June
'06
20. The Motor and Propeller
These pictures, supplied by Solarboat show how the electric motor is fitted into the propeller shaft. It actually sits above the shaft and drives a toothed belt in a 1 to 2 reduction ratio. The motor itself is less than 300 mm. across. This should present no problem in the engine compartment, designed as it has been for a diesel engine.
However, there is a slight problem with the propeller. Solarboat recommend up to 375 mm. diameter and pitch prop. whereas the this boat has been designed with a 300 mm. diameter, 250 mm. pitch in mind. Reducing the diameter of propellers in electric motors greatly reduces their efficiency. There is a little clearance, say about 50 mm., between the hull bottom and the propeller, but Paul Fisher of Selway-Fisher Design says that it is important to maintain a clearance of 10 to 12% of prop diameter. To increase the shaft angle to accommodate a larger diameter prop would reduce its efficiency too, quite apart from altering the geometry of the skeg and rudder. So it might be better to accept a smaller prop. At 320 mm. the clearance would be down to 30 mm. which is just under 10%. The shaft angle could be altered to give an extra 2 mm. of clearance without changing the fundamental dynamics of the rudder, so that is what I will aim for: 320 mm. prop diameter, 320 pitch and clearance of 32 mm.
From
the Selway-Fisher plan.
The occasion of the 60th. anniversary of the accession of the King of Thailand to the throne has produced an interruption to work on the slipper launch. As there is to be a Royal Barge Procession in Bangkok this month, which is a spectacle of some rarity, I will be dropping over there to take a look at their boat building and handling skills.
21. Sheathing the Hull
Prior to applying the glass to the hull I went over it one more time looking for small pin holes or areas of depression in the epoxy fill. These were roughed over with a scratching tool, and or filled again if the defect warranted it. Some of the fill was close enough to the surface not to need refilling, but far enough below the surface to have missed the sanding disc. After scratching they were acceptable for bonding with the next layer of resin which would be used to attach the fibreglass.
Before
and after scratching a staple fill area.
I have decided that the boat will eventually be painted a dark emerald green, so a light weight weave of fibreglass will be needed in order not to show the weave pattern through the paint. I have chosen to go with 4 oz. glass.
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Problems shows higher resolution shots as
well.