Random Insanity.

This skid fin should be located at about 60% of the static waterplane (aft of the bow), to be an effective starting point. This will place is some flow at the forward end of the contact patch when she's up and scooting.

Also a wedge style of sectional shape will be much more effective, if you can get the rudders to bite on solid water. Those double zero NACA sections are nearly useless at these speeds and in turbulent flow. Any angle of incidence at all and they're stalled.

Lastly, reconsider the complexity of moving the rudders forward. Basically, you're just reversing what you already have. In this forward location, all the things mentioned will greatly amplify their steering effectiveness. You probably wouldn't need an anti-skid fin and you could probably get by with exactly what you have now (NACA foils, same plan form, etc.), without dropping them deeper.

In short, rudders aren't the best choice for steering, particularly compaired to vectored thrust steering, but you have to work with what you have, unless you'd like to try articulating the wheels themselves. A gimbal mount and steering wheels, now that would be something. You could kiss off the rudders all together, for less drag and way better steering.

 

PAR, with all due respect, I have to disagree with this part of your proposal.
This picture shows why:



IMO, by moving the rudder forward it is true that it becomes hydrodynamically more efficient because it works in the undisturbed flow.
But the lever arm nearly halves because the new rudder position is too close to the CoG.

So the turning moment, M = F * Xr, might remain quite the same (hoping that it won't decrease), while the sideways force F will certainly increase.
The end result - boat not steering any better and instead just drifting sidewys even more than before. Especially without the skid fin. Ergo, all the efforts and money spent for nothing.

This is just my two-cents worth, of course, and at this point I guess I have said all that I could and had to say on this issue. You guys decide.
Wish you folks the best luck with this project, and I hope to soon read some good news about the outcome.

Cheers

 

I query why the rudders are placed only behind the washing machine, er, paddlewheels, might as well place another one along the centreline in undisturbed water. At speed it will have much more bite than the other two chewing foam, though at slow speeds would be pretty useless without much flow over it.

 

I personally have been keeping quiet to let the pros duke it out amongst themselves, and trying to balance out what sounds practically doable against the stated situation.

As I have stated once already, repositioning the rudders to ahead of the wheels is grossly impractical. The drive shaft and differential housing create serious space issues, and I can't envision there being enough clearance for any steering mechanisms in their immediate vincinity.

The old system was well above and clear of the shaft and differential where it passed through the transom. somewhere in the neighborhood of 3" - 5".

However, this last round of debate has given me an idea that I'll discuss later. I'm going to be taking a whole slew of pics of the boat and I will present a thought or 3 then.

Thanks so far to everyone who's been weighing in. Its not falling on deaf ears.

 

All right, so in the time I've been away, I did get a few things sorted out. The first thing I found out was my trusty and faithful camera died, and it died a horrible death. Frustrating and the worst kind of inconvenient.

After a healthy temper tantrum over my camera choosing the worst moment to crap out, I went to the next best thing. I started taking measurements.

The rudder form is virtually square at 13 x 15 inches, for a total area of 195 sq inches per rudder. (390 sq ins for both)

Knowing that, what kind of area, shape, etc. should the replacement rudder have, and how would the steering fin design complement that?

Furthermore, what would moving the rudders farther outboard, instead of forward of the paddle wheels do to the steering?