Rudder Design

Good day to all of you. I have a 57ft wooden fishing trawler with LWL 0f 52ft under construction.t I have got a point that I need to design and build a rudder which conforms to best practices in the business. The chord length is estimated to be 2.6ft. I have got three designs in mind. The NACA-0012 foil, Thistle rudder, or combination of the two. That is adopting the trailing end of the Thistle to the NACA foil.
I have read that Dave Geer wrote articles on rules and formula/calculations to determine the size and shape of rudders. I have checked the archives of proboat.com but that information is no longer available. In my part of the world sometimes it is very difficult to get technical/engineering information.

I would appreciate if someone out there can PM me with information on rudder design by Dave Geer.

Thank you for reading this post.

Nabb

This is the second of three articles by Gerr on rudder design and has all the calculations. Good luck!

OK, apparently the attachment is a little big at 6.7 (PDF) when only 2.0 is allowed. I will try attach in a PM failing which I will email it if you give me an address.
Cheers

Much of what Geer proposes in his articles is doable however similar results can be achieved with the typical plate rudders if you add flow directing plates and wedges to the trailing edge of the rudder. It's a very common modification on many of the commercial fishing boats in the Pacific Northwest. Attached is a picture of our rudder mods. On a bigger rudder I would add a third flow directing plate and probably change the wedge design a bit.
Tom

Rudder design

That was a very neat modification to the rudder. Thanks for the picture, apparently people would not ascribe to this design if it does not improve steering. May I know the size of the propeller you've got on there and also the LWL of the boat.

Thank you.
Edington

The rudder on my own timber boat is similar to Krogenguys rudder.
I built it myself when I built the boat.
It has an overflow plate, an underflow plate and a fish tale wheic does help the steering especially when manoeuvring.
See a photo of rudder in my album .
Cheers
Benn

The prop was 28 by 19 3/4, the WL was 39' 11", Dspl was about 41000 w/ fuel and water. Suggest you visit any commercial yard that works on purse seiners or draggers, should find lots of rudders w/ wedges and flow plates.
Tom

My rudder swings a total of 90 degrees and I am not wanting more rudder performance nor do I have drag producing appendages in the propwash.

Krogenguy what is your rudder swing? And yes the to the fact that many large commercial boats have wedges on their rudders. It looks to me like the large size of commercial boat rudders are hung on even larger boats. To start a motion on a very heavy boat takes all the rudder force one can get I assume. I do wonder how much swing these commercial boat's rudders have? My 90 degree rudder works great but the boat's hull design may have a lot to do w it. On a different hull I may be wanting a specialized rudder also.

My guess was about 35 degrees. There was a big improvement in slow speed maneuvers and I did not detect any reduction in cruise speed performance although I do think there was an improvement in autopilot tracking performance. Don't know many boats with 90 degree rudder swing - must be an interesting linkage unless we're talking about total swing i.e. 45 each side of center.

Yes 90 degrees (total) and I experienced a big improvement when I went to 45 degrees (total 90). I think it was when I put in the hydraulic system. I just changed/adjusted the position of the slave cylinder and used the attach point to the rudder horn that was closer to the rudder post. Dumb luck it was in the perfect position.

As to the drag who knows how much but I'm think'in the rudder total drag probably more than doubles. The prop wash down there is pretty fierce I'm think'in. The prop wash is trying to push the rudder backwards and since the rudder is attached to the boat .....

Yes I can see how the auto-pilot would like it. Instant response as the wedged rudder is sorta deployed both ways when going straight ahead ... excluding the P factor element.

Rudder design

Tidahapah, Is there any chance you can post the picture of your rudder. I am seriously considering this profile. I have to design a rudder with a chord length of 2ft 6in (30 in) and a span of 40+ inches because I have got a prop of 40" x 27" for a 57 footer.
Would it be advisable to use double plate in that configuration since I do not want the round rudder stock to run outside down the height of the rudder blade. I think this might cause turbulence.

All comments welcomed.

Nabb

As I said it was in my album in my profile but here it is.

Tidahapah, thanks for the picture. Anyway by the time I was able to open your album (with other pictures) I had already posted a reply.