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Old 10-21-2019, 09:21 AM   #1
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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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Old 10-21-2019, 10:38 AM   #2
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I've set up props and in some cases could not get the 15% clearance. Sometimes no issue at all. The only times it was a problem was prop noise at high speed, and that means like 25kts. Same boat at 18-20kts, prop noise was not bad at all. And this was clearance like 10%.

If you go to a 23x25, I bet you will need to knock out some pitch.

I also would not get too wrapped up in slip numbers. At slower speeds the slip will be high, nature of the beast.
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Old 10-21-2019, 10:45 AM   #3
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I know low speed slip will always be higher. But would going to a bit more blade area (either more diameter or a 4 blade) with less pitch possibly be more efficient at the lower speeds? In theory it should show a bit less slip. And speed, slip, etc. at WOT is pretty irrelevant for my use as my typical cruise is a solid 9 kts below WOT speed.

Speed-wise, if I'm doing better than about 18 - 19 kts, it's for the once or twice a season brief WOT check. My typical cruise is in the 17 - 18 range. Cruising faster than 18 kts or so is just asking for too much continuous output from the 454s IMO. As far as speed at WOT, Chris Craft lists 28 kts at 4000 RPM as built. I've personally seen a bit over 27 kts from it.

Maybe I'm best off saving the $$$ and just throwing the 22x25s I've already got on there? Assuming WOT slip stays constant and I gain 200 RPM at WOT, the boat should actually be a hair faster with those on in addition to being kinder to the engines.
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Old 10-21-2019, 11:33 AM   #4
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Going to larger diameter or otherwise getting more blade area (like more blades) is a double edged sword. More blade area will give you less slip, and from that the efficiency is better. BUTTTT... More blade area gives more skin friction on the blades and that hurts efficiency. For any operating speed for a given boat, there is an optimal prop. You can spec a prop for low speed. But change speed and now that prop is no longer optimal.

You worry about slip at 17-18kts, and worry about getting your 4200 full power. Optimal prop won't be the same at those two conditions.

If you run 17-18 and never go near full power, I would not worry about it making 4200. This is not some highly strung out turbo diesel. On those we get picky about making desired rpm.
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Old 10-21-2019, 11:37 AM   #5
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When I mention WOT RPM, it's mostly from the "don't overload it and cook the exhaust valves" perspective. If WOT ends up at 3700, then cruising at 3300 is pushing it pretty hard I'd think. Actual boat speed at WOT is pretty much irrelevant to me.

But that makes me think it might be worth just throwing the 22x25s on and see how it runs next year. And then decide if it's worth buying more props.
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