Re: What props will fit a 1959 Johnson 18HP?
The existing prop on your motor can be used in the interim, but as you said you want to keep the throttle settings low as it may lug your engine.
Your 18 shares propeller interchangeability with the 10-horse 1951-1957, the 15 through 1956, 18s & 20s spanning 1957 ? 1976, and the 25 1969 ? 1976.
I'm going to quote some older OMC part #s. Who knows what they would cross reference to today.
The standard prop on 18, 20 & 25s generally in that period was part # 383629, a 3-blade, 9? diameter 10? pitch prop. OMC offered a free exchange when they were new for alternate props on those motors.
First was part # 592959 that had the same specs (3x9x10) but was intended for extremely weedy conditions. That won?t help you in your case.
Next was part # 379717, a 3-blade, 9? diameter 9? pitch prop (which just happened to be the standard prop for the 1951-57 10-horse). The 9? pitch was intended for 14? ? 16? boats with heavy loads when used on the 15-18-20-25. That sounds like the ticket for you.
Speed range for the 10? props spans 20 ? 30 mph as a peak speed. The 9? pitch will span 10 ? 22 mph peak speeds (hey, on a heavy boat...) You want a prop selected not especially for more speed, rather you want one that will allow your motor to reach it?s peak intended rpm range. If you use the shorter pitch on a light, fast boat and you?d over-rev the thing and possibly grenade the engine.
I notice on the i-boats link provided by Tashasdaddy that there is also a shorter pitch prop in stock. It is a 3 x 9.25 x 7. If your boat is quite big or especially heavy that one might also be worth considering. Still it might be easiest to just find a junked 1951-57 10-horse and grab the prop. The 1958 & later 10s had a different lower unit and shared props with the 1964-73 9.5 horse motors - not appropriate for your motor.