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rslifkin 10-21-2019 08:21 AM

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?

Ski in NC 10-21-2019 09:38 AM

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.

rslifkin 10-21-2019 09:45 AM

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.

Ski in NC 10-21-2019 10:33 AM

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.

rslifkin 10-21-2019 10:37 AM

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.

rslifkin 05-24-2020 08:55 AM

So I've got an update on this one. I threw my spare 22x25s on over the winter. Unlike previous years, I painted them this time around (Pettit zinc stuff and then Black Widow on top). I may have lost a little due to the paint adding thickness and I didn't sand out the brush strokes (just knocked down any drips / runs), so I figure I may see a little improvement with some more running time as the paint wears a little.

Took the boat out for a test run yesterday and from what I'm seeing, slip in the 16.5 - 18 kt range is still about the same (low to mid 30s), just a bit lower boat speed per RPM with less pitch. They turned just on the high side of 4200 at WOT, but I only just barely touched 25 kts, when I know she's done 27+ with these props in the past. So I'm looking at just over 25% slip at WOT, which is less than great. I also seem to be getting a little more slip in the 6.5 - 7 kt range, as the RPM increase to get that speed is more than expected with a 1 inch pitch change. Engine responsiveness felt much better and then engines seemed good and happy. Took a little less throttle to do 17 kts as well, so probably better fuel-wise.

Oddly, despite being theoretically the same props other than pitch, the 22x25s visually look to have a little less blade area, which might explain a couple of the things I found on my test run. That might also explain why if I pushed the throttles up kinda quickly getting on plane it didn't feel like there was quite as much bite. The engines wound up more easily, but the boat seemed to accelerate more smoothly, rather than immediately giving a kick with the first bit of throttle push.

So I'll run these props for the season and see if anything changes as the paint wears, but I'm going to start keeping my eyes out for something in 22x25 with a little more blade area to try for next year unless anyone has a thought on something that might work better.

EDIT: Just realized I never mentioned it anywhere in here, but I've got 2.57 reduction gears, so my shaft speeds aren't all that high for a gas powered planing hull, which would explain why it seems to want a good bit of prop. Interestingly, despite my boat having 22x26 equipoise 3 blades originally, Chris Craft used 23x25 (same props otherwise) on some of their other, older 38 footers with 454s. Kinda makes me wonder why, although 23" diameter would be very tight on my hull.

Nomad Willy 05-24-2020 10:58 AM

At first I was thinking you need more blade area.
But what I think would be time well spent would be to explore ideal pitch/dia ratios and how it applies to your prop speed, power and boat speed. There is a range that is considered ideal and anything out of that range will be less than ideal.

Some applications are better outside the optimum range. A tug boat needs mostly thrust at basically zero fwd speed. A small outboard engine needs to perform fairly well with a wide variety of speeds and loads. The OB will be clamped to a wide variety of boats not being a part of any one boat.

An example of why I mention the above paragraph is that you (unlikely though) may not have the necessary prop clearance to get your ideal dia/pitch ratio due to clearance limitations.

Irish Rambler 05-24-2020 12:34 PM

First thing I would do is to measure the distance between the tip of the prop and the bottom of the hull.
This must be no more than 10% of the props diameter.
Then note your gearbox ratio, engine revs at WOT, how many bearings between gearbox and the prop and the weight of your boat. Record this information.
Then get in touch with Victoria Propellers, Vicprop, and use their calculator to determine the correct prop for you boat.

rslifkin 05-24-2020 01:05 PM

I've always heard no less than 15 percent clearance for a planing hull. I'm right at 16 percent now with the 22 inch diameter props. For pitch and diameter I'm very close according to the vicprop and Michigan wheel calculators, although both say that for a 3 blade I should have a little more diameter and less pitch if possible. Based on being in the range for wot rpm I'm not way off or anything.

It seems like the props just don't push the boat quite as well as they should even though the engine loading seems fine.

Nomad Willy 05-24-2020 01:29 PM

First for that “push” you need power.
Are you getting rated rpm? At any other rpm you’re shy on power.

rslifkin 05-24-2020 01:42 PM


Originally Posted by Nomad Willy (Post 882247)
First for that “push” you need power.
Are you getting rated rpm? At any other rpm you’re shy on power.

Yeah, I'm making rated rpm with the 22x25 set. Spec is 4000 - 4400 at wot. When I ran it up yesterday I saw just over 4200. Realistically, I don't care how fast the boat goes at wot, although I know it should be closer to 27 than 25 kts. It's cruise performance that matters more.

Nomad Willy 05-24-2020 07:16 PM

Have you called an talked to the man at Michigan Wheel?
I think the guy I talked to really knew his stuff. And pleasant to talk to too.
And have you checked your tachometers?
If this is an engine w breaker points you may need to set the point gap snd dwell. The point cam wears a fair amount.

Also re the rated rpm when a range of engine speeds is given there’s the question of exactly what rpm is best. Is (in your case) the 4000rpm ideal and the rpm’s from 4000 to 4400 a leway or range to work with? Is 4400 in essence a red line ... 4400 being the engine speed never to exceed? Or is 4000 to 4400 a range plain and simple. And then perhaps 4250rpm would be ideal and then maybe not. Seeing a power graph may shed light on precisely what rpm is ideal For WOT rpm on a boat.

I’ve run my boat at WOT of 3100. 100 over rated power. The boat has never ran that good since. This is not very objective but the engine “seems” more relaxed at all rpm’s.

You are where I’ve been for some time. Boat running fine, not perfect but in a way that would be OK for probably years. Well you’ve got some stones to uncover - if you feel inclined.

I still think you may be ahead w more dia and less pitch. May be more noise from prop clearance but it may not be much.

I’m amazed that the Michigan Wheel’s prop calculator would predict only 100rpm change with a 1” pitch change. Seems to me it should be a greater difference.

rslifkin 05-24-2020 09:13 PM

I haven't talked to Michigan Wheel yet, but I probably should at some point.

Going from fairly, but not perfectly clean 22x26 props to freshly painted 22x25s with what looks to be a hair less blade area gained me 500 rpm, so yeah, 100 rpm for an inch is definitely way off, even though I know I was about 150 rpm off what the bigger props would do if they were freshly tuned and cleaned. According to the Michigan Wheel calculator, I should turn 4200 at 27 kts with the 22x26 props, but I know that has never been the case on this boat. According to Chris Craft, as built, the boat was good for 22 kts at 3500, 28 kts at WOT at 4000 RPM. I've never seen 28 out of it, but I remember years ago seeing a bit over 27 (can't remember which set of props that was one). As far as test weight this weekend, it was full fuel and water, probably 15 gallons in the holding tank. 2 people and a normal load of stuff (not loaded for an extended cruise).

For the WOT rpm, they just say 4000 - 4400 as a range in the book, no preference to either end of the range. So I figure I want to see them turn at least 4000 with the boat loaded heavily, 4100 would probably be better. I did calibrate the tachs last season, both are well within 1%. I've never been able to find a power curve for these engines (Mercruiser MIE 340). For the very similar Crusader 454 / 350hp combo, Crusader spec-ed 4200 - 4400 as WOT RPM with a max continuous rating of 3400 (which Mercruiser doesn't publish, but between the Crusader recommendation and the 80% rule, I treat 3400 as max continuous).

Ignition is electronic, Mercruiser Thunderbolt IV, timing freshly set and confirmed to advance correctly. The engines are running great and don't seem down on power. They feel much better with these props, much more responsive to small throttle changes up on plane and aren't as deep into the carb secondaries at, say 17.5 kts, so they should burn less fuel with these props. Calculated slip in that speed range is pretty much identical with the 2 sets of props.

I figure I'll give it a bit of runtime and then go for another WOT run to see if anything changes as the paint on the props gets a few hours on it. I'm not expecting much, but if I gain a few extra RPM at WOT, that might give me headroom to add a bit more blade area without needing to drop pitch. Then the question becomes whether to stick with a 3 blade and go to 23x25 or to go to a 4 blade (but not one of the massive blade area ones) in 22x25. You've got the key in the sense that the boat runs fine as is, there are just indications that it could be better.

FF 05-25-2020 06:16 AM

IF only noise is the problem usually an extra 1/2 inch of glass laid up inside above the props will tame much of it.

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