This weekend I had a chance put the prototype hull in the water to see how it handled. I posted a no-frills video of it below:
During the first trial I ran the boat without any additional weight. I had some trouble steering it. I attributed this to the rudder control being inverted from what I was expecting and the boat sitting high in the water. I fixed the rudder controls using a reverse switch on the transmitter. I added about 20 lbs of dive weights to get the boat to sit lower in the water. After these changes I was able to make small steering corrections easily using the rudder.
Getting the boat to make more drastic turns turned out to be difficult. I tried putting one propeller in forward and the other in reverse but this didn’t seem to have the amount of turning power I expected. Powering just one propeller forward with the other off worked better but still was no match for the winds that were blowing at the time. A combination of one propeller, rudder and luck seemed to be the only winning combination. More testing will be needed to determine the exact cause for the trouble but my current theories are:
- Unequal drive power between the two motors. If this is the case, a closed loop control system can be used to fix it.
- Insufficient power. For the tests I used an old 9.6V NiMH R\C battery pack to supply power to both motors. Given that the voltage sagged to below 5V during operation, it is likely that the battery pack’s ESR was too high to supply full power to both motors when I was trying to drive them at full speed in opposite directions. This problem can be eliminated by using a different type of battery or by using separate batteries for each motor. Using separate batteries has the additional bonus of redundancy.
- Insufficient propulsion. The motors / propellers may need to be changed to provide more thrust.
Other things I noted during the testing:
- A bumper should be added around the perimeter of the boat to protect the fiberglass when I run it into the dock or pilings.
- A cage should be constructed to protect the rudders / props when the hull is set on the ground or when the stern collides with a dock / piling.
- The RC Transmitter is not well suited for a boat. The port motor control uses a throttle control stick meant for an airplane; the throttle does not automatically return to zero.
- The hull maintained dry despite water washing over the top of the boat occasionally.
- The addition of 20 lbs of weight made the boat more stable and less tippy.
- The port drive motor needs adjusted. It made a lot of noise and seemed to have less power than the starboard motor. A post inspection found the port motor shaft to be out of alignment with the propeller shaft tube. I didn’t notice this upon first running the boat so the motor support member made need additional support.
- We may want to consider placing the thrust elements midships on the next version to make turning in place easier.