4.2M rib with 25HP...what prop??

jpn · 12 October 2010, 11:09

I got a china made 4.2m rib and put my 25HP mercury 4stroke....I know it would be ideal to put a bigger motor, but it really is just for the kids to buzz around so I do not want more power....

Now it has a 10x11p prop, at wot it gets on plane 3 adults and full gas tank...but I can only achieve 4800rpms, I will have to change props...any suggestions on prop size appreciated.....

engine rev limiter is 6300 rpm.....ideal WOT rpm is 5500-6000rpm.....

9D280 · 12 October 2010, 12:54

I'm in two minds as to whether that sounds right or not. I had a Suz 2-stroke 25 on an Avon SR4, and ran a 13" prop on it.

The usual approximation is 200rpm per inch of pitch (assuming you keep the diameter the same), implying you could drop to 9"...

What rpm does it do one up?

njensen · 12 October 2010, 13:26

Going to 9" would give you WOT rpm at 5200-5500. That would be a top speed at 18-19 knots with 10% assumed prop slip (gear ratio 1.92:1)

jpn · 13 October 2010, 01:56

Quote:

Originally Posted by 9D280

I'm in two minds as to whether that sounds right or not. I had a Suz 2-stroke 25 on an Avon SR4, and ran a 13" prop on it.

The usual approximation is 200rpm per inch of pitch (assuming you keep the diameter the same), implying you could drop to 9"...

What rpm does it do one up?

I can do 5000-5200 1 up fully trimmed out.....

Thanks for the input guys...i appreciate it....

9D280 · 14 October 2010, 10:19

So one up you could probably drop to 12"...... either way it sounds like you are a bit over pitched.

18-19 knots sounds ballpark right for that kind of setup. Mine did 21 knots absolute flat
out with a tail wind, but I usulally cruised at 18-ish.