I get the thing about efficiency on an in-line arrangement, but for me there are other issues side by side motors would have that in-line doesn't. Assymetry is an obvious one, but as two side by side engines should rotate in opposite directions, and most model engines don't, then I'm not sure how much the torque thing would be effected by having two rotating masses trying to roll the boat (I've not looked recently, but about twenty years ago I bought the engines for a twin boat. I chose Irvine marine 40's, as at the time it was the only engine available to me with a reverse crankshaft option. Pop in the new crank, turn the piston and cylinder through 180, one backwards operating engine. Still have them, never built the boat
). The motive for me really is just about fun with a different installation, with the advantage of neutral torque if I can get it, plus there's the grin of the sound of a twin, and the unique chopper note in-line's give as the pulse from the front prop hits the rear one
. If I was after full efficiency I'd be doing everything here very differently.
I've been testing the motors for the last week, but both on pullers at the moment, pusher props are seriously limited here with only one viable one around, APC 9x6. One is on the way, so I'll know more soon. It'll be interesting to see if the Irvine is more meaty than the TT, it has an early plastic carb and as it's only just been run in I'll have to see if it's any good, I had to replace these on the two marine engines as the throttle barrel thingy became sloppy in the carb bodies allowing air to pass, rubbish tbh.
There's a bigger potential problem I need to suss first regarding the pusher. I'm not sure if either engine can take a pushing force on the crank, it can mean that the prop driver runs on the front of the crankcase, wearing it down, eventually the con rod can be hit by the crank web which will equal a total loss. If this is the case I'll have to machine a thrust bearing arrangement (big prop driver running through thrust bearing to collar on crank case) QED.
Today I think I sussed the fuel system. There's not enough space between the engines, or an attractive way for me to put two tanks in the pylon, so the tanks are going in the rear of the 'fuselage' section, so the head is only about 4". This means a long fuel run, and whilst the engines coped with this I found the drain back after priming an annoyance so I've made a couple of non return valves. They work fine, but I need next to try it under way, so a rough build is on the cards, maybe a boogie board. How the weight shift through consuming fuel effects the boat may come to light too perhaps.