Operating Cost Comparison

[Portager]

I am trying to compare the operating cost of two very different boats (displacement is about 4 to 1, one uses jet drives and the other uses surface props, ...). Normally, if maximum speed is the same operating cost is considered to be proportional to displacement, but I think that regardless of speed operating cost per hour of operation would be a function of applied horsepower. If both boats cruise at 80% of rated power, the operating cost per hour would be proportional to rated power. What do you think? Is there a flaw in my logic?

Cheers;
Mike Schooley

[Willallison]

Are you talking overall operating costs, or simply fuel consumption?

[Portager]

I'm really interested in overall operating cost, which for these boats if going to be mainly fuel. One boat weight 8.5 tonnes and does >55 knots on 2 850 HP engines. The other is 90 tonnes and does 45 knots with 2 3,700 HP engines.

I guess the maintenance cost of 2 3,700 HP engines will be higher than 2 850 HP engines, but less than 4.35 (i.e. 3700/850) times as high.

Cheers;
Mike Schooley