Suppose you have a polis very far from the others and surrounded by enemies, with the harbour constantly blocked (even for years) no matter what you do. You can free that harbour only for some hours, maybe 1-2 day but in the end it remains blocked.
1) Calculate how many resourses you need to do an up.
2) Control how many resourses you get for deleting a paddle speedboat in the interested polis. The amount is different if you have or not fireworks.
3) Calculate how many paddles you need to reach the amount of resources you want supposing you obtain resourses by deleting paddles.
4) Build paddles in other polis or buy them.
5) Send paddles to the interested polis. When they arrived if the harbour is blocked they disperse ( scatter) but then they return into the polis after 4 hours.
6) Delete the paddles.
7) Use the premium trader to convert sulfur into what resource you want and up the building.
Why paddles? Because they have a very low manteinance cost, so big accounts can mantain a lot of paddles for a very long time.
And because they cannot be destroyed if attaccked ( of course if there are no first or second line)
Of course you can produce paddles with firework and delete them in a city without so you lose as few resources as possible, and you can integrate with resources from other methods like benevolences points or pirates resources.
If you opponent is so active that is able to attacck and scatter you paddles every 4 hours going in vacation mode for 2-3 day can solve the problem so when you are back you can immediately use your paddles. Of course in the other polis you have to be free to produce and move paddles.
As a disadvantge is the long production time.
I tried to use this strategy to up a residence to lv 10. I stocked about 3 milions resources in about 20.000 paddles, and with a few other resources from benevolency and about 350/400k resources already accumulated in the polis ( mostrly protected by warhouses) I finally made the up even if it requires about 1 month preparations.