How does a cut down prop make you faster??

[Kaotic]

I'm curious to know why a cut down prop will give you more pitch. For example, if one where to cut down a 447 prop to a 445 prop how does the pitch increase?

I'm having a hard time wrapping my head around this concept. From what I've read a 445 and 447 have 1.4" of travel through a solid piece of material (theoretically of course) and I understand that you can multiply the diameter to the pitch and get an overall pitch number. So if one where to cut a 447 down to a 45mm diameter you then take that 45 and multiply it by 1.4 correct??

Does the pitch change the further away you get from the hub?

Thanks!

[785boats]

For example, if one where to cut down a 447 prop to a 445 prop how does the pitch increase?
I'd like to know the theory behind that myth too.

[Fluid]

It is not really a 'myth', it is just terribly misunderstood. You do not change the pitch of a prop by reducing its diameter - that can't happen without bending the blades. One reason to do it is this:

You have an x447 which is as large a diameter as your motor/boat will handle, but you want more speed. You take an x450 and cut it down to 47mm diameter:

x447 = 2.59" of pitch
x450 cut down to 47mm diameter = 2.75" of pitch

You now have a 47mm prop which your motor can probably turn but it has 6% more pitch. It is just a way to fine tune a prop to your specific application if a commercial prop is not available. In the case of a 45mm prop you have lots of pitch alternatives for the same diameter:

x445 = 2.48"
x545 = 2.65"
x645 = 2.83"

But if you wanted a 45mm prop with 2.59" of pitch you could cut down an x447 to get there. Note that you will want to cut the larger prop so it has the same blade shape and size as the smaller prop - just cutting down the tips to reduce diameter is not enough.


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[785boats]

That makes more sense.
I guess the misunderstanding comes from the fact that most of the time it is usually written as, for example, "I cut down a 47mm prop to 45mm to give it more pitch" which always just sounded wrong to me.
Thanks.

[dasboata]

On Sep 15, 2014, at 9:09 AM, Dasboata@aol.com wrote:


on a 447 there is a 1.4 pitch ratio 47 MM X 1.4 = 65.8 pitch.. when you cut them down the pitch ratio goes up !!
In a message dated 9/15/2014 8:57:37 A.M. Eastern Daylight Time,,,,,,,,,,,,, your not grasping what the pitch ratio is ??

[785boats]

Where on the blade is the pitch angle measured to work out the ratio from? Half way out from the hub? 2/3 the way out?
Or is it measured at a few points and averaged? How do they work it out on a progressively pitched prop such as we use?

[dasboata]

Where on the blade is the pitch angle measured to work out the ratio from? Half way out from the hub? 2/3 the way out?
Or is it measured at a few points and averaged? How do they work it out on a progressively pitched prop such as we use?I measure at 3 places then average together but 2/3 or a bit lower is fine

[785boats]

Thanks for that, Chris.
I've always wondered how that was done.
I've seen on full size props that a 4 blade was done at 1/2 distance & a 3 blade was done at 2/3. Just couldn't find where a 2 blade model prop was done.