Peter, I’ve had to learn my way through the choice maze for a 1:1 gearbox/transmission connecting an engine to a straight prop shaft for my ST21 build. As you may be seeking the same thing, the following may be of assistance. I should stress that what follows is self-evident to experts, but was a series of OMG revelations to an amateur like myself.
Firstly, when in forward motion, the propeller is trying to push the shaft through the boat, and the only thing stopping it is the gearbox/transmission. So the gearbox needs a thrust bearing strong enough to take the lateral and torsional loads. If it hasn’t, the geearbox or the engine’s crank shaft will quickly be destroyed. Marine gearboxes have appropriate thrust bearings, but most non marine ones do not. For example the ZF Hurth 45C marine gearbox that I’m using has one, but the snappy little 1:1 boxes produced for souped up go carts, and that I nearly used, do not. To use one, I would have had to insert a standalone thrust bearing between it and where the prop shaft entered the boat. If you choose to use an automotive engine with its’ own gearbox, or one of the hot go cart boxes, then you’ll need to insert a thrust bearing that is firmly attached to the hull itself.
Secondly, most marine boxes are designed to be bolted directly on an engine, either by using an off the shelf bell housing (and sometimes with off the shelf adapter plates as well), or by having a marine engine builder modifying an off the shelf bell house and/or adaptor plates. So for example, if you go with a common automotive or marine engine and ZF Hurth 45C combination, chances are there will be off the shelf bell houses and adapter plates available. If you use a non marine box, then you’ll need a length of shaft connecting the box to the thrust bearing, then another length of shaft connecting this to the prop. The shaft connecting box to the thrust bearing is likely to require a universal joint at both ends, to cope with the vibrating movement of the engine on its mounts.
Hope this is of some help. The stringers look the goods.
Currently building Jacques Mertens ST21 "Skinnydip". Boating adventures: Splash testing and using 'Skinnydip, as a basis of further building refinement; Adams 44’ sailing sloop "Great Sandy" (cruising and maintaining); Iain Oughtred Feather Pram "Mini Dip" (building); Jacques Mertens R13 "Wood Duck" (built and due for maintenance).