Talador
Again this is a situation where keyboard in gear before brain.
The shaft is 8mm diameter. It curves down from the gearbox starting at 20 degrees and is around 15 degrees at the waterline and in line with flow where it mounts to the prop. Are you starting to work it out? The waterplane where it enters the water is a narrow ellipse - drag coefficient much much lower than a circle. It then becomes a long slender rod essentially in line with flow. Its cost in drag is well under 1% of overall drag.
My drive systems are race proven. During a river race the prop and shaft contend with all manner of moving and fixed objects from tenacious weed, floating logs, shallow submerged logs, shallow gravel bars, rock dykes, sand bars etc. The compliantly mounted curved shaft and folding prop survive that quite well. And I can tell you at the finish line after a 100km day if you find a bit of tenacious weed wrapped on the prop it is not a nice feeling to know you have been spinning that mother through the water for the last two hours now knowing why. With the side mounted prop it is a simple matter to reach down and inspect. Without the ability it could mean a swim or a long paddle to the nearest shore to remove fouling.
Last month I did 200km of the Murray River from Albury to Mulwala:
Just a sample. Close to the best exercise on earth. In the last 20km where the river slows due to the weir that forms Lake Mulwala the weed is prolific. It gets broken off by power boats and floats to the surface. A good portion that gets caught on the my prop blades will release when the blades fold but I still had to stop 8 times, reach down and manually remove weed wrapped so tight it would not release during the last 2 hours.. At the top end of the lake the land is flooded and studded with millions of dead trees. The original channel could be 3km from the nearest shore - a long paddle through dead trees with a disabled prop just to clear fouling.
The offset thrust costs a little over 1% in extra drag to keep course. The steering control has friction so it stays where i set it. On a lake in calm weather I can go the 1.5km length of the lake without course correction.
Rick
On 28/03/2015, at 1:04 PM, taladorwood@yahoo.com.au [harryproa] <harryproa@yahoogroups.com.au> wrote:
Rick, I know you are sick of hearing from me, but I really enjoyed the pictures of your pedal boats.
I know you ran the drive shaft and mounted the prop on the side for ease of construction, but the round drive shaft and surface interference from the side mount of the prop may be the biggest single sources of drag at speed. It is why they fair the wires on biplanes, more parasitic drag from the wires than the wings.It is just a thought, but with the prop mounted forward, below the hull with a faired mount, below the waterline, you would save a lot of drag. With a retractable rudder I wonder if you couldn't get it to run straight enough that you could retract the rudder for long periods of time.
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Posted by: Rick Willoughby <rickwill@bigpond.net.au>
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