You must have the wrong prop, do you know what it is? Mine goes 40 mph and cruises at 30 easily.

maybe motor mounted too low?

you should bang right up on plane cruise at 30 mph @ 5000rpm. Trimming up should top out around 5800 rpm and just under 40mph

I have 14 x 20 SS prop.

I always thought a 14x20 would be the best prop, I had a 14x19 and a 14x21, I used the 21p most of the time because it brought down the rpms a little, but with the 21p prop my top speed was 38-39, I got 40 with the 19p.

My prop is 15x21 ..What would be the benefit of the bigger slower prop ? I think i'm going to go by the prop shop tomorrow . The top end at 30 hasn't really bothered me though ..it seems like im in a no wake zone more than not . but it would be nice to get a little more speed

I would try a 14x19 or 14x20, the 15 inch prop is too much for the 140 to turn on this size boat.

Its not as much a question of more speed, its that you are putting a lot of strain on your motor and it will wear out much quicker, or just fail.

If you go larger diameter, you need to step down in pitch normally due to a larger prop hooking up better. Larger diameter props have MANY advantages IMO, but top speed is not one of them. Me personnally, I was always willing to sacrifice 1-3 mph top end to get all the other advantages that a larger diameter prop provided. Chances are you need about a 15x18 if you stick with the larger diameter. And lugging an engine by over propping is one of tge #1 causes of powerhead failure. Many think over propping is good as it keeps the rpm's down, but in reality you're KILLING your powerhead and putting unneccesary stress on your lower unit.

ok ... so I went and talked to the guy at the prop shop and he said I wanted the top end RPM's in the specified range . On my Johnson 140HP 4 stroke the manual says full throttle range is 5600 to 6200 RPM. I took the boat out today and at full throttle I hit 5300 rpm and my top speed was 31.4 mph . Im right below the range but not much . Speed is not that important to me but damaging my power head is . With the above #s do you guys think I am over propped or maybe just a little ans shouldn't worry about it ?

Another way to look at it is you are 900 rpm away...with my 14x19 I spun right at 6200 at WOT 40mph GPS measurted, the 14x21 was around 5900 WOT, 38 or so mph.

You will be much happier with one of those props at all rpms, your boat will pop out of the water when you take off, and I bet you cut your fuel usage by 1/3 or more.

Tomorrow Dave (the new owner of my DF-140) and I are going to the lake to run it. If he doesn't need the 14x19 prop, I will be glad to send it to you to try out.

We have the 14x21 on it now, I am thinking he will need a 23p or 25p as his boat is considerably lighter than a V-20.

Maybe a swap can be done if the 19 or 21 works for you.