Props are expensive, they tend to be designed to spin really fast, and there efficiency, although it can be up to about 90% is typically more like 50%. At least for anything I can afford. So for displacement speeds I keep thinking that fins might work out. OK they tend to use a more complex mechanical system but, they wouldn't have to move very fast = less friction. They could be easily DIY.
i'm thinking something like this
but not a planing hull.
Its called a swashdrive but thats kinda a misnomer cause there is no swash plate in it.
On any drive system the business end is going to end up taking a beating eventually. So I keep thinking replacement cost. Fins could be wood, ply or whatever, props I gotta buy, and they're spendy. So the cheaper option is the fin. Chalk up one for fins
Considering efficiency the typical prop sucks, 50% loss to friction and cavitation is nothing to be all that proud of. OK it is possible to build a prop with about 90% efficiency but its a huge prop, ends up looking more like an airplane than a boat. Fins on the other hand already look like half an airplane prop, and the half part means they only stick down half as far into the water, IE shallower draft. Chalk up two for the fins.
Mechanicals, The prop wins this one hands down. It spins, couldn't be much easier. Fins on the other hand have to oscillate on two axis, definitely more complex and definitely not as efficient. I'm thinking any mechanical system looses about 1% pr gear/chain/belt connection so a prop with a couple bearings and a reverse gear might have 6 or 7% losses in the mechanicals. The fin drive on the other hand has roughly twice as many, but they move slower, which does seem to make a difference in the longevity of the parts. Props are more efficient mechanically but they have parts that wear faster. Fins would have more parts that move slower. I'm thinking a fin might move at 100 rpm where a prop would spin at 1000 rpm +. I still think the prop wins this one but the slower moving system does have its merits.
weight, Prop wins this one as well. The fin system is complex and ends up weighing a lot more.
Anyway just some late night thoughts on drive systems. I'm thinking eventually we'll come up with something better than props and who knows, maybe fins will be it.
I like the idea that they already have some good fin based stabilization systems out there. Seems kinda reasonable that they eventually develop a good fin drive system as well.