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That's the graph for the motor. It reduces at low speeds because the losses are proportional to current/torque but output power drops as rpm decreases, so at half the rpm but the same torque the percentage loss doubles. But as I tried to explain this doesn't matter for a motor driving a propeller, because you never get into the bad efficiency low-rpm/high-torque corner -- when you follow the prop torque curve down as rpm drops the efficiency actually gets a little better, see the figures I gave.

Which I wouldn't do if I had "lithium-ion" batteries (e.g. NMC chemistry) instead of LFPs. But then I don't think anyone with a boat should touch NMC or similar with a bargepole of any length, even a ten-foot one wouldn't be long enough if they caught fire... You'd hope that insurers will see sense about this, especially with the increasing use of batteries for off-grid/feed-in applications in home -- nobody there suggests that you should not charge them (LFP!) from solar or the mains when you're not in the house, so why should boats be any different? Unlikely to happen, any more than it does in houses with off-grid/feed-in setups with batteries... 😉 It's been pointed out to the BSS (via their technical committee) that though this advice is correct for the batteries in many portable devices (including power banks, ebikes, escooters...) which do pose a fire risk if overcharged, it does not apply to correctly designed inbuilt LFP batteries on boats.

From a discussion on the Electric Narrowboats group on Facebook, for those who don't use it but might be interested... ============================================================================== There's still a lot of misinformation/misunderstanding about prop/motor choice including oft-repeated statements like "they're different to diesels" or "low torque at high speed changes things", so I'd like to try and clear this up. There's no fundamantal difference; to get the full rated power out of a motor (electric or diesel) you need to match the prop size to the maximum power rpm. Overpropping means you can't reach full power/rpm, the rpm stops rising when propeller torque (proportional to rpm^2 -- power rises as rpm^3) hits the maximum motor torque. For a typical electric narrowboat, to keep prop size up and noise down you don't want to go much over 1000rpm at full power, and for a motor rated at 14~15kW/130-143nm (e.g. Torqmar, Engiro) you end up with something close to an 18" x 12" prop (or 16" x 12" 4-blade, which I've got and so has Paul Sumpner -- and this was also the conclusion of the recent Ortomarine trials. (Exact equation : Nm = 9550 * kW/rpm) This is why not just the headline power of the motor matters, but also the rated rpm and torque -- many of the motors being sold for direct drive on canal boats are really too high-speed/low-torque, possibly because high torque at lower speed needs a bigger and more expensive motor, or the main target market for the manufacturer is faster seagoing boats where higher speed props are fine. The reason "low-speed torque" doesn't matter is that propeller torque drops rapidly at lower rpm -- and this also affects efficiency. I've attached the efficiency plot for the Engiro 12013 (14kW/130nm/1080rpm), and this is what happens as rpm drops: 1080rpm : 14kw/130nm/91% 850rpm : 7kW/80nm/92% 650rpm : 3kW/46nm/92.5% 500rpm : 1.4kW/27nm/93% If you put the same size prop on a lower-torque higher-speed motor, this will limit the power and rpm to below the rating of the motor. For example, a 15kW 1500rpm motor (e.g. Bellmarine 15/20kW) has 95nm of continuous torque, so with this prop will top out at 920rpm, at which point power will only be 9.2kW. Of course you can push more than this for short periods (e.g. short-term rating (30 seconds?) is 127nm), but that doesn't help you on a river (see below). For many people with motors like this, such overpropping will be a better choice than the much smaller noisier "egg-whisk" matched prop for normal cruising, so long as you're aware that you can no longer achieve the full rated power -- a bit more than 60% in this case. Which will only matter if you ever need it, for example when travelling upriver against a current for any distance. The more overpropped the motor is, the more that maximum power/rpm drops off -- put an 18" x 14" on the Bellmarine 15kW and it'll top out at about 8.5kW/95nm/850rpm, again fine (and quiet!) for normal canal cruising, but now down to not much over half the rated power should you need it on a river. This downside of overpropping matters more on electric boats than on diesel boats since they have much less power to begin with -- an overpropped Vetus 10kW might end up only able to sustain about 7hp, which is very definitely on the low side for anything other than still water. Of course all this only applies to direct drive motors, but that's what most electric canal boats use for various very good reasons...

African or European?

Several decent beers every time we were there, but I don't recall exactly what. Possibly due to the amount consumed... 😉

Was friendly the last couple of times we were there, with decent pub grub and beer. Not many customers on midweek evenings when we were there, but landlady said it could get packed on Friday/Saturday so if you want to eat there I'd check ahead... 😉

Well colour me surprised... 😉

That's why I know it'll be raining on Saturday... 😞 https://www.facebook.com/Smiffs.W6

I can only assume you think this would make the canals "suitable for navigation" because you can use your fabulously effective anchor and winch to go up and down them? 😉

Of course not, because the government's own figures show that the biggest overall benefit from the waterways is to the much larger number of people who use them for other purposes -- walking, cycling, fishing -- which is how the DEFRA grant is justified. Though strictly speaking they don't have to be *navigable* for those users, meaning working locks and paddles and so on, just kept in water. OTOH it's quite possible that the cost of just this (keeping them navigable, as opposed to in water for the non-boater users) *is* paid for by boaters, including license fees for both private boaters and hire companies, the 9% slice from most marina fees, and direct payments to CART from EOG/farm and online moorings.

<sigh> Ignore the fact it was a literal reply to a snide comment your bestie made, and throw some more fuel onto the fire... 😞 Onto the naughty step again, maybe you'll be happy there together...

I didn't say they *couldn't*, just that they're bred for slow heavy load pulling, but the heavier the animal is the less well suited it is to moving more quickly, and the more quickly it gets exhausted and overheats when doing so -- as I'm sure you're well aware of, even elephants can get a move on if they have to but they're not really built for it... 😉 Getting under bridges is certainly an issue in some places, it's one reason cobs or donkeys were often used on the canals, but whether it is on the Lancaster I don't know.

It's unlikely they'd have used heavy draught horses, these are bred for pulling heavy loads (ploughs, wagons) at walking pace, not trotting at 8-10mph.