The tomtom app seems to keep screen alive ok. And testing this morning Skydeamon does when simulating but it definately did not work while navigating yesterday.

SkyDemon already does this, and always has. The only exception might be if your device is especially stubborn (has software designed to ignore when apps ask the device not to go to sleep) or possibly with the Lollipop operating system.

Not sure if this would be possible (on android) but it would be useful to have an option to prevent tablet going to sleep and lock screen (in my case I need a pin to unlock) while while navigating