How to make UK Supply system on WW1?

I was wondering if someone in the community know how to replicate that system. I have an scenario, and made two "ground unit: other". One is the "merchant ship" with Def 10, Crit 5, HP 1 and the "supplies" with Def 9, HP 100 and Crit 100. Also there are submarines with 10 attack, 5 defense, Crit. 7 and HP. 7, the usual.

However when the subs attack the merchant ship carrying the "supplies" BOTH units survive. I need that only the supply unit survives, not the "merchant ship".

1. Have a land unit act as supplies with high cost 0 range, with average defense (6) and high hp (100) and can only be transported by a special "supply ship". You want high HP so that it is difficult to kill for enemy.
2. Have a supply ship , say - 10 def 1 hp. It's high def is so that it defends first over other units and 1 hp so it dies instantly.
3. Have a special "defender" unit to defend supply ship. This defender unit must have higher defense than the supply ship (11+).

Now if the supply ship get attacked, the supply ship will defend first if there's no higher defense unit and dies instantly. If the supply ship dies, you can't transport supplies anymore.
If you have the defender unit, it will defend first because it has higher defense than the supply ship.

Finally, have somewhere that you want to carry the supplies to in order to suicide it.

Thank you Withc-Doctor.