Prop help for my Southwind on steroids

No problems with the prop logic, I struggle with the 1/4" clearance between the prop and hull, that just seems like a problem in the making?   I would feel better if it was at least 1/2" clearance but I am no prop expert.

DenDen wrote: It seems that I either need a 430 13 x 13 or a 474 13 x 12 5/8. Both at .080 cup. What is the expected RPM difference between a 1 inch change in pitch? If it is as low as 200 RPMs, then these two props are basically the same. If it’s as high as 1000 RPMs, then the difference between these two props is 300 RPMs. My understanding is by changing the cup. You could turn either one of these props into the other. So my big question is why do they make so many props so close together? I am still confused, but getting close. Is it better to start with a big prop and trim it down or start with a little prop and build it up?

You would need to know your torque curve and where your current prop fell rpm wise to even make a guess at rpm’s per inch of pitch. Your understanding of cup is flawed at best and while it is possible to change cup or trim down the diameter of a cnc propeller it is not free and is now way to live.   You want the fresh cnc goodness’s.   I’ve got a boat that runs 5000 with a borrowed 431 and I bought it a fresh 475 hoping to get a few more rpm out of it - it’s 61 years old so I am hoping to give it a little easier time.
- you’ve been given the best advice you are going to get knowing what you know, but a prop and go boating
- or wait a few weeks and I’ll tell you the difference between the two props on a completely different boat with a completely different engine

Thanks for the update Ken!  Seems like a speedy boat 

That all sounds about right - if for some reason I was dead set on overloading an engine with too much prop, or just in a position where it was the only prop I had to run and needed to get some recreation in, then I would make some adjustments to prevent knocking/destructifyering.   Specifically consider dialing back a couple/few degrees of timing. Could also give it a bit more fuel jetting and or higher octane gas - particularly if you are going to pull watersports.   450 dollar experiments need not become 4500 dollar season enders.

The 51 mph at 4300 surprises me.  You upgraded power to a bit over 300 hp so that I guess explains it.

I bet some of you were wondering what happened to Den Den.
Since I went to Lake Norman came home sent my prop out and had it decupped, I have not had the boat in the water. It may be a couple of weeks before the final test.
But I have been thinking about purchasing a prop and the procedure to go through.
It all seems to be a guessing game of sorts at the expensive the guy buying the Prop
I say that because the only thing a person can do is look for recommendations from people who are supposed to know what prop you need.
In my case, I was recommended at one time a 13 x 11 , a 12,5 x 11.5, a
13 x 12.625, a 13 x 13, and a 13 x 14.
All I had to do was make the choice and see what worked.
Kind of a guessing game? especially since the boat was a south wind 18 which there are very few of.
I’m pretty confident I’m going to be close to what I wanted going the route I chose. I also know I could’ve probably taken another route and ended up maybe the same place.
It seems to me there should be a better way to sell propellers, but I do understand the difficulty in returning props that are wrong or perform poorly.
That’s all I have right now. I will post my findings when I get the boat in water again.
I hope everyone is having a great summer.

Well said!!!!
I couldn’t agree with you more.

DenDen wrote: In my case, I was recommended at one time a 13 x 11 , a 12,5 x 11.5, a 13 x 12.625, a 13 x 13, and a 13 x 14.

I think I hear ol' blue eyes in the background singing " I did it my way"