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First, I don't see how 1800 RPMs is to slow for a Yanmar 4JH3E, but that wasn't one of your questions.
The problem with the pods is lack of variety with props. I think there are only two or three, all small diameter, and all skinny bladed.
For a displacement boat, the most efficient prop will be the largest diameter you can fit, with the most blade area you can find, running at the slowest RPM you can make it go, with a large pitch. Think TUG BOAT prop.
All that being said, there is probably no good reason to change the prop you already have. My calcs put your prop RPM at 674 with 1800 diesel motor RPM, and that seems like a pretty efficient prop speed to me.
Now using only the calc's you posted, you are going to need 6750 Watts (9hp) to maintain 6 knots. For that I would definitely go with the 12kW water cooled motor paired with a controller that will output 300Amps so that you are usually only running them at half capacity.
For the belt drive figure out and get the closest ratio that will give you 674 prop RPM at the most efficient Motor RPM (consult an efficiency curve chart for your electric motor). You can always change pulleys later to change the ratio to suit your needs. Seems I remember a youtuber with a sailboat that wound up changing his all the way down to 1:1 (direct) to be happy. I think that channel is "The Neds".
So running near 7kW for for an hour would use 7kWh (surprised, I know) and that equates to about 2 1/4 hours (with no safety factor) with a single 48V 314Ah battery that would need to be able to discharge at 130A continuously. That's only about 16 miles. Add a same size battery in parallel and double your time and range and halve your battery discharge rate. I have seen where there are larger cells, like 600 or 800 Ah or some such crazy thing, but I would prefer the redundancy of multiple parallel smaller batteries (up to 4), than just 1 large one.
One youtuber (Slight Adrift) listed his stats as actual measured 6850W @ 6knots (which is really close to your theoretical 6750W @ 6knots).
But when he slowed to 5 knots he measured only 3840W, and only 1920W @ 4 knots.
So basically, if your boat had real similar usages, then slowing down to 4 knots would give you a range of about 38 miles and 8.3 hours (for each 16kW battery), with about 24 miles and 4.2 hours at 5 knots and 16 miles and 2.3hours at 6 knots.
To get the full output from your 3.5kW generator, you would need a 70Amp 48Volt battery charger. If you had that you could probably get almost 5 knots without draining your batteries.
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