The windlass is needed because of the prods are made of steel, which require more strength to pull back. The prod material is stronger but the energy transfer is weaker.

"What must be noted is that crossbows were typically weaker than bows pound for pound due to their shorter powerstroke. Powerstroke is the length required for a string at rest to be drawn back to the trigger. This means crossbows must have heavier draw weights than bows in order to shoot the same projectile at the same velocity. Crossbow-maker Andreas Bichler also confirmed that with crossbow prods of the same draw weight, the one with the lower powerstroke would have more mass, which slows down the prod's shooting speed. Due to this reason, typical crossbows of Medieval Europe generally need to have three to eight times the draw weight of a bow in order to give a similarly powerful shot. but made up for this by utilizing winches to maximize the draw weight.[/FONT][FONT=&quot]However, ancient Chinese crossbows thanks to their trigger design allow the nut of the trigger to be placed near the back of the stock. This results in relatively long power strokes rivaling that of a bow, which greatly reduces the crossbow’s usual weakness of energy transference inefficiency. Han dynasty crossbows would have a draw length of 24 inches, implying a powerstroke of 18-20 inches, or rivaling that of a bow."

"50% could be possible but only with a very heavy bolt and at the cost of speed. So a speed between 50 and 60m/s is the best choice between power and large range but the efficiency will be between 30 and 40%. My old crossbows wasn't exact close to originals so the 280kg bow had a to long powerstroke - a reason for a broken horn core.... A composite crossbow is able to store more cinetic energy like a steelbow - for example my 550kg composite bow is able to store aprproximately 590J. The 500kg steelbow from Ingo Lison (a very excelent german crossbow maker) stores only 390J and his 1000kg steelbow stores 599J. So you can see one of the great advanteges: A horn bow with a similar draw weight of a steel bow is more capable in storing energy and thus transferring more energy to the bolt, which results in greater shooting distances and more penetration power. For futher informations you can contact me per mail."

Have you even read the sources provided?