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Choosing correct prop for 56 Runabout

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bartelma
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Choosing correct prop for 56 Runabout

Post by bartelma » Sat Jun 30, 2012 11:59 am

My 1956 runabout has a prop number of 654 on the tag which is 12X12X1 but over the years it was switched out to a 350 chevy with all the older Chris-Craft mounting hardware (I assume from a 283 or 327) and prop 656 which is 12X12.5X1.

I want to order another prop since this one is a little beat up but not exactly sure which one to get with the engine change. Anybody have an idea?

Thanks,
Matt
1956 Chris-Craft Runabout

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mfine
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Post by mfine » Sat Jun 30, 2012 2:31 pm

First...How did it perform with the current prop? Did it get on plane well? What was the max RPM at full throttle with a light load? Any other performance issues?

Second, what area are you trying to improve? Acceleration on to plane? Top speed? Reduce slip with a heavy load?

Finally, is there room to go to a 13 or 14 diameter, or are you limited to 12?

One more thing, which model runabout (and length)?

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Post by Don Ayers » Sat Jun 30, 2012 4:52 pm

Also, you might consider a 4 blade. You will throw rocks at the three blades after you run a modern 4 blade.
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Post by bartelma » Sun Jul 01, 2012 3:18 pm

It performs well with the current prop and gets up on plane rather quickly with 2 or 6 people on board obviously a little more throttle for six. It will pull to 4500 on the old mechanical tach (accuracy?) though I have not really hammered on it yet since I am having an overheating issue on one side of the engine. Going to post that question after this.

This current prop is nicked up a bit so I thought I would get a new one for a spare. Just didn't know if one of these props is more ideal for the 17' runabout since it was changed to a SBC 350.

I only have 1.5" of clearance from the prop to the hull so I don't thinkI can go much bigger in prop diameter.

Don -That four blade sounds interesting. Does that load the engine harder? Any recommendations on where to look for one?

Thanks!!
1956 Chris-Craft Runabout

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Don Ayers
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Post by Don Ayers » Mon Jul 02, 2012 7:11 am

the 12 X 12.5 sounds a little light to me.

I did a 350 replacement on a 18' Sports years ago we put a 4 blade 13X13 on it. a world of difference. Engine turns to 4400.

Once you have a modern 4 blade dialed in you will not want to go back to a three blade again.

I found a OJ Force 4 blade 13X13 RH online for $375 new.

http://www.boatownersworld.com/Props/oj ... ellers.htm
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Post by mfine » Mon Jul 02, 2012 7:27 am

I would agree with the 13x13 rec as a good place to start. Especially with a 4 blade, you should get on plane well with less slip and also limit the max RPM's you will ask the engine to put out. 4500 is probably the max you want to see (full throttle light load) unless the engine was built to rev higher. If you were looking for max speed you could probably go with a higher pitch like a 13x14 or 12x15 but most of us are not looking to go much over 40 mph very often so the 13x13 is probably the better all around choice.

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Post by drrot » Mon Jul 02, 2012 8:37 am

Don Ayers wrote:the 12 X 12.5 sounds a little light to me.

I did a 350 replacement on a 18' Sports years ago we put a 4 blade 13X13 on it. a world of difference. Engine turns to 4400.

Once you have a modern 4 blade dialed in you will not want to go back to a three blade again.

I found a OJ Force 4 blade 13X13 RH online for $375 new.

http://www.boatownersworld.com/Props/oj ... ellers.htm
Be careful ordering from that site. Only props listed in the whire background are tapered with a keyway like old boats need. Props with a green background are splined for newer ski boats.
Jim Staib
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1947 Penn Yan 12' Cartopper WXH474611
1950 Chris-Craft 22' Sportsman U-22-1532
1957 Chris-Craft 26' Sea Skiff SK-26-515
1968 Century 17' Resorter FG-68-174

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Post by rdapron » Mon Jul 02, 2012 10:34 am

Once you have a modern 4 blade dialed in you will not want to go back to a three blade again.
Could not agree more with Don. On my own boat I run a modern stainless steel 4 blade - it is a sweet wheel.

rob

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Post by bartelma » Mon Jul 02, 2012 1:31 pm

Awesome feedback thank you!!! Off to order a four blade.
1956 Chris-Craft Runabout

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Don Ayers
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Post by Don Ayers » Mon Jul 02, 2012 2:29 pm

Also;

Call Todd at Acme marine and talk to him. They have some of the best props and many fine tuned choices for RH blades. If you buy direct they also have a free exchange program so you can try more than one and get the boat dialed in.

http://www.acmemarine.com/

I talked with him this AM as I am needing a 4 blade for my project.
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rdapron
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Post by rdapron » Mon Jul 02, 2012 3:30 pm

Don, you might want to do some research on the SS props that are available. The main advantage is due to the inherent strength of the material they can design props with thinner blades, especially in the root area next to the hub. This results in being able to run more blade area (either diameter, pitch or more aggressive cup) than a typical bronze or nibral wheel which results in increased efficiencies seen in lower slip numbers.
rob

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Don Ayers
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Post by Don Ayers » Mon Jul 02, 2012 3:43 pm

Rob;

Can you point me in the right direction with regard to who makes these in SS?
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rdapron
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Post by rdapron » Mon Jul 02, 2012 8:03 pm

Don, the wheel I am running is made by Cutter. I found it an ebay in unused condition for a 150 bucks - it was a no brainer deal to give it a shot and I have been really pleased with the results. If I were to choose from new wheel I would look real hard at a OJ Legend 4 blade in SS. I would suggest calling Todd again at Acme and discuss. The old t-36 props of CC were good designs in their day, but running an old prop in my mind is like putting bias ply tires on your car today. There has been progress made in the technology of propellers.

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Post by Wood Commander » Mon Jul 02, 2012 9:02 pm

I can testify about what Rob says, I've ridden in RED HEAD, and it runs smoother than silk!
Bret

1953 35' Commander "Adonis III"

1970 23' lancer project

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Don Ayers
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Post by Don Ayers » Tue Jul 03, 2012 12:33 pm

rdapron wrote:Don, you might want to do some research on the SS props that are available. The main advantage is due to the inherent strength of the material they can design props with thinner blades, especially in the root area next to the hub. This results in being able to run more blade area (either diameter, pitch or more aggressive cup) than a typical bronze or nibral wheel which results in increased efficiencies seen in lower slip numbers.
rob
Rob, I called and talked with Eric at OJ. They have not made the 4 blade SS for 6-7 years. He was very helpful and I ordered a 13 X 13 4-blade Force Legend

BTW, the Acme guy was helpful as well but I went with OJ because they have a trade out program and the guy really worked with me in that he did not hang a time frame on me for return/switch out. The most the Acme guy gave me was 45 days. I'm not exactly sure when I will be ready for water test so this difference was what I needed to decide who to go with.
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Post by Paul P » Tue Jul 03, 2012 10:26 pm

A 12x12 indicates a 95 Hercules, which I ran for years with a small bump up in pitch to a 12x12.5. Going to a small block V8 in the same boat I would use the same prop some of the small block V8 runabouts used, which is a 12x15. You will be stunned at the difference. It may not be good for six people but with two aboard you will be in the 45-miles per hour range, perhaps more depending on the ultimate power of the V8 motor.

The commenst on the 4 blade props are undoubtedly good for smoothness and efficiency, but I have not used a 4 blade on such a small boat. Most of the time I am looking for speed in a small boat and a 3 blade traditionally seems to trump a 4 blade every time unless you are comparing antique to modern.

If you go to a 4 blade you will need to back off 1 or 2 inches of pitch too. When I ran 23x25 (3blade) props on my 38 and changed to a bran new $1,000 23x24 4-blade the motor would not spin it up to max rpm, and I eventually had to have them repitched (ugh$$) to 23x23 (4-blade). The stock 3-blade was 23x25 by comparison.

If by chance you are in a 17' runabout with a 283 or 327, I would try the ebay special 12x15 3-blade for a third of the price of a new 4-blade. If you don't like it you can easily sell it as they are frequently traded.

Another comparison is my 20' fiberglass Sea Skiff, which came stock with a 13x12 prop (210-horse 327F motor). I am running a 13x13 cupped now. That is a 16-degree deadrise with a mini keel and your boat (unless it is a sea skiff) is about 0-degree deadrise,so my bet is the 12x15 3-blade.

best,

Paul
1956 17' CC Sportsman, 300-hp
1957 17' CC Sportsman, 95-hp
1966 20' CC fiberglass Sea Skiff, 210-hp+
1973 23' CC Lancer inboard project, 427/375-hp.
1966 38' CC Commander Express, 427/300-hp(2)

So many boats.........so little time.....but what a way to go!!

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Post by Don Ayers » Fri Jul 06, 2012 4:29 pm

Just got it the new 4 blade from OJ. A thing of beauty
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Post by mfine » Fri Jul 06, 2012 4:34 pm

Nice Don, it even looks fast sitting in a box!

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Post by Don Ayers » Sat Jul 07, 2012 5:31 pm

OK, was worried about the negative rake of the modern prop but in my case I have plenty of clearance
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Paul P
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Post by Paul P » Mon Jul 09, 2012 12:00 pm

Beautiful prop Don. Share some specs for us, you know we need a little red meat now and then, photos are nice but...... :-)

Let us know when you are able to do a water test.

Paul
1956 17' CC Sportsman, 300-hp
1957 17' CC Sportsman, 95-hp
1966 20' CC fiberglass Sea Skiff, 210-hp+
1973 23' CC Lancer inboard project, 427/375-hp.
1966 38' CC Commander Express, 427/300-hp(2)

So many boats.........so little time.....but what a way to go!!

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Don Ayers
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Post by Don Ayers » Mon Jul 09, 2012 4:15 pm

This is a 13 X 13 RH. All OJ props have a built in cup and this one is .090.

After talking with the OJ prop tech we decided this would be the best place to start. They have an exchange program and can also take out some of the cup to get the right RPM.

Very helpful people.

I hope to do a water test in the fall.
Don Ayers
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Paul P
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Post by Paul P » Tue Jul 10, 2012 10:02 am

The 13x13 is used on a LOT of boats, will be fun doing a water test. I also talked with ACME during some research on what kind of prop to use on an upcoming project, and they were very nice to work with. I got good info also from the naval architect at Michigan Wheel, helping do some hull evaluation and whether I should be running a 1.5 reduction or just going with a direct drive. Water testing is fun, I am still having some fun with my 20' Sea Skiff and all the ebay props I bought, checking GPS speed and rpm.

Just doing the math you may find this of interest, and only time will tell the difference between keyboard desk jockeying and actual water testing, it is looking like 45-miles per hour at 4500 rpm.

My formula is 60(mph)/63360 X x/58500

The 58500 comes from a 13" pitch x 4500 rpm = 58,500 inches per minute

In the formula above the resultant is 55.39 miles per hour at 4500 rpm but must be down-graded by what I think would be 15% for slippage and/or hull drag, (-8.31) with a final number of (drumroll please) 47 miles per hour.

The 4-blade has more drag rthan a 3-blade simply due to the fact that there is another blade in the water and even with good design ACME can not defy the laws of physics. But on the other hand it has more bite so even though it may consume some horsepower via drag, it may make up for it with less slippage. In any case if the motor will spin that prop to 4500 you'll be moving pretty quick.

Factoring in a cup on that formula would make the 13 a 13.5 or so, and that would push you near 49 mph, but it all comes back to power and drag. Should be fun. If you have any data from ACME let me know as I always want to refine my method of calculations. I made some assumptions on weight and hull type when I assigned the 15% drag factor.

Paul
1956 17' CC Sportsman, 300-hp
1957 17' CC Sportsman, 95-hp
1966 20' CC fiberglass Sea Skiff, 210-hp+
1973 23' CC Lancer inboard project, 427/375-hp.
1966 38' CC Commander Express, 427/300-hp(2)

So many boats.........so little time.....but what a way to go!!

bartelma
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Post by bartelma » Thu Feb 21, 2013 8:21 am

Paul, I see you have a 17 foot 300hp sportsman in your list of boats and was wondering what prop you are running on that?

I am in the middle of a full restoration on this boat and putting in a new engine that will be up around 300 hp and making lots of torque so I am back to thinking about what prop to run. I was thinking about 13X14 or 13/15

Thanks
1956 Chris-Craft Runabout

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