Ship detection is a crucial surveillance problem for the country with long coastlines. The problems are such as border security, illegal fishing, and harbor protection. To overcome those problems, radar and satellite surveillance have been used widely. However, radar and satellite are very expensive. Particularly, satellite is easily affected by the cloud which makes difficult for detection. Moreover, radar is also sensitive to the noise clutter generated by uneven sea surface. We propose ship detection using Wireless Sensor Networks (WSN). We derive a ship speed estimation formula using three cooperative sensors to increase the number of node efficiency. We use our formula to calculate speed of the ship. Initially, sensors detect a ship generated wave and send the timestamp to the Fusion Center (FC). FC collects the timestamps from three sensors and estimate speed of ship. We also compare our propose formula with four sensors. We make an experimental setup with the distance between sensors being 2 meters. The experimental results show that estimation speed error of using our formula (53.33%) is lower than four sensors formula (183.33%).