Do you use your engine 10 minutes per day to get in/out of the slip?  Do you use it 48 hours straight on offshore passages with no wind?

The practicality of electric drives depends entirely on your requirements.

Some notes for Werner

1.) all elec does not work if you want to motor (but is OK if you just use it to backout and enter the slip)

2.) the series elec diesel ( elec motor with a generator to recharge bat's) is a bad choice. = just use more fuel!!! = bad ,bad, bad

3.) the current parallels are not so great! but it is what is out there.

some thoughts, and an inquire to Mike; to make a hybrid system cost effective you need the following; in my opinion

a.) fewer parts

b) better efficiency 

c.) genset like op

A grand Idea sprung to mind reading this question, and would consist of the following specs

I.) Diesel engine eff improved by 3 to 15 % over conventional diesel engine drive designs (just by how the drive is done = no gears or clutches as we know them)

II.) elec motor restricted to 5 to 10 KW bat cap. (less than most) with eff = straight elec systems. = back out and motor into slip via elec. and = bigger bat bank for house use and a built in gen set.

III.) no clutches but could be (engine & elec to prop) (engine to prop) or (elec to prop) or engine to elec regen with no prop motion) with the reverse to prop from elec motor only.   

IV) no transmission as we know it, No starter, no alternator, no clutch all replace by one drive part = less parts and just a little more than the cost of a bigger diesel engine. (true if the elec controller is not to much)

This is a patentable Idea; so I can't share. but all of the above is not to hard and is possible, and I am amazed no one has done it yet, as it is dirt simple and very very Navy like.it is much like but not the same.

http://www.steyr-motors.com/marine-diesel-engines/2-4-and-6-cylinder/full-hybrid-propulsion-system-diesel-and-electric/

But the shortfalls would be:  full power would be limited to an hour or so, then back to the diesel engine only. and you would need 2 to 4 8D bat's = a bit more weight, and lastly operation would be more effort unless done electronically.

One footnote

On Mikes notes; of added speed under low prop RPM, this is true (well I have seen it for myself) under low rpm engine while under sail speed increase was much more than the power input from the engine. So that part I can say is a positive for the current state of elec boat motors. 

Norm,  

About 5 years ago, I read a news story about a startup company that wanted to build a starter/alternator/battery charger working off AC/7 kw. 115 vAC backup generator for your house.  It would be offered to the big 3 automakers.  Never made it to market.  I guess it was not as easy as they thought.

Re the hybrid info from Mike.  A year ago, I sent Mike some follow-up questions.  He never replied.  Below are the questions I sent.

I also have questions,  you say a modest battery bank is 15000 watt hours.  Wow!  How many batteries is that?

A 6v golf cart battery we usually say has 200 amp-hour capacity, or 2400 watt hours at 12.3 volts.   It sounds like 6 such batteries to make 15000 wh.  Right?

But the useful capacity is only about 25% of the nameplate rating.  Therefore, 3750 wh.  Drawing at 600 w means 5-6 hours on a full charge.  Is that right?  I'm trying to put  your electric drive in perspective.  Is this something for day sailors, or for long-distance ocean crossings?

Solar panels:  I use the rule of thumb that on board panels can make about 250 watt hours per day per 100 watts of panel.  If the drive is used at 300 watts, 24 hours per day, then it would need 1500 watts of panels (5 hours per day) to keep up with it.  I know one W42 owner, Drake on Paragon, who has seventeen 100-watt panels.  With that many panels, and with 12 golf cart batteries, he could do it, but on a W32, I find room for 200 watts of panels max, and only two batteries. 

I love the idea of using the drive as a generator to recharge the batteries using almost 75% of a 38 hp engine's available power to make more than 4 kw.   But 4.3 kw at 14 v means more than 300 amps of charging current.   What kind of wiring, what charge controller would one need for that?   Accidental or intermittent interruption of so much current could be a real safety problem.  I think an elaborate safety system would be needed.

Werner: Based on your comments (4kt min travel) the only option for you is Bud's bata! please see item #2 on my posting; you are proposing to do the worst possible solution = engine to gen 90%, gen to batteries 98%, batteries to elec motor 98% elec motor 95% or .9*.98*.98*.95 = 82% of what you would get from the engine buy itself. or about 20% more fuel usage. If you add solar you may get that number down to just 10% more fuel use.

Dick: You are correct a great idea is very hard to get out there (mine has almost nothing to do with the electric gen/motor, nor much to do with the diesel but everything to do with the tranny.)

a small math note: 300watts x 24hrs = 7200 watt hours not 1500 so your # of panels is 7200/250 = about 29ea 100 watt panels = wow that is allot. and if the elec motor is 98% eff (the best) the 300watt/745.699872 watt/hp *.98eff = 0.394 Hp or about the power of a cheap troling motor or about 1 kt in calm water. Dick I would like to thank you, for the comment on getting back to you, it helps in that, it is a comment on the company and will keep me from working with them. but for my design the bat would be 48Volt and the correct motor would be 26 to 28Hp knocking the amp's down 2.8Kw/48v = about 60 amps which is allot at 48v

Werner just in case you still like elec motors (please try a project I did not do but wanted to do) a troling motor at the bow (could swing down from the sprit with switches for fwd off rev in the cockpit and have the motor mounted at 90Degs to the boat) which would make a great bow thruster and if aligned with the boat, a way to back out of the slip or up to the anchor. would be cheap, and along your idea. so 20% more fuel (per work hp but there would not be to many 600watts * 15 min = 150 watt hours = 1 panel) would get you better boat handeling in the harbor and not much cost .est less than 1k for the whole elec setup.(troling motor long shaft, battery and one solar panel) labor may be an extra 1k if you do not do it yourself. P.s. I would make it so the battery could be moved up to the motor but stored aft with the panel when not in use. (wire loss thing)
lastly for regen from the prop please search this site and reread Mike's note: I don't think it can happen, Unless you are in a storm and surfing above 7kts