Appendage resistance

Appendage resistance is the added calm-water drag from everything outside the bare hull: rudders, bilge keels, shaft bossings, A-brackets, struts, propeller shafts, stabilizer fins, and bow-thruster tunnels. In the ITTC-1978 procedure it is handled by a form factor on the appendages or measured directly, since appendages sit largely in the boundary layer where local velocity is below the free-stream value, so model-scale drag does not scale by Froude alone. Appendage drag on a twin-screw ship typically adds 5 to 15 percent to bare-hull resistance, and shaft-and-bracket arrangements dominate that figure on fast fine-form hulls.