design - engine placement - ELI5

[billgewater]

OK. So why do light aircraft have the prop at the front eh? Please don't respond to this. I do agree with the conservative normal arrangement on boats.

[Sailmonkey]

Originally Posted by boatpoker

I've surveyed about 30 Lagoon cats (power & sail) of various sizes and never seen one with the rudder forward of the propeller. To which models do you refer ?

Here’s a lagoon with the sail drive behind the rudder. This particular example is the 450.

[Sailmonkey]

Originally Posted by Cheechako

I've not seen a boat with a "pullpellor". Normally the prop pushes the shaft hard against the transmission and the tranny is built for that. One reason is just that; it's built to take the thrust. The other reason to not have the prop in front is it will hit things and catch things...

Here’s a pair (quad actually) of tractor props. Volvo Penta IPS drives have them. They increase efficiency by keeping the props in a clean flow of water…..but you better be able to afford to maintain that efficiency!!

[Seapig]

To sum up --as I understand it from the posts-- the main advantages of the prop at the stern is that depending on the curvature of the hull it allows for a straighter and thus more efficient propulsion. It allows for greater water flow over the rudders which makes for more responsive steering. The placement of the engine closer to amidships is better from a center of balance and effort standpoint.



However if the aim is to move the engines sternward as in a condo-cat with a saildrive than the advantages come at a cost, ie the infamous saildrive. ( A device invented by the devil for the purpose of collecting souls.) Given the propeller could be secured to and supported by the keel it would probably be more, not less protected than it's traditional location and definitely more so than the aforementioned device of the devil. It probably would be less hydro-dynamically efficient, how much would require testing, computers, and lots of lasers. Any good scientific project has to have lots of lasers.


By the way, if one could support the forward point of a propeller shaft to the keel or maybe have the propeller contained within the keel if there any reason that one couldn't use a good old fashioned universal joint to get better alignment. I am looking to be educated here, not start a fight.



Trivia tidbit-- regarding small airplanes with pusher props. The props are (or at least were when I was flying) made of wood because they tolerated the differential pressure zones created by the fuselage. Metal props tended to fatigue and fail spectacularly.

[team karst]

Originally Posted by boatpoker

I've surveyed about 30 Lagoon cats (power & sail) of various sizes and never seen one with the rudder forward of the propeller. To which models do you refer ?

My props are well aft of the deep spade rudders.
It has terrific low speed handling. And, i transitioned from a twin inboard power boat that spun tite circles just like the sailcat.