Retrofit paralell hybrid diesel

Hi, longtime lurker and few posts. Thank you all for the wealth of information I've gained by frequenting the forums over the years.
I've moved from assembling my own pedelec with a Ping battery 10yrs ago to owning a Twizy, then a Leaf and now I'm contemplating making a paralell hybrid from an existing boat.

The desired outcome would be to be able to run on electricity alone at about 3-4knots for 2h. By running on the diesel engine (with a gear engaged), the electric motor would regenerate power back into the batteries.

I haven't purchased a boat yet, but the model I'm most focused on is the Albin 25 DL and are plentiful on the market. Easy to push through the water and outfitted with a small diesel as the original engine (12hk). https://www.albin25.eu/index.php/mega-gallery

The boat has a fixed axle and external rudder. After the reversing gear/pressure bearing, the axle is exposed and there's room for a small electric motor to be mounted in paralell. I was thinking about using a belt drive and pulleys to get the necessary gearing.

The battery is going to be a gen1 leaf pack, of a size determined by the motor+controller combo (I have a full original leaf pack in the garage).

Boats are particularly unforgivning in the power/speed department, and I think 3,5-4knots is the highest I might be able to achieve with a small electric motor.

But there's a few issues I haven't sorted yet:

1. While freewheeling, i.e running the diesel and the batteries are 100%, won't I be generating a lot of voltage that I need to dump somewhere in order to not damage the controller? Or is this taken care of by having the controller powered on but in neutral?

2. I was hoping to use a small, but high efficiency motor so I can air cool it. But then again, while this compartment is cool since it's below the waterline, there's no real ventilation. Leaving only convection to cool the motor. Is this ok, or is the efficiency high enough and the duty cycle low enough to not cause overheating or thermal runaway?

3. Can you suggest any motor+controller combo that would suit my needs? 1kw to 3kw.

Any other suggestions are also welcome. Cheers.

If you are going to use belts and pulleys, can you use an electric clutch to engage/disengage motor from the propeller shaft?

Is this more efficient than a huge electric trolling motor?

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

thoroughbred said:

If you are going to use belts and pulleys, can you use an electric clutch to engage/disengage motor from the propeller shaft?

Is this more efficient than a huge electric trolling motor?

Click to expand...

In theory yes, but it would use more space and I'd have to acquire one that was marine grade (corrosion resistance and whatnot) adding cost and complexity. I'd rather not have it if it's not necessary.

Huge trolling motor? Most trolling motors are made with 2-blade or 3-blade propellers with a steep incline to resist seaweed, I doubt I'd find even a 48v version of a Minn Kota that would haul a 1500kg boat efficiently.

John's suggestion of the thunderstruck-ev kit was good, I've looked at it before. But the sealed version.