Propellor, Etc.

The propellor is a tricky thing to get. The simple solution is to buy a new one - around $600. The problem with this is that you have to specify the diameter and pitch. Diameter is fairly straight forward, as big as possible but allowing sufficient clearance between it and the hull to avoid cavitation problems. This gives me 13 inches diameter as a maximum. The pitch is more difficult because it depends on hull speed, the propellor RPM and a factor called "slip".

Hull speed for a displacement boat can be calculated as factor multiplied by the square root of the waterline length in feet. The factor is somewhere betwwen 1.0 for a barge and 1.4 for a slippery hull shape. Working on a factor of 1.3 and LWL of 15.25 gives a hull speed of 5.1 knots.

The formula that relates boat speed a, propellor pitch and RPM is:

        speed = RPM x pitch x slip / 1215 (1 knot = 1215 inches per minute)

Rearranging this to calculate the pitch:

       pitch = (speed x 1215) / (RPM x slip)

Putting numbers in gives:

      pitch = (5.1 x 1215) / (1000 x 0.4) = 15.5 inches.

I started looking for a secondhand  13 x 15.5 prop and found one on ebay and bought it for $120. This prop is 11.5 x 15, smaller in diameter but close in pitch. I will use this prop to make some test runs and then buy a larger new one with the correct pitch and then sell the first one on ebay.

Here is the propellor and shaft sitting temporarily in the boat.

At the other end of the prop shaft are some bearings and the electric motor. The bearings are visible fore and aft of the two floors. The plywood bracket holding the motor isn't quite the right shape and I will make a new one. The motor will drive the prop shaft with a poly vee belt around two pulleys giving a 4:1 reduction - motor speed 4000 RPM, prop shaft 1000 RPM.

The pulleys are easy enough to make (aluminium and I have a big enough metal turning lathe) and I have found a place that will cut the keyways in them for a reasonable price. I could cut the keyways on my lathe but it is tricky to set up and slow to do.

Another accomplishment over the past 2 weeks has been replacing the motor on my table saw. I started ripping timber to make the gunwales and capping strips for the cockpit coaming and the table saw would only run for a few seconds before it tripped the circuit breaker. I thought it would be easy! 2HP motor is a common size and would be available off the shelf. Took the old motor to a shop and found that it might be easy to fix (good) and that they didn't have a replacement motor in stock (bad). It turned out that the old motor was dead and the new one took a week to arrive! Of course the holes in the new motor's feet were in different places to those in the old motor, sigh!

Much nicer than the old motor. Maybe I should buy a new V belt - the old one is looking a bit tatty!