New Rudder Design

[amayotte]

Hi,

I am about to build a new rudder and was wanting to make it a balanced one. Can some people give me their opinion as to much rudder I should keep in front of my rudder shaft in order to balance it?
I have read that one should do more that 20% cord but I was planning on putting the shaft 30% cord behind the leading edge. How does that sound?

Thanks,
Adam

[Chrysler20%26]

On My To Try List: Add a rudder to out-board, an remove old rudder. I will so if it workes or not when the ice is off the bay.

[Guillaume C.]

take a look at the forum at boatdesign.net you should find an answer

remember that the positive and negative pressure are not centered on the foil, depending on foil section they may very well be at about 30%, wich would result in totally neutral rudder... does not seem very interesting to steer!

20&26: are you speaking of adding rudder to outboard motor or simply an outboard rudder? The later would work fine but the first one would be a very bad idea!

[Chrysler20%26]

Guillaume C. Yes I am in the proses of adding a rudder to a out-board motor. "Why do you think that is a bad idea": no one has doen it, but why not. My rudder would move up & down, down for deep water, an by not locking the moter down it would kick up if it hit somthing. I think the out board with rudder added would be as strong as the rudder I now have. WHY would this not work????

[amayotte]

take a look at the forum at boatdesign.net you should find an answer

remember that the positive and negative pressure are not centered on the foil, depending on foil section they may very well be at about 30%, wich would result in totally neutral rudder... does not seem very interesting to steer!

20&26: are you speaking of adding rudder to outboard motor or simply an outboard rudder? The later would work fine but the first one would be a very bad idea!

Thanks for the hint Guillaume. I decided to go back and re analyze the foil I was going to make (Naca0014) and it shows a center of pressure at 25%.
So I would have ended up with a very unstable and unpredictable rudder which would require 100% attention.
Thanks so much for the heads up.
However, now that I know the Cp is at 25% I wonder what would be a comfortable place to put the rudder shaft? 15% or even 20%?

One other thing I have come across after reading through Boatdesign.net
There was a fella on there (http://www.boatdesign.net/forums/boat-d ... 18688.html, Rick Willoughby) who describes that, in his experience, the most efficient Naca dsign is Naca0020

So, I might try that.

Thanks for all the help,
Adam

[Guillaume C.]

Here's a little website with some interesting infos on foil http://www.worldofkrauss.com/foils/1714

I dont know wich section I would use for a rudder tough, but keep looking at boatdesign.net, there are some professional naval architect hanging out there. They generally have "senior member" under their name and they ocasionally give out seriously great infos!

I remember having read a passage in a boat design book about rudder balance, but do not remember the % given. I'll try to find it out this evening. Tough at first glance I would be more tempted by 15%.

20%26: the forces acting on a rudder can be quite impressive, it could easily be argued that it is the part that break the most often on a sailboat. Take the 1979 Fastnet as exemple, at least 3 rudder broke that day. They are supported by (generally) stainless steel tube having 2-3 feet of support into the hull, it is relatively easy to calculate the resistance and it is huge in theory, and still in reality they sometime bend or break.

There is NO WAY your outboard will support the load, first the screw holding it to the boat wont be nearly strong enough,if you attach it differently then the outboard will for sure kick back when turning, if you find a way around that, ultimately if going in rough weather the lower unit itself will most likely bend or loose the lower part due to a break at bolts level.

Sorry if I sound harsh, just want to save you some time and an engine!

[Chrysler20%26]

Thank you Guillaume C., I now know that adding a rudder to an outboard motor will not work. I did get the outboard to turn, it was frose up do to 30 years of salt water. So I can now stear with the outboard if the main rudder falles. thank you agin