rudders, straightedged or curved

Thorvald

Member
405
0
Puget Sound
I just popped the rudder off my old Garden yawl last night and I want to improve it. Much as I appreciate the last guy that rebuilt it, as it kept the boat going for another thirty years or so, it wasn't very sleek. it had a trailing edge about 1 3/4 wide and square. I improved it a bit but it still has a profile like an elephants ear. A couple of my old yacht design books that date back to the CCA era mention that a straight trailing edge may be a better, more efficient shape. I'm not feeling bound by tradition here, just want to improve where I can. My only concern is that I will change the balance of the boat. The book I have says that the straight egde may result in moving the center of lateral resistance aft. This is not what I would want to do. If anything I would move it forward to have a bit more weather helm, not less. I really want to improve this rudder as it has always had a vague, heavy feel to it, and I had just come from a T-bird which of course had a lovely light helm with plenty of good feedback when I bought this boat, so if I could improve the helm on the yawl it would be a wonderful thing. Rod Stephens said that the rudder should be free enough that when you stepped aboard the tiller would sway back and forth a bit . I forget the exact statement but it was something like that. I may be able to help that with a zerk fitting on the heel casting and another on the tube through the horn timber so I can get some grease in them once in a while but I'm really keen to improve the rudder shape as well. A nice helm is very important. No es cierto? Thoughts?

 

Bob Perry

Super Anarchist
31,915
1,228
Thorvald:

A quick look thru any current sailing magazine will shpow you modern p[rofile rudders that generally work very well.

Old rudders like yours have a bottom end that tapers the chord down to pretty much nothing. This may be good for reducing drag but it also reduces lift so you "elephant's ear" rudder can't turn the boat efficiently.

It's a pretty shape though so if your boat has no handling problems at this time I'd leave it as is. If you find the boat difficult to steer then a change in planform should be tried.

I don't think a change in the rudder will effect the balance of the boat. It will change the feel of the tiller though. If done properly you should need less rudder angle to achieve the same balance. That would be ideal.

Why don't you consider loading up your wagon with the plans and making the trip up to my beach shack. I'll make tea and we can stare at your drawings with knowing looks on our faces and nod a lot.

Maybe we can pencil in a new profile and discuss trailing edges. Have you read my GOB article on rudders? That might help you.

 

Ajax

Super Anarchist
14,605
2,893
Edgewater, MD
When you say "elephant ear", do you mean something like this?

/monthly_10_2011/post-42428-067699800%201317911992_thumb.jpg

 

Attachments

  • P30rudder.jpg
    P30rudder.jpg
    67 KB · Views: 4

sailak

Super Anarchist
2,868
46
AK
Not the same as your boat, but Garden changed the rudder design on the 1950's Rawsons (as well as rig) as part of an update for the late 1970's. The improved design was noted to have less weather helm. I know owners with the classic curved have reported improvements using the more modern section, but nothing quantitative as far as I know. There has be discussion at length on the subject at http://groups.yahoo.com/group/rawsonownersnet/

New:

rawson-30-brochure-1.jpg


Old:

http://www.aleutianexpress.com/sitebuildercontent/sitebuilderpictures/.pond/Rawson_30_ITA_10-95_new_Awl-Grip__Copperpoxy_bottom.jpg.w180h271.jpg[/img

 

Bob Perry

Super Anarchist
31,915
1,228
Slow:

I am pretty certain that the Rawson 30 as originally designed had unacceptable weather helm. The addition of the bowsprit is the obvious tip off.

Increasing the planform of the rudder over the original "elephant ear" (I like that term) type would have resulted in more lift and less rudder angle required to balance the boat.

Ajax:

Now I know you are not reading my GOB articles!

The rudder you show is a C&C "scimitar" rudder shape. Not sure the boat in the pic is a C&C (looks like it could be a Pearson) but the rudder is a clone of the C&C shape. This shape was used by C&C for years. I asked Rob Ball about it once. The idea behind that profile was to reduce the chord at the root, i.e. where the rudder blade meets the hull, so that interference drag would be reduced. Unfortunately the gains made in reducing the interference drag were offset by the loss of lift. Remember, no drag=no lift. Today designers take the top of the rudder right up to the hull so that flow from the high pressure side of the rudder cannot escape over the top of the rudder, thus reducing lift. The hull acts like an end plate helping to keep the flow attached to the rudder blade.

But, when you look at those old C&C boats you have to admit that shape was sexy. I have replaced rudders like that on two C&C 39's with more conventional shaped rudders with very good results.

It's fun to go back and look at design features that were in vogue and read about why the designers thought they worked. Remember "eliptical loading". That resulted in some interesting keel shapes.

 
Last edited by a moderator:

Poda

Anarchist
631
0
Toronto
Slow:

I am pretty certain that the Rawson 30 as originally designed had unacceptable weather helm. The addition of the bowsprit is the obvious tip off.

Increasing the planform of the rudder over the original "elephant ear" (I like that term) type would have resulted in more lift and less rudder angle required to balance the boat.

Ajax:

Now I know you are not reading my GOB articles!

The rudder you show is a C&C "scimitar" rudder shape. Not sure the boat in the pic is a C&C (looks like it could be a Pearson) but the rudder is a clone of the C&C shape. This shape was used by C&C for years. I asked Rob Ball about it once. The idea behind that profile was to reduce the chord at the root, i.e. where the rudder blade meets the hull, so that interference drag would be reduced. Unfortunately the gains made in reducing the interference drag were offset by the loss of lift. Remember, no drag=no lift. Today designers take the top of the rudder right up to the hull so that flow from the high pressure side of the rudder cannot escape over the top of the rudder, thus reducing lift. The hull acts like an end plate helping to keep the flow attached to the rudder blade.

But, when you look at those old C&C boats you have to admit that shape was sexy. I have replaced rudders like that on two C&C 39's with more conventional shaped rudders with very good results.

It's fun to go back and look at design features that were in vogue and read about why the designers thought they worked. Remember "eliptical loading". That resulted in some interesting keel shapes.
Bob - just out of curiosity, is there any reason why you didn't cover rudders in your "According To Perry" sections in Yacht Design According to Perry? You seem to cover pretty much everything else and seem to have quite the opinion on the topic ;) Any links to the GOB article so I can print it off and glue it to the back of the book as my own "According to Perry: Rudders" addendum? :lol:

 

Bob Perry

Super Anarchist
31,915
1,228
Pods:

Not sure why I didn't do rudders in my book. We did run into a space problem with the book and that may have been why. Maybe.

I'm sure if you go to the GOB site you could find the article. It was a good one because I involved a team of other designers to chime in with their thoughts on rudders.

I'll do rudders in a blog.

I don't think I have an "opinion" on rudders. Maybe I do. They are really not complex. There are some trade offs to be made with stock size and rudder blade thickness that can be controlled by the materials used and the actual size of the rudder is a bit of a guess. I base my rudder sizes on my previous work and what I see working today. We have gone thru a lot of whacky rudder shapes over the years but if you look at top performing boats today rudder shapes are pretty consistant. My opinion on rudders for cruising boats is: make them big and make them on the fat side so they are forgiving and have plenty of room inside the blade for a stout stock.

The interesting thing about the GOB article on rudders is that the team of designers contributing to the article all agreed on what makes a good rudder. I was actually hoping for some controversy but I didn't get it.

 

Hike Bitches!

Super Anarchist
7,362
156
Solomons, MD
Ajax, the elephant ear is more like that Rawson...attached its full length, except for maybe a prop in the middle & it flops around like an elephant ear. You need to swing by the Boat Show and stop at the Good Old Boat tent and get your happy-ass a subscription...STAT. The magazine is not super cheap, but it really is a magazine geared towards old 4KSB's like ours and not the multi-million dollar carbon shit floating around on the pages of SW. I've been really tickled by Bob's articles over the last several issues..just geeky enough for some good reading, and if I need help falling asleep, I read the hull shapes one again and again and again, since I can never wrap my head around that subject..especially when he starts talking about the contorted IOR shapes & measuring stations. :D :unsure:

The deal at the boat show usually is if you renew/get a subscription, they give you a full year of back issues on CD for free. I started subscribing in early 2009 I think, so I've been filling my 'library' with CDs from years prior to that. :D

 
Last edited by a moderator:

Bob Perry

Super Anarchist
31,915
1,228
Brane:

May I please use that pic in my blog on rudder shapes. It's a good example of the "elephant's ear".

 

Bob Perry

Super Anarchist
31,915
1,228
Brane:

Ok. It is a good example of a really outdated rudder design and considering the age of the boat it makes perfect sense.

 

SemiSalt

Super Anarchist
7,752
269
WLIS
Not sure the boat in the pic is a C&C (looks like it could be a Pearson) but the rudder is a clone of the C&C shape.
Pearson 30. Very familiar to us East Coast types. Some other Pearson's were similar.

A couple years ago, I read an story on the web by a guy who swapped one of the C&C scimitar rudders for a more conventional spade. The boat may have been a Redwing. Just looking at it, the new rudder looked heaps better.

 

Bob Perry

Super Anarchist
31,915
1,228
Semi:

Looking at that Pearson rudder you can easily see that the top 30% of that blade is not going to do much work. The foil will just get fatter and fatter in thickness ratio as the chord is diminished. There will be a high increase in drag, at all rudder angles, without much benefit in lift. Today, to my eye, that rudder looks really bizarre. However, those Pearson 30's were nice boats and in the PNW effective on the race course for a while.

 
that elephant ear rudder sure looks salty though.

I gotta read your book, a lot of the design of sailboats Ive been able to figure out with my engineering backround, but I have no idea about some of this stuff. by 'lift', you mean lifting the boat in general or just the rudder?

 

Bob Perry

Super Anarchist
31,915
1,228
Brane:

The way I look at it and I think most designers agree, the rudder is a foil with a high pressure side and a low pressure side when you apply an angle of attack. Like a keel. That's why when you apply too much rudder you can stall the rudder because the flow is no longer attached to the lifting side. I think I covered it pretty well with the help of some of My fellow designers in the GOB article. Rudders have to be efficient over a much broader range of attack angles so they don't use the same foils that you might choose for a keel. They typical rudder foil will have max thickness at 30% of chord while keels usually have their mnax thickness around 40% of chord. The fatter leading edge delays stalling longer.

But some of those old rudders just flowed with the origanic lines of those old wineglass section boats. They were pretty if not efficient. I guess most of them were efficient enough.

 
Top