Makes more torque than a 180° crankshaft would, or a V-twin for that matter. The firing pulses are evenly spaced 360° apart instead of an odd firing 180 crankshaft (like a Ninja 250, 300, and 650)-which has zero torque to speak of, and the 90° V-twin's pulses are 270°, also an odd-firing engine.
Suzuki had a V-twin outboard. Flop. It vibrated quite a bit, due to the odd firing pulse-NOT because of the balance of the engine (the 90 deg V-twin doesn't suffer too much from imbalance vibration. I have run one and was nowhere near impressed with it. At low speed it felt like a 9.9. Once it was revved it felt more like a 25 should, but had a really good midrange, 3500-4500rpm. Then it kind of signed off up top. It was also a bigger (physically) motor-at least it looked and felt "big" to me.
One manufacturer tried a 180 deg crank in one of their motors and it had such little torque down low that it would not have been real popular. That's what I am told. I did not see one run and I didn't question an engineer's story. He talk, me listen.
Keep in mind that the intake and exhaust systems also are affected differently in outboards than a bike. The firing pulses also affect the intake manifold, so a V-twin with a single carb is typically going to have some roughness to it's idle at lower engine speeds (say, below 800 rpm). This is due to reversion in the intake manifold. A pair of carbs would solve it, but also add complexity, weight, and cost.
A 180 deg crank has a large split between the firing of one cylinder and the firing of another; same for the intake pulses. There is such a large split that it runs a little rough with a single carb; hence most of the parallel twins with 180 deg cranks have dual carbs-to help smooth out the idle.
Then there's the horizontally opposed twin. To make one live, at 6000 RPM, would require a ton of reinforcing material and perhaps even a counterbalancer. They are hard on crankshafts and blocks, they are usually large (wide) in comparison to a V or parallel twin, and keeping them together at extended operation at those engine speeds has been challenging without a lot of heavy parts. More weight, more complexity, bigger, not a good option for an outboard.
Hmm...big bang parallel twin Have someone make a cam. It could make good torque and be real responsive but my goodness I could not imagine the vibration! I'm pretty sure it'd shake my boat apart.
There's a lot more to it but that's the nuts and bolts of it.
