JR Potts, AUS 173d AB (Updated: 8/8/2015):
As one can expect, one problem after another had to be overcome. Such issues arose as how the one-man crew could breathe when submerged, how to navigate with or against the current, propulsion techniques, how to submerge/resurface, basic buoyancy and how would the operator see anything once inside the submerged Turtle especially at night. Bushnell not only had to conceive a device that never existed but he also had to design and build it himself.
Bushnell proceeded by building into the hull six smallish thick-glassed windows to provide natural lighting and considered using a candle while the vessel was submerged. Experiments with the candle proved inadequate because of the confined spaces - the candle flame would eventually consume the operator's oxygen supply. He wrote Benjamin Franklin about this dilemma and Franklin, intrigued with the problem, suggested using bioluminescent foxfire fungi found on decaying trees. During the natural decaying process the fungi would produce a bluish-green "glow" in the dark. This produced just enough illumination for the operator to read his compass and depth meter.
With internal navigation resolved, propulsion proved the next major hurdle. Sails, oars and poling were the current types of propulsion used on water so there was very little to go on in terms of powering a submarine. In the Turtle, the operator became the horsepower output and a hand crank was devised to spin two propellers, one for vertical movement and the other for forward progress. This was the first recorded use of a screw propeller on any sea-going vessel. To submerge the Turtle, Bushnell needed a process for the water to enter a bilge tank at the bottom of the vessel. Conversely, to surface he installed a hand pump to push the water out into an outside spare tank. In case of an emergency he installed up to 700 hundred pounds of lead that could be quickly released to surface the vessel.