To do it properly, yes it is that "complicated". But it's not rocket science.
Everything else is a guess. They may be an educated guess, but the correct way to do it is with a working, calibrated tachometer.
But to argue my own point, prop selection is not a science so much as it is a guess. The tach just give you more education so that you can choose more effectively.
More pitch=less RPM. Less pitch=more RPM. Simple.
If your motor is specified as having a maximum RPM of, say, 5500 for example, and you have a 15" pitch on it....with no tach, how do you know what RPM you're currently turning? It could be 3800. It could be 6500.
Under propping is just as bad...if not worse...than over propping.
You want your maximum RPM to be right on what your motor specifies. If it's specified 5500 RPM, you'll want to prop it to 5500 or real close. The motor will thank you with a long life. Overloading the engine with a prop that has too much pitch overheats the engine, carbons up the pistons and cylinders and heads, sticks the rings, and generally hurts it in the long run. Too little pitch tends to stretch rods, overheat in some cases, stresses the engine and the drive system, etc.
Good story. When I worked at a dealer, a customer came in and bought a really nice 860 War Eagle duck boat, rigged with a Yamaha F90 4 stroke. 25hp and up do not come from the factory with a prop, so it's up to either the dealer or the customer to select a prop. He wanted a 15" Turbo 1. 15" is not nearly enough pitch. I told him that when he picked it up and advised him that if something were to happen to the motor due to incorrect prop, that Yamaha could decide not to honor the warranty (not likely, but they could based on the warranty statement). He said he'd been doing all the research and said that 15" was what everyone suggested. There was no tachometer in the boat, he did no purchase one-said he didn't need it since he'd been running boats all his life (but never had a 4 stroke....)
Couple weeks later guy shows up saying his motor is like running rough when he's got the throttle pinned, and sometimes it wants to cut the speed back to about 1/2 of what it's supposed to do. He says his max speed is right around 28 mph, which he accepted. I knew already what the problem was, so I took off with him to the river and we ran the boat. I also took my portable inductive tachometer. I was driving, he was holding the tach. I pinned the throttle and we were on plane almost instantly. Impressive planing time. By 25 mph, we were at 6400 RPM. max is 6000. At 28, we were on the 6600RPM limiter. Tach was bouncing between 6600 and 6400, and I could hear the motor missing. He says that's exactly what he was talking about, something was wrong with it. Turning point (at the time) suggested 15", but the F90 was a new motor...which hadn't been on the market long. I called the shop and asked them if they had a 19" in any sort. Parts guy had one in stock, Solas 4 blade aluminum...which I didn't really want to use, but we loaded up, drove back to the shop, and I told him we'd switch the prop and head back to the river. 20 min later we're back in the water with the Solas. Pinned the throttle, it was a little slower to get on plane but once it was up, it stayed up at a much lower RPM (right around 3500 vs 4200). Then we ran down river at full throttle, him holding the tach. We were right at 6100 full throttle, which is acceptable to go over a hundred with a light load. But the big news is that his speed picked up from 28-ish to around 37 or 8. 10mph improvement. And the motor was MUCH quieter, smoother, he absolutely loved the way it ran. And to this day still does, though, he switched from the 4 blade 19 to a 20" 3 blade stainless Turbo which is a shade faster.
The tach, if it reads correct, does not lie. If you find your hull and your motor on a setup bulletin, how do you know yours is loaded the same? How do you know that your motor is at the same height? Same fuel? Same elevation, air pressure, humidity, temperature, etc? All these things play into it, some more than others. Elevation is a big part. Higher elevation=less oxygen=less power=you would need less pitch.
