rudder profile and carbon stock

[andrewfennell]

Hi,

I have been looking through the archives for info on rudder and stock design (for balanced spades). So far I haven't found what I'm looking for.

On my 43' trimaran (weight 4.5t) the new rudder needs to be 0.5mx1.5m deep.

This will double the loads on the stock (old rudder same chord but only 1m deep).

The old stock is SS OD=53mm ID=29mm on a rudder with max thickness 13% of chord (chord is 520mm)

To double its strength I would need to increase the thickness of the foil section. What is the generally accepted max foil thickness (as a percentage of chord) for a performance boat with a similar aspect ratio rudder design?

Many thanks for your thoughts.
andyf

[andrewfennell]

My apologies for having a subject which doesn't fit the content. Subject should have read 'Stock diameter vs rudder foil chord size'....

[Eric Sponberg]

Andrew,

I am an advocate of relatively thick rudders, for precisely the problem you point out. About the smallest I go is a 15% foil section. I have just finished a new rudder for a boat out west that has a 20% foil section, with a carbon fiber skin and carbon fiber rudder stock.

In multihull design, one is generally caught between a rock and a hard place because you'd like to go thin for low profile drag, but the loads can be really high, even as small angles of attack, because of the high boat speed, necessitating the need for a thick rudder so that a larger stock will fit inside the blade.

My opinion is that for the same planform area and depth of rudder, the added profile drag of a fat section over a thin section is minimal. And there are two added advantages of the fat section. 1) More modern foil shapes have a pronounced "drag bucket", or a region within a few degrees of zero angle of attack, where the profile drag is exceptionally low. A fat section has a minutely greater drag coefficient, but also a much wider drag bucket than a thin section, on the order of twice as wide. This means that at small angles of attack, a fat rudder can actually have less profile drag than a thin rudder at small angles of attack. 2) A thin foil section will stall at a much lower angle of attack than a thick section. Rudders operate at high angles of attack frequently, and you don't want them to stall. So a fat section is better, in my opinion.

In C.A. Marchaj's book, "Aero-Hydrodynamics of Sailing", he discusses the virtues of streamlining, and points out that a foil section even up to 25% thickness still has beneficial effects for the reasons I have just described.

So, if you have to increase rudder thickness to gain the strength in the stock that you need to support the load, don't be afraid of going fat, up to 20-25% if need be. It won't hurt you.

Another thought: don't lose too much sleep over which foil section to use. Pick one with the maximum thickness position at the 40% chord position. This includes the 63-series and 64-series of foil sections, as well as the LS(1) and GA(W) sections. I personally favor a modified GA(W) section.

I hope that helps.

Eric

[andrewfennell]

Eric,
many thanks for your prompt reply. Weirdly enough I have just been trying to get my head round an article you wrote for Pro BoatBuilder on Keels and Rudders (recently forwarded to me by a friend). So to then have you answer my post was a pleasant surprise!

May I wish you and the other Boatdesign.netters a happy new year!

andyf