The base of Hydrofoam was designed using existing models out there as a basis which all seem to follow the same basic shape for both the top platform and sponsons.

I'm using 3MM depron so I added in carbon fibre strips 2MM thick across the length of the fuselage to support it.

The elevon servo is sunk through the fuselage to the bottom so only the head is on the top.

The engine mount is just depron layers built up. I just guessed with the size of the fins I figure tall is best for increased stability.

The receiver also sits on the bottom inside a balloon which comes through the top. Both the servo bottom and RX are sealed in with a pod water proof.

Hydrofoams are designed for use on the ground as well as water and flying so I added some carbon fibre skid plates to protect the depron.

All the control lines are fitted using my normal carbon fibre + heatshrink pipe method. All electronics with exception of the motor (which doesn't care about water) are sealed either by glue or inside balloons.

The last thing to do was test how it sat in the water. It's important to know your propeller won't hit the water and that it points slightly upward to help lift out of the water and lower the speed required to get airborne.

As you can see the model sits very high and hardly displaces any water at all, which at just 140 grams AUW isn't so surprising. Although you can't see the pod on the bottom where the RX is I did check and its a good 2CM off the water level, even though it's water proof is best to avoid contact if you can.