Originally Posted by Nomad Willy
I’ve talked about a cruder version for years. Just two rudders. One on either side of the propwash.
Just like w airplanes the propwash if passing over part of the boat can be a drag. The rudder is pulling the boat backwards when underway. The relatively high velocity of water over the boat’s rudder causes a lot of drag and that drag is pulling backwards on the boat. Imagine trying to hold onto a rudder holding it in the propwash or go 25 knots and thrust your rudder down into the water alongside. It’s a drag man
Also the propwash is not straight. It cones in on aircraft and may do so on a boat. And rudders that aren’t flat plates will experience some interplane drag similar to biplanes. Two pilings in a river have more than twice as much drag of one if they are close to each other.
Thanks very much for posting Lepke.
Good points. And seems like there's some merit to a ducted prop, essentially a rudder that wraps around the prop. Planes have these, too. However, in boating, not sure what the tradeoffs are.
Also, the flat rudder may not be the most efficient. The fishtail rudder certainly has better handling and my understanding that it doesn't significantly affect the speed..... but may at high speeds. Perhaps better for trawlers. The NACA 23000 wing in aircraft is similar, fat front and skinny back, but is not symmetrical, and has great performance characteristics.
Now, here's a question...... why don't we see more hydrofoil trawlers? Seems like once you get it up, it becomes fast and efficient?