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Catbootsegler
04-24-2017, 02:31 AM
Dear all,
I am looking to modify my rudder design on my own catboat. Mainly to get the pressure at the tiller reduced. Now, before I enter such an issue I started to study all the available books on catboats and there designs. I found that the famous Mr. Fenwick Williams stated, that the length of the barndoor rudder should have a length of 1/7th to 1/6th of the total boat length. But he gave no reason why ? Probably only for the aesthetics? Or does it have technical reasons? I could not find any explanation of this century old design, why the rudders of catboats have to be or to look like a barndoor?
Two explanations I found by myself are:
1. The early designers already found out about the difficult steering behaviour of catboats and thought to ease the control of the boats by using larger the surface area of the rudder !-?
2. In order to get the center of lateral resistance as far as possible backwards without increasing the draft, the possibility is left on the rudder resulting in a version with a surface that is as far as possible stretched backwards.
But as I am not a boatbuilder these are my speculations.
The modifications that I am looking at are two ways:
- try to prebalance the rudder surface area, that would reduce pressure at the tiller.
- possibly shorten the rudder would enable me to incorporate a rudder profile, which would be very difficult to achieve on large rudder blades.
I would appreciate any input on this question.
Thank you in advance
With nautical regards
Peter Plate

Peerie Maa
04-24-2017, 05:32 AM
First off, Catboats have been around for so many years that if the rudder could be improved, it would have been by now.

Try thinking about a different solution, like a tiller rope.
http://www.colnesmack.co.uk/_Media/at-the-helm_med.jpeg

Works best with a rotating brass sleeve just behind the tillers hand grip, doubles the helmsman's power whilst allowing him to sit further out.

Thad
04-24-2017, 05:45 AM
Are you not talking about weather helm? The way to reduce weather helm is to reef, that's what the tiller is telling you.

Jim Ledger
04-24-2017, 05:50 AM
A catboats barn door rudder can be thought of in terms of an aircraft wing. Think of an old WW2 B-17...it had a huge wide wing providing tremendous lift at a low speed, that's your catboat rudder. You don't have to move the rudder very far to effect a lot of sideways force. The similarity extends to stall speed, a big wing can fly very slowly and with a lot of angle of attack before the wing loses its effectiveness and stalls. Stalling occurs in wings and rudders when the smooth flow of water or air over both upper and lower surfaces becomes impossible and the resulting turbulence destroys the effective lift of the surfaces. A deep, narrow rudder will stall long before a shallow, wide rudder, not on the rudder angle, but on the amount of force generated. In other words, to generate the same sideways force the deep narrow rudder has to assume a greater angle, getting much closer to its stall angle than the barn door.

Reynard38
04-24-2017, 06:02 AM
Good thread. Would a catboat barndoor rudder benefit from a foil section? The B17 certainly did.
A wide chord wing like the B17 was also quite thick. Maybe thicken up the section and use a NACA profile.
Would be an interesting experiment. As a new owner of a FW catboat I'm interested.

Catbootsegler
04-24-2017, 06:34 AM
A catboats barn door rudder can be thought of in terms of an aircraft wing. Think of an old WW2 B-17...it had a huge wide wing providing tremendous lift at a low speed, that's your catboat rudder. You don't have to move the rudder very far to effect a lot of sideways force. The similarity extends to stall speed, a big wing can fly very slowly and with a lot of angle of attack before the wing loses its effectiveness and stalls. Stalling occurs in wings and rudders when the smooth flow of water or air over both upper and lower surfaces becomes impossible and the resulting turbulence destroys the effective lift of the surfaces. A deep, narrow rudder will stall long before a shallow, wide rudder, not on the rudder angle, but on the amount of force generated. In other words, to generate the same sideways force the deep narrow rudder has to assume a greater angle, getting much closer to its stall angle than the barn door.

Thanks Jim for the explanation, so was the reason for the barndoor to achieve only a low rudder angle? Would be interested on how the rudder surface area would have to be calculated.
On my catboat for years standard barndoors had been attached. All their owners also had reported strong tiller pressure. I am having already a modified version with the barndoor only like 6 inch below waterline and supplemented with a narrow foldable blade that reaches deeper than the standard barndoor would. This was to allow me to move the center of lateral resistance to a certain degree. But again at wind of a good 3 bft. a friend of mine (70kg, sportive) cannot hold the tiller any longer, not to speak of my wife, with my 1,85m and 105 kg I can hold it but the force is - extreme! The rudder angle then is already quite big, IŽd say between 10 and 20°! That is a lot and we make then 4.5-5.3 knts. Of course reefing helps but at bft 3 to add the 1st reef? I think there should be a solution with the rudder design. thanks anyhow will keep watching out for solutions.

Peerie Maa
04-24-2017, 06:38 AM
Good thread. Would a catboat barndoor rudder benefit from a foil section? The B17 certainly did.
A wide chord wing like the B17 was also quite thick. Maybe thicken up the section and use a NACA profile.
Would be an interesting experiment. As a new owner of a FW catboat I'm interested.

A catboat rudder is not a wing, it is the flap behind the tail fin to follow the aircraft analogy through. So you need to research flap design, rather than wing theory.

Catbootsegler
04-24-2017, 06:42 AM
Hi Reynard,
thats something I would like to find out too. I believe that there would be a lot in using a NACA profile. I now of 2 examples where rudder profiles had been attached on catboats. 1. The Com-Pac Horizon, which is a modified Halsey Herreshoff-design had altered there original folding simple sheet metal rudder to a profiled "easy -on-helm" rudder. Another version I know of the German builder "Fricke & Dannhus".

Catbootsegler
04-24-2017, 06:46 AM
yes, but I differentiale weather helm in the two factors: weather helm and tiller force. At bft. 3 already extremely strong. Of course reefing helps , but at wind 3?

Ian McColgin
04-24-2017, 06:49 AM
Given the shape of the hull and skeg with the rudder fully hung, it's not a wing at all. More like a flap. So some taper, sure. No foil.

If you could go down, making an easier helm would be easy. But you can't and still have a catboat.

First, learn how a catboat sails. It's a big low angle sail which means that the real center of forward effort is to leeward of the boat's centerline. And it moves further out the farther off the wind you're sailing. Like mounting an outboard right amidships fore and aft and well outboard athwartships. Or rowing with one oar.

Most people over-trim their sails which induces a dynamic where she wants to weathercock up. Ease the sheet and ease crew weight back. Learn how to adjust luff, foot, and head tensions for the point of sailing and wind strength. You should get to the point where on a beat to a reach you can make small course changes just by moving about a little.

Williams' rule was suitable for his normal shapes, which are more or less on the Cape Cod Catboat model: B = 1/2L; D = 1/10L. These numbers are quite general, nothing magic.

There are three things you can do. All are in addition to learning the boat. Again, 99% of the tales of catboat weather helm and hard helm come from people who have not learned the type.

The first and universally best: Make a longer tiller. Leverage solves so much. A 2:1 line as Nick shows is restful for all boats.

Second: Some have had luck with end plates. These don't let you reduce the amount of rudder plane area but the do increase the bite meaning you don't need to pull so far.

Thirdly and most radical: Chop out some of the deadwood and move some blade area forward of the rudder post. This balanced rudder is much much easier to steer. A balanced rudder is very efficient as a freely hung spade rudder. Less efficient if making a slot at the aft end of the dead/wood as the shape induces turbulence. But the trade off might be worthy if designed by a genius. If you're a hydrodynamic genius, go for it.

Of the solutions, learning to sail is the most cost effective. Get a copy of "The Competitive Cat, Racing Small Gaff-Rigged Catboats" by Bill Welch. Despite the title, it's really about sailing these unique boats correctly and will introduce you to a wonderful world of making it look good while being actually easy.

G'luck

Reynard38
04-24-2017, 07:34 AM
Thanks Ian, makes sense. You too Peerie.

While on the subject of foil vs. no foil what about the center board? Do the Marshalls use a foil section on their boards?

Jim Ledger
04-24-2017, 08:13 AM
Thirdly and most radical: Chop out some of the deadwood and move some blade area forward of the rudder post. This balanced rudder is much much easier to steer. A balanced rudder is very efficient as a freely hung spade rudder. Less efficient if making a slot at the aft end of the dead/wood as the shape induces turbulence. But the trade off might be worthy if designed by a genius. If you're a hydrodynamic genius, go for it.


G'luck


Be advised before trying this that in many boats it will be the rudder post you'll need to carve away, not the deadwood. You don't want to compromise the rudder post as it's a major tie between the transom and keel.

Ian McColgin
04-24-2017, 08:19 AM
Jim's point is well taken. The engineering of this bright idea is almost as complex and risky as getting the hydrodynamics right.

Peerie Maa
04-24-2017, 08:24 AM
Jim's point is well taken. The engineering of this bright idea is almost as complex and risky as getting the hydrodynamics right.

You would probably get more benefit from end plates, which by controlling the tip vertices will increase the effective aspect ratio and area. Also so easy to undo if they do not yield enough improvement.

Reynard38
04-24-2017, 09:32 AM
What if instead of carving away the rudder post you instead moved the forward edge of the rudder aft? Extend the pivot point several inches aft of the transom and then the balance portion of the rudder does not need to cut into the deadwood.

End plates would be a quick and easy thing to try.

Agree with Ian on technique being a major part of curing weather helm. I sailed on a large catboat last summer and was surprised how little pressure it took. The owner knew what he was doing.

Gib Etheridge
04-24-2017, 09:32 AM
I'll probably be laughed at, but I love ideas. Here's mine.

One could create a balanced or nearly balanced rudder by building a frame aft of the transom that allowed pivoting the rudder at or just forward of its fore and aft length (center of effort?).

Gib Etheridge
04-24-2017, 09:46 AM
Thinking about it some more I picture an "A" frame supporting the upper pivot and a single frame extending the bottom of the keel to support the lower end.

The "A" frame could also act as a limiter to prevent the rudder from swinging too far, not that it would be all that inclined as a balanced assembly.

Reynard38
04-24-2017, 09:56 AM
This would also move the CE of the rudder aft adding to its effectiveness as well as allowing room for a balance.
Whether or not it would be enough to justify the added complexity would be a matter of opinion I suppose.

If the balance were notched around the upper frame to allow the balance portion of the rudder to extend up to the tiller the aesthetics wouldn't be affected too much.

So in chronological order;

1 Learn to sail a cat.
2. Add end plates if westher helm is still an issue
3. Move pivot points aft and add a balance if, like me, you enjoy building and experimenting!

Ian McColgin
04-24-2017, 09:58 AM
Balanced rudders are fun but they create huge shear strains on the rudder post. In practice one rarely sees a lead of even 15% for a sailing boat, less for a fast power boat.

Another bright idea that could take some strain off, again at some performance cost, is a trim tab. Set it opposite the angle of the rudder to let the pressure on the tab (back at the end, good leverage) push the rudder.

The more I look at these ideas, the more I realize that buying Dr. Welch's book is the most cost-effective and performance-effective solution. Nothing beats sailing right.

Thad
04-24-2017, 10:03 AM
Reef! And ease the sheet, perhaps.

"Speed and performance are acquired gradually as you sail and maintained only with concentration and effort. Conditions are always changing and opportunities are always presenting themselves. Sailing is the art of being conscious of these things and using them or letting them go as you please." -- Bernie Huddleston, Advanced Catboat Sailing for Beginners, Catboat Assoc. Bulletin #104

Reynard38
04-24-2017, 10:05 AM
Trim tabs work well on aircraft. On my Jurassic jet we have no hydraulic assist to the elevator or ailerons. We move the tabs via cables. The tabs move the control surface. And that's at 440 knots!

This would be another fun thing to try, and would not require any mods to the hull.

Gib Etheridge
04-24-2017, 10:06 AM
Seems to me that speed and performance would be enhanced by a balanced rudder. I really don't know, it just makes sense to me.

rbgarr
04-24-2017, 01:51 PM
IIRC, the cut of a catboat's sail along the gaff and mast should be loose enough to allow for three fingers to be inserted perpendicular to the spar without wrinkles showing in the panels, creating a full enough sail that it can't be strapped in tight and thus will work best when slacked off so the boom crosses the aft corner of the transom when going to weather. Is that what Welch's book suggests? (The book isn't available in our library system and I don't want to buy it.)

willin woodworks
04-25-2017, 06:49 AM
Frankly, Ian's response about learning to sail your boat is the best advice. Sail trim, weight trim and practice will benefit you more than fiddling around with chopping out deadwood.

When I was a kid at sailing camp on The Cape we regularly left the rudders at the dock and sailed Mercurys, Bay Birds, and Beetles around a triangle course using nothing but sail trim and weight trim. Taught us a lot about the mechanics of sailing q small boat and has done well over the years racing and crusing.

Welch's book is a good read and worth the time.

Extending the tiller would be my first technical modification. Leverage is a wonderful thing.

Ian McColgin
04-25-2017, 07:14 AM
The rule of thumb - or rule of three fingers - is a starting point but at least on the cats and other gaff rigs I've sailed has limitations.

For a Marconi sail, a nice rule of thumb is haul up till you have vertical creases along the luff. Start sailing and ease the halyard till the creases just disappear. A gaff rig's more complicated because peak halyard tension and throat halyard tension interact to result in changing peak outhaul tension and that can't be adjusted under way. And the higher the peak angle, the more radical this interaction becomes. On Marmalade, the head tension (along the gaff) is slack enough that you could make pleats. Normally hoisting is sway up both peak and throat keeping the gaff about level. Once the throat is up and reasonably tight we belay that and raise the peak. As the peak is raised the throat is pulled upwards and you get increased tension on the luff and along the head.

Getting this set correctly for the day is a challenge only mitigated somewhat by the wonderful rigidity of modern sail materials where there really is a standard luff and outhauls tension for almost all conditions. But playing the peak is the real art. Here the illustrations in Dr. Welch's book are quite wonderful for showing how to move the draft around in the sail for a maximum drive depending upon wind strength and point of sailing. Even a heavy boat like Marmalade can find an extra half knot or more easing the peak an inch or two (this through a 4:1 halyard) off the wind and trimming the peak a bit to weather.

And yes, almost all Cape Cod cats don't want the boom hauled in to over the transom. Most folk complaining about catboat weather helm do just that. The exercise of sailing without the rudder is most excellent and I am happy to say that kids I see in Knockabouts and Beetles sailing in Lewis Bay, Hyannis Port, Bass River . . . all of the places that do real sailing . . . still do just that. Can't teach these things quite so deeply in a Laser -- not to denigrate a really fun, hot, and competitive little boat in its own right. It's just a boat that teaches different lessons. The well rounded sailor should learn both.

Addendum: You can fix the weather helm and within limits you can reduce the heavy helm while sailing but physics is against you for bringing the total stress down to anything like the light touch you may have with a deep narrow blade. To make the boat turn or to hold it on a course in a good breeze even with little actual angle to the helm still means overcoming the water pressure at the back end of the rudder which may be three feet or more behind the rudder post. That's the price of shoal draft. Some of those cool Chesapeake boats and Florida shallows boats have nifty balanced rudders but they also have very different hulls and rigs and the balanced rudder is not readily adapted to the Cape Cod catboat fat hull and massive sail area.

Tom Lathrop
04-25-2017, 07:28 AM
Frankly, Ian's response about learning to sail your boat is the best advice. Sail trim, weight trim and practice will benefit you more than fiddling around with chopping out deadwood.

When I was a kid at sailing camp on The Cape we regularly left the rudders at the dock and sailed Mercurys, Bay Birds, and Beetles around a triangle course using nothing but sail trim and weight trim. Taught us a lot about the mechanics of sailing q small boat and has done well over the years racing and crusing.

Welch's book is a good read and worth the time.

Extending the tiller would be my first technical modification. Leverage is a wonderful thing.

This is absolutely the best teaching tool in learning what affects what and how to control it in a sailboat. Many catboats are so dependent on the rudder for balance that it is not possible to do this but any small boat with a split rig should be able to sail without a rudder.

Most traditional catboats sacrifice lots of sailing qualities to be what they are. Bolger was known to design in a CB or DB kind of foil into a shallow barn door rudder to get more control without all that stress on your bones. Sharpies, sprits'ls and other extreme shallow water craft take the opposite approach to a barn door by having a very shallow, very long and highly balanced rudder that has its own set of issues. Traditional catboats don't really give a hoot whether the rudder or CB is a foil or not because they don't usually have a level of performance efficiency to appreciate such go fast stuff.

Capt.Fred
11-23-2017, 09:43 PM
The barn door on a catboat seems to cause problems rather than steering solutions. An over sized rudder indicates a rig that has trouble sailing without undo effort. I have a 19 foot gaff cat with a normal boat rudder. She self steers to weather and like any sailing boat requires a bit of effort and diligence running. here's a photo and a couple short videos. https://flic.kr/p/HbcMAe https://flic.kr/p/ALWThQ https://www.youtube.com/watch?v=0G7auDMHoBY
Capt. Fred Saas

Ian McColgin
11-23-2017, 10:54 PM
Capt.Fred has a lovely little boat. She appears to have more hull shape in common with New Jersey catboats, a bit narrower and often a keel rather than a centerboard. This gives a shape and depth that a somewhat deeper and shorter rudder can work with. But then the under twenty footers that are Cape Cod porportions (B=1/2L) such as boats made by Arey’s Pond or Marshall have a not too large barn door. These are little boats, like Capt.Fred's, in the one to two ton range - that are not heavy helmed.

Especially not heavy helmed to weather, not unlike Capt.Fred's boat or any well trimmed well sailed catboat. That's why Bill Welch in "The Competetive Cat" is able to argue so compellingly for learning how to make course adjustments to weather by crew weight distribution.

I've set Marmalade (all 6tons+ of her with 620 square feet of sail) on a tight beat due compass east at Point Gammon and then not touched the helm all the way to the Stage Harbor bell, about 15 miles. There's nothing wrong with a barn door rudder if the sailor knows what they are doing. There's everything wrong with any rudder if they don't.

sailcanoefan
11-26-2017, 09:05 AM
I built my sailboat according to the plans, including rudder.
The rudder was that kind of rudder, more likely a ''round barndoor''.
This rudder was excellent and pleasant at tiller.

However I've change it for kick up rudder, last summer 2017. Why?
Since my boat has very little draft, I can anchor in 12 inches of water and walk to the beach.
The barndoor rudder was not the right type of rudder for beaching the boat cuz it was scratching bottom.
It was necessary to remove it and put it away for mooring time.
The kick up rudder was a good alternative for beaching my 19 ft sailboat.

See photos below.

I made the kick up rudder from a plan I found from Google image, I like it very much at first sight.

I downloaded the plans and started to draw plans to full size on paper and built the 4 pieces composing the rudder unit. Fiberglass and epoxy all surfaces.

I was very happy at first sail last July with this new rudder. I keep the barndoor rudder as spare part in case something bad happens to the kick up rudder.

Barndoor rudder
6493

Kick up rudder
6494

Ian McColgin
11-26-2017, 09:20 AM
I'd not have called sailcanoefan's original rudder a barn door. As we view it around here, the barn door rudder is not lower than the heel of the skeg. Anyway, the kick-up looks like a great solution though you may find that blasting across a shallow bar with the rudder kicked partly up puts a huge strain on the cheeks. I've broken such units before I learned how to be careful.

Some solve the kick-up problem of this sort of boat by using gudgeons on both rudder and boat with a long rod connecting so the whole unit can ride up. If the underhang is short enough, you can go with long pintals. Meg's rudder angles to maybe 6" lower than the worm shoe. The long captive pintal allows that much lift. You can about see the gap in the upper unit here.

https://scontent.fbed1-2.fna.fbcdn.net/v/t1.0-9/15541663_1487614574600866_1779895097953259469_n.jp g?oh=e1c19f939b3e0d38d3404f1682b26dbe&oe=5A98F510

Ian McColgin
11-26-2017, 09:24 AM
Here they are test fitting the lower P&G. I have run aground but in such soft mud that the rudder did not lift at all. It just dug in.

https://scontent.fbed1-2.fna.fbcdn.net/v/t1.0-9/15073555_10207857661974580_8494524996556177619_n.j pg?oh=296ee5e8e451681524882a2317df2c4a&oe=5A951C40

Hreoaj
11-26-2017, 10:11 AM
Mr. Gartside has foiled his centerboard and rudder it would appear
https://store.gartsideboats.com/collections/dinghies-and-daysailers/products/copy-of-12ft-cat-boat-c-f-a-desgin-186

sailcanoefan
11-26-2017, 10:26 AM
Here they are test fitting the lower P&G. I have run aground but in such soft mud that the rudder did not lift at all. It just dug in.

I see what you mean Ian. Looks like a good solution to your rudder.

My kick up rudder has an auto-release cleat witch lift up when rudder blade hits bottom, rock or any solid object.
When this happens, higher tension is applied to ''down string'' attached to the auto-release cleat, witch then is released from it's position by lifting up, then the rudder simply lift up, leaving very small scratches to paint on the blade.
This happened twice last season. experience was positive.

6495

ahp
11-26-2017, 11:10 AM
My experience with cat boats is very limited but I will put in my three cents worth anyway.

I once sailed a 15 foot Marshall cat in a strong breeze tiller was more than my 12 year old son could manage.

The strong weather helm can be reduced on a broad reach by raising the centerboard part way. That moves the center of lateral resistance aft, assuming you have a swinging centerboard and not a dagger board.

Professional watermen seemed to always go for simple. A balanced rudder would be better, but not so simple.

Ian, what do you think?

Canoeyawl
11-26-2017, 11:51 AM
Helm forces always change with conditions but keep in mind that the force vector at the mast is the big part of the deal. You can just sight down the boom to visualize the line of force trying to turn the boat. If you really want to "cure" that weather helm, move the mast aft, or add another mast further aft. (Kidding)
This is the norm for catboats, and it is just a compromise made for the convenience of shoal draft, ease of handling, stability and safety while keeping the gear out of the workspace on the boat. It is all part of the evolution of a workboat, and in my opinion the advantages of a catboat are worth the compromise. If you were working pots or gear it is a thing of joy when you can just let go the helm and your boat will snap into the wind and wait patiently for you to decide what to do next.
Wheel steering with a worm gear (mechanical advantage) is the old school cure, but it doesn't change the boat other than easing the work for the helmsman.

Ian McColgin
11-26-2017, 12:11 PM
Many boats that are not cats have heavy helm. Spend a few hours racing a Wianno Senior.

The rudder's force can be considered as a vector located from the rudder post but the force on the helm is essentially the location of the center of force on the rudder itself. Whatever X pounds of force vectored from the rudder post is your goal for turning the boat. If the center of force on the rudder blade is closer to the rudder stock, then that leverage will be more easily countered by the tiller (keeping it simple) than if the center of the same net force is further aft, further away from the rudder stock. You can get your required force from a narrow deep rudder or a broad shallow rudder but you need that turning force. If, as with catboats, you have a strong depth limit, you need a rudder that's bigger fore and aft and you will then require much more leverage on the tiller to control it.

So what to do? Besides suitably robust steering gear, it comes down to sail trim and boat trim. The former is not just in and out. Peak tension makes a spectacular difference and not just off the wind. Size matters as well, which is why being able to reef easily matters a lot - an unfortunately much neglected factor. Boat trim for the lighter cats will include crew placement and for all cats will involve centerboard depth.

Catboats are considerably more sophisticated and interesting to sail correctly than you'd think from just one sail. More than many rigs, you do not just sail a cat with the rudder.

FF
11-28-2017, 02:52 PM
It is possible the sail has changed shape. Usually draft moves aft when a sail gets older and that causes a strong weather helm as well. You could take pictures from boom to gaff when hard on the wind and show a sailmaker. Frank

ahp
11-28-2017, 03:47 PM
Catbootsegler, I don't like any boat with a strong weather helm. It is tiring, but there is another reason. It slows the boat down. If you are pulling the tiller to windward to keep the boat going in a straight line, you are also causing the rudder to push the stern to weather also. The rudder is a lifting plane, and not a very good one if a barn door rudder. Lift causes drag. Any aero engineer knows that.

SHClark
11-28-2017, 05:36 PM
There are two issues.
The balance of the boat.
The balance of the rudder.
They are related but very different.
The balance of the boat is what makes the boat want to head up or bear off. It is a function of the center of effort of the sails, the center of pressures of the centerboard and rudder and the symmetry of the heeled hull.
The balance of the rudder is a function of the length of the tiller forward of the axis of rotation ( or pintles) and the center of pressure of the immersed rudder blade. The cat boat Barn door rudder has a very wide shallow blade which requires great force to hold in position.
Catboats also have wide shallow hulls which tend to become very asymmetrical when heeled. That combined with a rig that moves the center of effort well to leeward when the sheets are started compounds the effect.
A deep narrow rudder can provide enough side force to keep the boat on course, and require less effort at the tiller because the distance between the pivot axis and the center of pressure is much less. It is heresy to suggest that a rudder could be as deep as the centerboard since the centerboard will be down when you are sailing, there is no percentage in having the rudder be a whole lot less deep. Unless you plan top slide over flats and sandbars with the board up.

One thought has always intrigued me is the thought of using the last few inches of a barn door rudder as a trim tab. This would operate like the trim tab on a self steering gear. It would turn the opposite way you want the rudder to turn and should provide enough force to keep the rudder itself at the required angle of attack to counter the weather helm of the catboat.
Putting a rudder on the back of the rudder is like what your front wheels do when backing up a trailer. You would end up with a second linkage on the tiller which drove the trim tab, but in theory the loads would be quite manageable and you could sail the boat without ever having to pull on the big stick.
Standing by to receive the opprobrium of wise men.
SHC

gilberj
11-28-2017, 06:19 PM
Phil Bolger was a fan of the cat rig (one sail/one mast) as being simpler, and posited that by lengthening out the boat, most of the handling issues could be eliminated.......

Ian is the resident expert here, though some others have experience in smaller cats. It is always more complicated to get the best out of a simple tool....takes more time and practice....there are hidden subtleties, to be learned.

I'd say the correct answer was one of the first responses. If the force of the weather helm is getting really difficult, it is probably time to reef. It seems a lot of people are not confident about reefing. They avoid being in situations where it might be advisable, and often carry too much sail too long. I'd say practice reefing till it is really second nature. Cats are not supposed to heal over more than a few degrees. They are built wide to stay on their feet......nearly rail down and you are way too far.

A very big rudder with a large lever needs a lot of force. I think most of the bigger cats had wheel steering for this reason....I doubt a drop down rudder would work well on a traditional Cape Cod style cat. The forces of the really long boom and large sail are considerable. A high aspect rudder ( like a drop-down rudder) has relatively little turning force, until you get to faster boat speeds. I have a drop down rudder on my Meadowlark, and am blessed because I have such fine control using sail balance. Were it not for the sail balance it would not work, as presently configured. Most designers consider a little weather helm to be important for most efficient sailing to windward. I think this is really only the case if the rudder and keel/CB/skeg lifting fin are close or connected. The total fin/rudder combination become a more efficient shape. A lot of sailors consider a bit of weather helm to be a safety valve of sorts, that if the boat is overwhelmed it will round up and feather itself.

Traditional sharpies of course has very shallow balanced rudders. The balance part, leading the rudder post removed some/most of the rudder feed-back force. It does not actually reduce the rudder force, just the tiller force. The same would be true of a rudder with a steering tab. The steering tab, steers the rudder, needs only a little force, while the rudder steers the boat. Again all the total forces will remain essentially the same and everything should be robust.

SHClark
11-29-2017, 05:39 PM
If you took the immersed area of a barn door rudder and rotated it 90 degrees (and made it into a proper airfoil) it would provide as much lift as the barn door rudder. Area is area, improving the aspect ratio will make the deeper rudder more efficient, which means that it will produce adequate force at a smaller angle of attack and with less drag. This is basic stuff. The consequences of having a deep rudder on you catboat may be unacceptable. Having a rudder that is no deeper than the skeg is really nice for operating in shallow water, and this probably trumps all other concerns, but from a straight hydrodynamic point of view, a deeper skinnier rudder would make the boat sail better and would be easier on your arms and back. The stylistic concerns are valid as well, a catboat without the rudder back there would look like something is missing, and not being able to stand on it would make furling the mainsail without a dinghy difficult.
SHC

gilberj
11-29-2017, 11:36 PM
Area and area are certainly similar...lift is lift.... it is all in the moments....force x distance....
High aspect foils have different characteristics than low aspect area foils. In this case the relatively slow speed performance is enhanced by the low aspect ratio foil. At higher speeds more of the load will be supported by the person at the helm, even if the actual rudder angle is nearly midship.....there can still be considerable load. A high aspect ratio foil provides little lift a low speeds. It may be operating in a stalled condition much of the time at slow speeds, particularly if there is much ambient load on it. I notice this with both my leeboards and with the rudder, and I have taken some pains to improve the slow speed performance with minimal increase in drag. At higher speeds the high aspect foil may out-perform the barn door, but always high speed is not most peoples reality with sailing.
This can really work when the whole design is considered.....as per MrGartside's dinghy. The cat rig is not nearly as extreme...the boom is shorter, the sail taller....etc. it is a cat boat but not on the Cape Cod pattern.
Ian had to rebuild his rudder a few years back....I'd guess that aside from it just not being acceptable on a real Cape Cod style cat a high aspect drop rudder on Marmalade would not work well at all....my opinion.....

Tom Robb
12-01-2017, 04:08 PM
Barn door rudders don't have any area below the hull. None I've seen anyway. Would an end plate help help lift? Does the area of that plate have a limit beyond which there's too much drag for the hoped for lift benefit? And does the shape of the plate matter much? Some passenger aircraft have rather small skinny ones but boats don't typically travel at high mock (SP?) numbers.
I thought I'd try one just to see how well it works and would rather not waste too much time or materials if the effort turns out to be futile. The boat is an 8' cat rigged pram.

Ian McColgin
12-01-2017, 04:32 PM
Areys Pond built an interesting shoal draft boat with what are really more like end wings, not just an end plate. Very elegant. It was in WoodenBoat some years back. So yes, some sorts of end treatment can be good. Many are boasted of with no actual scientific reason.