Prop help for my Southwind on steroids

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

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.

Thanks for the update Ken!  Seems like a speedy boat 

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

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.

Boy, look at all the activity on this thread now

To clarify, not every hand finished prop is a flexy paperweight- just the ones that run away up top. Can lend you one ken, if you want it!

There’s no doubt that the smaller bladed cast/hand worked props are often faster than an acme or cnc OJ - particularly on a smaller/lighter older boat like mustangs/skiers and ski nautiques older than the 2001’s.   They have less drag and often help bring up the nose. The only boat i have ion the water today is rocking a federal - but it sure does take the joy out of really hammering on it - can’t throw $450, or multiple 450s at everything in the barn .

I’ve spent the equivalent of a pretty good modern wakeboard boat rebuilding small blocks on steroids - all the best steroids - that I have broken messing around for more speed- it was usually props that were just a bit too big, or brought down the rpm a bit too much that broke them.   More rpm - less load, less stress - less heat- less detonation- less pieces of metal in the bilge.

Nice work Ken!  Did you lap the props when you switched them?  If not then I am not so sure the data is reliable!  (that is meant to be a funny by the way)  

Certainly- the vortec 350 can get to stock bbc hp numbers pretty easily just not at the same rpm - which is why you wouldn’t want to run the same propeller - good advice above.   The ellis prop is a bit of an unknown,   Tunneldrive hull guys purportedly love them to reduce vibrations which could be a cupping thing - in my experience cupped props are even more useless at higher rpm’s than lower rpm’s.

Either way don’t fear the rpm I wouldn’t prop that boat for anything that it can’t spin at least 4900 rpm - assuming the cam is healthier than a stock excalibur grind i’d probably be shooting for 5100 rpm plus

I forgot to add to the above post that the new CNC props wake up old boats in ways I never thought possible.  Noticeable and favorable differences for sure.

I think the basics are, the first number, 12 or 13 is the diameter of the prop, second number is the pitch but I was told it is how many inches of travel through the water with one rotation.  12 x 15 = 12" diameter x 15 = 15" travel per rotation.  Less pitch = more RPM, so a 12 pitch will spin more RPM than a 15 pitch.