Something that can sometimes present problems, especially to beginners, is mounting electric motors in a hull. Almost all motors, certainly every one of the 380 and 540 variants, have ready tapped holes in the output end of the casing, nearly always to a standard spacing. You can buy ready-made mountings to fit these tapped holes at model shops, and it’s not a particularly difficult item to make yourself. Ready made mounts are usually just a simple right angle aluminium bracket, and in addition to the motor fixing holes, there are usually holes drilled in the bottom to fix the mount onto a suitably shaped wood block, which should be glued securely into the bottom of the fibreglass hull.
Bonding things like strips or blocks of wood really securely to the inside of a GRP hull isn’t difficult, but like anything else there’s a right way and a wrong way to go about this. The main pitfall when gluing anything to fibreglass is inadequate preparation of mating surfaces before applying the sticky stuff.
You should abrade or roughen all surfaces here, the wood as well as the GRP. The best way to do this is with a small piece of fairly coarse abrasive paper, not on a block in this case, but just held in the fingers. Done this way, it’s easier to get right into undulations, and any nooks and crannies. Afterwards brush and blow away any dust, then wipe the abraded area with a solvent moistened rag to make doubly sure before applying any adhesive.
The simplest way to line everything up properly is to screw the shaped wood block onto the underside of the motor mount and offer this to the hull, so that final trimming can be carried out to make sure that the motor and prop shaft line up exactly, and everything fits together nice and snugly. Then bed the whole assembly onto either epoxy glue or my own preference for this particular job, a polyester filler paste such as P-38, applied fairly generously to both hull and the wood block under the motor mount. Press down hard enough to squeeze adhesive out at the sides. You can use a slightly moistened finger to smooth out stray blobs around the edges before the stuff hardens.
After allowing everything to set really firmly, then you can unscrew the mount. Fill any gaps around it, and generally tidy things up with neat fillets of P-38 or epoxy glue around all the joint between the wood block and the hull. If you are using a mount that incorporates some form of reduction gearing, like the Olympus drives mentioned earlier, more or less the same advice applies, the only difference being that these mounts are moulded plastic rather than metal. If you decide to use anything other than the popular 380 & 540 types, one of the electric motors sold by Marx for example, you’ll probably have to use a motor mount made by that manufacturer. In many cases though, these motors come with mounts already fitted, so all you need to do is to make the usual wood mounting block bonded into the GRP hull.
And finally, one more very important tip. When you install motors, batteries, speed controllers and the like, never forget that you’ll probably want to take them out of the hull again some day, so remember to provide adequate access at the building stage, to any items that may require maintenance at some time in the future.
Try not to use too small a motor for your model, it’s always better to have too much power rather than not enough. You don’t have to run at full power all the time, and speed controllers are so good these days that it’s a simple matter to throttle back.…Read More →