How to calculate the power of a pleasure boat engine?

In administrative horses (tax horses), steam horses or kilowatts, the power of the boat engine is expressed in multiple ways. To calculate the annual francization and navigation tax (DAFN), only the power of the engine expressed in tax horses interests us.

Experts in the boating of boat-immamarculation.com decipher for you all the ways of measuring the engine power of your boat and their usefulness.

The power of the steamer's boat engine

The horseshoe (CV in French) is a unit of measurement invented by James Watt, Scottish engineer at the end of the 18th centurye century. In order to assess the power of the steam machines of his time, he estimated it according to the physical capacities of a draft horse to draw a charge.

The steam horse is commonly used but is not one of the units of powers recognized by the international unit system.

This is why on the new certificates of recording of pleasure ships, delivered since 2019 in France, only the powers expressed in KW and CV (CV for administrative or tax horses, see below) are indicated

The abbreviation of the horsepower is different depending on the country:

• resume For French -speaking countries
• hp For HorsePower in English -speaking countries
• pk to Paardenkracht in the Netherlands, hence the unit used on ICP (International Certificate of Pleasturecraft) issued by the Watersport Vetbonde

Beware of the confusion between the CV abbreviation which designates vapor horses but also administrative CV. Although there is no equivalence between the two units, for example a Suzuki DF115 CV vapor engine has a power of 11 administrative CV. The order of magnitude makes it possible to identify which type of CV is used.

The power of the boat engine in kilowatts

We readily use this real power unit expressed in kilowatts (1kW = 1000 W) for a boat engine because it is the reference worldwide.

The Watt is the unity of international power, so named in honor of James Watt. A horsepower is equal to 0.736 kW and 1 kilowatt (KW) is equal to 1.36 CV steam. There is therefore a possible equivalence between CVs and KWs. On the other hand, there is no possible equivalence between a power expressed in steam or KW with administrative CVs.

In France, only VNM (motor nautical vehicles, seas scooters) are subject to a calculation depending on the power expressed in KW and not in administrative CV.

The power of the fiscal horsepower engine

Expressed in CV, the tax horse (or administrative horse) is an unit of measure used in certain countries for the establishment of the registration certificate of a land or maritime vehicle. Please note, the calculation of the tax power of a car and a boat engine are completely different. Likewise, the formula used for the administrative power calculation of a boat engine in France and in another country, is also different. This is why the power inscribed on a “dichiarazione di potza” of a boat engine purchased in Italy, does not correspond to French calculation.

In France, for a boat engine, whether it is a sailboat or a motor boat, the tax calculation method takes into account several parameters: number of cylinders (N), bores in millimeters (A), race in millimeters (C).

The bore is the internal diameter of the piston. The race is the piston's travel distance inside the cylinder. They are multiplied by a different constant depending on the engine fuel (diesel or petrol):

fiscal power of a diesel engine = (315 x n x a2 x c) / 100,000

fiscal power of a petrol engine = (45 x n x a2 x c) / 10,000

Why should the administrative power of my boat be known?