<<How many hours? How much thrust? All of this will be a probabilities/ personal comfort assessment, right?>>

  Certainly.  With zero thrust and zero hours being an option. 

  Which one could argue is more of an option with an HP than with most other boats because of the ability to go to neutral in any wind and direction, convert to bare poles, and be able to take waves "astern" safely with wave-piercing bows, slowed and prevented from pitchpoling by trailing series drogues.

  But let's say you need a good four hours of 30 horsepower on tap.  My friend needed an hour, but we'll say two, such as if you have to reposition a few times in an anchorage during a big blow, or you need to motor continuously for a few hours while on anchor because the hook is dragging and you can't set a second. 

  We want four hours of storage because  a) we can't assume we're at 100% when the storm starts, and  b) we can't assume full discharge from full charge.

  That's roughly a 90 KWH battery.  With a gravimetric energy density of 110 watt-hours per kilogram from your wikipedia article, that's 1.800 pounds -- way too much weight for a weight-sensitive multihull.  Even if you cut that in half... wow.  It's a lot more realistic for a ballasted monohull, particularly if you can keep the weight low and use it to add to stability.

  If you went with the more-flammable lithium-ion battery, you could get a 1,200 lb 85 KWh battery from Tesla for 3.8 hours at 100% charge, or let's use a more realistic 2.3 hours assuming we're running between 20% and 80% charge in order to not stress the battery.

  On the other hand, if the boat is an ex40 or one of the cruisers, driven by a 40 horsepower honda outboard on the tender, it will burn 3.9 gph at wide-open-throttle.  Or 100 pounds of fuel.  Which means you could store 200 pounds of fuel for an eight-hour stint.  And/or just go get more when you run out.  With a 33% increase in horsepower as well.

  It's tempting if you want to be cheap, as light as possible, and lazy in terms of innovation.  Also lazy in terms of repairs -- outboards are about as simple as it gets if you want to drop them off at a repair shop.

        - Mike

 

|   There's a lot to be said for having some serious thrust available for several hours straight.

Is there not more to be said for having a safer designed boat to start with?

How many hours? How much thrust? All of this will be a probabilities/ personal comfort assessment, right?

I am not enough of a sailor to suggest a critique of your given example. But I am not sure the best lesson learned is MORE POWER! Are you sure that it is?

|   You may never need it, but that said, most cruisers I know have at least one or two stories to tell, often involving a boat getting dragged in a crowded anchorage (either their boat or another).

That's an interesting point. Perhaps the answer is always have more power on tap.

|   But...  I don't think I'd go with lithium batteries on a cruiser that leaves sight of land.
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|   Even though the LiFePO batteries are more stable than lithium-ion, they still burn, and they don't go out until the boat is submerged. Which is a problem if you're depending upon that boat. 

Firstly, the risk of a lithium battery fire is very small. Especially a bespoke hand made and well instrumented system like you or I would build.

Second, the best way to put out a lithium fire is water, because cooling is the most important thing. So, I disagree. It is rather easy to stop a lithium fire on a boat, cause water.

Epoxy and foam burn, and they won't stop once started.
Catching lithium on fire is, perhaps, about as easy or hard as catching epoxy and foam on fire.

Keeping lithium batteries cool is stupid easy on a boat surrounded by water and with super cheap battery monitoring chips, it is cheap, light, and easy to know the exact health/state of your battery packs.

I am not trying to debate your personal comfort level. I am suggesting your personal concerns seem a bit miscalibrated to me.

| For a Tesla on the highway, you can step away and get picked up by AAA.  For a boat in the middle of the ocean, or a crowded anchorage, the situation becomes more complex.

One builds faith and understanding in the technology through safe exposure over time. Yes, do not rush to cross an ocean in a new bespoke boat and power system. Try it close to land first, perhaps.

But, I bet, in the final history of man, more boats will be lost to out of control hydrocarbon fires than will ever be lost to lithium battery fires.

|   I /really/ want to skip the internal combustion motor altogether, but for now, there's not really a good substitute.

I agree. But the gap is closing and fast. As I eye the time to decide the power system for my boat, and the closing gap, I am pretty sure the gap will be well closed before I must decide.

But I am often wrong.