Hi Bill,
I know I won't convert you (or your boat), but I wanted to try to clear up some misconceptions.
A prop that delivers thrust will not be very efficient at producing torque from the water stream (aka produce usable power). So you are going to need an adjustable prop that can either figure which mode it is in for itself or have a user selectable switch/lever/button. My suspicion is that will be expensive in the extreme.
The long range cruiser could certainly benefit from this right up to the time he drops anchor. Then you have to fall back on solar, wind, or a genset.
We agree that the ideal driving prop won't necessarily be the best charging shape. But ANY prop is going to turn to some extent when the boat's in motion. No fussy prop required. So a more realistic expectation is - will a nice daysail that inefficiently spins the prop still regenerate the electrical energy required to motor out from and back to your dock or mooring? And in many cases the technical answer is already yes.
If you concede that battery tech is not going to cut it for the foreseeable future then you are forced to embrace a genset driving an electric motor. So what is the advantage? Pumping it into the batteries for use later is a 20% reduction in efficiency. You might as well just stick with a gas driven prop. Your genset would have to be at least as powerful as your electric motor. Presumably you will want to have the capability to achieve hull speed. If that is true then you would need a genset that was approx 10% larger to account for the gas motor to electric motor efficiency. Does not make sense to me.
I'm only trying to make the case for
hybrid systems, which would always include a generator. So in my scenario, the required battery capacity would be for only 30 to 60 min of motoring, with the expectation that generator-running is necessary to support extended motoring at hull speed. There will be conversion loss, but there's some efficiency recovered in that the generator IC engine is optimised for running at generating speed and load.
You're only looking at one part of the efficiency game. Yes, for propulsion only, the direct drive IC engine has an edge on efficiency... particularly if you're running it at it's most efficient point (eg 80% of hull speed). At different RPM the efficiency goes down.
Now let's say you need to run the engine at anchor to top up the batteries, or to run the AC or a fridge, (or a hair dryer). With a large engine turning a small alternator, you're now not very efficient at all.
The whole efficiency equation should also include energy cost, and system maintenance, and if the batteries are topped up between sails by shorepower, or solar or wind generation, then the typical boat user may not have to run the IC generator at all to support daysailing and race nights.
Another overlooked advantage of electric drive - it's far easier to quickly blip an electric motor for short powerful blasts in forward or reverse, compared to wrestling gearshift and throttle, and overcoming the higher inertia of a spinning engine. Docking should become alot easier. Notice I said
should...
Assuming a miracle happens and batteries somehow get smaller, lighter, with more AH….. a miracle, You would find yourself with a high voltage electric motor on a sail boat. To get the required HP you would need wires as thick as your arm using 12 volts. It only becomes feasible when you get above 150-200 volts. Higher is better and some are quoting 460 volts used in motors available off the shelf. Not sure I’d be manufacturing these just because of the law suits that would ensue when people start electrocuting themselves. A galvanic reaction at these voltages would be dynamic to say the least. Stray currents in the water killing folks swimming near the boat (hay that could be me!!). A whole host of issues to failsafe. No doubt this would be expensive to engineer if even possible.
I suspect you're overestimating the electrical power required.
Typical rule of thumb is to estimate 2.5 kW per ton of boat to do hull speed, and only 500w/ton for cruising at 70% of hull speed. For sailboat use, the systems seem to be designed around 144v DC max, with 48 and 72 v DC systems being more common. So from a voltage perspective, it's no more lethal or tricky to manage than shorepower, with currents averaging well under 100A.
Magic batteries aren't required. Hybrid power for boats is feasible now, and will only get more attractive as the advancements in auto EV batteries trickle down.
(further apologies to the jetski engine aficionados for usurping the thread. But you already know why jetski motors won't be appearing on sailboats anytime soon
)
