get a mass average temperature. The temperature shall be taken in the engine inlet air stream or with 150mm of the air inlet horn to the air cleaner (inside the engine cowl, if furnished).


Temperature of coolant medium shall be measured within 150mm of the sea water inlet. Water jacket temperatures in liquid-cooled engines shall be measured at the inter-cooler inlet if applicable,or at an outlet point of the engine specified by the manufacturer. Temperatures in air-cooled engines should be measured at point(s) specified by the manufacturer, such as spark plug(s) or cylinder heads.


Oil temperatures shall be measured at point(s) specified by the manufacturers.

5.5 Readings

Observed dynamometer load, speed, ambient air pressure, wet and dry bulb readings, and fuel consumption data (if recorded) shall be taken simultaneously, and shall be the average of at least two stabilized sustained values which do not vary more than 1%. A measuring interval of not less than 30 seconds shall be used when measuring speed and fuel consumption.

5.6 Recordings

Power test data to be recorded for test documentation:

6 Presentaton


A single value statement of power shall be accompanied by a statement of the corresponding speed. Alternatively power and speed may be represented as a power curve. Declarations shall indicate that the power is Crankshaft Power or Propeller Shaft Power, whichever is applicable.


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%. If the highest power exceeds the declared power by more than 6%, both powers shall be stated for that model.

7 Manufacturing tolerance

The corrected power at rated speed of any individual marine propulsion engine or propulsion system must not deviate more than ±10% or 0.45kW, whichever is greater, from its declared power, except that for governed engines or systems of more than 100kW [134-HP] the tolerance shall be ±5%.

(End transcription of the ICOMIA 28-83.)


For outboard motor enthusiasts, this standard clarifies several frequent questions. First, the "rated power" is to be the corrected power after careful measurements at the "rated speed" which is to be the mid-point of the full throttle range recommended (3.1.1). Thus, if a manufacturer declares the full throttle range to be 5000-5500 RPM, then the rated power should be derived from measurements made at 5250 RPM.

The "rated power" is to be measured at the propeller shaft. (3.3.1)

The "rated power" is recommended to be presented so that there is no more than a 6% difference between the rated power and the peak power which occurs in the full throttle speed range. (6.2). For example, an engine rated at 225-HP should not have more than 238.5-HP at any speed in the full throttle speed range. In general, the measured horsepower will be greater with engine speed. This means that if the full throttle speed range is chosen to be a wide range, for example 5,000 to 6,000 RPM, it is possible that the engine's peak horsepower will occur near 6,000 RPM, but the rated horsepower must be that obtained at 5,500 RPM. The difference between these two horsepower ratings is recommended to not be greater than 6%. It appears that manufacturers have some room to fiddle with the engine horsepower rating by adjusting the RPM range which they declare to be the the full throttle speed range of the engine. Rating of the engine horsepower in this way accounts for some of the claims seen in which engines of a particular brand and rating can test at significantly higher horsepower in actual field tests.

The tolerance for variations in manufacturing permits engines to deviate not more than ± 10% from the "rated power" (7.0). Thus an engine sold as a 150-HP engine could produce as much as 165-HP or as little as 135-HP due to variations in manufacturing tolerances. Note that the more restrictive tolerance mentioned (± 5%) applies only to "governed engines" (i.e., engines running under a governor) whose rated power is greater than 134-HP (100 kW). It is my understanding that outboards are ungoverened engines in the context of this standard, and thus all individual production units are permitted a ± 10% tolerance in actual propeller shaft horsepower compared to "rated power."

DISCLAIMER: This information is believed to be accurate but there is no guarantee. We do our best! It is highly recommened that true copies of the document presented here be obtained from ICOMIA.

This article first appeared November 16, 2003.
Portions are Copyright © 2003 by James W. Hebert. All rights reserved.
Permission to reproduce for non-commercial, personal use is granted.

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