rudder fairing

That type of rudder should have more bite. But, more damage could result in a grounding.

A couple of thoughts about building a new rudder:

1) Do the boat design books make any mention about the proper weight of the rudder? Any consequence if the new rudder is more bouyant? The original wooden rudder is mighty heavy, especially soaked with water.

2) It seems quite difficult to fiberglass a rudder watertight. Even the manufacturers can't seem to build fiberglass rudders that don't eventually suffer water saturation, swell and split.

There is an article by a surveyor towards the bottom of this link saying he sees waterlogged foam rudders everyday.

http://old.cruisingworld.com/plastic1.htm

The old woodie rudder is OK by me. The rudder and keel keeps the boat tracking even heeled way over. She never rounds up on you.

I have to laugh when I sail on boats that are out of control at 25 degrees heel.

two cents worth of glass dust.
Read about and seen some rudders that come into the yard that are what I call Foss style rudders.
They are hollow fiberglass box-blades into which a stainless steel tube with a modicum of flat and usually short crosspieces welded to the tube
is suspended in the space. Then urethane foam is poured in and expanded.

I believe these rudders are made with polyester, but could be wrong.
If the blade has an episode that breaks the skin or loosens it on its shaft, water is going to get in.
Urethane foam at best is 85% closed cell. If it gets crushed it becomes a sponge.
This is not same same 'closed cell' terminology used to describe some foam rubbers. Or pvc closed-cell.

Technically not even epoxy is waterproof. There is a lot of water action and pressure on a modern free standing rudder.


I believe the ideal is to have a neutral buoyancy rudder.
So I've read. That is, the rudder should neither float nor sink when put in the water.
Good luck.


Don't know that a neutal buoyancy rudder could be deasigned for the barn-door type we have.
Get close with epoxy, pvc foam and tricky new fiber.
I'm pretty sure that the bronze and honduras mahogany original is close to perfect as it gets for our boats.
But because of the construction there is little we can do about hydrodynamically fairing a foil onto the sides of the blade.
Also its 1/2 moon shape makes it imco difficult to lay out a true foil.
The tri-angular Constellation shape would also be difficult to lay out, as the length of the foil to the trailing edge constantly changes from top to bottom.

Some kind of wing shape can be worked into a glass and foam or glass and plywood rudder.

I was wondering if a shaped blade would make the Commander or Ariel sail a little better.
Flat blades tend to 'stall'. Wooden know nuttin bout that!

Rudders and fairings, just the topics we were messing around with today as we began building the S-Boat rudder, which is both faired and foiled.

The foil shape of the S-Boat rudder allows the horizontal curve of the keel to continue to the back side of the rudder. The front edge of the rudder is 2.25" thick, the same thickness as the back side of the faired stern-post and is reduced to about 3/4" on the trailing edge. Without the stern-post fairing there would be an enormous gap between the rudder's radiused forward edge and the keel. The Ariel with it's thin flat rudder and correspondingly narrow stern-post doesn't really need to be faired in my opinion. The gap is really pretty small, about the same as an S-Boat with all it's delicate fairing. I bet the Ariel's narrow rudder runs quite slippery thru the water. I also believe that the rudder's thinness would minimizes the effectiveness of a foil shape.

Thanks Ebb, you got me thinking.

Thanks for keeping this bit of esoterica going.

It seems a perfect logic to whale's tail the rudder off the back of the keel post.
And that would mean, as you say, that the front half of the rudder foil is the boat itself.
Right! The hull is the leading edge.
That leaves the long skinny part of the NACA foil to translate to the blade.

And that I think is going to be a lot of fun
I know you're making a new S-boat rudder, lucky you, what a gas that'll be!

For litlgull there will be a rendition out of bronze rod and welded plate. [nothing neutral about that weight!]
The stuffing will be mid-weight green closed-cell pvc foam
with a skin of glass and epoxy.

The keel 'post' where the rudder is mounted is approximately 2 1/4" wide.
The boat came with a pretty awful homebuilt that was about 1" thick.
This is what I ended up with:

Used a NACA 0012 section [because it looked right]
and worked up the percentages from the nose to the maximum chord depth and to the tip end.
It's at 30% distance from the imaginary nose of the blade/wing that the chords of this Ariel rudder will be generated. The blade begins at two inches wide.
At this point the 0012 foil becomes streamlined and slender. Probably terminating at about 1/4" width. Slippery might also describe it!
Use doorskin to make a set of ten concave foil sections to check the rudder's final shape. But also get a preview of what the blade is going to look like. The 'triangular' rudder that is foiled from top to bottom and
rudder post to trailing edge is going to be a trip

The rudder dimensions is a scaled up copy of the dotted line Constellation found on Alberg's lines on a drawing in the Association Manual.
He has the lower corner well rounded. I've been wondering if that should be minimized.
Modern marine engineering likes things squared off.
I've also tilted UP a bit this substantial extension of the keel, the bottom of the rudder.
Looking forward to grounding and such.
The original halfmoon rudder must be nearly immune from grounding damage. [or getting immobilized by mud - not a good thing!]

Don't know about your S-boats, but I've toyed with the idea of fairing the keel to the rudder with strips of bronze or plastic. In other words attach long strips to the end of the keelpost that would close off the eddy making space between said post and the rudder shaft.. Too much work - but if you are racing..........


Wood is possibly the best material for a rudder if planning on surviving a nasty grounding.
I just know that glass and foam will get totally wasted, while a wood rudder will probably still be useful if still attached, and mostly fixable as well.
Riffsawn white oak and bronze...sheeeeesch!

Anyone interested in FIN-FOILING their rudders here are a couple sites.
Turns out that the 0012 foil is commonly used for our purpose.
I did not know/remember that all NACA foils are thickest at 30% back from the leading edge, so any section can be used, but seems boat workers unanimously choose the 0012.

google> Foil FAQ
www.boat-links.com/foilfaq.html
Good practical intro to the mystery. Includes the NACA foil chart.

google> Part 1 - Duckworks Magazine
http://www.duckworksmagazine.com/06/...oils/index.htm
Here a topographical construction approach is detailed with zoom photos... where plywood (probably meranti) is stacked and glued.
Then the ledges are filled with epoxy/silica gel to produce non-stepped curves.
Seems to me this is a very accurate, strong, easy method of getting the foil correct on both sides.
IE if you have stacked correctly. NO GRINDING.
Excellent for anybody contemplating a glass coated plywood rudder. The process can certainly be adapted to build an A/C rudder.

You will discover you are reading Warren Messer articles on Rudder and Daggerboard Foils.
You need to be computer litterate to be able to program an A/C rudder.
See the preceeding paper: 'Designing NACA 0000 Foil Sections' for his topographic method of generating veneers /layers of plywood into a glue-up that will generate a foil.
Beyond me, but it is possible for a fool to make a set of concave 'try' patterns using the basic percent grid numbers available
when shaping the 70% keel-hung foil rudder by hand and the front of the brain.

[ Triton wood rudders are originally made only with the top bent part of the ruddershaft: the part that goes from the tiller head - through the tube - then curves around the top of the propeller aperture. Below that the rudder is all wood with no metal shaft. There are however two sets of cast bronze gudgeon/pintels that do the hinge work. There is no heel fitting.
A version of this in glass coated Meranti BS1088 could be made for the A/C. No heavy 1" rod for a lower piece of shalt.
The bottom of the rudder would need a custom fitting to marry with the existing heel casting - and the single strap gudgeon.]

Believe that the NACA foil could be applied to both the original 'D' profile rudder and the later version that is found behind the as-drawn rudder in the A/C Manual (Pg 144) and on all Alberg's later Cape Dory designs.
Imco it's not feasible to foil shape the original two/three board rudder as built.
In any other remake the foil ought to be seriously considered.

Ok,

I KNOW I have mentioned this elsewhere, but I read this thread and see that I have not shared it HERE where it may matter most.

When I was delivering 'Ariel Spirit' (A-3) years ago, I learned that there was an aspect of rudder design one will overlook at their own peril.

She is a fine little ship, and my first mate and I greatly enjoyed sailing her. I had done a pre-purchase look see for her new owners and found all kinds of great work Keith had done.

One of the modifications he had made was a rebuilt rudder that was very well made, but tapered to a much finer edge then I have seen on any other A/C.

This is not a good picture, I was trying to document the great folding ladder... not really much to see of the rudder here.
Attachment 7038


Now, I did not notice any weather helm issues (the rudder carries a little more surface aft then OEM). What I did notice was a tendency for the tiller to vibrate at speeds over about 4 knots. It was not awful, and probably would not make anyone unhappy with the design... but it was a distraction (to me).

I believe the cause was not the shape, but the fine trailing edge. It seems to me that the vibration was the result of the eddies formed as the water crossed the fine trailing edge.

Just 2 more cents.

Hey Craig,
I was futzing around with page one posts while you were talking over here.
You know I really listen to what you say!
I'd have to see that rudder.

This vibration issue is a valid point.
I've read others that also complain about this phenomena.
Not only the Ariel/Commander (can't remember) but other boats with slender or badly proportioned rudders.
Craig speaks from experience.


YET I wonder if doubling the thickness of the rudder at its leading edge (at the keelpost) as proposed would thicken the NACA foil scantlings enough so that no vibration could happen.
Because I think water flow should be fair off the hull to the rudder surface.
And the rudder surface flow should reflect the flow from the hull.
If there is a jump in water flow, perhaps vortexes are produced that cause the vibrating.

Of course there still may be water disruption occuring if we increase the rudder thickness and not the circumference of the rudder shaft.* An all-round thicker rudder cannot turn in the existing bearing of the heel fitting. The thicker rudder has to be pared back at the keelpost to allow it to turn and that creates a disruption in smooth flow.
But even so, I wager that a thicker leading edge, twice the thickness of the original, with a NACA foil shape the flow takes to the trailing edge, you'd have a blade that would not make a fuss at hull speed.
I feel that the NACA airfoil formula should be used for shaping an upgraded rudder for our boats. Don't think the blade can be carved by eye.

[The NACA partial foil coming off the back of our keel actually stays thicker longer than the original 1" thick flat wood blade as it goes toward its trailing edge. The blade could be shaped to a less thin end. But compared to the blunt edged flat sided blade, imco the foil stays in effect thicker in the center of the blade, until, of course, it starts to trail. There are thinner and fatter NACA foil sections, maybe there's one more appropriate for the A/C. Imco if we start the blade foil at, say, 2" at the keel post, the blade will be too substantial in section to vibrate even a t a fine trailing edge. The flow of water along the hull should continue along the rudder blade - and trail off the blade - without eddies or vortex. There could a problem with the keelpost-rudder seam
- but at 6 knots is this a real concern? In troubled water and going downwind, probably.
Also, how well does the rudder shaft seat in it's shoe?
Is The gudgeon strap in place and snug?
Are there new O rings in the sleeve bearing at the tiller head?
Is the tiller arm sloppy at the hingebolt of the tiller head?
These might not be causes but contributors to vibration.]
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*I've pointed out this before too.

When you look at the original Alberg drawing (assume it is) Page 144 in the Manual.
you notice he has a drawn in a nearly two inch diameter rudder shaft on top of the heel fitting.
It had to be Pearson that decided to go with the skinny Tritonesque rudder for the A/C. But the Triton keelpost is very thin, matching the width of its thin wood plank rudder.

And I bet the supplier Pearson used was already set up to make these very traditional wood/bronze rudders at a lower cost than Pearson could do it. Pearson should have had the cojones to invent the first frp rudders to match the frp boats they were making.

Does anybody know where the first fiberglass rudders began appearing? What they are like, how they are lasting?
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Way later EDIT: If we are foiling the surface of the A/C rudder, it appears that the original ear shaped rudder will have to be fudged quite a bit.
Good fudge needs a good eye.
Foils are gererally used on roughly rectanglar shapes: wings, keels and rudders. So there has to be a considerable fudge needed to trail the edge off the rounded A/C blade. Don't think flat sides & mere rounding makes the best rudder. Every A/C came that way from Pearson.
The trucated triangular shape of the Constellation style seems easier to do - but keeping to a mathematical formula would be nearly impossible. Even the straight trailing edge rudder will have grossly fudged foils toward the top of the rudder. Foils are about flow over the surface and leading air or water cleanly off two surfaces so that it doesn't cavitate - or stall - and cause the boat to loose control. Especially in fast water- or straight down-wind. Assume the trailing edge is a single flat (not rounded) dimension along its whole length (1/4" wide, blunt and sharp)
Problem may be unsolvable for the original rudder shape - and why, during the 5-6yr production run of Ariel/Commanders, no foiling was done (so far as I know.)
......However, if we are making NACA 0012 formula foil templates for the triangular rudder (on the height of the rudder, 6" wide stations - remember to lay out templates horizontal with the waterline, not to the angle of our keel) you can find practical help by looking up foiling (keels and rudders) on google. imco