I have plans and most of the components for a diesel genset but haven't moved forward with it yet. Too many other things going on and the boat is fully functional for the way I currently use it (day sailing mostly). I have a brand new Kubota 722 3 cyl diesel that was removed from a brand new mini-excavator for full electric conversion. It is rated for 12kW at 2200 RPM. The engine with flywheel weighs about 180 lbs. It is the same base motor Beta Marine uses for their 20HP unit. For the generator head I have another of the same motor as for propulsion, a Motenegy ME1616. Used as a generator it should be able to output better than 10kW. I won't need that much but it also has cooling which will be important if I want to run at sustained speeds of 6-6.5 (like going up the Detroit River). To get the charging voltage needed the motor has to turn at about 2200 RPM (0.026V/rpm for ME1616) which is right in line with the diesels rated spec. I am planning a direct drive of the generator using a spider type coupling and an electronic speed controller. From my initial power estimates I thought I would need 8-10kW to get to 6+ knots. After getting real data it appears to be more like 5-7 kW. So maybe I could get by with a 5 or 6kW genset with a smaller diesel like the Kubota Z482 (2cyl version of the 722). But that is only 15 lbs lighter and a couple inches shorter so not a big difference. The 3 cyl will deliver the needed power at lower RPM with less noise and vibration and I already have it. And the generator head will need cooling which the ME1616 has. I still need a rectifier and voltage regulator both of which will likely need cooling to run at high output for 6+ knots. Why a diesel hybrid drive? Could a larger battery provide the wanted performance? It depends on the use of the boat. For day sailing and the occasional weekend trips the battery is fine and all that's needed (11kWh usable capacity, 20-25 miles range at 5 knots). But if I want to do things like transit the Erie or Welland canal or go up the Detroit River to L. Huron I need to sustain motoring at speed for 8-10 hrs a day. The battery for that would be 4 or 5x larger than what I have and would take a very long time to recharge. More than I would have overnight at a dock unless I had a very large (and expensive) charger and access to a lot of shore power to run it. My current 1.5kW charger would take 7hrs to fully recharge my battery (11 kWh useable capacity). I could double that with a larger charger on a 30A shore power hookup but that's about it. So I could get the charge time down to 3-1/2 hrs. Not bad. But to motor all day at 6 knots (which you just about need in the Detroit River or Welland Canal with current and ship traffic) I would need about 50kWh and that would take 17 hrs to recharge on one shore power hookup (at 3kW charging). It would also be 1000 lbs of batteries that would cost about $12,000. And that's a good price. Solar is absolutely something I plan to add to the mix but at 300-500 watts (and that's optimistic) it has no hope of keeping up with the charging needs for passages that require all day motoring at speed. To be fair there are not too many of those. But if those are in the mix of what you want to do you need more power for sustained speeds. With the right wind you can sail a lot in the Detroit river but it's not really an option in the Welland Canal (or the Soo Locks if I get that far - fingers crossed someday). And in the Erie the mast comes down for several days of motoring. So speed is not the problem. I can reach hull speed with the electric. It's sustained speed and the power reserves required for that and the replenishment of those reserves. Also 4 or 5x of the same battery I have (which is quite compact for the power) would require a lot of space.

There are basically two ways to use a genset in an electric hybrid drive. You can use an AC genset to provide power to the same chargers you use for shore power charging of your traction bank, or you can use a DC genset to charge the traction battery directly.

AC generators are more readily available, you can use them to power AC house loads directly, the higher output voltage means less amps through the genset output wiring. If you want to propel the boat on the generator at sustained speeds of 0.85x to .90x hull speed (about 6 knots for me) you will need enough charger capacity to put that much power from the genset through to the batteries/motor. And you have an extra stage where you loose efficiency (going through charger). But it can certainly be done. For 6 knots my boat needs about 5000 Watts into the electric motor. So I need 5000 Watts from the battery charger. There are some efficiency losses so the input to the charger needs to account for that. My charger has a conversion efficiency of 95%. So I need 5250 Watts from the genset to get 5000 at the motor. But I probably don't want to run my system at peak output continuously. 65% is more comfortable and easier on the components. That would mean a charger capable of 7700 Watts output and a genset with 8000 Watts output. That would give comfortable cruise at 6 knots continuously without running the system components at their max levels.

My DC genset as described above will output 48VDC directly to the battery at up to 10kW. So for the 5000 watts needed to drive the boat at 6 knots it will be at about 50% capacity. I will have some losses in the rectifier and regulator but none from the need to run through the charger. And I don't need as high capacity of a charger. For charging from typical shore power I have available (120V, 30A) I cant make use of a charger with a capacity larger than about 3200 Watts and if I want to keep comfortably in the capacity of the shore power system its more like 2400 watts. Right now I have 1500 watts capacity which seems fine in terms of charge time. But I am not needing to do a full charge overnight. My typical use on a daysail is 2000Whs. I can recharge that in about 90 minutes. But to maintain high cruise speeds the power reserve provided by the genset is the most practical solution I have found.

There are lots of suitcase type generators available that have continuous output of 1500 to 2000 Watts. One of these should be able to drive the boat (through the charger) at 4 to 4.5 knots. This would be set up to run on deck NOT BELOW. Most run on gasoline but there are dual fuel units that can run on propane. You get a little less output on propane but it has some advantages. No stale gas in the carburetor, no worse on the boat than gasoline (for fumes) and changing a propane bottle at sea seems a much easier task than re-filling a gas tank on one of these generators. From reports of people using these on similar sized boats I should be able to motor at 4 knots for 8 to 10 hrs on a 20lb bottle of propane. This is not a permanent solution but workable. It won't give the power reserve needed to motor continuously at 6 knots as would be needed on the Detroit River. It would probably be fine for motoring through the Erie Canal. And I could run at 5 knots with one of these and only need to add about 1KW from the battery. That might be OK on a canal passage if I had a hookup and time to recharge overnight.