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!

Steering Wheel

I've been making the steering wheel. This has fitted in well around the Christmas interruptions and my lathe, drill press and CNC mill are in a cooler workshop - it is very hot here at the moment!

I want a wheel with a ring and spokes with handles - see the mockup in the previous post. I had already made the hub and its aluminium bush. This is visible in the left hand photo below. There is a slot through one end of the bush with a pin through the axle to stop it turning. Also in this photo are the pieces of the ring temporarily assembled with their loose tenons. The six segments of the ring were cut out on my small CNC mill; they are oversize to provide for the eventual clean up. They had to be flat on the outside with square corner so they could be stood on edge in a jig while the slots for the loose tenons were milled. The right hand photo shows the sticky mess after gluing all the pieces together.

With the glue set I roughly cut the outside to shape and trimmed off the excess of the tenons. On the lathe the ring was held on the outside of the chuck jaws while the outside was carefully finished to size and one flat side faced off. Then a "jam chuck" was turned in a piece of scrap timber; this has a recess turned in the face which is a tight fit on the outside of the ring. With the ring hammered into the recess the inside and the other flat face can be turned to size. The LH photo below shows ring and hub  on the assembly board.

The RH photo below shows the setup for drilling the holes in the hub and ring. These holes have to be accurately drilled or the spokes simply will not fit. The phot shows the hub being drill. It is clamped to the jam chuck used on the lathe which now has a ring of holes in it exactly 60 degrees apart. The jam chuck can rotate on a bolt which holds it to a vertical board on the drill press table. The board also has the same 6 x 60 degree spaced holes. At the 6 o'clock position a drill is locking the rotating chuck to the fixed support while one hole is drilled. Then the chuck is turned by 60 degrees and the next hole drilled. The ring was drilled with it jammed into the chuck.

 The LH photo below shows the wheel assembled with pieces of dowel and no glue. It went togehter very easily - it is far easier to get those holes right than to position and size the photos on this page! I turned 6 handles with a hole the same size as the dowel and then glued the whole wheel together.

Now I am debating whether to stain it or leave it natural.