Calculating propeller pitch, revs, slip and drive train damage

For calculating speed, engine revs divided by the gearbox reduction ratio, times the pitch,

give the theoretical speed, less slip, which on displacement boats runs at 20 to 30 per cent.

The sum is theoretical, because water gives way. The difference between the calculated speed and the speed actually made is slip: around fifteen per cent on planing boats, twenty to thirty on displacement boats. Anyone falling well short of the calculated speed has a problem, the wrong propeller, fouling or too much load aboard.

The drive train and the propeller are the parts that suffer first in a grounding. Rudder blade, shaft, propeller and gearbox all take damage, often with consequential damage to bearings and seals. Running aground is the most frequently reported cause of loss in boating.

Grounding is usually covered by comprehensive hull cover; check whether consequential damage to the drive train is included. The machinery cover extends the hull cover to engine damage that hull cover on its own will not pay for. One limit stays: design and material defects in the part directly affected are excluded, while consequential damage arising from them is insured.

Worked out for your type of boat

Related