There would be many ways to calculate that but the easiest (and probably most accurate) is to use a rule of thumb provided by my friend and boat guru David Wyman, who tells me that a typical marine Diesel engine outputs 20 HP for every gal/hr it burns. So for Spray this means the engine outputs 30 HP as we cruise. Doesn't seem like much considering Spray weighs 18,000 lb and contains everything we need to live almost comfortably.
To put that into metric units (SI units to you techies), Spray's diesel engine outputs 22.4 kiloWatts (kW) as we cruise (1 HP = 0.746 KW).
[Some technobabble : When we speak of the diesel engine's output power we mean the power of the rotating shaft of the engine. Unfortunately, some of that power is then lost in Spray's transmission, prop shaft bearings, and especially the propellor. No matter, we'll just hold all those losses as a constant and only compare shaft power of various drive systems.]
----- Warning !!! ----- Tedious Physics Calculations Ahead !!! ------
[How efficient is Spray's diesel? Well, a gallon of diesel fuel contains about 136 million joules of chemically stored energy (that's a lot!). So a 100% efficient diesel engine burning 1.5 gal/hr would produce 204 million joules of energy per hour, which is 57,000 joules per second. As you know, a joule per second is a Watt, so our 100% efficient engine outputs 57 kW. But Spray's diesel only outputs 22.4 kW, so instead of being 100% efficient its only 22.4 / 57 = .39 or 39% efficient. Doesn't sound very good but I'll bet its better than your car engine does.]
So we conclude that Spray"s diesel engine is about 39% efficient as it cranks out 22 kW of power while we cruise. In Part 2 of this analysis we'll look at the option of using an electric motor that is 90% efficient to power Spray. Wouldn't that be great?
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