How does water physics work in KSP

[TheFlyingKerman]

I am building an ISRU seaplane for Laythe. With two junos for the 8 ton plane I can't get it to pick up sufficient speed to takeoff.

My questions:

1. What hull form give the least drag for a given immersed volume?

2. Does aerodynamic surfaces work with increased lift when immersed?

[bewing]

Water drag is the same as air drag except bigger. So any high drag aero parts (eg. mk2 parts) are also going to have high drag in the water.

Yes, aero surfaces have increased lift under water.

One big problem is that you can't  "rotate" your plane for liftoff when it's on the water. You can use canards and hydrofoils to raise the nose a little bit -- and that usually helps you pick up some speed. But using a conventional design with rear horizontal stabilizers is usually a non-starter.

Using a couple of small elevons or small wing parts below the water level as hydrofoils can help a lot. 

 

[TheFlyingKerman]

There are some major differences. Parts can be partially submerged in water. Also boats work in a lower speed regime than planes.

I tested the hydrofoil idea. It works to provide a lot of lift, but its drag is so high that I am better off without.

[mystifeid]

1 hour ago, TheFlyingKerman said:

I tested the hydrofoil idea. It works to provide a lot of lift, but its drag is so high that I am better off without.

Test and test again. Two pairs of canards, a panther and 400 units of fuel gave this upside down rover a range of 300+km at 50m/s and with some more experimentation I'm sure I could have done better. (Check out the throttle position). Take the canards away and the range and speed become impossible. I've used also canards on other boats with good results. Try using the authority limiter of the canards to get the boat out of the water and then adjust for cruising trim. Further adjustment is necessary if the CoM moves as fuel is burnt.

Edited August 27, 2018 by mystifeid