Maiden Voyage and Test Run Numbers.

There have now been a number of launches in my local lake ad I am delighted with the outcome. There is still a lot of work to do on the woodwork and the final battery arrangements and controls. This short video (thanks Paul) shows the maiden voyage.

I am disappointed by the whining noise from the motor; it is a 3 kW BLDC motor from Golden Motor in China. I will build a sound insulated cover for it and hope this will quiet it down. Apart from that it works well.

By weighing the vehicle and trailer with and without the boat I find that the boat with the temporary batteries weighs about 430 Kg. With me (about 70 Kg) at the front of the cockpit the boat floats level about 30 mm above the DWL. I worked out that the area on the DWL is about 4.4 sq metres so the volume of the 30mm thick slab is about 130 litres or 130 Kg. The total weight (displacement) of the boat when at the DWL is 430 + 70 + 130 = 630 Kg. This is close to the figure of 620 Kg given by the designer.

With the boat floating 30 mm high I made some test runs and took measurements of speed, amps, volts and prop RPM. See below. From the measured numbers on the left I calculated the power required in watts and equivalent HP.

NOTE. This table is a revised version of the original; The measured current was low by a factor of 3.33 and the the calculated speed and slip have been removed. 

The power required is lower than I expected and I believe this is, to some extent, because the boat is floating high so less water to displace and less wetted area. I caught some weed on the prop and had to paddle the boat to the bank so I could clear it. Sitting on the gunwale with a single ended paddle I managed about 1 knot so the boat is easily driven.

Water line length is about 15.25 feet so hull speed is around 5.25 knots. In the video the boat is travelling (right to left) at between 4 and 5 knots.

The prop is second hand from EBay. It is 11.5” diameter x 15” pitch. The calculated slip is much higher than the 40% number I had in my mind and this puzzles me. The calculated slip decreases as prop RPM increases. I don’t understand this at all!

It was (still is) my intention to replace the EBay prop with a better one after making some test runs. I was thinking 12” or 13” diameter and more pitch. On the other hand the EBay one works so need to know what would be gained by changing it and to what.