The propeller tube should be the first thing to fit into the model, and using the plans / instructions it’s time to get the ruler out and start marking up the hull.
It is wise double check that any such markings are correct otherwise things may not function correctly when installed, as the old saying goes “measure twice, cut once”.
The good thing with a quality GRP hull is that they normally come with markings on them when the propshafts and rudders would go, and this is a great start, however I would still recommend checking all your measurements.
If twin screws are fitted, then equally spaced from the centre-line unless otherwise stated in the instructions. If you fail to do this then the model will still operate but you run the risk of encountering some odd sailing characteristics.
When making holes through plastic hulls it is wise to cover the area around the hole with masking tape to prevent any accidental damage to the surface while drilling/cutting. The rudder tube hole can usually be drilled to the exact size but it is always a good idea to start with a smaller pilot hole first. The propeller tube, usually being at an angle to the hull bottom, will need an elliptical hole. The obvious way to make this is to drill a hole to match the tube in the middle of the desired opening then use a round file to get the desired shape. The aim should be for a snug fit without any suggestion of the hull being deformed when the tube is at the correct angle.
Long unsupported tube lengths can be prone to vibration which destroys motor/propeller shaft alignment and creates noise, wear and a loss in performance. A simple half bulkhead, or even just a block of wood glued between the tube and hull would prevent such problems.
Fixing the propeller tube into the hull can be done at this stage, again read the instructions and see it makes sense for your model. Suitable adhesives ought to be specified but you may have to make your own choice. Epoxy types will bond well to metal tubes and GRP (Glass Reinforced Plastic) or wooden hulls.
Motor Mounting
It is vital that the motors are secured firmly into the model and correctly aligned with the propeller shaft.
There are many ways to install the motor, but a typical one is using a mounting bracket which is secured by screws to a wooden block fitted to the bottom of the hull. The block is shaped so that the motor and propeller tubes will be aligned. To ensure that this is so a rigid connector, (usually a close fitting tube of the same length as the intended coupling), is slipped over these two shafts.
Couplings
The coupling between motor and propeller shafts comes in many forms and sizes. No one type is inherently better than the others and all can be badly installed to cause no end of problems.
By far and away the most common type of couplings are those using a ‘Universal’ type of joint. The central part is plastic with brass inserts fitted to the ends which make the connection with the shafts. A grub screw usually connects the coupling to the motor shaft whilst the propeller shaft can be plain and secured with a grub screw or threaded and must have a matching threaded insert. It is vital that these inserts are the correct size and type for the motor and propeller shafts. Incorrect inserts will result in vibration and almost certainly fail at the most inopportune moment!
Now the question you have wanted to raise: if the coupling can accommodate misalignment, why go through all the trouble to get the motor shafts as near perfectly in line in the first place? Well, these couplings can only accept a limited amount of angular displacement before they start to protest, usually no more than 10-15 degrees. Rotation is possible at larger angles but significant power loss will occur and the couplings have even been known to show their displeasure by disassembling!
