Changing Props?

[Good Grief]

I have a 98, 290 sundancer with Mirage 15.75x15 props (48-19841 15P & 48-19838 15P) purchased it that way. Not long ago I read a prop thread where an individual calculated what the effect of changing props would be.

SeaRay recommends 13.75x21 props.

I'm trying to determine the "hypothetical" difference prior to investing in changing the props.:huh:

The boat has twin 4.3l mercruiser (190hp) and Alpha 1 Gen2 O/D's.

There is a little inaccuracy in the actual vs. indicated RPM. When surveyed there was 50 RPM difference between actual and indicated at 4600RPM. When accelerating to get on plane I have seen up to 5000 RPM and immidiately backed off.

Thanks in advance!:smt038

 

[jdamackenzie]

My mechanic is an ex boat racer and has valuable opinions on props and speed. When we bought our 270DA she had steep-pitched, small-diameter props, just like yours. Many people think that more pitch equals more speed. The steep props on our DA made her want to fall off plane at anything less than 3/4 throttle and, although the engines would reach 4,500 RPM with "zero" vacuum, our speed was not that great. I took his advice, which was the same as SeaRay's but he didn't know that, and now I get much better "climb-out" higher speed and cruise at 3,400 RPM with no tendency to fall off. My guy says that increasing diameter increases lift at the stern so the boat planes sooner, planes at smaller throttle openings and I still have 5" of vacuum on both sides for greater engine life. In my case, the mechanic was able to trade props through his supplier to get the ones he recommended (we are in the pacific northwest so there are many shops who specialize in fixing props).