I acquired a demo 150 Yammie 4-stroke last fall, which came with a 13 3/4 x 19 prop. I wasn't sure what my old engine used but I noticed that the engine was a little sluggish out of the hole but very fast. I chalked this up to difference between the 4 stroke and my previous 2 stroke.
This weekend, just for fun, I tossed on my old prop... a 13 3/4 x 17. Immediately, I could feel a huge difference on "take off", but the boat was significantly slower... (New prop: 52 mph @ 5000 rpm, old one: 44 mph @ 5500+
) This was slower than my old two stroke (also a 150) and I felt as though I was at risk of over-reving the engine.
I recall the installer saying that he had mounted the new engine "one hole higher" as it was rubbing a bit on the steering gear.
After this little experiment, I concluded that for day-to-day use (generally shorter trips with a load of dive gear) the 17 pitch might be best, but if I was heading out on a longer cruise where speed and fuel consumption might be more important, I would toss the 19 on.
So a couple of questions: Is this a reasonable approach? Or would I be better to get an 18 and split the difference? Should I consider dropping the engine a hole and see what happens?
I keep the boat on a trailer, so it's about a 3 minute job to change props...
Your thoughts would be appreciated!