I clamp my cylinder motors in place so: 1) make a center drilled actual size dummy motor and put the clamp on it. 2) install a long music wire for the prop shaft thru the dummy motor and temp alignment bulkheads on either side of where the motor goes. 3) Glue/screw/fix the motor clamp to the hull. 4) remove the music wire, the dummy motor and temp alignment bulkheads. 5) install the real motor in the clamp, slide your prop shaft in🎉

Hi bikerjohn
I make a solid joiner out of brass rod and drill and ream out to fit the motor and propshaft on my lathe.
Connect all up and adjust the shaft and fix the motor mount into alignment. When happy glue the shaft into position (I use plastic padding and roughen/degrease the shaft. Leave to set. Replace the solid joiner with a flexible coupling of your choice. If you have a lathe you can make your own.
This should ensure a correctly aligned motor and propshaft suitable for the fastest of craft. I also check my propshaft to make sure they are not damaged and run free and true. A flat pane of glass is useful to roll the propshaft to check. If it's not change it as it is unuseable.
dave976

For all of my boats I use a double u-joint shaft. Most of them are the Dumas "dogbones" and never have had a problem. The Nylon shaft seems pretty tough.

Lew

If you want to get your motor and propshaft aligned
perfectly, I suggest you purchase one of these
(link below).
They have various sizes to cover any motor/shaft variation, don't know what I would do without mine.

https://www.mobilemarinemodels.com/alignment-tool-71-c.asp

Cheers, Pete

I have that coupler on my Typhoon, Dave
I also bought it from Rich at Mack.
It is a little noisy at high RPM.
Trev

When you have done that a coupler is still preferable.
Have experimented with several types, the one on the extreme right shows the most promise.
The one next to it with the spiral cut works well, but is bulky and the shaft must be slide out to fit.
This one can be assembled is situ, is robust and allows a small amount of misalignment and vibration thus cuts noise down. Available through our friends at Aliexpress and Bangood
The others all have failings. The traditional Hookes joint style vibrate, wear and are noisy

I always align and fix the motor height and angle using a solid joint between the motor and prop shafts. When the model is ready for motor trials, the solid joint is removed and replaced with a coupler made by Krick. They have two types of these, a standard and heavy duty parts. There is a flange to fix on each shaft. These are then joined by a splined, slightly flexible plastic joint which allows for slight mid-alignment and vibration. They are very useful units.
The picture is of a heavy duty connector.
Peter

best way is to use a solid connection and shim motor position to suit
see my blog
https://model-boats.com/blogs/111186#112090

Def use a U-Joint Coupler if possible and actually a double articulated coupler is ideal of there is much mis-alignment.
At times I have doubled up(back-to-back) individual U-Joint couplers to take up any variances.
Depending on which type motor mount, either hinged, or home-made, always allow for adjustment in the aligment.

This is a coupler from Mack rc industries. 5mm motor shaft to 3/16" prop shaft. It is hardened steel and made for high speed operation. I think it's a good choice and if you are slightly off on alignment it will take up any movement and greatly reduce vibration. Hope this helps,. Dave RR 👍

This depends on how you have the motor and the shaft mounted. If you have a flexible coupling between them, this is usually not a big problem. Your eyeball should be good enough. If is a fixed direct drive then a precise method is pretty much requiring.
Lew

I slide a piece of tube over the coupling, this holds the shaft and motor in alignment. Do this before fixing the shaft and motor mount permenantly in place. Once you are happy, glue shaft tube and motor mount in place. Use shims if needed under the motor mount...