I have a 3HP outboard which I too only use when I have to.http://www.ebay.com.au/itm/New-3-0HP-In ... 3145744745
We intend to TI the length of the River Murray, 1,476 miles, in 2015.
The Murray is not wide enough to tack in most parts and I aint pedalling that length.
If you do go for the motor, here is the mount I fabricated out of aluminium with blind rivets and epoxy. The aluminium 1" tubes, reinforced with wooden dowelling, go in the rear scupper holes, right through to the bottom. I tie those poles to the cleats where the rear elastic ties. This gives some movement to the mount so I have rubber glued to the pads that sit on the transome. Even though I car top, I don't bother to remove the mount as it is very light.
The motor works well hanging off the side and easily controlled from the rear seat.
I had the motor fitted last week when we did the upper reaches where there is some rapids. Only used the motor to test in the deeper water. The 3 HP is plenty of grunt and maximum speed is obtained at just over half throttle. It uses 1 litre per hour and you are right, it is messy compared to propane, especially when filling under way. That frame on the back takes additional fuel giving me about 5 hours. And no battery to recharge at the end.
Of course the best time is when you shut down the motor and enjoy the peace of sailing.
I have a new Mercury 5HP 4stroke I intend fitting to my TI. I like the look of your mount. I was going to have an stainless steel mount fabricated and be bolted thru the hull in same location as yours. However I like your the idea of not drilling holes thru the hull. My mount will have to be in s/steel due to the power of the engine. The feet which rest on the hull would of course have to be longer to take/spread the additional load of a 5HP engine (just been running it in in a tub in the back garden!). By the way where do you store your wheels when at sea?
Reason I now want/need an engine is I have had two rudder failures last year. Trust me if anyone ain't had one yet your in for a fright esp. being blown downwind and with the tide pushing at (2 knots) towards rocks solo in a TI, not a chance in hell of peddling even in a light wind. I just managed (peddling like ##### ) to run it up a beach packed with holiday makers 100 feet before the rocks. There was a good swell running which turned the boat slightly side-ways at the last moment causing the aka safety pin to pop. Sunny day in a dry suit ensured I was soaking in sweat! Got out the boat trying to look cool as if I had done it all on purpose
The first break was due to the plastic rudder pin breaking (whilst at sea!). The second break (thankfully I had a co-pilot this time he was able to paddle steer from the rear seat) again happened at sea, and was due to one of the two s/steel bolts which hold the rudder assembly to the boat shearing
I have since mounted (behind the rear seat port side) a back up rudder from a small dingy using a Hobie sidekicker mount cut down (engine will be on the starboard side).
I go out in the English Channel near Dover (the narrowest point and tide can reach up to 3 knots (4 knots on the French side) so your soon swept away when things go wrong hence the need for an engine. Also one of the busiest shipping lanes in the world link shows shipping traffic in near real time http://www.marinetraffic.com/ais/
ps I sail out of Folkestone