What SD said. Those numbers square pretty well with my experience using a 40# Motorguide and a Group 27 (115AH) flooded batt. There's a ton of variables in play, however, so YMMV.
For example, the Minnkota Endura 50 is an analog troller -- the 5 FWD/3 REV is your clue. That is to say, it achieves lower-than-maximum speeds by putting a bloody rheostat (dimmer-switch resistor) in line with the motor. The result is that analog trollers use nearly the same juice at any speed setting
-- at low speeds, they burn off surplus amps as heat.
Digital trollers (like the Maxxum or Great White series) use electronic speed control. These can yield up to 3x improved run time at reduced speeds over analog models, tho at WOT there's no real advantage. They typically have infinite speed settings.
Another variable is discharge rate -- how fast you are pulling amps measured as a fraction of the battery's capacity. For most trollers, that's well inside the 8-hour discharge rate. Most batteries are labeled with their 20-hour discharge rate, which may be half again the 8-hr rate. What that means is the faster you discharge a battery, the smaller it behaves. So you should discount that 115 AH to maybe 80 AH as a derated capacity; then, as SD proposes, halve that to 40 AH for your functional capacity, and that means you get 1 hour of run time at 40A draw.
Now, there's a school of thought that says it's false economy to nurse batteries this way. That by limiting yourself to 50% DoD or less, you are leaving half your useful capacity on the table. Sure, driving a battery to its full 80% DoD may halve its life span -- but you will have gotten twice as much work out of it
, so you come out even in the dollars-per-mile calculus. A decent deep-cycle lead antimony batt should withstand 1500 cycles between the eighties (80% DoD to 80% SoC). These contrarians, whom we might call the "Rented Mule" school, propose it's better to utilize more of the battery's capacity and accept a shorter service life. The advantage is, you carry around less battery mass in the meanwhile. And entropy being what it is, batteries decay even when babied or unused ... so why not ride em hard & put em away wet, buy new ones when these die?
A 30W mono- or polycrystalline solar panel will recharge your Group 27 in about 1 sunny day following 1-2 normal days' use, so it is a good match. You probably should add a charge controller, just a simple one, to keep from boiling the battery dry once it is charged. I got this one on eBay
for $15 and like it just fine.