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Choice Of Horsepower

Many examples of trawler yachts are grossly overpowered for cruising service. This means the full power steaming speed of these vessels tends to be in the high speed length ratio (v/√ >1.34) zone where the propulsive economy is poor.

Owners soon find in practice a cruising speed of (v/√L >1.2) is the most economical and reduce engine rpm to achieve this.

The trouble is that the power requirement at this cruising speed is typically only 50% of that at v/√ =1.34. In other words the engine is operating for most of the time at only 50% or less, of its rated power where its specific fuel consumption tends to be higher.

If we take an actual example of the published figures for a Caterpillar 3408c DITA engine, this will be clearly demonstrated.

RATING SPEED v/√L BHP @ RPM Spec. Fuel Consumption
Full Power 1.34 402 @ 1800 0.365lb/BHP-per Hour
Cruise Power 1.2 200 @ 1800 0.388lb/BHP-per Hour

We have found that a better approach is to calculate the vessels power requirement at v/√L = 1.2 add on the accessory demands such as alternators, stabilizers etc and then multiply this power by a suitable power margin, typically 20-25%, to determine the max continuous rating of the engine.

The engine will then operate in the cruising mode at 75-80% of its max continuous rating instead of 50%.

Using this approach the powering of the vessel could be by using a CAT 4306 DIT engine for which the following rpm and consumption figures would apply.

RATING SPEED v/√L BHP @ RPM Spec. Fuel Consumption
Full Power 1.3 250 @ 1800 0.352lb/BHP-Hr
Cruise Power 1.2 200 @ 1800 0.357lb/BHP-Hr

As can be seen a smaller engine has been chosen which is run at a cruising rpm of 1667 i.e. 92% of the max continuous rpm where the drop in specific consumption is less. Obviously the approach we have described cannot be a hard and fast rule because so many factors must be taken into consideration. Some owners, due to the environment their vessel will operate in may require a larger power margin. Also there is an argument that a larger engine may have a longer ultimate life. However, operating an engine at 50% or less of output power places it in the “high carbon” area which no engine likes.

Last Updated (Saturday, 29 August 2020 03:18)