VP100b prop diameter

[gljnr1983]

Hello guys i have got a dinked & pitted 13x12 on the(22ft medium v 1800kg) boat just now so was going to order a new 14x12 to hopefully get less slip at low rpms(trolling-cruising),i dont do much over 3000rpm when cruising so any one think the diameter increase will benefit me?without causing any extra vibration and the likes with the added weight spinning.

 

[Silvertip]

Re: VP100b prop diameter

Buying a prop strictly on its diameter is a mistake as is basing the decision on what your normal cruise speed is. Fact is, the engine manufacturer has specified what the recommended wide open throttle rpm is for that engine. The prop you install must allow the engine to reach that WOT rpm with what you consider an average load. Failure to reach that rpm caused by too much prop PITCH means the engine is lugging which also means the throttle must be open farther than it needs to be so fuel economy is bad. Too little PITCH means the engine can over rev. Yes -- I know you never run at WOT or rarely do but if the prop is wrong at WOT it is wrong across the entire RPM band. You buy a prop for your outdrive using a selection chart and the pitch based on several WOT rpm runs. You then select PITCH that puts RPM in the proper range. Diameter will take care of itself. If you look at selection charts, as pitch goes up, diameter comes down a bit and vice versa.

 

[jestor68]

Re: VP100b prop diameter

Let's keep this simple.

An inch of diameter has a similar effect as an inch of pitch in terms of changing rpm.

Increasing your diameter by 1 inch will reduce your WOT rpm by about 200 rpm, while keeping the same pitch.

The larger diameter prop will produce more static thrust and provide better cruising performance. It may not run quite as much top speed due to more surface area causing increased drag at higher speeds.

So, if your WOT rpm can afford to lose 200 rpm and still be within the recommended range, your're good.

If you need to keep the WOT rpm the same, you will need to reduce pitch as you increase the diameter. For example if you go to 14" of diameter, you need to drop to 11" of pitch in order to keep the same WOT rpm.

Your 3978 pound boat needs the largest diameter prop that will fit on it (15"?) in order to maximize performance.