Generally do very few passages. In the past 2-3 per year over 1500nm. Now expect no passages with longest uninterrupted run ~500nm. Trend is to change cruising grounds then do very short hops (25-50nm) every few days. Often stay in one place for over a week. When ever feasible anchor out.
How many others here fall into the same or similar cruising pattern? If you do something else, what?
Having studied this problem for a while, I’ve arrived at the conclusion that, within acceptable tolerances*, a figure of 1 kWh per nm for propulsion makes a good starting point. So, using the middle of the 25-50 nm range, we get 37.5 nm. Add a few percentage to that to cover errors and omissions, and a comfortable number appears to be 40 kWh to cover most cruising needs, with maybe 60 kWh covering all the “short hops” running.
I’ve never gotten a clear read on how much of a budget to use for house power, parasitic loads, and what appears to be referred to as “hospital power” (a term I’m not familiar with). However, being comfortable with guessing, I’m going to budget 3 kW of continuous consumption for those loads. With 7 kW consumed for propulsion while underway, a total of 10 kW, with the appropriate amount of squinting, feels like an acceptable number (I’ll address those error bars).
Looking into generator consumption, it appears that a 16 kW generator at 3/4 load (12 kW) consumes about 1 gph. So running at 1 gph of fuel consumption should get us 7 nm, with a 20% budget for total consumption errors (12 kW produced vs. 10 kW estimated for use). Note that this doesn’t necessarily mean running at 7 knots. That 1 kW/nm depends heavily on hull shape and displacement, and might mean 5 knots in an inefficient boat.
So, running on generator for long runs, it looks like 3/4 load will produce sufficient energy to go seven miles on one gallon of fuel. That sounds like amazing efficiency, but not so far out that it’s ridiculous (how far will your boat go on 1 gallon at slow speeds?). Given that, just over 70 gallons of fuel will be consumed in one of the “long” runs of 500 nm.
I hear a lot of grumbling in the back of the room, so I’ll say this: as Hippocampus described above, a lot of this depends on hull design. But it FEELS to me like this model is a good one; a workable one.
Now the trick is to find a hull that is a) livable (because that’s the key here; we’re not racing, we’re cruising); and b) moves through the water an an acceptable speed on 7 kW of motor power. To the question of what constitutes an “acceptable speed,” well that’s left to the skipper, and how long they want to spend at the helm to get where they’re going.
And, to address those error bars further, even if I’m off by 100% on the propulsion consumption, and it’s really 2 kW/nm, that’s still reasonable efficiency. Generator sizing is down to a tolerance for how long you want to run the generator to power the batteries, not how long you want to be underway (which is the case with diesel-driven boats; the engine is running ALWAYS while underway, and the generator is running sometimes when stopped).
Thoughts?
* A generalization, and depends entirely on hull shape, displacement, drag, etc., etc., etc. There are wide error bars in the curve.
