You don't have to travel at 1mph on an electric boat, and I suspect that very few boaters do since (see spreadsheet I posted) you could do 2mph on about 750W (400rpm at the prop -- all those numbers are for an 18" x 12" prop for both diesel and electric boats), and that's exactly the power figure/rpm I've seen quoted by some electric "slow boaters" who are happy to dawdle along enjoying the scenery... 🙂 However that's also a bit slower than a Beta 43 at tickover (900rpm engine with a 2:1 gearbox), so you'd be holding up a following diesel boat even if they've dropped back to tickover... ☹️ The efficiency numbers including losses really show why the claims of "a series hybrid is more efficient because the diesel engine in it is running at optimum power and rpm" -- typically quoting efficiency figures of 30% or more -- are just not true in real life. The engine might be doing that, but at the output of the generator (including all losses) maximum efficiency is usually about 25%, see the measured results from the Victron generator report. Then there's the loss in the inverter used to convert this to DC, I've used 90% efficiency (as measured for my Quattro 48/10000). Then there's the efficiency of the motor controller and motor, I used 93% for each (taken from actual data sheets). Put all that together, and even at a not-especially fast cruising speed (3mph/600rpm) with 2.5kw at the prop, power out of the batteries is 2.9kW, power out of the generator to recharge them is 3.2kW, power in from the fuel is 13kW -- that's <20% overall efficiency from fuel to prop, nowhere near the 30+% often claimed. Depending on whose figures you believe (17% or 22%) for the Beta 43 at this speed, that's about the same -- what it certainly isn't is "much more efficient". And if you go faster -- especially going against the current on rivers! -- the diesel is definitely more efficient, which I expect will come as a shock to many electric aficionadoes... 😉 So the "green" gains with electric/hybrid drive come from going more slowly (*big* gains with electric), lower fuel use at tickover and zero when stopped (e.g. for locks), and the fact that power from solar can be used instead of it all coming from the generator. And these gains can be very big, in some cases (but not mine) people don't use a generator at all, at least in summer -- though this is pretty much impossible to avoid in winter. But the biggest gain is nothing to do with this, it's "silent cruising" -- IMHO that's the real reason for going electric/hybrid... 🙂 Ludicrously inefficiently, somewhere around 5%... 😬
