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Which prop do i need?

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Which prop do i need?

Old 06-28-2005, 08:45 PM
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joman2055
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Default Which prop do i need?

I tried running my dad's very old Prather Tunnel Cat that he gave me but it was lurching in the water. It would stay a consatant RPM and then lurch forward and stop and repeat but it would go about 15-20 feet sometimes without stoping. This is an outboard tunnel.
He said the prop was cavitating (spelling?) he said it was probably just spinging in the water in like its own little hole. and i guess his friend told him we need a 440 prop but i have no idea what that is. While on the lake we tried a smaller prop and it seemed to have no effect.
Thanks
Old 06-28-2005, 08:55 PM
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piper_chuck
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Default RE: Which prop do i need?

What engine? What props did you try? Where is the center of the prop shaft relative to the sponson bottoms? Cavitating could be a prop that's too small, but it also could just be that the engine is too high, or perhaps at the wrong angle.
Old 06-28-2005, 09:18 PM
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joman2055
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Default RE: Which prop do i need?

It is a K&B 3.5 CC (.21) The engine was a low as possible and the Prop shaft was flat with the hull of the boat. I dont know the size of the props. what size do you recomend? How are they meassured anyway? Yup im a newb. lol
Thanks
Old 06-28-2005, 10:21 PM
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Default RE: Which prop do i need?

Unlike on airplanes, where they are labeled by diameter and pitch, boat props just have numbers from the manufacturer. There may be a pattern to their numbers, but I've never thought about it.

It's possible that you've got the correct prop on it, but just don't know. Remove the prop and look for any names or numbers. You could see something like Prather 215 in tiny letters on the hub where the nut contacts it. You may also see something on the hub between the blades. Don't invest in a new prop until you know what you've got and have confirmed it's not the right size. I believe, from memory, that an Octura X438 or x440 is a common prop to start with. I used a Prather 215 stainless on the one I just got running.

A note about new props. They are not ready for use as they come from the manufacturer. They need to be sharpened and balanced. If you're not prepared to spend a couple hours doing this (you will need a balancer, files, and very fine sandpaper), get your prop from a hobby shop that does boats, or a boating supplier, and pay the extra $ to have them do it.
Old 06-28-2005, 10:58 PM
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Ron Olson
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Default RE: Which prop do i need?

A quick schooling on Octura props as when you're talking a 440 prop that's from Octura.
They use the metric system on thier props as it seems to be easier to work with for them even if they are located in Illinois. What you have is the "X" series, a low lift prop. Using the size described, the first 4 is 1.4 times the diameter of the prop in millimeters. The diameter, 40 is 40 mm. I have an Octura prop sheet in front of me that says that the pitch is 56mm or 2.198". In theory, the boat would move the 2.198" per revolution but there is a slippage factor.
Anyway, try a smaller prop like an X-437 or X-438 on it and follow Chuck's advice.
Old 06-28-2005, 11:45 PM
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joman2055
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Default RE: Which prop do i need?

Ok i went and gathered all 3 props that i have. Two appear to be the one that came with it and another one of the same size. They look identical and neitehr have any numbers or letters. One is silver and one looks like it is brass. The other one is an Octura X442. Should i put the X442 on and see what happens?

I also have no idea how to balance and or sharpen a prop and from the looks of it non of these were ever sharpened or balanced. What exactly does sharpening and balancing do?
Old 06-29-2005, 12:27 AM
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Ron Olson
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Default RE: Which prop do i need?

The X-442 won't fit under the cavitation plate without modifications.
Look elsewhere on this forum, therer should be some postings on how to sharpen and balance props. I really need to do an article on it and pin it to the top or if someone else would like to do one it would be great.
Old 06-29-2005, 07:09 AM
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Default RE: Which prop do i need?

ORIGINAL: Ron Olson

The X-442 won't fit under the cavitation plate without modifications.
Look elsewhere on this forum, therer should be some postings on how to sharpen and balance props. I really need to do an article on it and pin it to the top or if someone else would like to do one it would be great.
Ron, I found a few links and started a thread. http://www.rcuniverse.com/forum/fb.asp?m=3120686

I started a thread with some links to prop balancing that were in one of your posts earlier this year. I also included a link to an article I found on after run oil. I took a stab at organizing it like a FAQ. If you think this format works, feel free to pin it and edit my original post with any additional info you think needs to be there (to keep everything at the top). If others come up with additions, post them and I can add them to the top.
Old 06-29-2005, 02:23 PM
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joman2055
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Default RE: Which prop do i need?

Thanks for the help. Next time i go to the LHS i will get a 215 prop and have them sharpen and balance it.

I have one more question. Where can i get just the lower unit for a K&B 3.5cc outboard? My dad dropped the boat and broke the cavitation plate off and right now its all ghetto JB Welded together.
Old 06-29-2005, 03:33 PM
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Default RE: Which prop do i need?

You can get new lower unit halves from K&B. Here's the page showing the outboards: http://www.mecoa.com/kb/marine/marine_outboard.htm

Use this page to identify the model it most closely resembles and then follow the links for parts and ordering. You can order just the lower unit halves, or a complete assembly. I suggest taking yours apart before you order to see what condition the flex shaft, nylon tube, and prop shaft are in. You might as well order a spare shaft while you are at it. Consider the flex shaft to be a regular maintenance item, and grease it after every session at the lake. I use waterproof bearing grease that's used for boat trailers. If you don't do this, they will rust quickly.

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