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.

Getting Close to a Test Run

There has been a bit of "two steps forward and one step back" progress over the last few weeks. That said I'm hoping to put the boat in the water sometime in the next week or two.

The boat is on the trailer. This is a fiddly business with the boat being lifted on and off the trailer while the axle is moved to balance the whole thing with the right amount of load on the tow ball. The keel rollers and bolsters have to be adjusted so the boat will clear the mudguards. Once this was done the supporting V-block on the winch post fouled the pulling eye and winch hook so the winch post had to go back to the maker for modification (a two hour drive each way). Here's what the boat looks like on the trailer.

The modified winch post.

And the aft end of the boat.

Fitting the rudder seemed to have more backward than forward steps! I fitted the bracket to the skeg while the boat was still upside down. It is held in place by 5 8mm metal tread screws. I drilled the holes in the skeg oversize and filled them with thickened epoxy before the paint went on. With the bracket clamped in place I drilled and tapped holes into the epoxy for the screws. Then coated the mating surfaces with Sikaflex marine sealant, dipped the screws in epoxy and screwed them in.

With the boat up on the trailer I went to put the rudder in place and found there was too much rudder blade to allow it to lift enough for the thread end of the rudder shaft to get into the hole in the bracket. Two ways to solve this problem: take the bracket off and refit with the rudder in place or cut some off the top of the rudder blade. The bracket was supposed to be permanent and I knew it would be difficult to remove the screws so I cut a strip off the top of the rudder and cleaned up the shaft where the weld had been. The ruuder blade is 6mm stainless steel and so the paint was damaged. In the photo above it is waiting for some more topcoat.

When I put the rudder in place the threaded part of the shaft wouldn't pass through the hole in the bracket - 16mm hole in bracket 20mm thread on rudder shaft. . Drilling out the hole in the bracket on the boat was impossible; it is 10mm thick stainless steel and hand held drills are not up to this sort of job. The bracket had to come off and getting the screws out was more difficult than I expected. The technique is to heat the screws to soften the epoxy. A soldering iron works for small screws but even my big tinsmiths soldering iron held in place for 10 minutes wouldn't do it. I had to get under the boat with my high temperature gas torch and apply a frightening amount of heat. Eventually the screws were out but the Sikaflex still held on quite well. Surprisingly the paint work wasn't damaged at all. Drilling the bracket on the drill press took only a few minutes and refitting it was messy but quick - lying under it with Sikaflex and epoxy falling on me was not nice!

Inside the boat the motor, prop shaft seal, bearings, etc are all in place. I tested the electrical side of things on the workbench so the batteries and control box only had to be lifted into the boat and hooked up.

The batteries are second hand car starting batteries totaling 48 volts and will only be used for a few short test runs while I measure the current and propeller shaft RPM at various boat speeds. Once the current is known I will make an informed decision about the battery capacity (amp hours) needed for a day's cruising. Once the prop RPM and boat speed are known I will decide what to do about the prop. The one on the boat at the moment is 11.5"diameter (I would like 13") and 15" pitch (I think I need 17" or 18").

The box above has volt and amp meters, a keyed on/off switch, a forward/reverse switch and a speed control knob. Again this is only for testing the boat, the final controls will be more permanent and hopefully simpler. The green cargo straps will hold a cover down over the batteries to stop them moving about.