Powercat prop sizing

I would like to get some second, and hopefully, educated opinions about prop sizing for the Skoota.

I ran the boat into the Victoria Prop tool and it only returns a top speed of above 16 knots and we expect more like 20 or a bit more.

Here is a screenshot from Vic Prop. Let me know if more info is needed. I have one prop now. Need another.

13 1/2X15-K is what I have, want a good second option and wondering why VicProp has my top end so low.

 

It all depends on what algorithm the program is using for resistance estimate, and whether your slender demihull fits within the application limits. This is the difficult part of the excercise. There simply is no one-fit-for-all hull shapes, it requires adequate knowledge to select. The propeller algorithm is a different creature; if you have a realistic speed input, the outcome in terms of propeller configuration will be quite realistic.

If you think your speed estimate is more realistic, then you may input false values in the hull dimensioning (begin with lower displacement values), until you get the program to come up with that speed. Then the output prop data will be closer to your speed estimate. But as always, crap in => crap out.

 

they told me the calculator was wrong for my effort when I called them

 

Oh oh! Time to drag out your copy of Gerr's propeller handbook. Or follow @baeckmo 's suggestion.

Work is force times distance, and power is work over time. So if you know your weight, and speed at a given power and rpm you can work backwards to determine first, resistance, then which prop gives that much thrust at that power and rpm. Which is what the VicProp calculator does. It calculates top speed using the Crouch Formula, then finds the prop that gives those values. If it's giving too low a top speed you should input a higher Crouch Constant. Although that calculator only goes up to 210. Maybe your boat performs better than that.