How do you determine rudder size?

One of the hydroplane boats I picked up is missing the whole rudder setup. So I went to one of the local boat shops to buy a new rudder setup. I was hoping to pick their brain as to what size rudder I would need and some other info about rudders, but they really seemed like your typical parts store employee. Couldn't really give me any info on how to determine my rudder size needs, had a hard time determining what I needed without the part number etc. And I fully admit I am a newbie when it comes to this type of stuff, so I need some help. I tried doing a google search to find the answers, found a few forum questions like this, none of the ones I found had any real answers to my specific question. I did find some info on rudder torque, but a lot of that was based on a known speed and such, but since I am building this boat drive system I don't really have any numbers for speed yet. Should I build it for a desired, hopefully attained speed, and then deal with it at lower speeds until I get to the higher speed? Should I design it for a safe lower speed to test things with, and then as I increase the speed of my boat, change the rudder to compensate for that? I thought there was some way to determine rotor size based on prop size, but I haven't been able to find that info anywhere.
If anyone has any info on this I would greatly appreciate it.
Jay

Hi Jay,

Can't help you directly with boat rudders, but I do have some experience with aircraft and hovercraft rudders.

On hovercraft, most people install rudders that are too small and then end up turning them so far that they stall in the airflow. On aircraft, you have to set up a system that is effective at low speeds but doesn't cause problems at high speeds.

The answer to these problems is generally to limit the angle of turn of the rudder and to make sure there is some force feedback to the driver. Ie., the driver gets a good feel for the force acting on the rudder, so he naturally avoids dangerous control inputs.

A rudder can be "fully floating" (the whole assembly rotates) or a trailing edge on a fin. If its the former, then where the pivot is affects the forces felt. There is also a limit through which you can safely turn it before it stalls. Its about +/- 14 degrees in air, but that may be different in water. If its a trailing edge on a fin, it can be turned through a larger angle, but balancing the forces is more difficult.

Nick

PS. On the hydroplane there may be other options to consider, such as turning the whole drive, the way an outboard motor does. Also, is the rudder operating in the flow from the propeller or in the free stream?

Nick, thanks for the info.
I picked up two boats at the same time. One already has a rudder, but also has a drive system that might be able to be rotated, but like mentioned, it already has a rudder. On that one the rudder is in line with the prop.
View attachment 1

The other boat that is missing the rudder has a long shaft for the drive that goes through the center of the boat so the whole drive can't be turned. There is already a hole for the prop, which is slightly back from the prop and offset slightly to the side.

If you look closely you can kind of see the hole for the rudder to the right side of the prop.

Jay,

As the two boats are similar, I'd be inclined to just copy the rudder you have and use it on the boat that's without as a starting point.

The shrouded prop on the other boat will sap power, although it will increase low speed thrust. The drag from the shroud will be pretty high at anything over a fast walking pace.

I'd go for the smallest rudder you can get away with, out of preference make it deep and relatively narrow, as that will make it more efficient. It needs to be a symmetrical aerofoil shape for low drag, but the actual shape won't be overly critical; just aim for the thickest part to be about 30% back from the leading edge. The pivot point needs to be about 20% to 22% back from the leading edge, in terms of area (i.e., try and get about 20% or so of the total rudder area in front of the pivot).

The prop will probably benefit from being changed, but it might be worth trying it with the existing prop and shaft first to see how it goes.

Jeremy

So, should I just start off with a rudder similar to the one on the second boat and test with smaller ones until they become uneffective?