The Crashed Ship Site allows you to add 5% to each of the stats / bonuses of each of your newly built ships per level of the Crashed Ship Site. And it does this without any additional cost to your ships, by allowing you to harvest alien technology from the crashed ship.
Stats increased are: Speed, Hold Size, Stealth, and Combat
The Bonus is added (not multiplied) to all other bonuses and the sum of all bonuses is applied. For example: a level 3 crashed ship site (providing 15% to all ship related bonuses) combined with level 5 shipyard (provides 5% increase to ship speed) would result in a 20% increase to ship speed.