Prop help for my Southwind on steroids

I have a 1975 Southwind 18 I recently rebuilt the boat and the motor.
Originally it had a 307 with 225 horse and Ellis 13 x 13 propeller
The boat seems to be all original as it looks like nothing was ever damaged or replaced.
With the old 307 the boat would run 4000 RPMs and be at 40 mph.
With the new 350 that we stepped up a bit the boat easily overspeeds the motor.
When you run it up to 5000 RPMs and pull the throttle back to stop it there you’re going about 45 mph.
The only other thing I might mention is that the clearance between the propeller and the hull is about a quarter of an inch or less. And that is how it came from the factory.
I am thinking there’s someone out there that has done this before and has some recommendations
Please help

Den

Foolish question - would replacing the shaft with a maybe 1" longer one make sense, would increase clearance and looks like plenty of prop to rudder clearance now.  1/4" seems crazy close to me, I guess Tim's suggestion of dropping to 12.5" would do the same but wonder if you'll still turn close to or over 5k rpm.

Got it thanks for the explanation.

The boat performs exactly like it did with the 307. At 4000 RPMs 40 mph. That seems logical. As for the Tach, when we were timing the motor up we had a Tach on the timing light and it matched all the way up, so that’s been verified.
I’ve been talking to a fella. He’s real knowledgeable about ski boats. I won’t mention who he is but believe me he knows. Told me that the distance from the prop to strut Can be no more than the diameter of the shaft. In this case, it’s 1 inch. That’s where we are.
I had another thought, a barefoot nautique running a 454 probably has somewhere in the neighborhood of the same horsepower that I’m talking about with my 350.
I heard they were not known to be powerhouses   The hulls of the boats were originally the same so I wonder what they were using for a prop. Of course back in those days it would’ve been an old cast prop, and not a CNC. Pretty much what I’m running, but who knows.
What does anyone think of Ken‘s recommendation of 13 x 13? I guess that’s a pretty common prop.
My only other option I think is to go with the 13 x 14. And I’m afraid that might be Too Tall.
Does anybody have one of those props wants to sell it. I would really like to go on vacation with one of each.
Remember, we are talking right hand turn.

Den Den

There was a boat company in Sacramento, Keaton Boats, they built a 18' Barefoot Keaton for a while that looked a little like the early ski Nautiques.  It was also known as the Keaton Utility if you try and look it up.  

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.  

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.

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

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)  

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.

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!

Boy, look at all the activity on this thread now