Universal joints

[Torpman]

I am building a 1/24 scale twin motor, high speed, PT-109. I want to use u-joints for the motor-shaft-prop connections. Any suggestions/vendors for the high speed u-joints?
Thanks

[JimClark]

High Speed? Are you trying to go faster than a scale speed would be?

[Torpman]

Thanks for your response. I don’t know what the scale speed of the PT-109 should be. I’ve read several articles as to how it could be calculated but there seems to be a wide divergence of opinion. According to the PT specs, the speed of the boat was 43 knots (approx. 50 mph). One formula (boat speed x square root of the scale (1:24)) provides a scale speed of approx.. 10 mph.
I would prefer to run much faster – something in the rage of 25 mph.; please see attached link for a video of a Dumas 48” model:

https://www.bing.com/videos/search?q=dumas+48%22+pt-109+video&view=detail&mid=A36677F7ECCE5229F7FCA36677F7ECCE5229F7FC&FORM=VIRE

The higher speed would require relatively strong and balanced joints and I could use some sealed ball bearings to carry most of the load
I’ve been thinking about using 2 u-joints per shaft; one would connect the motor to the drive shaft and the second would be attached to the drive shaft and connect to the propeller shaft just forward of the mounting bracket that supports the propeller. In this configuration, the longitudinal axis of the prop shaft would be parallel to the deck. Alternatively, I could use one u-joint and have the drive shaft set at an angle of 5 – 7 degrees. I would appreciate any suggestions that you could offer.
Thank you for your time and consideration.

[Peter A]

Why not just use flex shafts to achieve the configuration you want? Much easier and more reliable.

[royalcasa]

Why not just use flex shafts to achieve the configuration you want? Much easier and more reliable.

Flex shaft don’t do well in reverse

[royalcasa]

9586A71C-AEDF-4F75-9EAC-EC074A5F83DC.jpgHow about this

[785boats]

The universal joints posted above by royalcasa are probably the most widely used in scale modelling. I think that nearly all my scale models have them. But I wouldn't run them at anything over about 15000 revs. They can fly to bits. Trust me on that.
Metal ones are available also.
https://www.ebay.com.au/b/Unbranded-...71/bn_86179779

But with any universal joint it is most efficient to set the motor & shaft up so they are in perfect alignment. Don't have any angle between motor & prop.

[royalcasa]

The universal joints posted above by royalcasa are probably the most widely used in scale modelling. I think that nearly all my scale models have them. But I wouldn't run them at anything over about 15000 revs. They can fly to bits. Trust me on that.
Metal ones are available also.
https://www.ebay.com.au/b/Unbranded-...71/bn_86179779

But with any universal joint it is most efficient to set the motor & shaft up so they are in perfect alignment. Don't have any angle between motor & prop.

The is true I only use them on my scale boats not my FE

[NativePaul]

Then what is the point in having one? Just take care when lining up your shaft or motor mount (whichever you fit last) like you would a FE, and you don't need one.

[785boats]

Agreed Paul.
A solid straight coupler is the best, but requires the alignment to be perfect or the vibrations induced can be horrendous.
The universal joint removes the vibration from any slight misalignment.

Where an angle is required, these are the most efficient an quiet. I've made up some in the past myself for a scale runabout, where due to space, the motors had to lie flat creating an angle with the prop shaft.

https://www.offshoreelectrics.com/pr...?prod=ose-3345