What battery capacity runs 40 amps for 2 hours?

About 197 Ah in AGM or 104 Ah in LiFePO4. The energy actually used is 80 Ah in both cases. The gap is the Peukert effect at that discharge rate plus the depth of discharge guideline, which on lead acid means half the nameplate was never available to begin with.

Two reasons that compound. LiFePO4 has a Peukert exponent near 1.02, so it delivers essentially its rated capacity even at a heavy draw, while AGM at 1.15 gives up a real fraction of it. And lithium is planned at eighty percent depth of discharge rather than fifty. Together those turn a 197 Ah lead acid requirement into roughly 104 Ah of lithium.

Only if you genuinely run the motor at full power for the whole trip, which almost nobody does. A published maximum draw is measured at maximum thrust. Sizing from it is conservative and expensive, and measuring your own average draw across a normal day with a clamp meter usually shows a figure well under half the rating.

In parallel at the same voltage, close to it, and slightly better than double on lead acid because the pair is discharged at a gentler rate relative to its combined capacity. In series for a higher voltage motor it does not: two 12 volt batteries wired in series make 24 volts at the same amp hours, which buys thrust rather than time.

That is usually the real constraint rather than the money. A 100 Ah lead acid battery runs around 60 lb and a 100 Ah LiFePO4 pack around a third of that, and on a small boat or a kayak the weight decides the answer before the price does. The exact figure is a manufacturer specification for each battery rather than a general rule.

Roughly ten to thirteen percent of the bank capacity per bank is the lead acid convention, so a 200 Ah bank wants somewhere around 26 amps. Lithium accepts far more and twenty percent is a comfortable onboard figure. What matters more than the amps is that a lithium pack gets a charger with a lithium profile.