Merc 30 4stroke

My 2001 mercury 30 4stroke will not turn up rpm. It bogs at 4k and will suck a tank of gas in no time at all. All 3 cyl are firing, and it accelerates smooth up to 4k when in the water. Out of the water it turns up about the same but backfires

Looks like you need to drop pitch roughly 200 rpm per inch of pitch. Stay with the same brand and style of prop for more consistent results. Tune for max rated rpms at WOT. Probably up around 5 to 5500...check engine spec. So that's 5 or 6 inches of pitch drop and that's a lot. Course running 4000 on a 5500 rated (if that's the number) is a lot too. Engine is lugging and that can account for high fuel consumption.....like driving your car in the mountains vs on level plains. Diameter will get larger slightly but don't worry about that. Hole shot will be much much better also as will ability to maintain a steady pace, especially right at planing speed and running in rough water.

You may not want to go the full 200. Reason being is that your boat will perform much better and more of the hull will get out of the water as speed increases; hull drag decreases due usually to less wetted area and higher transom position. Since props can be bought online for about $100 and probably less in that hp range, I might shoot for a 4" pitch drop and see what happens. It's really a SWAG until you run the prop on your boat with your operating conditions. That is the only way you know for sure if you have the right setup.

However, "setup" is most important as type of boat, hull condition, load, engine height, trim position if you have it and so on. My comments are based upon your only problem being a lot of boat for 30 hp requiring a change in the "transmission gear ratio" to handle the load that you have........like downshifting your car to handle a hill to keep the engine from lugging.....same thing. Difference is there is no tranny of sort and you only have one gear.

Oh, and this all assumes the engine is it tip top running condition as it is 13 years old.


Mark