Extra Diesel(or other suggestions) for Leopard 40 (2020) to cruise.

[Cuvee]

I would first estimate the existing maximum fuel range for your boat. Then determine if that is sufficient for your cruising area and the likely distance between reliable fuel supply.

The advertised fuel consumption curve for your 29hp Yanmars are 2l/h @ 2000 rpm (per engine), 3l/h @ 2300 rpm, 4l/h @ 2600 rpm, 6l/h @ 3000 rpm, and ~6.8l/h @ 3200 rpm.

With 360L tankage and running 1 engine at a time, you've got 180 hrs (~1 week) motoring range at 2000 rpm. You likely know your motoring speed as a function of engine rpm. Perhaps you get 5 kts at 2000 rpm with 1 engine. With 360L tankage, that gets you ~900 nm range with flat seas and no head winds.

Add two 20L jerry jugs so you have 10% fuel reserve.

You'll also need to subtract for fuel consumption from generator during this motoring time. Won't be much with engines running unless you need genset to run an AC watermaker.

You should also confirm the height of the fuel pickup tube in the tank as that might reduce the useable tankage.

Definitely add AIS for collision avoidance and Doppler Radar for squall avoidance.

Regarding the genset, what brand is it, how many hrs, and is it in a dry locker (lifespan can be a challenge with gensets located in bow lockers). My Cummins genset located in an aft cockpit locker is still going strong at 10500 hrs.