Going by the data from the calculations in sheet the electric motor seller provided Jim (the OP) on page 3, that hypothetical battery bank of 1917 Ah would be ~400 Ah at 48 volts, which would/should give around 35 NM at 4 knots speed and 50 NM with "reduced speed" (3.3 knots). On par with dinosaur juice. This with 9500 lbs displacement which is considerably more than you (Arne) and David are working with.

There are solar panel and electric motor efficiency numbers available on the internet, the former being I think around 20% and the latter at 85-95%?

No arguments here that the issue with electric motors is energy storage, that's the downside, albeit one of the only ones. By combining solar, wind and regeneration from the prop it's not that crazy anymore, given that after the initial investment (and future upkeep) of mainly batteries, those are free.

And let me also say that yes, if your intent is to motor for 12+ hours for whatever reason then forget about electric motors, I'm talking about sailors here. With a 2 hour range you already have a lot of flexibility. Pretty much the same as with a yuloh or sculling oar, except it's faster and you won't be out of breath when you run out of juice.

All of this said I'm still leaning towards keeping my diesel engine, because the electric alternative is quite expensive upfront and I'm also somewhat lazy and not that zealous about saving the planet... With any boat designed from scratch the largest combustion engine I'd like to have would be an outboard, something like the 6 hp Tohatsu often mentioned here. But now I have what I have, so...

A 4-stroke petrol engine runs with around 25% efficiency (..give or take quite a bit...). That means that a litre of petrol should produce around 2.3kWh mechanical energy. A single ten-litre can therefore stores about 23kWh. In 12V battery parlance that means 1917 Ah  -  and that is with loss-free batteries (which don’t exist...)

In plain English; ten litres of fuel fed to David’s or my Tohatsu 6, will move our boats somewhere between 30 and 50 Nautical miles, with a bit reduced speed.

David is right that for only motoring in and out of harbour, the electric engine would be great, with access to plug-in power at the berth. However, it doesn’t pay back the extra cost of replacing the existing petrol engine with an electric one.

The only place where I think using electric propulsion would be a practical and economical advantage, is on short-distance shuttle ferries. All other use will be based on idealism, ‘electric enthusiasm’ or special needs, such as quietness (birdwatching etc.).

Arne

They also come with less parts. The big one is the transmission and coupling. The quiet torque also has pretty much everything mounted to a motor shaped single unit that has motor mounts made to fit in most diesel replacement scenarios. The control parts also look less ready to install or at least less water proof than in the quiet torque. So the one is a quick easy replacement while the other is very much a DIY start of a kit that requires the buyer to search for more parts and study what ratios should be used etc. Very much getting what one pays for.

I think you summed up the reason for the difference in prices very well. I'm not knowledgeable enough in electrical matters to presume I could assemble a system by myself, like the owner of Suma. See the blog, "Sailing Suma". 

Anonymous wrote:

I've been eyeballing these:

https://www.thunderstruck-ev.com/sevcon-brushless-sailboat-kit-5kw.html

https://www.thunderstruck-ev.com/sevcon-brushless-sailboat-kit-10.5kw.html

Not sure how they compare with the QuietTorque but price is way lower and they're also 48 VDC brushless motors.

They also come with less parts. The big one is the transmission and coupling. The quiet torque also has pretty much everything mounted to a motor shaped single unit that has motor mounts made to fit in most diesel replacement scenarios. The control parts also look less ready to install or at least less water proof than in the quiet torque. So the one is a quick easy replacement while the other is very much a DIY start of a kit that requires the buyer to search for more parts and study what ratios should be used etc. Very much getting what one pays for.

Oscar Froberg wrote:

3. Replace with outboard engine
Pros:
- Inexpensive
- Easy maintenance
- Easy to replace anywhere anytime
- Combined with 2 (removing the footwell) would gain lots of space inside
- Get rid of prop+shaft (no leaking & improve sailing performance)
Cons:
- Horrible in a seaway with prop coming out of the water
- Aesthetically non-pleasing

At this point I'm torn between 1 and 2. Due to the hull shape (double-ender) an outboard well isn't really an option, otherwise 3 would be almost a no-brainer.

Well, based on my experiences of an outboard as an auxiliary motor that would be my choice every time. I had an outboard on my 9 meter Newick trimaran, and as you will know on the 10 meter 'Footprints' I have nothing but positive comments to make about a good outboard, and this is based on both coastal and ocean cruising. I think the only choice would be the 9.9hp high thrust Yamaha with remotes and self raising. I understand Tohatsu also make a 6hp high thrust motor which would suit smaller boats. The motor on Footprints was in a well but it was right at the transom. I don't recall that the prop coming out of the water in rough seas was ever a problem, but I would motorsail in those type of conditions anyway. I can see though that fitting an ouboard to a double ended vessel could have it's challenges, but not impossible I think to come up with a workable solution.

If you have an old, difficult to access diesel engine then I think that electric drive will seem very appealing, so it really comes down to cost and practicality.

it will be interesting to follow Jim's installation of electric drive, which is what this thread is all about, and I am sure he will share the details with us. It will great to know how the challenge of sufficient battery charging, hence range, is dealt with on a smaller vessel size. Jim's will be a real world experience which will ultimately allow all of us to better weigh up the pros and cons of the three main drive options. Maybe Alan on Zebedee has got us all beat with only a yuloh?!