The farmer you meet near the hay bales will give you a bag. (just one)
The hovercraft runs on manure.
The hover board (hovercraft) runs on manure.
The fuel rod powers the saucer Excalibur. The hovercraft runs on manure.
The manure is used to power the hovercraft hidden in the attic of the mill. (see related question)
You have to power it up with manure which you have to get from the farmer near the secret hatch to Mordred's Hideout
In Astro Knights Island on Poptropica, the fuel for the hovercraft can be found in the fuel canisters located next to the dock where you first arrive on the island. You need to collect the fuel canister and use it to power the hovercraft to cross the toxic swamp.
You first get the manure then you make the mill turn right then entert the top and go to your bag and hit use on the manure
You use it to power the hover board (hovercraft) in the attic of the windmill. (see related question)
In the field between the castle and the mill. You only get one bag, and it is used to power the hovercraft.
Go to the mud and you'll find it there. As soon as you fuel it, it stays right there in the swamp.
Once you add the manure to the hovercraft, it automatically goes to the point where it is used : in the mud swamp to the right of the mill.
To find the hovercraft on Astro Knights Island, you need to rescue the princess and receive the Coordinates Decoder. Once you have the decoder, you can use it to uncover the hovercraft's location on one of the planets. It's located on a frozen planet called Tundara.