some essencial resources on engine knoledge for Day skipper and yachtmaster engine and boat checks Engine checks Fuel economy Volvo marine engine https://youtu.be/u5Y2T-NTLkc?list=PLHwEfFYvn0F04wGIjls6-D4RL8V6WDPSr another volve engine https://youtu.be/oHqrq3ptxC8?list=PLHwEfFYvn0F04wGIjls6-D4RL8V6WDPSr fouled props changing impeller