Which propshaft?

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30 September 2021 at 11:38 #98070
Roo Hawkins 1
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@roohawkins1
Hi I am building a spearfish kit powered by a 4250-700kv motor. Which propsaft should I use m4 or m5.
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30 September 2021 at 11:38 #4766
Roo Hawkins 1
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@roohawkins1
30 September 2021 at 11:54 #98071
Charles Oates
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@charlesoates31738
I use M5 every time, for any model with significant power like your Spearfish.
30 September 2021 at 19:31 #98074
ashley needham
Participant
@ashleyneedham69188
That Charles is just so right. M5 for anything that powerful.

Ashley
1 October 2021 at 05:07 #98081
harry smith 1
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@harrysmith1
Hi Roo

WE use M4's in our boats.

As for your motor pick we are using Hobbyking's 3648-1450kv with a 120Amp ESC, a Bolt 4S 5000mah 65-130C Lipo and a CNC 42/45mm 2blade prop.

Dam fast on 4S and the motor and ESC will handle up to 6S.

But it's fast enough for me and my mates.

Harry
1 October 2021 at 12:42 #98087
Charles Oates
Participant
@charlesoates31738
Hi Harry, I use those too, powerfull but not as much torque, I stand by my 5 mm choice.
1 October 2021 at 13:21 #98090
Richard Simpson
Moderator
@richardsimpson88330
I think if you did some calculations you might be surprised at just what a power a 4mm stainless steel shaft is capable of transmitting. I think the biggest challenge is when you start to get long shafts and very high revs where there may be a potential for flexing and hence failure due to stress reversals.

If it was a short shaft I would certainly not bother with anything over 4mm but a longer unsupported shaft might need a little more rigidity.

Balance is one of the biggest killers when you start to get into high revs particularly when it comes to couplings. It always amazes me that you can buy beautifully machined high tolerance couplings with a grub screw inserted into one side!
1 October 2021 at 14:29 #98091
ashley needham
Participant
@ashleyneedham69188
For me. It’s the larger sized bearings on a m5 shaft that tick the box, but if Harry uses m4 then obviously they must be ok..on the proviso they are not too long to avoid shaft flex.

Agree on the balance thing. My new shiny craft had a balance issue on the 6 inch shaft/solid coupling front, so I changed the solid coupling to a silicone use one and it is super smooooth. Ditto the re-motored jet unit on Stingray.

Ashley
2 October 2021 at 06:23 #98096
harry smith 1
Participant
@harrysmith1
Hi Ashley

Sounds like you had an alignment problem between the motor and shaft !!!

Harry
2 October 2021 at 09:44 #98098
ashley needham
Participant
@ashleyneedham69188
I used a solid coupling, so the alignment was ok, it was just that the shaft was not 100% true…as seen when rolling down a glass sheet. ALMOST, but not quite, and at 20,000 rpm…that’s an issue with a solid coupling, hence the flexible one substituted!

In operation , there doesn’t seem to be any vibration from the prop, so the shaft is true-enough, and now no vibration can be felt on the motor mount.

Ashley
2 October 2021 at 12:11 #98100
Roo Hawkins 1
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@roohawkins1
Thanks for all your replysettings and help.I think I will get a m5 one and be on the safe side. Roo
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