Rudder

[Alan Wheeler]

Sometime during our winter period, which will be mid year for us here in NZ, we will be lifting the boat for all the usualls. But one project I would like to do, is rebuild the rudder. It doesn't really have to be rebuilt, but the previuose owner went 18months without changing anodes, and so our rudder has a few holes in it. It is just a sheet of flat steel plate. Maybe some will remember, but I talked about this a long time back. I asked about the difference between the steel plate and a shaped rudder. I think it was Gord, mentioned about the rudder generating 10% of the lift. And having done a little reading myself, I think a shaped rudder would help in weather helm and better control and a greater steering range before stalling. Am I kinda on the right track here???
Anyways, my idea was to use the steel plate as my template and weld ribs to it to give some shape. Then form thinner steel plate around the forms to create a new rudder shape. It will most likely be heavey, but being hollow, I think the floatation will offset the dry weight.
OK, so here are my questions.
Is this the way to do it. OR!!!!
Do I need a particular pattern to follow.
How do I derive the diamensions required as far as the thickness at the leading edge and trailing edge and the contour.

Thanks guy's

[NoTies]

You're correct Alan. Power boat rudders rely on diverting prop thrust to steer ,which is not ideal for a sailboat. Hydrodynamically designed rudders will decrease effort & increase performance by acting in much the same way as a sail, generating lift to leward of the turn. (What'shisname) Henley wrote a very good article about this a few years back but I can't find the damn thing. A good outfit like M&G in Nelson should have calculations available. I would definitely recommend using good engineering support.

[ssullivan]

The shaped rudder should help out a great deal, as a flat one will act like a brake in the water.

Think of what happens when you are astern in a canoe steering by dragging a paddle. A flat rudder isn't a heck of a lot different than that. The shaped rudders do create a lift, as you said.

You're a scientific guy... are you going to calculate the weight of materials vs. displacement of the rudder to make sure you have something workable before you start out?

[GordMay]

You’re contemplating a highly technical design and construction.

Most sailboats use foil shapes based upon studies undertaken by the National Advisory Committee for Aeronautics (NACA). This shape is published in table form as NACA-00XX, where XX is the thickness expressed as a percentage of the chord.

*NACA - National Advisory Committee for Aeronautics

For a basic diagram of Foil Sections Goto:
http://cruisersforum.com/photopost//...php?photo=1616

Here’s some light reading on the subject of Foil Design, and Rudders:

1. How to Build Rudder Blades & Centerboards
and
2. How to Loft Airfoil Sections ~ By J.R. Watson (Gougeon Brothers)
http://mothboat.tripod.com/CMBA/Building/foils.htm

“The Totally Free Airfoil Primer” ~ by John Dreese
http://www.dreesecode.com/other/aflprimer.pdf

“Keel & Rudder Design - by David Vacanti (‘Professional Boatbuilder’ magazine)
Goto page 76 et al
http://www.proboat-digital.com/proboat/200506/

“Summary of Airfoil Data” ~ by Ira Abbott et al
http://naca.larc.nasa.gov/reports/19...report-824.pdf

World's Cheapest Foil Chart for NACA Section Profiles
http://www.sitexpress.com/slrmbc/foil_chart.htm

HTH,
Gord

[BC Mike]

Use the old rudder to determine area and balance. Make your new rudder from Kauri. Use a steel plate inside. The leading and trailing edges can be parallel, or the leading edge can slope aft. If you want more lift use a NACA 0015 type foil, less drag use 64-015 type foil. I have photos showing how to build a transom hung rudder. Can send direct if you like. Frank Bethwaites book High Performance Sailing will tell you about foils and recaptured drag, rudder angle ( no more than 10% ) and that the rudder does 10% of the lifting work. Other design books will tell you to make the balance ( the amount of rudder in front of the pivot point ) no more than 17%. The Nature of Boats by David Gerr has good info. That would be my first choice of books. The first mesurement you need to determine is the leading edge to trailing edge. All the foil calculations are taken from that. The thickness will tell you which foil number to use. If you can work out the basic size I can tell you how to draw the foil and bung it together.
Michael

[Alan Wheeler]

Thanks Guy's, Gord,once again, your blood is worth bottling mate. Mike, it's been awhile scince you were here in NZ eh. Scince then, the oldtimers cut down most of our Kauri trees and they are impossible to get. The result is that it would be cheaper for me to cast the rudder in solid gold than Kauri
Nah, just kidding, we import Kauri now believe it or not.

[BC Mike]

There are stacks of it from old floor boards near Onehunga. Rimu is too brittle and hard, Kahikatea would probably be okay. Good idea to use a hard wood on the leading and trailing edges. Cover the whole thing with three layers of cloth and resin. I think Tanekaha is hard and good old pine should work for the main section. There is not a lot of difference is stiffness between pine and the hardwoods at 65mm thick and the cloth makes it all quite stiff. Let me know if you need help drawing the foil.
Michael

[GordMay]

Alan lusts after my "links", not my precious bodily fluids.
Not long ago. most of what flowed through my veins WAS previously "bottled" - these days it's not worth bottling. My "favourites" bookmarks might be worth something, if anyone could organize them into a usefull compendium.

Would'nt a foam form (FRG covered), over Wheels' existing steel plate, make for easier shaping, than wood - and perhaps better ballance (as in rudder bouyancy, not for/aft post centreline)?

Gord

[BC Mike]

Balance is a function of the amount forward of the pivot point. Wood floats and is easily shaped with an electric hand planer. Shaping foam requires a special tool and is likely a lot more work, and definately will not be as easy to get a straight trailing edge. Wood is stronger than foam. There are many foam rudders, they are often heavier than word because they require more cloth and resin. Shaping a wood rudder takes about two hours, the least time consuming part of building a rudder.
Michael

Wheels, "Good Old Boat" Sept/Oct 2002 has a great detailed article on rudder design and construction. I will try to scan it and email to you. If that doesn't work, I can fax it, or, maybey someone local has a copy. Just let me know if you want the info.

[Alan Wheeler]

YES PLEASE!!!
Try scanning and Email if you can. I think my Email address is available in my profile. Let me know if it is not.
Fax is not so easy for me.
I don't mind you mailing it if you want, I have some time.

I will give it a shot. I have to send a package to NZ later this week anyway, so if all else fails, I will mail it to you.

[Boracay]

It is possible that your rudder was not built to the designer's plans.
If this is the case then correspondence with the designer could be beneficial.
If it was then they may have a good idea on how to improve it.

[southernman]

just a question someone might be able to help me with is anodes on a steel yacht - on average how often would you have to change them, and what material do people recommend for the anodes. Ours havn't been changed for 12 months but they are showing signs of wear now. Also on a 38ft yacht how many anodes should you have and where? Lots of questions that I keep getting different answers for but value your opinions.

Some other news - our cat plans arrived today for our new boat - ye ya - may the building begin. The Cad cut panels in containers arrive in a few weeks.

cheers

Southernman

[Alan Wheeler]

Well actually, being a Hartely Tahitian, I was wondering if either you or Greg may have had an idea or even a plan for a rudder for one of these. I wasn't sure if you had built a Hartley or not, but I do know Greg did.
I know it was comon to build steel plate rudders, but I imagine Richard Hartley would have also had a proper design available.
I don't want to have to buy the compleate boat drawings set just to get the design for the rudder.