Thread: Prop Question
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Old 10-21-2019, 08:21 AM   #1
rslifkin
Guru
 
City: Rochester, NY
Country: USA
Vessel Name: Hour Glass
Vessel Model: Chris Craft Catalina 381
Join Date: Aug 2019
Posts: 797
Prop Question

I know the rule of thumb on prop tip clearance is no less than 15% (with 20% preferred for planing hulls), but in a real-world situation, what can you get away with before noise / vibration become an issue?

My current (stock) props are 22x26 and the boat is a little over-propped. WOT range for the engines is 4000 - 4400, but Chris Craft propped to the 4000 end of the range. So with full tanks, a few people, some stuff and 33 years of age, it doesn't quite hit 4000.

My spare set of props are almost identical Federal Equipoise props, but in 22x25. So the easy button would be to just throw those on and that should get it into the range. IIRC from when I saw the boat run with them at one point a bunch of years ago, they'll get about 4200 at WOT with similar top speed.

But I've had the thought of a little more blade area (either a 4 blade with a bit more blade area or more diameter) being beneficial, as the WOT slip numbers are good (based on the speed the boat ran when new and what I've seen it do, WOT slip is around 16 - 17% at 27 - 28 kts), but the lower speed slip numbers are pretty bad. At an 18 kt planing cruise, slip is in the 30 - 33% ballpark. And cruising along below hull speed at about 6.5 kts, we're looking at about 40% slip.

The Michigan Wheel prop calculator indicates that the 22x26 should turn 4200 and 22x25 should give me 4300, so the calculator is a little off, but in the ballpark. With either target RPM, it gives an optimal diameter of 24" for a 3 blade, and either 22 or 23" for a 4 blade (23" for 4200 target, 22" for 4300 target).

Now here's the snag: With the 22" props on there now, I've got 3.5" of clearance to the hull, so about 16% of prop diameter. If I go to a 23" prop, I'd be down to 3" or 13% of diameter. Which by the rules is too tight. But I've heard of people running tighter than that with no issues.

I'm thinking of keeping my eyes out for a suitable set of used props this winter and just throwing the 22x25s on if I don't find anything. I do have a line on a reconditioned set of the same Equpiose props in 23x25 for a good price, but there's the diameter issue and a 23x25 might need to be de-pitched to a 23x24.

So what would you guys recommend?
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