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Three blade v. four blade prop....


CliffB

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I have an old Skool three blade prop on my '89 'Bu Skier (with 260hp Mercruiser, bone stock). I don't see any markings on it, but it looks exactly like what you'd expect as the original prop on a 23 year old ski boat.

I have two prop-related questions:

1. Does a four blade prop offer a performance advantage over a three blade? Or is it more just personal preference?

2. This winter I'm planning on replacing the iron heads with aluminum heads with better flow/valves/springs along with a better aluminum intake manifold. I'll probably also change the cam to match the heads better. Overall, this should take about 55lbs out of the boat, and add another 50-60hp/tq. So, in other words, a meaningful improvement in performance. My question is, I imagine I'll need a different prop to really take advantage of this gain in hp/tq...so what prop would be recommended? I'm guessing it's a four blade with a bit more diameter and a steeper pitch, but I really have no idea.

Any help/advice is appreciated.

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Depending on the heads/cam/intake you install, you could end up pushing the h/tq curve higher and therefore actually hurting skiing performance.

Overall, four blades will give you more grip in the water. I don't know enough about inboards to say which one will work (just getting into the inboard realm).

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By many accounts, with today's CNC'd props which allow better blade size than cast, a four blade will hurt performance, especially at the upper end. When you're laying down too much power for a propr to grip its time to look for more traction which also comes with more friction. With that engine, you won't be hurting for traction. New carbon pro on my lake with the 409 still runs three blade and is the preferred factory prop of all late model direct drives other than correct craft.

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I would recommend calling either OJ or Acme.

Prop preference depends on a number of factors like how you use the boat, how much weight your are running, altitude where you run the boat, etc. Again call the experts they will ask you a number of questions and hopefully get you in the right prop.

  • Like 2
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As far as the cam and intake goes, try to mate them to the desired rpm range you're expecting. Mix that with the prop you desire and you should be good to go !

Things get pricey real quick, but you may think about adding roller rockers also. They relieve so much friction on the valve train, allowing the engine to "whiz" much more effortlessly.

Steve B.

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Call ACME for a recommendation... but I will tell you this from my personal experience. I like my four blade for slower speeds like surfing. The three blade I have has a little more pull to it but it hits the RPM band slightly higher and that just happens to be a "noisy" range for my particuliar engine. (2500 on a GT-40) It also tends to cavitate a bit more. This usually occurs during a turn and I really have no explanation for it. The three blade is perfect for skiing because it yanks me out of the hole so quick and is more responsive to the PP. A lot of this has to do with pitch and diameter which will be different between the 4 and 3 blade props. Also, there tends to be a fair amount of trial and error when optimizing props. I have talked to ACME, bought their recommendation and then was happier with another prop that I had selected (they have a great return policy but I need a spare anyway). Is there something you are looking for or just want to get the most out of the engine because you can? Clearly, if you just want a little more out of the hole going with a 13x12 3 blade will do that. I live at altitude and have went down as far as a 13x10.5 on my old boat (265 HP Mercruiser Magnum). It really helped compensate for the HP reduction. Top end gets reduced so you have to know what you want. We only ski at a max of 32 MPH so top end was never an issue. -Marc

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Thank you gents, I really appreciate the information and experience.

It sounds like there's no huge/obvious performance advantage to be had out of a four blade, particularly for skiing. Let me describe what the boat is used for primarily.... Mostly, it's just cruising around the lake with my kids doing about 25mph.....we pull over, throw over the anchor, put on some John Denver or Supertramp (yeah, I'm old......), do some fishing and have lunch, then pull anchor and do a little inner tubing. If there's some flat water and no other boats around, my (slightly incompetent) wife will give me a pull on the ski at around 28-30mph and I'll make some cuts until it looks like she's headed in the wrong direction. It's exciting skiing because she likes to turn hard when I'm on the inside of the turn in the middle of a cut, so I almost sink.....and then accelerate to around 50mph in the span of about a second! We have a wake board on board and we might give that a try sometime just to see if anyone can actually get up on the thing.

Once in a while (twice this season) we'll do a top speed run to chase down a fast boat (at a safe distance) in a drag race sort of thing...just for a thrill for the kids (and me, a bit...). That's rare, and only with no other boats within a mile or more, and not for more than a minute perhaps. As is, the boat seems to have loads of grunt out of the hole, and will accelerate hard to about 46mph on flat water. I work hard to keep excess weight out of the boat so she's quite light weight. Replacing the heads/cam/intake is really for getting weight out of the boat, and secondarily maybe a bit more top end. It's also just plain fun to hot rod an American V8. At some point I'll put a set of four weber down draft IDA's on there with a plexi bubble added to the dog house for visual effect. As is, the engine doesn't seem to want to rev past about 4,300. I'd also install some good quality roller rockers, perhaps stainless due to the marine environment.

I'll give Acme a call and seek their opinion. I also sounds like there's some trial and error to it as well. I should have a spare so I'll start exploring the options a bit based upon the shared experience here.

Thanks again!

Edited by CliffB
  • Like 3
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Sounds like a good time ;). I'd build the motor first, take it out and see how it performs with the stock prop. Then call ACME or OJ and talk with them. Also look on CL and forums for used props you can usually pick one up for pretty cheap if you are patient.

I changed out my stock stainless 13X13 prop to an ACME 515 (which is another 3 blade prop 13x12 and a different cup than my stocker), ACME recommended this prop to me given my usage of the boat. I did notice a significant difference in the hole shot getting a skier up, and gained a few mph on the top end (but I never go that fast except that one time to see what the difference was). The biggest reason I wanted to change a prop is to go from SS to brass. If I hit something I'd much rather have the brass prop take the brunt of the damage, where a SS prop will be quite a bit more resilient and possibly increase the chance of shaft damage.

I found a 515 on CL for $200

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Do your engine work then get a baseline with the stock prop. Find out where your max rpm ends up and go from there. I'm not sure what the ideal max rpm should be on your engine but 4300 seems low. My guess is the engine work you do will push the rpm to where you want it (4800?). If that's the case then you have to decide whether a new prop is worth the cost. A three blade CNC OJ or Acme prop will likely give you a bit more low end and less vibration because they are more efficient and balanced better. My guess is that you will end up with a 13x12 if you choose to go that route.

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Cliff, I had a 90 skier, so I may have more annecdotal evidence than other posts. Had a 13x13 3 blade for years...worked great. A co worker who worked at OJ before they moved outta CA gave me a 13x13 4 blade.

What I observed was....at 6 '4" and 220-230, the boat would grunt a tad and ever so slight drag me with the 3 blade, the 4 blade had a slightly better hole shot (no feeling of being drug). At WOT, the 4 blade was 1-2 mph slower. SInce it is hard to measure gas usage, your guess would be as good as mine.

BUt theoretically, you are increasing your frictional forces by 25% when adding that additional blade.

What I did notice, was the difference in wakes. For my buddies who were skiing 15 and 22 off, the wakes was noticeable softer and or fluffier. At the shorter line lenghts, there was no difference between the 2.

After years of playing around with props and the like, I found a great prop guy in Lodi, CA...He took that 3 blade and cupped the prop for a better hole shot and to lower the rpms at ski speed of 34 mph. So much better that I used the 3 blade all the time and had the 4 blade as a back up.

Your increase in HP should be interesting, as I have always thought it would be a hoot to put the new SC enginer in a Skier....a nealry 300 HP increase should do something.

Call Acme or OJ, cause we are all just WAGing it, Acme has great reps who know their stuff...I have talked to them about my LXI, and they are by far the most knowledgeable...by far.

Edited by jkendallmsce
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When you get that conversion done, you will need a new prop. I did basically what you are looking at (fast burn heads, Performer RPM intake, ZZ4 cam, free flow exhaust) and it required a "better" prop (cavitation). If you want to keep the top end intact with that parts combo, a three blade CNC prop properly tuned will work, Acme 515 (maybe with some extra cup) depending on your altitude. A four blade prop will hit a wall at 49-50 mph whereas a 3 blade won't. You will most likely need a freer flowing spark arrestor.

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Cliff, I had a 90 skier, so I may have more annecdotal evidence than other posts. Had a 13x13 3 blade for years...worked great. A co worker who worked at OJ before they moved outta CA gave me a 13x13 4 blade.

What I observed was....at 6 '4" and 220-230, the boat would grunt a tad and ever so slight drag me with the 3 blade, the 4 blade had a slightly better hole shot (no feeling of being drug). At WOT, the 4 blade was 1-2 mph slower. SInce it is hard to measure gas usage, your guess would be as good as mine.

BUt theoretically, you are increasing your frictional forces by 25% when adding that additional blade.

Were your 13x13 3 and 4 blades cast or cnc'd?

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I had a 13x13 3 blade, 13x13 4 blade, both cast, and now a 13x12 3 blade CNC. (Acme 515) The CNC prop is better everywhere. It has better holeshot, top end, and keeps speed more steady and with less throttle too.

Edited by ColinP
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Both were nibral.

That's just the material. The point is, when nibral props were cast, the accuracy and balancing only allowed for a blade size which is substantially smaller than a cnc'd nibral prop. In other words, a cnc 3 blade has more blade surface making it more comparable to a 4 blade cast. Which is also why "stock" cast 3 blades were almost always 13x13 and then with the advent of cnc they went to 13x12 because they could get the necessary drive out of a bigger bladed 13x12 while improving holeshot using a 12 pitch as opposed to a conventional 13. Best way to see this work is put a 13x13 cast on a boat and see what it will spin up to. Put a 13x13 cnc prop on there and it will be down several hundred RPM because of the blade size difference.

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That's just the material. The point is, when nibral props were cast, the accuracy and balancing only allowed for a blade size which is substantially smaller than a cnc'd nibral prop. In other words, a cnc 3 blade has more blade surface making it more comparable to a 4 blade cast. Which is also why "stock" cast 3 blades were almost always 13x13 and then with the advent of cnc they went to 13x12 because they could get the necessary drive out of a bigger bladed 13x12 while improving holeshot using a 12 pitch as opposed to a conventional 13. Best way to see this work is put a 13x13 cast on a boat and see what it will spin up to. Put a 13x13 cnc prop on there and it will be down several hundred RPM because of the blade size difference.

That is why I told Cliif to call OJ or Acme, as they actually know what they are talking about, vs a WAG or some vague annecdotal opinion.

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Actually, I do know what I'm talking about. It ain't no WAG. Need a picture of a cnc prop next to a cast? To your suggestion, I'd suggest OP call Bill Weeks at ACME. Anecdotal "opinion"? haha.

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Actually, I do know what I'm talking about. It ain't no WAG. Need a picture of a cnc prop next to a cast? To your suggestion, I'd suggest OP call Bill Weeks at ACME. Anecdotal "opinion"? haha.

I know that you know you think you do....take a deep breathe, and put the frisbee down.

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I had a 13x13 3 blade, 13x13 4 blade, both cast, and now a 13x12 3 blade CNC. (Acme 515) The CNC prop is better everywhere. It has better holeshot, top end, and keeps speed more steady and with less throttle too.

Colin, you're right, and your results are typical. Some folks don't appear to understand the large performance distinction between the same diameter and pitch prop, one cast and one CNC'd. If the OP still running a cast prop, he, like you did, will see a large performance upgrade by going to a CNC'd prop, though four blade CNC not necessary. I think I mentioned it already, when the lightest slalom boat on the market (carbon pro) with the 409 still uses a 3 blade, tells you something about the traction a cnc 3 blade has! No need to give up the top end performance by using a 4 blade.

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I know that you know you think you do....take a deep breathe, and put the frisbee down.

Call Bill. Tell him there's someone on the Malibu Crew saying a CNC prop has more blade surface than a cast prop and that's why people have such a performance upgrade with a CNC and that you'd like to straighten him out.

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That's just the material. The point is, when nibral props were cast, the accuracy and balancing only allowed for a blade size which is substantially smaller than a cnc'd nibral prop. In other words, a cnc 3 blade has more blade surface making it more comparable to a 4 blade cast. Which is also why "stock" cast 3 blades were almost always 13x13 and then with the advent of cnc they went to 13x12 because they could get the necessary drive out of a bigger bladed 13x12 while improving holeshot using a 12 pitch as opposed to a conventional 13. Best way to see this work is put a 13x13 cast on a boat and see what it will spin up to. Put a 13x13 cnc prop on there and it will be down several hundred RPM because of the blade size difference.

:plus1: on this. Not much need for the four blades on a ski boat today. Don't know how it would effect a wake boat.

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Call Bill. Tell him there's someone on the Malibu Crew saying a CNC prop has more blade surface than a cast prop and that's why people have such a performance upgrade with a CNC and that you'd like to straighten him out.

No need to get all wee weed up. I am certainly not looking to as you say "straighten any one out"..I'll leave that to the experts. And who is Bill?? Should I know Bill??

For your on own well being....put the frisbee down, breathe deep and a few "serenity now"s will do wonders...may change your whole outlook on life....

Edited by jkendallmsce
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