posted 11-08-2004 06:47 PM ET (US)        

---

I have a 17' Montauk with a 90hp Nissan 2 stroke. The motor originally came with a 13.25x17 prop. It is a Michigan Wheel exchangeable hub series, or Merc. Flo-torque series. The dealer was both a Merc/Nissan dealer and the stamped number on the prop is identicle to the one on my buddy's boat with a Merc 90 4-stroke. His prop is black, mine is white. In any case, I had purchased a 13.75x15 back in march that I have been using when going out into the ocean. Back in June I installed a Stingray Hydrofoil. I had recently put the 17" prop back on and have noticed a few interesting things. Both props are aluminum.

1. The 17" prop creates much more lift despite the Foil.
2. The boat porpouses now when trimmed up past 0 (plate parallel to hull). Did not do this when I had the 17" but no Foil.
3. The 17" prop does not cause the engine to vibrate as much in the crossover range. (1200-1800 rpm)
4. Fuel economy seems to be about the same for both props and Foil, although I am running the boat much faster now than before.
5. Max rpm's seem to be lower than the last time I ran with the 17" but I used to be able to trim her up higher than I can now with the Foil.

According to the MW website, it would seem that not only is the there a difference in pitch and diameter, there may be a bit more rake on the 17" prop than on the 15". That would account for the increased lift. Now, the lift itself does not seem to be a benefit, at least not in running through rough water. It would seem to me that the increased lift should reduce surface contact, so it should improve economy and reduce engine lugging. So, if this is true, then the amount of stress on the motor from either the 17" or 15" prop should be the same. The 15" prop does allow the motor to rev a little higher, up to where it is at proper temp. so as to keep the cylinders and plugs a little cleaner. What is interesting though is that the 15" had always ran rough, as if missing, in the crossover range when accelerating after even a few minutes of idle. I would have to accelerate slow, and let it "cough" out for about 10-15 seconds before it would smooth out and accelerate. I assumed that maybe it was caused by the foil at low speeds. The 17" does not do this, neither with or without the foil. Offcourse, when I changed out the props a few weeks ago, I also closed the pilot screws 1/8 of a turn each to lean it out just a bit to compensate for increased load at idle. So, it could have just been a bit on the rich side and now I will need to put the 15" back on to test with the new pilot settings. Otherwise the 15" is about perfect for trolling for Salmon, and the 17" is great for trolling for Stripers.